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Posted to cvs@httpd.apache.org by sl...@apache.org on 2002/09/22 22:04:12 UTC
cvs commit: httpd-2.0/docs/manual/misc index.html.en perf-tuning.html.en rewriteguide.html.en index.xml perf-tuning.xml rewriteguide.xml index.html perf-tuning.html rewriteguide.html
slive 2002/09/22 13:04:12
Added: docs/manual/misc index.html.en perf-tuning.html.en
rewriteguide.html.en index.xml perf-tuning.xml
rewriteguide.xml
Removed: docs/manual/misc index.html perf-tuning.html
rewriteguide.html
Log:
Convert rewriteguide, perf-tuning and misc/index to xml.
Submitted by: Tim Gerundt <ti...@gerundt.de>
Revision Changes Path
1.1 httpd-2.0/docs/manual/misc/index.html.en
Index: index.html.en
===================================================================
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
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--><title>Apache Miscellaneous Documentation - Apache HTTP Server</title><link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /><link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /><link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link href="../images/favicon.ico" rel="shortcut icon" /></head><body id="manual-page"><div id="page-header"><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p><p class="apache">Apache HTTP Server Version 2.0</p><img alt="" src="../images/feather.gif" /></div><div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div><div id="path"><a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.0</a></div><div id="page-content"><div id="preamble"><h1>Apache Miscellaneous Documentation</h1>
<p>Below is a list of additional documentation pages that apply
to the Apache web server development project.</p>
<dl>
<dt><a href="custom_errordocs.html">How to use XSSI and
Negotiation for custom ErrorDocuments</a></dt>
<dd>
<p>Describes a solution which uses XSSI and negotiation to
custom-tailor the Apache ErrorDocuments to taste, adding the
advantage of returning internationalized versions of the
error messages depending on the client's language
preferences.</p>
</dd>
<dt><a href="descriptors.html">File Descriptor use in
Apache</a></dt>
<dd>
<p>Describes how Apache uses file descriptors and talks
about various limits imposed on the number of descriptors
available by various operating systems.</p>
</dd>
<dt><a href="fin_wait_2.html"><code>FIN_WAIT_2</code></a></dt>
<dd>
<p>A description of the causes of Apache processes going
into the <code>FIN_WAIT_2</code> state, and what you can do
about it.</p>
</dd>
<dt><a href="known_client_problems.html">Known Client
Problems</a></dt>
<dd>
<p>A list of problems in HTTP clients which can be mitigated
by Apache.</p>
</dd>
<dt><a href="perf-tuning.html">Performance Notes - Apache
Tuning</a></dt>
<dd>
<p>Notes about how to (run-time and compile-time) configure
Apache for highest performance. Notes explaining why Apache
does some things, and why it doesn't do other things (which
make it slower/faster).</p>
<div class="warning"><strong>Warning:</strong>
This document has not been fully updated
to take into account changes made in the 2.0 version of the
Apache HTTP Server. Some of the information may still be
relevant, but please use it with care.</div>
</dd>
<dt><a href="security_tips.html">Security Tips</a></dt>
<dd>
<p>Some "do"s - and "don't"s - for keeping your Apache web
site secure.</p>
</dd>
</dl>
</div></div><div id="footer"><p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div></body></html>
1.1 httpd-2.0/docs/manual/misc/perf-tuning.html.en
Index: perf-tuning.html.en
===================================================================
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
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This file is generated from xml source: DO NOT EDIT
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--><title>Apache Performance Notes - Apache HTTP Server</title><link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /><link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /><link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link href="../images/favicon.ico" rel="shortcut icon" /></head><body id="manual-page"><div id="page-header"><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p><p class="apache">Apache HTTP Server Version 2.0</p><img alt="" src="../images/feather.gif" /></div><div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div><div id="path"><a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.0</a></div><div id="page-content"><div id="preamble"><h1>Apache Performance Notes</h1>
<div class="warning"><strong>Warning:</strong>
This document has not been fully updated
to take into account changes made in the 2.0 version of the
Apache HTTP Server. Some of the information may still be
relevant, but please use it with care.</div>
<p>Orignally written by Dean Gaudet.</p>
<p>Apache 2.0 is a general-purpose webserver, designed to
provide a balance of flexibility, portability, and performance.
Although it has not been designed specifically to set benchmark
records, Apache 2.0 is capable of high performance in many
real-world situations.</p>
<p>Compared to Apache 1.3, release 2.0 contains many additional
optimizations to increase throughput and scalability. Most of
these improvements are enabled by default. However, there are
compile-time and run-time configuration choices that can
significantly affect performance. This document describes the
options that a server administrator can configure to tune the
performance of an Apache 2.0 installation. Some of these
configuration options enable the httpd to better take advantage
of the capabilities of the hardware and OS, while others allow
the administrator to trade functionality for speed.</p>
</div><div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#hardware">Hardware and Operating System Issues</a></li><li><img alt="" src="../images/down.gif" /> <a href="#runtime">Run-Time Configuration Issues</a></li><li><img alt="" src="../images/down.gif" /> <a href="#compiletime">Compile-Time Configuration Issues</a></li><li><img alt="" src="../images/down.gif" /> <a href="#trace">Appendix: Detailed Analysis of a Trace</a></li></ul></div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="hardware" id="hardware">Hardware and Operating System Issues</a></h2>
<p>The single biggest hardware issue affecting webserver
performance is RAM. A webserver should never ever have to swap,
swapping increases the latency of each request beyond a point
that users consider "fast enough". This causes users to hit
stop and reload, further increasing the load. You can, and
should, control the <code class="directive"><a href="../mod/mpm_common.html#maxclients">MaxClients</a></code> setting so that your server
does not spawn so many children it starts swapping.</p>
<p>Beyond that the rest is mundane: get a fast enough CPU, a
fast enough network card, and fast enough disks, where "fast
enough" is something that needs to be determined by
experimentation.</p>
<p>Operating system choice is largely a matter of local
concerns. But some guidelines that have proven generally
useful are:</p>
<ul>
<li>
<p>Run the latest stable release and patchlevel of the
operating system that you choose. Many OS suppliers have
introduced significant performance improvements to their
TCP stacks and thread libraries in recent years.</p>
</li>
<li>
<p>If your OS supports a <code>sendfile(2)</code> system
call, make sure you install the release and/or patches
needed to enable it. (With Linux, for example, this means
using Linux 2.4 or later. For early releases of Solaris 8,
you may need to apply a patch.) On systems where it is
available, <code>sendfile</code> enables Apache 2 to deliver
static content faster and with lower CPU utilization.</p>
</li>
</ul>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="runtime" id="runtime">Run-Time Configuration Issues</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="../mod/mod_dir.html">mod_dir</a></code></li><li><code class="module"><a href="../mod/mpm_common.html">mpm_common</a></code></li><li><code class="module"><a href="../mod/mod_status.html">mod_status</a></code></li></ul></td><td><ul><li><code class="directive"><a href="../mod/core.html#allowoverride">AllowOverride</a></code></li><li><code class="directive"><a href="../mod/mod_dir.html#directoryindex">DirectoryIndex</a></code></li><li><code class="directive"><a href="../mod/core.html#hostnamelookups">HostnameLookups</a></code></li><li><code class="directive"><a href="../mod/core.html#enablemmap">EnableMMAP</a></code></li><li><code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code></li><li><code class="directive"><a href="../mod/prefork.html#maxspareservers">MaxSpareServers</a></code></li><li><code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code></li><li><code class="directive"><a href="../mod/core.html#options">Options</a></code></li><li><code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code></li></ul></td></tr></table>
<h3><code>HostnameLookups</code></h3>
<p>Prior to Apache 1.3, <code class="directive"><a href="../mod/core.html#hostnamelookups">HostnameLookups</a></code> defaulted to <code>On</code>.
This adds latency to every request because it requires a
DNS lookup to complete before the request is finished. In
Apache 1.3 this setting defaults to <code>Off</code>.
However (1.3 or later), if you use any <code>Allow from domain</code>
or <code>Deny from domain</code> directives then you will pay for
a double reverse DNS lookup (a reverse, followed by a forward
to make sure that the reverse is not being spoofed). So for the
highest performance avoid using these directives (it's fine to
use IP addresses rather than domain names).</p>
<p>Note that it's possible to scope the directives, such as
within a <code><Location /server-status></code> section.
In this case the DNS lookups are only performed on requests
matching the criteria. Here's an example which disables lookups
except for <code>.html</code> and <code>.cgi</code> files:</p>
<div class="example"><pre>
HostnameLookups off
<Files ~ "\.(html|cgi)$">
HostnameLookups on
</Files>
</pre></div>
<p>But even still, if you just need DNS names in some CGIs you
could consider doing the <code>gethostbyname</code> call in the
specific CGIs that need it.</p>
<p>Similarly, if you need to have hostname information in your
server logs in order to generate reports of this information,
you can postprocess your log file with <a href="../programs/logresolve.html"><code>logresolve</code></a>,
so that these lookups can be done without making the client wait.
It is recommended that you do this postprocessing, and any other
statistical analysis of the log file, somewhere other than your
production web server machine, in order that this activity does
not adversely affect server performance.</p>
<h3><code>FollowSymLinks</code> and <code>SymLinksIfOwnerMatch</code></h3>
<p>Wherever in your URL-space you do not have an <code>Options
FollowSymLinks</code>, or you do have an <code>Options
SymLinksIfOwnerMatch</code> Apache will have to issue extra
system calls to check up on symlinks. One extra call per
filename component. For example, if you had:</p>
<div class="example"><pre>
DocumentRoot /www/htdocs
<Directory />
Options SymLinksIfOwnerMatch
</Directory>
</pre></div>
<p>and a request is made for the URI <code>/index.html</code>.
Then Apache will perform <code>lstat(2)</code> on
<code>/www</code>, <code>/www/htdocs</code>, and
<code>/www/htdocs/index.html</code>. The results of these
<code>lstats</code> are never cached, so they will occur on
every single request. If you really desire the symlinks
security checking you can do something like this:</p>
<div class="example"><pre>
DocumentRoot /www/htdocs
<Directory />
Options FollowSymLinks
</Directory>
<Directory /www/htdocs>
Options -FollowSymLinks +SymLinksIfOwnerMatch
</Directory>
</pre></div>
<p>This at least avoids the extra checks for the
<code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code> path.
Note that you'll need to add similar sections if you
have any <code class="directive"><a href="../mod/mod_alias.html#alias">Alias</a></code> or
<code class="directive"><a href="../mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> paths
outside of your document root. For highest performance,
and no symlink protection, set <code>FollowSymLinks</code>
everywhere, and never set <code>SymLinksIfOwnerMatch</code>.</p>
<h3><code>AllowOverride</code></h3>
<p>Wherever in your URL-space you allow overrides (typically
<code>.htaccess</code> files) Apache will attempt to open
<code>.htaccess</code> for each filename component. For
example,</p>
<div class="example"><pre>
DocumentRoot /www/htdocs
<Directory />
AllowOverride all
</Directory>
</pre></div>
<p>and a request is made for the URI <code>/index.html</code>.
Then Apache will attempt to open <code>/.htaccess</code>,
<code>/www/.htaccess</code>, and
<code>/www/htdocs/.htaccess</code>. The solutions are similar
to the previous case of <code>Options FollowSymLinks</code>.
For highest performance use <code>AllowOverride None</code>
everywhere in your filesystem.</p>
<h3>Negotiation</h3>
<p>If at all possible, avoid content-negotiation if you're
really interested in every last ounce of performance. In
practice the benefits of negotiation outweigh the performance
penalties. There's one case where you can speed up the server.
Instead of using a wildcard such as:</p>
<div class="example"><pre>
DirectoryIndex index
</pre></div>
<p>Use a complete list of options:</p>
<div class="example"><pre>
DirectoryIndex index.cgi index.pl index.shtml index.html
</pre></div>
<p>where you list the most common choice first.</p>
<p>Also note that explicitly creating a <code>type-map</code>
file provides better performance than using
<code>MultiViews</code>, as the necessary information can be
determined by reading this single file, rather than having to
scan the directory for files.</p>
<h3>Memory-mapping</h3>
<p>In situations where Apache 2.0 needs to look at the contents
of a file being delivered--for example, when doing server-side-include
processing--it normally memory-maps the file if the OS supports
some form of <code>mmap(2)</code>.</p>
<p>On some platforms, this memory-mapping improves performance.
However, there are cases where memory-mapping can hurt the performance
or even the stability of the httpd:</p>
<ul>
<li>
<p>On some operating systems, <code>mmap</code> does not scale
as well as <code>read(2)</code> when the number of CPUs increases.
On multiprocessor Solaris servers, for example, Apache 2.0 sometimes
delivers server-parsed files faster when <code>mmap</code> is disabled.</p>
</li>
<li>
<p>If you memory-map a file located on an NFS-mounted filesystem
and a process on another NFS client machine deletes or truncates
the file, your process may get a bus error the next time it tries
to access the mapped file content.</p>
</li>
</ul>
<p>For installations where either of these factors applies, you
should use <code>EnableMMAP off</code> to disable the memory-mapping
of delivered files. (Note: This directive can be overridden on
a per-directory basis.)</p>
<h3>Process Creation</h3>
<p>Prior to Apache 1.3 the <code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>, <code class="directive"><a href="../mod/prefork.html#maxspareservers">MaxSpareServers</a></code>, and <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> settings all had drastic effects on
benchmark results. In particular, Apache required a "ramp-up"
period in order to reach a number of children sufficient to serve
the load being applied. After the initial spawning of
<code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> children,
only one child per second would be created to satisfy the
<code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>
setting. So a server being accessed by 100 simultaneous
clients, using the default <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> of <code>5</code> would take on
the order 95 seconds to spawn enough children to handle
the load. This works fine in practice on real-life servers,
because they aren't restarted frequently. But does really
poorly on benchmarks which might only run for ten minutes.</p>
<p>The one-per-second rule was implemented in an effort to
avoid swamping the machine with the startup of new children. If
the machine is busy spawning children it can't service
requests. But it has such a drastic effect on the perceived
performance of Apache that it had to be replaced. As of Apache
1.3, the code will relax the one-per-second rule. It will spawn
one, wait a second, then spawn two, wait a second, then spawn
four, and it will continue exponentially until it is spawning
32 children per second. It will stop whenever it satisfies the
<code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>
setting.</p>
<p>This appears to be responsive enough that it's almost
unnecessary to twiddle the <code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>, <code class="directive"><a href="../mod/prefork.html#maxspareservers">MaxSpareServers</a></code> and <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> knobs. When more than 4 children are
spawned per second, a message will be emitted to the
<code class="directive"><a href="../mod/core.html#errorlog">ErrorLog</a></code>. If you
see a lot of these errors then consider tuning these settings.
Use the <code class="module"><a href="../mod/mod_status.html">mod_status</a></code> output as a guide.</p>
<p>Related to process creation is process death induced by the
<code class="directive"><a href="../mod/mpm_common.html#maxrequestsperchild">MaxRequestsPerChild</a></code>
setting. By default this is <code>0</code>,
which means that there is no limit to the number of requests
handled per child. If your configuration currently has this set
to some very low number, such as <code>30</code>, you may want to bump this
up significantly. If you are running SunOS or an old version of
Solaris, limit this to <code>10000</code> or so because of memory leaks.</p>
<p>When keep-alives are in use, children will be kept busy
doing nothing waiting for more requests on the already open
connection. The default <code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code> of <code>15</code>
seconds attempts to minimize this effect. The tradeoff here is
between network bandwidth and server resources. In no event
should you raise this above about <code>60</code> seconds, as <a href="http://www.research.digital.com/wrl/techreports/abstracts/95.4.html">
most of the benefits are lost</a>.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="compiletime" id="compiletime">Compile-Time Configuration Issues</a></h2>
<h3>mod_status and ExtendedStatus On</h3>
<p>If you include <code class="module"><a href="../mod/mod_status.html">mod_status</a></code> and you also set
<code>ExtendedStatus On</code> when building and running
Apache, then on every request Apache will perform two calls to
<code>gettimeofday(2)</code> (or <code>times(2)</code>
depending on your operating system), and (pre-1.3) several
extra calls to <code>time(2)</code>. This is all done so that
the status report contains timing indications. For highest
performance, set <code>ExtendedStatus off</code> (which is the
default).</p>
<h3>accept Serialization - multiple sockets</h3>
<p>This discusses a shortcoming in the Unix socket API. Suppose
your web server uses multiple <code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code> statements to listen on either multiple
ports or multiple addresses. In order to test each socket
to see if a connection is ready Apache uses
<code>select(2)</code>. <code>select(2)</code> indicates that a
socket has <em>zero</em> or <em>at least one</em> connection
waiting on it. Apache's model includes multiple children, and
all the idle ones test for new connections at the same time. A
naive implementation looks something like this (these examples
do not match the code, they're contrived for pedagogical
purposes):</p>
<div class="example"><pre>
for (;;) {
for (;;) {
fd_set accept_fds;
FD_ZERO (&accept_fds);
for (i = first_socket; i <= last_socket; ++i) {
FD_SET (i, &accept_fds);
}
rc = select (last_socket+1, &accept_fds, NULL, NULL, NULL);
if (rc < 1) continue;
new_connection = -1;
for (i = first_socket; i <= last_socket; ++i) {
if (FD_ISSET (i, &accept_fds)) {
new_connection = accept (i, NULL, NULL);
if (new_connection != -1) break;
}
}
if (new_connection != -1) break;
}
process the new_connection;
}
</pre></div>
<p>But this naive implementation has a serious starvation problem.
Recall that multiple children execute this loop at the same
time, and so multiple children will block at
<code>select</code> when they are in between requests. All
those blocked children will awaken and return from
<code>select</code> when a single request appears on any socket
(the number of children which awaken varies depending on the
operating system and timing issues). They will all then fall
down into the loop and try to <code>accept</code> the
connection. But only one will succeed (assuming there's still
only one connection ready), the rest will be <em>blocked</em>
in <code>accept</code>. This effectively locks those children
into serving requests from that one socket and no other
sockets, and they'll be stuck there until enough new requests
appear on that socket to wake them all up. This starvation
problem was first documented in <a href="http://bugs.apache.org/index/full/467">PR#467</a>. There
are at least two solutions.</p>
<p>One solution is to make the sockets non-blocking. In this
case the <code>accept</code> won't block the children, and they
will be allowed to continue immediately. But this wastes CPU
time. Suppose you have ten idle children in
<code>select</code>, and one connection arrives. Then nine of
those children will wake up, try to <code>accept</code> the
connection, fail, and loop back into <code>select</code>,
accomplishing nothing. Meanwhile none of those children are
servicing requests that occurred on other sockets until they
get back up to the <code>select</code> again. Overall this
solution does not seem very fruitful unless you have as many
idle CPUs (in a multiprocessor box) as you have idle children,
not a very likely situation.</p>
<p>Another solution, the one used by Apache, is to serialize
entry into the inner loop. The loop looks like this
(differences highlighted):</p>
<div class="example"><pre>
for (;;) {
<strong>accept_mutex_on ();</strong>
for (;;) {
fd_set accept_fds;
FD_ZERO (&accept_fds);
for (i = first_socket; i <= last_socket; ++i) {
FD_SET (i, &accept_fds);
}
rc = select (last_socket+1, &accept_fds, NULL, NULL, NULL);
if (rc < 1) continue;
new_connection = -1;
for (i = first_socket; i <= last_socket; ++i) {
if (FD_ISSET (i, &accept_fds)) {
new_connection = accept (i, NULL, NULL);
if (new_connection != -1) break;
}
}
if (new_connection != -1) break;
}
<strong>accept_mutex_off ();</strong>
process the new_connection;
}
</pre></div>
<p><a id="serialize" name="serialize">The functions</a>
<code>accept_mutex_on</code> and <code>accept_mutex_off</code>
implement a mutual exclusion semaphore. Only one child can have
the mutex at any time. There are several choices for
implementing these mutexes. The choice is defined in
<code>src/conf.h</code> (pre-1.3) or
<code>src/include/ap_config.h</code> (1.3 or later). Some
architectures do not have any locking choice made, on these
architectures it is unsafe to use multiple
<code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code>
directives.</p>
<dl>
<dt><code>USE_FLOCK_SERIALIZED_ACCEPT</code></dt>
<dd>
<p>This method uses the <code>flock(2)</code> system call to
lock a lock file (located by the <code class="directive"><a href="../mod/mpm_common.html#lockfile">LockFile</a></code> directive).</p>
</dd>
<dt><code>USE_FCNTL_SERIALIZED_ACCEPT</code></dt>
<dd>
<p>This method uses the <code>fcntl(2)</code> system call to
lock a lock file (located by the <code class="directive"><a href="../mod/mpm_common.html#lockfile">LockFile</a></code> directive).</p>
</dd>
<dt><code>USE_SYSVSEM_SERIALIZED_ACCEPT</code></dt>
<dd>
<p>(1.3 or later) This method uses SysV-style semaphores to
implement the mutex. Unfortunately SysV-style semaphores have
some bad side-effects. One is that it's possible Apache will
die without cleaning up the semaphore (see the
<code>ipcs(8)</code> man page). The other is that the
semaphore API allows for a denial of service attack by any
CGIs running under the same uid as the webserver
(<em>i.e.</em>, all CGIs, unless you use something like
<code>suexec</code> or <code>cgiwrapper</code>). For these
reasons this method is not used on any architecture except
IRIX (where the previous two are prohibitively expensive
on most IRIX boxes).</p>
</dd>
<dt><code>USE_USLOCK_SERIALIZED_ACCEPT</code></dt>
<dd>
<p>(1.3 or later) This method is only available on IRIX, and
uses <code>usconfig(2)</code> to create a mutex. While this
method avoids the hassles of SysV-style semaphores, it is not
the default for IRIX. This is because on single processor
IRIX boxes (5.3 or 6.2) the uslock code is two orders of
magnitude slower than the SysV-semaphore code. On
multi-processor IRIX boxes the uslock code is an order of
magnitude faster than the SysV-semaphore code. Kind of a
messed up situation. So if you're using a multiprocessor IRIX
box then you should rebuild your webserver with
<code>-DUSE_USLOCK_SERIALIZED_ACCEPT</code> on the
<code>EXTRA_CFLAGS</code>.</p>
</dd>
<dt><code>USE_PTHREAD_SERIALIZED_ACCEPT</code></dt>
<dd>
<p>(1.3 or later) This method uses POSIX mutexes and should
work on any architecture implementing the full POSIX threads
specification, however appears to only work on Solaris (2.5
or later), and even then only in certain configurations. If
you experiment with this you should watch out for your server
hanging and not responding. Static content only servers may
work just fine.</p>
</dd>
</dl>
<p>If your system has another method of serialization which
isn't in the above list then it may be worthwhile adding code
for it (and submitting a patch back to Apache).</p>
<p>Another solution that has been considered but never
implemented is to partially serialize the loop -- that is, let
in a certain number of processes. This would only be of
interest on multiprocessor boxes where it's possible multiple
children could run simultaneously, and the serialization
actually doesn't take advantage of the full bandwidth. This is
a possible area of future investigation, but priority remains
low because highly parallel web servers are not the norm.</p>
<p>Ideally you should run servers without multiple
<code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code>
statements if you want the highest performance.
But read on.</p>
<h3>accept Serialization - single socket</h3>
<p>The above is fine and dandy for multiple socket servers, but
what about single socket servers? In theory they shouldn't
experience any of these same problems because all children can
just block in <code>accept(2)</code> until a connection
arrives, and no starvation results. In practice this hides
almost the same "spinning" behaviour discussed above in the
non-blocking solution. The way that most TCP stacks are
implemented, the kernel actually wakes up all processes blocked
in <code>accept</code> when a single connection arrives. One of
those processes gets the connection and returns to user-space,
the rest spin in the kernel and go back to sleep when they
discover there's no connection for them. This spinning is
hidden from the user-land code, but it's there nonetheless.
This can result in the same load-spiking wasteful behaviour
that a non-blocking solution to the multiple sockets case
can.</p>
<p>For this reason we have found that many architectures behave
more "nicely" if we serialize even the single socket case. So
this is actually the default in almost all cases. Crude
experiments under Linux (2.0.30 on a dual Pentium pro 166
w/128Mb RAM) have shown that the serialization of the single
socket case causes less than a 3% decrease in requests per
second over unserialized single-socket. But unserialized
single-socket showed an extra 100ms latency on each request.
This latency is probably a wash on long haul lines, and only an
issue on LANs. If you want to override the single socket
serialization you can define
<code>SINGLE_LISTEN_UNSERIALIZED_ACCEPT</code> and then
single-socket servers will not serialize at all.</p>
<h3>Lingering Close</h3>
<p>As discussed in <a href="http://www.ics.uci.edu/pub/ietf/http/draft-ietf-http-connection-00.txt">
draft-ietf-http-connection-00.txt</a> section 8, in order for
an HTTP server to <strong>reliably</strong> implement the
protocol it needs to shutdown each direction of the
communication independently (recall that a TCP connection is
bi-directional, each half is independent of the other). This
fact is often overlooked by other servers, but is correctly
implemented in Apache as of 1.2.</p>
<p>When this feature was added to Apache it caused a flurry of
problems on various versions of Unix because of a
shortsightedness. The TCP specification does not state that the
<code>FIN_WAIT_2</code> state has a timeout, but it doesn't prohibit it.
On systems without the timeout, Apache 1.2 induces many sockets
stuck forever in the <code>FIN_WAIT_2</code> state. In many cases this
can be avoided by simply upgrading to the latest TCP/IP patches
supplied by the vendor. In cases where the vendor has never
released patches (<em>i.e.</em>, SunOS4 -- although folks with
a source license can patch it themselves) we have decided to
disable this feature.</p>
<p>There are two ways of accomplishing this. One is the socket
option <code>SO_LINGER</code>. But as fate would have it, this
has never been implemented properly in most TCP/IP stacks. Even
on those stacks with a proper implementation (<em>i.e.</em>,
Linux 2.0.31) this method proves to be more expensive (cputime)
than the next solution.</p>
<p>For the most part, Apache implements this in a function
called <code>lingering_close</code> (in
<code>http_main.c</code>). The function looks roughly like
this:</p>
<div class="example"><pre>
void lingering_close (int s)
{
char junk_buffer[2048];
/* shutdown the sending side */
shutdown (s, 1);
signal (SIGALRM, lingering_death);
alarm (30);
for (;;) {
select (s for reading, 2 second timeout);
if (error) break;
if (s is ready for reading) {
if (read (s, junk_buffer, sizeof (junk_buffer)) <= 0) {
break;
}
/* just toss away whatever is here */
}
}
close (s);
}
</pre></div>
<p>This naturally adds some expense at the end of a connection,
but it is required for a reliable implementation. As HTTP/1.1
becomes more prevalent, and all connections are persistent,
this expense will be amortized over more requests. If you want
to play with fire and disable this feature you can define
<code>NO_LINGCLOSE</code>, but this is not recommended at all.
In particular, as HTTP/1.1 pipelined persistent connections
come into use <code>lingering_close</code> is an absolute
necessity (and <a href="http://www.w3.org/Protocols/HTTP/Performance/Pipeline.html">
pipelined connections are faster</a>, so you want to support
them).</p>
<h3>Scoreboard File</h3>
<p>Apache's parent and children communicate with each other
through something called the scoreboard. Ideally this should be
implemented in shared memory. For those operating systems that
we either have access to, or have been given detailed ports
for, it typically is implemented using shared memory. The rest
default to using an on-disk file. The on-disk file is not only
slow, but it is unreliable (and less featured). Peruse the
<code>src/main/conf.h</code> file for your architecture and
look for either <code>USE_MMAP_SCOREBOARD</code> or
<code>USE_SHMGET_SCOREBOARD</code>. Defining one of those two
(as well as their companions <code>HAVE_MMAP</code> and
<code>HAVE_SHMGET</code> respectively) enables the supplied
shared memory code. If your system has another type of shared
memory, edit the file <code>src/main/http_main.c</code> and add
the hooks necessary to use it in Apache. (Send us back a patch
too please.)</p>
<div class="note">Historical note: The Linux port of Apache didn't start to
use shared memory until version 1.2 of Apache. This oversight
resulted in really poor and unreliable behaviour of earlier
versions of Apache on Linux.</div>
<h3><code>DYNAMIC_MODULE_LIMIT</code></h3>
<p>If you have no intention of using dynamically loaded modules
(you probably don't if you're reading this and tuning your
server for every last ounce of performance) then you should add
<code>-DDYNAMIC_MODULE_LIMIT=0</code> when building your
server. This will save RAM that's allocated only for supporting
dynamically loaded modules.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="trace" id="trace">Appendix: Detailed Analysis of a Trace</a></h2>
<p>Here is a system call trace of Apache 2.0.38 with the worker MPM
on Solaris 8. This trace was collected using:</p>
<div class="example"><p><code>
truss -l -p <em>httpd_child_pid</em>.
</code></p></div>
<p>The <code>-l</code> option tells truss to log the ID of the
LWP (lightweight process--Solaris's form of kernel-level thread)
that invokes each system call.</p>
<p>Other systems may have different system call tracing utilities
such as <code>strace</code>, <code>ktrace</code>, or <code>par</code>.
They all produce similar output.</p>
<p>In this trace, a client has requested a 10KB static file
from the httpd. Traces of non-static requests or requests
with content negotiation look wildly different (and quite ugly
in some cases).</p>
<div class="example"><pre>
/67: accept(3, 0x00200BEC, 0x00200C0C, 1) (sleeping...)
/67: accept(3, 0x00200BEC, 0x00200C0C, 1) = 9
</pre></div>
<p>In this trace, the listener thread is running within LWP #67.</p>
<div class="note">Note the lack of <code>accept(2)</code> serialization. On this
particular platform, the worker MPM uses an unserialized accept by
default unless it is listening on multiple ports.</div>
<div class="example"><pre>
/65: lwp_park(0x00000000, 0) = 0
/67: lwp_unpark(65, 1) = 0
</pre></div>
<p>Upon accepting the connection, the listener thread wakes up
a worker thread to do the request processing. In this trace,
the worker thread that handles the request is mapped to LWP #65.</p>
<div class="example"><pre>
/65: getsockname(9, 0x00200BA4, 0x00200BC4, 1) = 0
</pre></div>
<p>In order to implement virtual hosts, Apache needs to know
the local socket address used to accept the connection. It
is possible to eliminate this call in many situations (such
as when there are no virtual hosts, or when
<code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code> directives
are used which do not have wildcard addresses). But
no effort has yet been made to do these optimizations. </p>
<div class="example"><pre>
/65: brk(0x002170E8) = 0
/65: brk(0x002190E8) = 0
</pre></div>
<p>The <code>brk(2)</code> calls allocate memory from the heap.
It is rare to see these in a system call trace, because the httpd
uses custom memory allocators (<code>apr_pool</code> and
<code>apr_bucket_alloc</code>) for most request processing.
In this trace, the httpd has just been started, so it must
call <code>malloc(3)</code> to get the blocks of raw memory
with which to create the custom memory allocators.</p>
<div class="example"><pre>
/65: fcntl(9, F_GETFL, 0x00000000) = 2
/65: fstat64(9, 0xFAF7B818) = 0
/65: getsockopt(9, 65535, 8192, 0xFAF7B918, 0xFAF7B910, 2190656) = 0
/65: fstat64(9, 0xFAF7B818) = 0
/65: getsockopt(9, 65535, 8192, 0xFAF7B918, 0xFAF7B914, 2190656) = 0
/65: setsockopt(9, 65535, 8192, 0xFAF7B918, 4, 2190656) = 0
/65: fcntl(9, F_SETFL, 0x00000082) = 0
</pre></div>
<p>Next, the worker thread puts the connection to the client (file
descriptor 9) in non-blocking mode. The <code>setsockopt(2)</code>
and <code>getsockopt(2)</code> calls are a side-effect of how
Solaris's libc handles <code>fcntl(2)</code> on sockets.</p>
<div class="example"><pre>
/65: read(9, " G E T / 1 0 k . h t m".., 8000) = 97
</pre></div>
<p>The worker thread reads the request from the client.</p>
<div class="example"><pre>
/65: stat("/var/httpd/apache/httpd-8999/htdocs/10k.html", 0xFAF7B978) = 0
/65: open("/var/httpd/apache/httpd-8999/htdocs/10k.html", O_RDONLY) = 10
</pre></div>
<p>This httpd has been configured with <code>Options FollowSymLinks</code>
and <code>AllowOverride None</code>. Thus it doesn't need to
<code>lstat(2)</code> each directory in the path leading up to the
requested file, nor check for <code>.htaccess</code> files.
It simply calls <code>stat(2)</code> to verify that the file:
1) exists, and 2) is a regular file, not a directory.</p>
<div class="example"><pre>
/65: sendfilev(0, 9, 0x00200F90, 2, 0xFAF7B53C) = 10269
</pre></div>
<p>In this example, the httpd is able to send the HTTP response
header and the requested file with a single <code>sendfilev(2)</code>
system call. Sendfile semantics vary among operating systems. On some other
systems, it is necessary to do a <code>write(2)</code> or
<code>writev(2)</code> call to send the headers before calling
<code>sendfile(2)</code>.</p>
<div class="example"><pre>
/65: write(4, " 1 2 7 . 0 . 0 . 1 - ".., 78) = 78
</pre></div>
<p>This <code>write(2)</code> call records the request in the
access log. Note that one thing missing from this trace is a
<code>time(2)</code> call. Unlike Apache 1.3, Apache 2.0 uses
<code>gettimeofday(3)</code> to look up the time. On some operating
systems, like Linux or Solaris, <code>gettimeofday</code> has an
optimized implementation that doesn't require as much overhead
as a typical system call.</p>
<div class="example"><pre>
/65: shutdown(9, 1, 1) = 0
/65: poll(0xFAF7B980, 1, 2000) = 1
/65: read(9, 0xFAF7BC20, 512) = 0
/65: close(9) = 0
</pre></div>
<p>The worker thread does a lingering close of the connection.</p>
<div class="example"><pre>
/65: close(10) = 0
/65: lwp_park(0x00000000, 0) (sleeping...)
</pre></div>
<p>Finally the worker thread closes the file that it has just delivered
and blocks until the listener assigns it another connection.</p>
<div class="example"><pre>
/67: accept(3, 0x001FEB74, 0x001FEB94, 1) (sleeping...)
</pre></div>
<p>Meanwhile, the listener thread is able to accept another connection
as soon as it has dispatched this connection to a worker thread (subject
to some flow-control logic in the worker MPM that throttles the listener
if all the available workers are busy). Though it isn't apparent from
this trace, the next <code>accept(2)</code> can (and usually does, under
high load conditions) occur in parallel with the worker thread's handling
of the just-accepted connection.</p>
</div></div><div id="footer"><p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div></body></html>
1.1 httpd-2.0/docs/manual/misc/rewriteguide.html.en
Index: rewriteguide.html.en
===================================================================
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--><title>URL Rewriting Guide - Apache HTTP Server</title><link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /><link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /><link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link href="../images/favicon.ico" rel="shortcut icon" /></head><body id="manual-page"><div id="page-header"><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p><p class="apache">Apache HTTP Server Version 2.0</p><img alt="" src="../images/feather.gif" /></div><div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div><div id="path"><a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.0</a></div><div id="page-content"><div id="preamble"><h1>URL Rewriting Guide</h1>
<div class="note">
<address>Originally written by<br />
Ralf S. Engelschall <rse@apache.org><br />
December 1997</address>
</div>
<p>This document supplements the <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
<a href="../mod/mod_rewrite.html">reference documentation</a>.
It describes how one can use Apache's <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
to solve typical URL-based problems webmasters are usually confronted
with in practice. I give detailed descriptions on how to
solve each problem by configuring URL rewriting rulesets.</p>
</div><div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#ToC1">Introduction to <code>mod_rewrite</code></a></li><li><img alt="" src="../images/down.gif" /> <a href="#ToC2">Practical Solutions</a></li><li><img alt="" src="../images/down.gif" /> <a href="#url">URL Layout</a></li><li><img alt="" src="../images/down.gif" /> <a href="#content">Content Handling</a></li><li><img alt="" src="../images/down.gif" /> <a href="#access">Access Restriction</a></li><li><img alt="" src="../images/down.gif" /> <a href="#other">Other</a></li></ul></div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="ToC1" id="ToC1">Introduction to <code>mod_rewrite</code></a></h2>
<p>The Apache module <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> is a killer
one, i.e. it is a really sophisticated module which provides
a powerful way to do URL manipulations. With it you can nearly
do all types of URL manipulations you ever dreamed about.
The price you have to pay is to accept complexity, because
<code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>'s major drawback is that it is
not easy to understand and use for the beginner. And even
Apache experts sometimes discover new aspects where
<code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> can help.</p>
<p>In other words: With <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> you either
shoot yourself in the foot the first time and never use it again
or love it for the rest of your life because of its power.
This paper tries to give you a few initial success events to
avoid the first case by presenting already invented solutions
to you.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="ToC2" id="ToC2">Practical Solutions</a></h2>
<p>Here come a lot of practical solutions I've either invented
myself or collected from other peoples solutions in the past.
Feel free to learn the black magic of URL rewriting from
these examples.</p>
<div class="warning">ATTENTION: Depending on your server-configuration
it can be necessary to slightly change the examples for your
situation, e.g. adding the <code>[PT]</code> flag when
additionally using <code class="module"><a href="../mod/mod_alias.html">mod_alias</a></code> and
<code class="module"><a href="../mod/mod_userdir.html">mod_userdir</a></code>, etc. Or rewriting a ruleset
to fit in <code>.htaccess</code> context instead
of per-server context. Always try to understand what a
particular ruleset really does before you use it. It
avoid problems.</div>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="url" id="url">URL Layout</a></h2>
<h3>Canonical URLs</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>On some webservers there are more than one URL for a
resource. Usually there are canonical URLs (which should be
actually used and distributed) and those which are just
shortcuts, internal ones, etc. Independent of which URL the
user supplied with the request he should finally see the
canonical one only.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We do an external HTTP redirect for all non-canonical
URLs to fix them in the location view of the Browser and
for all subsequent requests. In the example ruleset below
we replace <code>/~user</code> by the canonical
<code>/u/user</code> and fix a missing trailing slash for
<code>/u/user</code>.</p>
<div class="example"><pre>
RewriteRule ^/<strong>~</strong>([^/]+)/?(.*) /<strong>u</strong>/$1/$2 [<strong>R</strong>]
RewriteRule ^/([uge])/(<strong>[^/]+</strong>)$ /$1/$2<strong>/</strong> [<strong>R</strong>]
</pre></div>
</dd>
</dl>
<h3>Canonical Hostnames</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>...</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<div class="example"><pre>
RewriteCond %{HTTP_HOST} !^fully\.qualified\.domain\.name [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteCond %{SERVER_PORT} !^80$
RewriteRule ^/(.*) http://fully.qualified.domain.name:%{SERVER_PORT}/$1 [L,R]
RewriteCond %{HTTP_HOST} !^fully\.qualified\.domain\.name [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteRule ^/(.*) http://fully.qualified.domain.name/$1 [L,R]
</pre></div>
</dd>
</dl>
<h3>Moved <code>DocumentRoot</code></h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Usually the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code>
of the webserver directly relates to the URL "<code>/</code>".
But often this data is not really of top-level priority, it is
perhaps just one entity of a lot of data pools. For instance at
our Intranet sites there are <code>/e/www/</code>
(the homepage for WWW), <code>/e/sww/</code> (the homepage for
the Intranet) etc. Now because the data of the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code> stays at <code>/e/www/</code> we had
to make sure that all inlined images and other stuff inside this
data pool work for subsequent requests.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We just redirect the URL <code>/</code> to
<code>/e/www/</code>. While is seems trivial it is
actually trivial with <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>, only.
Because the typical old mechanisms of URL <em>Aliases</em>
(as provides by <code class="module"><a href="../mod/mod_alias.html">mod_alias</a></code> and friends)
only used <em>prefix</em> matching. With this you cannot
do such a redirection because the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code> is a prefix of all URLs. With
<code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> it is really trivial:</p>
<div class="example"><pre>
RewriteEngine on
RewriteRule <strong>^/$</strong> /e/www/ [<strong>R</strong>]
</pre></div>
</dd>
</dl>
<h3>Trailing Slash Problem</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Every webmaster can sing a song about the problem of
the trailing slash on URLs referencing directories. If they
are missing, the server dumps an error, because if you say
<code>/~quux/foo</code> instead of <code>/~quux/foo/</code>
then the server searches for a <em>file</em> named
<code>foo</code>. And because this file is a directory it
complains. Actually it tries to fix it itself in most of
the cases, but sometimes this mechanism need to be emulated
by you. For instance after you have done a lot of
complicated URL rewritings to CGI scripts etc.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>The solution to this subtle problem is to let the server
add the trailing slash automatically. To do this
correctly we have to use an external redirect, so the
browser correctly requests subsequent images etc. If we
only did a internal rewrite, this would only work for the
directory page, but would go wrong when any images are
included into this page with relative URLs, because the
browser would request an in-lined object. For instance, a
request for <code>image.gif</code> in
<code>/~quux/foo/index.html</code> would become
<code>/~quux/image.gif</code> without the external
redirect!</p>
<p>So, to do this trick we write:</p>
<div class="example"><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^foo<strong>$</strong> foo<strong>/</strong> [<strong>R</strong>]
</pre></div>
<p>The crazy and lazy can even do the following in the
top-level <code>.htaccess</code> file of their homedir.
But notice that this creates some processing
overhead.</p>
<div class="example"><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteCond %{REQUEST_FILENAME} <strong>-d</strong>
RewriteRule ^(.+<strong>[^/]</strong>)$ $1<strong>/</strong> [R]
</pre></div>
</dd>
</dl>
<h3>Webcluster through Homogeneous URL Layout</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>We want to create a homogeneous and consistent URL
layout over all WWW servers on a Intranet webcluster, i.e.
all URLs (per definition server local and thus server
dependent!) become actually server <em>independent</em>!
What we want is to give the WWW namespace a consistent
server-independent layout: no URL should have to include
any physically correct target server. The cluster itself
should drive us automatically to the physical target
host.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>First, the knowledge of the target servers come from
(distributed) external maps which contain information
where our users, groups and entities stay. The have the
form</p>
<div class="example"><pre>
user1 server_of_user1
user2 server_of_user2
: :
</pre></div>
<p>We put them into files <code>map.xxx-to-host</code>.
Second we need to instruct all servers to redirect URLs
of the forms</p>
<div class="example"><pre>
/u/user/anypath
/g/group/anypath
/e/entity/anypath
</pre></div>
<p>to</p>
<div class="example"><pre>
http://physical-host/u/user/anypath
http://physical-host/g/group/anypath
http://physical-host/e/entity/anypath
</pre></div>
<p>when the URL is not locally valid to a server. The
following ruleset does this for us by the help of the map
files (assuming that server0 is a default server which
will be used if a user has no entry in the map):</p>
<div class="example"><pre>
RewriteEngine on
RewriteMap user-to-host txt:/path/to/map.user-to-host
RewriteMap group-to-host txt:/path/to/map.group-to-host
RewriteMap entity-to-host txt:/path/to/map.entity-to-host
RewriteRule ^/u/<strong>([^/]+)</strong>/?(.*) http://<strong>${user-to-host:$1|server0}</strong>/u/$1/$2
RewriteRule ^/g/<strong>([^/]+)</strong>/?(.*) http://<strong>${group-to-host:$1|server0}</strong>/g/$1/$2
RewriteRule ^/e/<strong>([^/]+)</strong>/?(.*) http://<strong>${entity-to-host:$1|server0}</strong>/e/$1/$2
RewriteRule ^/([uge])/([^/]+)/?$ /$1/$2/.www/
RewriteRule ^/([uge])/([^/]+)/([^.]+.+) /$1/$2/.www/$3\
</pre></div>
</dd>
</dl>
<h3>Move Homedirs to Different Webserver</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Many webmasters have asked for a solution to the
following situation: They wanted to redirect just all
homedirs on a webserver to another webserver. They usually
need such things when establishing a newer webserver which
will replace the old one over time.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>The solution is trivial with <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>.
On the old webserver we just redirect all
<code>/~user/anypath</code> URLs to
<code>http://newserver/~user/anypath</code>.</p>
<div class="example"><pre>
RewriteEngine on
RewriteRule ^/~(.+) http://<strong>newserver</strong>/~$1 [R,L]
</pre></div>
</dd>
</dl>
<h3>Structured Homedirs</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Some sites with thousands of users usually use a
structured homedir layout, i.e. each homedir is in a
subdirectory which begins for instance with the first
character of the username. So, <code>/~foo/anypath</code>
is <code>/home/<strong>f</strong>/foo/.www/anypath</code>
while <code>/~bar/anypath</code> is
<code>/home/<strong>b</strong>/bar/.www/anypath</code>.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We use the following ruleset to expand the tilde URLs
into exactly the above layout.</p>
<div class="example"><pre>
RewriteEngine on
RewriteRule ^/~(<strong>([a-z])</strong>[a-z0-9]+)(.*) /home/<strong>$2</strong>/$1/.www$3
</pre></div>
</dd>
</dl>
<h3>Filesystem Reorganization</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>This really is a hardcore example: a killer application
which heavily uses per-directory
<code>RewriteRules</code> to get a smooth look and feel
on the Web while its data structure is never touched or
adjusted. Background: <strong><em>net.sw</em></strong> is
my archive of freely available Unix software packages,
which I started to collect in 1992. It is both my hobby
and job to to this, because while I'm studying computer
science I have also worked for many years as a system and
network administrator in my spare time. Every week I need
some sort of software so I created a deep hierarchy of
directories where I stored the packages:</p>
<div class="example"><pre>
drwxrwxr-x 2 netsw users 512 Aug 3 18:39 Audio/
drwxrwxr-x 2 netsw users 512 Jul 9 14:37 Benchmark/
drwxrwxr-x 12 netsw users 512 Jul 9 00:34 Crypto/
drwxrwxr-x 5 netsw users 512 Jul 9 00:41 Database/
drwxrwxr-x 4 netsw users 512 Jul 30 19:25 Dicts/
drwxrwxr-x 10 netsw users 512 Jul 9 01:54 Graphic/
drwxrwxr-x 5 netsw users 512 Jul 9 01:58 Hackers/
drwxrwxr-x 8 netsw users 512 Jul 9 03:19 InfoSys/
drwxrwxr-x 3 netsw users 512 Jul 9 03:21 Math/
drwxrwxr-x 3 netsw users 512 Jul 9 03:24 Misc/
drwxrwxr-x 9 netsw users 512 Aug 1 16:33 Network/
drwxrwxr-x 2 netsw users 512 Jul 9 05:53 Office/
drwxrwxr-x 7 netsw users 512 Jul 9 09:24 SoftEng/
drwxrwxr-x 7 netsw users 512 Jul 9 12:17 System/
drwxrwxr-x 12 netsw users 512 Aug 3 20:15 Typesetting/
drwxrwxr-x 10 netsw users 512 Jul 9 14:08 X11/
</pre></div>
<p>In July 1996 I decided to make this archive public to
the world via a nice Web interface. "Nice" means that I
wanted to offer an interface where you can browse
directly through the archive hierarchy. And "nice" means
that I didn't wanted to change anything inside this
hierarchy - not even by putting some CGI scripts at the
top of it. Why? Because the above structure should be
later accessible via FTP as well, and I didn't want any
Web or CGI stuff to be there.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>The solution has two parts: The first is a set of CGI
scripts which create all the pages at all directory
levels on-the-fly. I put them under
<code>/e/netsw/.www/</code> as follows:</p>
<div class="example"><pre>
-rw-r--r-- 1 netsw users 1318 Aug 1 18:10 .wwwacl
drwxr-xr-x 18 netsw users 512 Aug 5 15:51 DATA/
-rw-rw-rw- 1 netsw users 372982 Aug 5 16:35 LOGFILE
-rw-r--r-- 1 netsw users 659 Aug 4 09:27 TODO
-rw-r--r-- 1 netsw users 5697 Aug 1 18:01 netsw-about.html
-rwxr-xr-x 1 netsw users 579 Aug 2 10:33 netsw-access.pl
-rwxr-xr-x 1 netsw users 1532 Aug 1 17:35 netsw-changes.cgi
-rwxr-xr-x 1 netsw users 2866 Aug 5 14:49 netsw-home.cgi
drwxr-xr-x 2 netsw users 512 Jul 8 23:47 netsw-img/
-rwxr-xr-x 1 netsw users 24050 Aug 5 15:49 netsw-lsdir.cgi
-rwxr-xr-x 1 netsw users 1589 Aug 3 18:43 netsw-search.cgi
-rwxr-xr-x 1 netsw users 1885 Aug 1 17:41 netsw-tree.cgi
-rw-r--r-- 1 netsw users 234 Jul 30 16:35 netsw-unlimit.lst
</pre></div>
<p>The <code>DATA/</code> subdirectory holds the above
directory structure, i.e. the real
<strong><em>net.sw</em></strong> stuff and gets
automatically updated via <code>rdist</code> from time to
time. The second part of the problem remains: how to link
these two structures together into one smooth-looking URL
tree? We want to hide the <code>DATA/</code> directory
from the user while running the appropriate CGI scripts
for the various URLs. Here is the solution: first I put
the following into the per-directory configuration file
in the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code>
of the server to rewrite the announced URL
<code>/net.sw/</code> to the internal path
<code>/e/netsw</code>:</p>
<div class="example"><pre>
RewriteRule ^net.sw$ net.sw/ [R]
RewriteRule ^net.sw/(.*)$ e/netsw/$1
</pre></div>
<p>The first rule is for requests which miss the trailing
slash! The second rule does the real thing. And then
comes the killer configuration which stays in the
per-directory config file
<code>/e/netsw/.www/.wwwacl</code>:</p>
<div class="example"><pre>
Options ExecCGI FollowSymLinks Includes MultiViews
RewriteEngine on
# we are reached via /net.sw/ prefix
RewriteBase /net.sw/
# first we rewrite the root dir to
# the handling cgi script
RewriteRule ^$ netsw-home.cgi [L]
RewriteRule ^index\.html$ netsw-home.cgi [L]
# strip out the subdirs when
# the browser requests us from perdir pages
RewriteRule ^.+/(netsw-[^/]+/.+)$ $1 [L]
# and now break the rewriting for local files
RewriteRule ^netsw-home\.cgi.* - [L]
RewriteRule ^netsw-changes\.cgi.* - [L]
RewriteRule ^netsw-search\.cgi.* - [L]
RewriteRule ^netsw-tree\.cgi$ - [L]
RewriteRule ^netsw-about\.html$ - [L]
RewriteRule ^netsw-img/.*$ - [L]
# anything else is a subdir which gets handled
# by another cgi script
RewriteRule !^netsw-lsdir\.cgi.* - [C]
RewriteRule (.*) netsw-lsdir.cgi/$1
</pre></div>
<p>Some hints for interpretation:</p>
<ol>
<li>Notice the <code>L</code> (last) flag and no
substitution field ('<code>-</code>') in the forth part</li>
<li>Notice the <code>!</code> (not) character and
the <code>C</code> (chain) flag at the first rule
in the last part</li>
<li>Notice the catch-all pattern in the last rule</li>
</ol>
</dd>
</dl>
<h3>NCSA imagemap to Apache <code>mod_imap</code></h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>When switching from the NCSA webserver to the more
modern Apache webserver a lot of people want a smooth
transition. So they want pages which use their old NCSA
<code>imagemap</code> program to work under Apache with the
modern <code class="module"><a href="../mod/mod_imap.html">mod_imap</a></code>. The problem is that there
are a lot of hyperlinks around which reference the
<code>imagemap</code> program via
<code>/cgi-bin/imagemap/path/to/page.map</code>. Under
Apache this has to read just
<code>/path/to/page.map</code>.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We use a global rule to remove the prefix on-the-fly for
all requests:</p>
<div class="example"><pre>
RewriteEngine on
RewriteRule ^/cgi-bin/imagemap(.*) $1 [PT]
</pre></div>
</dd>
</dl>
<h3>Search pages in more than one directory</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Sometimes it is necessary to let the webserver search
for pages in more than one directory. Here MultiViews or
other techniques cannot help.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We program a explicit ruleset which searches for the
files in the directories.</p>
<div class="example"><pre>
RewriteEngine on
# first try to find it in custom/...
# ...and if found stop and be happy:
RewriteCond /your/docroot/<strong>dir1</strong>/%{REQUEST_FILENAME} -f
RewriteRule ^(.+) /your/docroot/<strong>dir1</strong>/$1 [L]
# second try to find it in pub/...
# ...and if found stop and be happy:
RewriteCond /your/docroot/<strong>dir2</strong>/%{REQUEST_FILENAME} -f
RewriteRule ^(.+) /your/docroot/<strong>dir2</strong>/$1 [L]
# else go on for other Alias or ScriptAlias directives,
# etc.
RewriteRule ^(.+) - [PT]
</pre></div>
</dd>
</dl>
<h3>Set Environment Variables According To URL Parts</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Perhaps you want to keep status information between
requests and use the URL to encode it. But you don't want
to use a CGI wrapper for all pages just to strip out this
information.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We use a rewrite rule to strip out the status information
and remember it via an environment variable which can be
later dereferenced from within XSSI or CGI. This way a
URL <code>/foo/S=java/bar/</code> gets translated to
<code>/foo/bar/</code> and the environment variable named
<code>STATUS</code> is set to the value "java".</p>
<div class="example"><pre>
RewriteEngine on
RewriteRule ^(.*)/<strong>S=([^/]+)</strong>/(.*) $1/$3 [E=<strong>STATUS:$2</strong>]
</pre></div>
</dd>
</dl>
<h3>Virtual User Hosts</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Assume that you want to provide
<code>www.<strong>username</strong>.host.domain.com</code>
for the homepage of username via just DNS A records to the
same machine and without any virtualhosts on this
machine.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>For HTTP/1.0 requests there is no solution, but for
HTTP/1.1 requests which contain a Host: HTTP header we
can use the following ruleset to rewrite
<code>http://www.username.host.com/anypath</code>
internally to <code>/home/username/anypath</code>:</p>
<div class="example"><pre>
RewriteEngine on
RewriteCond %{<strong>HTTP_HOST</strong>} ^www\.<strong>[^.]+</strong>\.host\.com$
RewriteRule ^(.+) %{HTTP_HOST}$1 [C]
RewriteRule ^www\.<strong>([^.]+)</strong>\.host\.com(.*) /home/<strong>$1</strong>$2
</pre></div>
</dd>
</dl>
<h3>Redirect Homedirs For Foreigners</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>We want to redirect homedir URLs to another webserver
<code>www.somewhere.com</code> when the requesting user
does not stay in the local domain
<code>ourdomain.com</code>. This is sometimes used in
virtual host contexts.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>Just a rewrite condition:</p>
<div class="example"><pre>
RewriteEngine on
RewriteCond %{REMOTE_HOST} <strong>!^.+\.ourdomain\.com$</strong>
RewriteRule ^(/~.+) http://www.somewhere.com/$1 [R,L]
</pre></div>
</dd>
</dl>
<h3>Redirect Failing URLs To Other Webserver</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>A typical FAQ about URL rewriting is how to redirect
failing requests on webserver A to webserver B. Usually
this is done via <code class="directive"><a href="../mod/core.html#errordocument">ErrorDocument</a></code> CGI-scripts in Perl, but
there is also a <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> solution.
But notice that this performs more poorly than using an
<code class="directive"><a href="../mod/core.html#errordocument">ErrorDocument</a></code>
CGI-script!</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>The first solution has the best performance but less
flexibility, and is less error safe:</p>
<div class="example"><pre>
RewriteEngine on
RewriteCond /your/docroot/%{REQUEST_FILENAME} <strong>!-f</strong>
RewriteRule ^(.+) http://<strong>webserverB</strong>.dom/$1
</pre></div>
<p>The problem here is that this will only work for pages
inside the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code>. While you can add more
Conditions (for instance to also handle homedirs, etc.)
there is better variant:</p>
<div class="example"><pre>
RewriteEngine on
RewriteCond %{REQUEST_URI} <strong>!-U</strong>
RewriteRule ^(.+) http://<strong>webserverB</strong>.dom/$1
</pre></div>
<p>This uses the URL look-ahead feature of <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>.
The result is that this will work for all types of URLs
and is a safe way. But it does a performance impact on
the webserver, because for every request there is one
more internal subrequest. So, if your webserver runs on a
powerful CPU, use this one. If it is a slow machine, use
the first approach or better a <code class="directive"><a href="../mod/core.html#errordocument">ErrorDocument</a></code> CGI-script.</p>
</dd>
</dl>
<h3>Extended Redirection</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Sometimes we need more control (concerning the
character escaping mechanism) of URLs on redirects.
Usually the Apache kernels URL escape function also
escapes anchors, i.e. URLs like "<code>url#anchor</code>".
You cannot use this directly on redirects with
<code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> because the
<code>uri_escape()</code> function of Apache
would also escape the hash character.
How can we redirect to such a URL?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We have to use a kludge by the use of a NPH-CGI script
which does the redirect itself. Because here no escaping
is done (NPH=non-parseable headers). First we introduce a
new URL scheme <code>xredirect:</code> by the following
per-server config-line (should be one of the last rewrite
rules):</p>
<div class="example"><pre>
RewriteRule ^xredirect:(.+) /path/to/nph-xredirect.cgi/$1 \
[T=application/x-httpd-cgi,L]
</pre></div>
<p>This forces all URLs prefixed with
<code>xredirect:</code> to be piped through the
<code>nph-xredirect.cgi</code> program. And this program
just looks like:</p>
<div class="example"><pre>
#!/path/to/perl
##
## nph-xredirect.cgi -- NPH/CGI script for extended redirects
## Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved.
##
$| = 1;
$url = $ENV{'PATH_INFO'};
print "HTTP/1.0 302 Moved Temporarily\n";
print "Server: $ENV{'SERVER_SOFTWARE'}\n";
print "Location: $url\n";
print "Content-type: text/html\n";
print "\n";
print "<html>\n";
print "<head>\n";
print "<title>302 Moved Temporarily (EXTENDED)</title>\n";
print "</head>\n";
print "<body>\n";
print "<h1>Moved Temporarily (EXTENDED)</h1>\n";
print "The document has moved <a HREF=\"$url\">here</a>.<p>\n";
print "</body>\n";
print "</html>\n";
##EOF##
</pre></div>
<p>This provides you with the functionality to do
redirects to all URL schemes, i.e. including the one
which are not directly accepted by <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>.
For instance you can now also redirect to
<code>news:newsgroup</code> via</p>
<div class="example"><pre>
RewriteRule ^anyurl xredirect:news:newsgroup
</pre></div>
<div class="note">Notice: You have not to put <code>[R]</code> or
<code>[R,L]</code> to the above rule because the
<code>xredirect:</code> need to be expanded later
by our special "pipe through" rule above.</div>
</dd>
</dl>
<h3>Archive Access Multiplexer</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Do you know the great CPAN (Comprehensive Perl Archive
Network) under <a href="http://www.perl.com/CPAN">http://www.perl.com/CPAN</a>?
This does a redirect to one of several FTP servers around
the world which carry a CPAN mirror and is approximately
near the location of the requesting client. Actually this
can be called an FTP access multiplexing service. While
CPAN runs via CGI scripts, how can a similar approach
implemented via <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>First we notice that from version 3.0.0
<code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> can
also use the "<code>ftp:</code>" scheme on redirects.
And second, the location approximation can be done by a
<code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>
over the top-level domain of the client.
With a tricky chained ruleset we can use this top-level
domain as a key to our multiplexing map.</p>
<div class="example"><pre>
RewriteEngine on
RewriteMap multiplex txt:/path/to/map.cxan
RewriteRule ^/CxAN/(.*) %{REMOTE_HOST}::$1 [C]
RewriteRule ^.+\.<strong>([a-zA-Z]+)</strong>::(.*)$ ${multiplex:<strong>$1</strong>|ftp.default.dom}$2 [R,L]
</pre></div>
<div class="example"><pre>
##
## map.cxan -- Multiplexing Map for CxAN
##
de ftp://ftp.cxan.de/CxAN/
uk ftp://ftp.cxan.uk/CxAN/
com ftp://ftp.cxan.com/CxAN/
:
##EOF##
</pre></div>
</dd>
</dl>
<h3>Time-Dependent Rewriting</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>When tricks like time-dependent content should happen a
lot of webmasters still use CGI scripts which do for
instance redirects to specialized pages. How can it be done
via <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>There are a lot of variables named <code>TIME_xxx</code>
for rewrite conditions. In conjunction with the special
lexicographic comparison patterns <code><STRING</code>,
<code>>STRING</code> and <code>=STRING</code> we can
do time-dependent redirects:</p>
<div class="example"><pre>
RewriteEngine on
RewriteCond %{TIME_HOUR}%{TIME_MIN} >0700
RewriteCond %{TIME_HOUR}%{TIME_MIN} <1900
RewriteRule ^foo\.html$ foo.day.html
RewriteRule ^foo\.html$ foo.night.html
</pre></div>
<p>This provides the content of <code>foo.day.html</code>
under the URL <code>foo.html</code> from
<code>07:00-19:00</code> and at the remaining time the
contents of <code>foo.night.html</code>. Just a nice
feature for a homepage...</p>
</dd>
</dl>
<h3>Backward Compatibility for YYYY to XXXX migration</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we make URLs backward compatible (still
existing virtually) after migrating <code>document.YYYY</code>
to <code>document.XXXX</code>, e.g. after translating a
bunch of <code>.html</code> files to <code>.phtml</code>?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We just rewrite the name to its basename and test for
existence of the new extension. If it exists, we take
that name, else we rewrite the URL to its original state.</p>
<div class="example"><pre>
# backward compatibility ruleset for
# rewriting document.html to document.phtml
# when and only when document.phtml exists
# but no longer document.html
RewriteEngine on
RewriteBase /~quux/
# parse out basename, but remember the fact
RewriteRule ^(.*)\.html$ $1 [C,E=WasHTML:yes]
# rewrite to document.phtml if exists
RewriteCond %{REQUEST_FILENAME}.phtml -f
RewriteRule ^(.*)$ $1.phtml [S=1]
# else reverse the previous basename cutout
RewriteCond %{ENV:WasHTML} ^yes$
RewriteRule ^(.*)$ $1.html
</pre></div>
</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="content" id="content">Content Handling</a></h2>
<h3>From Old to New (intern)</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Assume we have recently renamed the page
<code>bar.html</code> to <code>foo.html</code> and now want
to provide the old URL for backward compatibility. Actually
we want that users of the old URL even not recognize that
the pages was renamed.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We rewrite the old URL to the new one internally via the
following rule:</p>
<div class="example"><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>foo</strong>\.html$ <strong>bar</strong>.html
</pre></div>
</dd>
</dl>
<h3>From Old to New (extern)</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Assume again that we have recently renamed the page
<code>bar.html</code> to <code>foo.html</code> and now want
to provide the old URL for backward compatibility. But this
time we want that the users of the old URL get hinted to
the new one, i.e. their browsers Location field should
change, too.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We force a HTTP redirect to the new URL which leads to a
change of the browsers and thus the users view:</p>
<div class="example"><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>foo</strong>\.html$ <strong>bar</strong>.html [<strong>R</strong>]
</pre></div>
</dd>
</dl>
<h3>Browser Dependent Content</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>At least for important top-level pages it is sometimes
necessary to provide the optimum of browser dependent
content, i.e. one has to provide a maximum version for the
latest Netscape variants, a minimum version for the Lynx
browsers and a average feature version for all others.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We cannot use content negotiation because the browsers do
not provide their type in that form. Instead we have to
act on the HTTP header "User-Agent". The following condig
does the following: If the HTTP header "User-Agent"
begins with "Mozilla/3", the page <code>foo.html</code>
is rewritten to <code>foo.NS.html</code> and and the
rewriting stops. If the browser is "Lynx" or "Mozilla" of
version 1 or 2 the URL becomes <code>foo.20.html</code>.
All other browsers receive page <code>foo.32.html</code>.
This is done by the following ruleset:</p>
<div class="example"><pre>
RewriteCond %{HTTP_USER_AGENT} ^<strong>Mozilla/3</strong>.*
RewriteRule ^foo\.html$ foo.<strong>NS</strong>.html [<strong>L</strong>]
RewriteCond %{HTTP_USER_AGENT} ^<strong>Lynx/</strong>.* [OR]
RewriteCond %{HTTP_USER_AGENT} ^<strong>Mozilla/[12]</strong>.*
RewriteRule ^foo\.html$ foo.<strong>20</strong>.html [<strong>L</strong>]
RewriteRule ^foo\.html$ foo.<strong>32</strong>.html [<strong>L</strong>]
</pre></div>
</dd>
</dl>
<h3>Dynamic Mirror</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Assume there are nice webpages on remote hosts we want
to bring into our namespace. For FTP servers we would use
the <code>mirror</code> program which actually maintains an
explicit up-to-date copy of the remote data on the local
machine. For a webserver we could use the program
<code>webcopy</code> which acts similar via HTTP. But both
techniques have one major drawback: The local copy is
always just as up-to-date as often we run the program. It
would be much better if the mirror is not a static one we
have to establish explicitly. Instead we want a dynamic
mirror with data which gets updated automatically when
there is need (updated data on the remote host).</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>To provide this feature we map the remote webpage or even
the complete remote webarea to our namespace by the use
of the <i>Proxy Throughput</i> feature
(flag <code>[P]</code>):</p>
<div class="example"><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>hotsheet/</strong>(.*)$ <strong>http://www.tstimpreso.com/hotsheet/</strong>$1 [<strong>P</strong>]
</pre></div>
<div class="example"><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>usa-news\.html</strong>$ <strong>http://www.quux-corp.com/news/index.html</strong> [<strong>P</strong>]
</pre></div>
</dd>
</dl>
<h3>Reverse Dynamic Mirror</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>...</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<div class="example"><pre>
RewriteEngine on
RewriteCond /mirror/of/remotesite/$1 -U
RewriteRule ^http://www\.remotesite\.com/(.*)$ /mirror/of/remotesite/$1
</pre></div>
</dd>
</dl>
<h3>Retrieve Missing Data from Intranet</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>This is a tricky way of virtually running a corporate
(external) Internet webserver
(<code>www.quux-corp.dom</code>), while actually keeping
and maintaining its data on a (internal) Intranet webserver
(<code>www2.quux-corp.dom</code>) which is protected by a
firewall. The trick is that on the external webserver we
retrieve the requested data on-the-fly from the internal
one.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>First, we have to make sure that our firewall still
protects the internal webserver and that only the
external webserver is allowed to retrieve data from it.
For a packet-filtering firewall we could for instance
configure a firewall ruleset like the following:</p>
<div class="example"><pre>
<strong>ALLOW</strong> Host www.quux-corp.dom Port >1024 --> Host www2.quux-corp.dom Port <strong>80</strong>
<strong>DENY</strong> Host * Port * --> Host www2.quux-corp.dom Port <strong>80</strong>
</pre></div>
<p>Just adjust it to your actual configuration syntax.
Now we can establish the <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
rules which request the missing data in the background
through the proxy throughput feature:</p>
<div class="example"><pre>
RewriteRule ^/~([^/]+)/?(.*) /home/$1/.www/$2
RewriteCond %{REQUEST_FILENAME} <strong>!-f</strong>
RewriteCond %{REQUEST_FILENAME} <strong>!-d</strong>
RewriteRule ^/home/([^/]+)/.www/?(.*) http://<strong>www2</strong>.quux-corp.dom/~$1/pub/$2 [<strong>P</strong>]
</pre></div>
</dd>
</dl>
<h3>Load Balancing</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Suppose we want to load balance the traffic to
<code>www.foo.com</code> over <code>www[0-5].foo.com</code>
(a total of 6 servers). How can this be done?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>There are a lot of possible solutions for this problem.
We will discuss first a commonly known DNS-based variant
and then the special one with <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>:</p>
<ol>
<li>
<strong>DNS Round-Robin</strong>
<p>The simplest method for load-balancing is to use
the DNS round-robin feature of <code>BIND</code>.
Here you just configure <code>www[0-9].foo.com</code>
as usual in your DNS with A(address) records, e.g.</p>
<div class="example"><pre>
www0 IN A 1.2.3.1
www1 IN A 1.2.3.2
www2 IN A 1.2.3.3
www3 IN A 1.2.3.4
www4 IN A 1.2.3.5
www5 IN A 1.2.3.6
</pre></div>
<p>Then you additionally add the following entry:</p>
<div class="example"><pre>
www IN CNAME www0.foo.com.
IN CNAME www1.foo.com.
IN CNAME www2.foo.com.
IN CNAME www3.foo.com.
IN CNAME www4.foo.com.
IN CNAME www5.foo.com.
IN CNAME www6.foo.com.
</pre></div>
<p>Notice that this seems wrong, but is actually an
intended feature of <code>BIND</code> and can be used
in this way. However, now when <code>www.foo.com</code> gets
resolved, <code>BIND</code> gives out <code>www0-www6</code>
- but in a slightly permutated/rotated order every time.
This way the clients are spread over the various
servers. But notice that this not a perfect load
balancing scheme, because DNS resolve information
gets cached by the other nameservers on the net, so
once a client has resolved <code>www.foo.com</code>
to a particular <code>wwwN.foo.com</code>, all
subsequent requests also go to this particular name
<code>wwwN.foo.com</code>. But the final result is
ok, because the total sum of the requests are really
spread over the various webservers.</p>
</li>
<li>
<strong>DNS Load-Balancing</strong>
<p>A sophisticated DNS-based method for
load-balancing is to use the program
<code>lbnamed</code> which can be found at <a href="http://www.stanford.edu/~schemers/docs/lbnamed/lbnamed.html">
http://www.stanford.edu/~schemers/docs/lbnamed/lbnamed.html</a>.
It is a Perl 5 program in conjunction with auxilliary
tools which provides a real load-balancing for
DNS.</p>
</li>
<li>
<strong>Proxy Throughput Round-Robin</strong>
<p>In this variant we use <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
and its proxy throughput feature. First we dedicate
<code>www0.foo.com</code> to be actually
<code>www.foo.com</code> by using a single</p>
<div class="example"><pre>
www IN CNAME www0.foo.com.
</pre></div>
<p>entry in the DNS. Then we convert
<code>www0.foo.com</code> to a proxy-only server,
i.e. we configure this machine so all arriving URLs
are just pushed through the internal proxy to one of
the 5 other servers (<code>www1-www5</code>). To
accomplish this we first establish a ruleset which
contacts a load balancing script <code>lb.pl</code>
for all URLs.</p>
<div class="example"><pre>
RewriteEngine on
RewriteMap lb prg:/path/to/lb.pl
RewriteRule ^/(.+)$ ${lb:$1} [P,L]
</pre></div>
<p>Then we write <code>lb.pl</code>:</p>
<div class="example"><pre>
#!/path/to/perl
##
## lb.pl -- load balancing script
##
$| = 1;
$name = "www"; # the hostname base
$first = 1; # the first server (not 0 here, because 0 is myself)
$last = 5; # the last server in the round-robin
$domain = "foo.dom"; # the domainname
$cnt = 0;
while (<STDIN>) {
$cnt = (($cnt+1) % ($last+1-$first));
$server = sprintf("%s%d.%s", $name, $cnt+$first, $domain);
print "http://$server/$_";
}
##EOF##
</pre></div>
<div class="note">A last notice: Why is this useful? Seems like
<code>www0.foo.com</code> still is overloaded? The
answer is yes, it is overloaded, but with plain proxy
throughput requests, only! All SSI, CGI, ePerl, etc.
processing is completely done on the other machines.
This is the essential point.</div>
</li>
<li>
<strong>Hardware/TCP Round-Robin</strong>
<p>There is a hardware solution available, too. Cisco
has a beast called LocalDirector which does a load
balancing at the TCP/IP level. Actually this is some
sort of a circuit level gateway in front of a
webcluster. If you have enough money and really need
a solution with high performance, use this one.</p>
</li>
</ol>
</dd>
</dl>
<h3>Reverse Proxy</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>...</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<div class="example"><pre>
##
## apache-rproxy.conf -- Apache configuration for Reverse Proxy Usage
##
# server type
ServerType standalone
Listen 8000
MinSpareServers 16
StartServers 16
MaxSpareServers 16
MaxClients 16
MaxRequestsPerChild 100
# server operation parameters
KeepAlive on
MaxKeepAliveRequests 100
KeepAliveTimeout 15
Timeout 400
IdentityCheck off
HostnameLookups off
# paths to runtime files
PidFile /path/to/apache-rproxy.pid
LockFile /path/to/apache-rproxy.lock
ErrorLog /path/to/apache-rproxy.elog
CustomLog /path/to/apache-rproxy.dlog "%{%v/%T}t %h -> %{SERVER}e URL: %U"
# unused paths
ServerRoot /tmp
DocumentRoot /tmp
CacheRoot /tmp
RewriteLog /dev/null
TransferLog /dev/null
TypesConfig /dev/null
AccessConfig /dev/null
ResourceConfig /dev/null
# speed up and secure processing
<Directory />
Options -FollowSymLinks -SymLinksIfOwnerMatch
AllowOverride None
</Directory>
# the status page for monitoring the reverse proxy
<Location /apache-rproxy-status>
SetHandler server-status
</Location>
# enable the URL rewriting engine
RewriteEngine on
RewriteLogLevel 0
# define a rewriting map with value-lists where
# mod_rewrite randomly chooses a particular value
RewriteMap server rnd:/path/to/apache-rproxy.conf-servers
# make sure the status page is handled locally
# and make sure no one uses our proxy except ourself
RewriteRule ^/apache-rproxy-status.* - [L]
RewriteRule ^(http|ftp)://.* - [F]
# now choose the possible servers for particular URL types
RewriteRule ^/(.*\.(cgi|shtml))$ to://${server:dynamic}/$1 [S=1]
RewriteRule ^/(.*)$ to://${server:static}/$1
# and delegate the generated URL by passing it
# through the proxy module
RewriteRule ^to://([^/]+)/(.*) http://$1/$2 [E=SERVER:$1,P,L]
# and make really sure all other stuff is forbidden
# when it should survive the above rules...
RewriteRule .* - [F]
# enable the Proxy module without caching
ProxyRequests on
NoCache *
# setup URL reverse mapping for redirect reponses
ProxyPassReverse / http://www1.foo.dom/
ProxyPassReverse / http://www2.foo.dom/
ProxyPassReverse / http://www3.foo.dom/
ProxyPassReverse / http://www4.foo.dom/
ProxyPassReverse / http://www5.foo.dom/
ProxyPassReverse / http://www6.foo.dom/
</pre></div>
<div class="example"><pre>
##
## apache-rproxy.conf-servers -- Apache/mod_rewrite selection table
##
# list of backend servers which serve static
# pages (HTML files and Images, etc.)
static www1.foo.dom|www2.foo.dom|www3.foo.dom|www4.foo.dom
# list of backend servers which serve dynamically
# generated page (CGI programs or mod_perl scripts)
dynamic www5.foo.dom|www6.foo.dom
</pre></div>
</dd>
</dl>
<h3>New MIME-type, New Service</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>On the net there are a lot of nifty CGI programs. But
their usage is usually boring, so a lot of webmaster
don't use them. Even Apache's Action handler feature for
MIME-types is only appropriate when the CGI programs
don't need special URLs (actually <code>PATH_INFO</code>
and <code>QUERY_STRINGS</code>) as their input. First,
let us configure a new file type with extension
<code>.scgi</code> (for secure CGI) which will be processed
by the popular <code>cgiwrap</code> program. The problem
here is that for instance we use a Homogeneous URL Layout
(see above) a file inside the user homedirs has the URL
<code>/u/user/foo/bar.scgi</code>. But
<code>cgiwrap</code> needs the URL in the form
<code>/~user/foo/bar.scgi/</code>. The following rule
solves the problem:</p>
<div class="example"><pre>
RewriteRule ^/[uge]/<strong>([^/]+)</strong>/\.www/(.+)\.scgi(.*) ...
... /internal/cgi/user/cgiwrap/~<strong>$1</strong>/$2.scgi$3 [NS,<strong>T=application/x-http-cgi</strong>]
</pre></div>
<p>Or assume we have some more nifty programs:
<code>wwwlog</code> (which displays the
<code>access.log</code> for a URL subtree and
<code>wwwidx</code> (which runs Glimpse on a URL
subtree). We have to provide the URL area to these
programs so they know on which area they have to act on.
But usually this ugly, because they are all the times
still requested from that areas, i.e. typically we would
run the <code>swwidx</code> program from within
<code>/u/user/foo/</code> via hyperlink to</p>
<div class="example"><pre>
/internal/cgi/user/swwidx?i=/u/user/foo/
</pre></div>
<p>which is ugly. Because we have to hard-code
<strong>both</strong> the location of the area
<strong>and</strong> the location of the CGI inside the
hyperlink. When we have to reorganize the area, we spend a
lot of time changing the various hyperlinks.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>The solution here is to provide a special new URL format
which automatically leads to the proper CGI invocation.
We configure the following:</p>
<div class="example"><pre>
RewriteRule ^/([uge])/([^/]+)(/?.*)/\* /internal/cgi/user/wwwidx?i=/$1/$2$3/
RewriteRule ^/([uge])/([^/]+)(/?.*):log /internal/cgi/user/wwwlog?f=/$1/$2$3
</pre></div>
<p>Now the hyperlink to search at
<code>/u/user/foo/</code> reads only</p>
<div class="example"><pre>
HREF="*"
</pre></div>
<p>which internally gets automatically transformed to</p>
<div class="example"><pre>
/internal/cgi/user/wwwidx?i=/u/user/foo/
</pre></div>
<p>The same approach leads to an invocation for the
access log CGI program when the hyperlink
<code>:log</code> gets used.</p>
</dd>
</dl>
<h3>From Static to Dynamic</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we transform a static page
<code>foo.html</code> into a dynamic variant
<code>foo.cgi</code> in a seamless way, i.e. without notice
by the browser/user.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We just rewrite the URL to the CGI-script and force the
correct MIME-type so it gets really run as a CGI-script.
This way a request to <code>/~quux/foo.html</code>
internally leads to the invocation of
<code>/~quux/foo.cgi</code>.</p>
<div class="example"><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^foo\.<strong>html</strong>$ foo.<strong>cgi</strong> [T=<strong>application/x-httpd-cgi</strong>]
</pre></div>
</dd>
</dl>
<h3>On-the-fly Content-Regeneration</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Here comes a really esoteric feature: Dynamically
generated but statically served pages, i.e. pages should be
delivered as pure static pages (read from the filesystem
and just passed through), but they have to be generated
dynamically by the webserver if missing. This way you can
have CGI-generated pages which are statically served unless
one (or a cronjob) removes the static contents. Then the
contents gets refreshed.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
This is done via the following ruleset:
<div class="example"><pre>
RewriteCond %{REQUEST_FILENAME} <strong>!-s</strong>
RewriteRule ^page\.<strong>html</strong>$ page.<strong>cgi</strong> [T=application/x-httpd-cgi,L]
</pre></div>
<p>Here a request to <code>page.html</code> leads to a
internal run of a corresponding <code>page.cgi</code> if
<code>page.html</code> is still missing or has filesize
null. The trick here is that <code>page.cgi</code> is a
usual CGI script which (additionally to its <code>STDOUT</code>)
writes its output to the file <code>page.html</code>.
Once it was run, the server sends out the data of
<code>page.html</code>. When the webmaster wants to force
a refresh the contents, he just removes
<code>page.html</code> (usually done by a cronjob).</p>
</dd>
</dl>
<h3>Document With Autorefresh</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Wouldn't it be nice while creating a complex webpage if
the webbrowser would automatically refresh the page every
time we write a new version from within our editor?
Impossible?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>No! We just combine the MIME multipart feature, the
webserver NPH feature and the URL manipulation power of
<code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>. First, we establish a new
URL feature: Adding just <code>:refresh</code> to any
URL causes this to be refreshed every time it gets
updated on the filesystem.</p>
<div class="example"><pre>
RewriteRule ^(/[uge]/[^/]+/?.*):refresh /internal/cgi/apache/nph-refresh?f=$1
</pre></div>
<p>Now when we reference the URL</p>
<div class="example"><pre>
/u/foo/bar/page.html:refresh
</pre></div>
<p>this leads to the internal invocation of the URL</p>
<div class="example"><pre>
/internal/cgi/apache/nph-refresh?f=/u/foo/bar/page.html
</pre></div>
<p>The only missing part is the NPH-CGI script. Although
one would usually say "left as an exercise to the reader"
;-) I will provide this, too.</p>
<div class="example"><pre>
#!/sw/bin/perl
##
## nph-refresh -- NPH/CGI script for auto refreshing pages
## Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved.
##
$| = 1;
# split the QUERY_STRING variable
@pairs = split(/&/, $ENV{'QUERY_STRING'});
foreach $pair (@pairs) {
($name, $value) = split(/=/, $pair);
$name =~ tr/A-Z/a-z/;
$name = 'QS_' . $name;
$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
eval "\$$name = \"$value\"";
}
$QS_s = 1 if ($QS_s eq '');
$QS_n = 3600 if ($QS_n eq '');
if ($QS_f eq '') {
print "HTTP/1.0 200 OK\n";
print "Content-type: text/html\n\n";
print "&lt;b&gt;ERROR&lt;/b&gt;: No file given\n";
exit(0);
}
if (! -f $QS_f) {
print "HTTP/1.0 200 OK\n";
print "Content-type: text/html\n\n";
print "&lt;b&gt;ERROR&lt;/b&gt;: File $QS_f not found\n";
exit(0);
}
sub print_http_headers_multipart_begin {
print "HTTP/1.0 200 OK\n";
$bound = "ThisRandomString12345";
print "Content-type: multipart/x-mixed-replace;boundary=$bound\n";
&print_http_headers_multipart_next;
}
sub print_http_headers_multipart_next {
print "\n--$bound\n";
}
sub print_http_headers_multipart_end {
print "\n--$bound--\n";
}
sub displayhtml {
local($buffer) = @_;
$len = length($buffer);
print "Content-type: text/html\n";
print "Content-length: $len\n\n";
print $buffer;
}
sub readfile {
local($file) = @_;
local(*FP, $size, $buffer, $bytes);
($x, $x, $x, $x, $x, $x, $x, $size) = stat($file);
$size = sprintf("%d", $size);
open(FP, "&lt;$file");
$bytes = sysread(FP, $buffer, $size);
close(FP);
return $buffer;
}
$buffer = &readfile($QS_f);
&print_http_headers_multipart_begin;
&displayhtml($buffer);
sub mystat {
local($file) = $_[0];
local($time);
($x, $x, $x, $x, $x, $x, $x, $x, $x, $mtime) = stat($file);
return $mtime;
}
$mtimeL = &mystat($QS_f);
$mtime = $mtime;
for ($n = 0; $n &lt; $QS_n; $n++) {
while (1) {
$mtime = &mystat($QS_f);
if ($mtime ne $mtimeL) {
$mtimeL = $mtime;
sleep(2);
$buffer = &readfile($QS_f);
&print_http_headers_multipart_next;
&displayhtml($buffer);
sleep(5);
$mtimeL = &mystat($QS_f);
last;
}
sleep($QS_s);
}
}
&print_http_headers_multipart_end;
exit(0);
##EOF##
</pre></div>
</dd>
</dl>
<h3>Mass Virtual Hosting</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>The <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code> feature of Apache is nice
and works great when you just have a few dozens
virtual hosts. But when you are an ISP and have hundreds of
virtual hosts to provide this feature is not the best
choice.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>To provide this feature we map the remote webpage or even
the complete remote webarea to our namespace by the use
of the <i>Proxy Throughput</i> feature (flag <code>[P]</code>):</p>
<div class="example"><pre>
##
## vhost.map
##
www.vhost1.dom:80 /path/to/docroot/vhost1
www.vhost2.dom:80 /path/to/docroot/vhost2
:
www.vhostN.dom:80 /path/to/docroot/vhostN
</pre></div>
<div class="example"><pre>
##
## httpd.conf
##
:
# use the canonical hostname on redirects, etc.
UseCanonicalName on
:
# add the virtual host in front of the CLF-format
CustomLog /path/to/access_log "%{VHOST}e %h %l %u %t \"%r\" %>s %b"
:
# enable the rewriting engine in the main server
RewriteEngine on
# define two maps: one for fixing the URL and one which defines
# the available virtual hosts with their corresponding
# DocumentRoot.
RewriteMap lowercase int:tolower
RewriteMap vhost txt:/path/to/vhost.map
# Now do the actual virtual host mapping
# via a huge and complicated single rule:
#
# 1. make sure we don't map for common locations
RewriteCond %{REQUEST_URL} !^/commonurl1/.*
RewriteCond %{REQUEST_URL} !^/commonurl2/.*
:
RewriteCond %{REQUEST_URL} !^/commonurlN/.*
#
# 2. make sure we have a Host header, because
# currently our approach only supports
# virtual hosting through this header
RewriteCond %{HTTP_HOST} !^$
#
# 3. lowercase the hostname
RewriteCond ${lowercase:%{HTTP_HOST}|NONE} ^(.+)$
#
# 4. lookup this hostname in vhost.map and
# remember it only when it is a path
# (and not "NONE" from above)
RewriteCond ${vhost:%1} ^(/.*)$
#
# 5. finally we can map the URL to its docroot location
# and remember the virtual host for logging puposes
RewriteRule ^/(.*)$ %1/$1 [E=VHOST:${lowercase:%{HTTP_HOST}}]
:
</pre></div>
</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="access" id="access">Access Restriction</a></h2>
<h3>Blocking of Robots</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we block a really annoying robot from
retrieving pages of a specific webarea? A
<code>/robots.txt</code> file containing entries of the
"Robot Exclusion Protocol" is typically not enough to get
rid of such a robot.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We use a ruleset which forbids the URLs of the webarea
<code>/~quux/foo/arc/</code> (perhaps a very deep
directory indexed area where the robot traversal would
create big server load). We have to make sure that we
forbid access only to the particular robot, i.e. just
forbidding the host where the robot runs is not enough.
This would block users from this host, too. We accomplish
this by also matching the User-Agent HTTP header
information.</p>
<div class="example"><pre>
RewriteCond %{HTTP_USER_AGENT} ^<strong>NameOfBadRobot</strong>.*
RewriteCond %{REMOTE_ADDR} ^<strong>123\.45\.67\.[8-9]</strong>$
RewriteRule ^<strong>/~quux/foo/arc/</strong>.+ - [<strong>F</strong>]
</pre></div>
</dd>
</dl>
<h3>Blocked Inline-Images</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Assume we have under <code>http://www.quux-corp.de/~quux/</code>
some pages with inlined GIF graphics. These graphics are
nice, so others directly incorporate them via hyperlinks to
their pages. We don't like this practice because it adds
useless traffic to our server.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>While we cannot 100% protect the images from inclusion,
we can at least restrict the cases where the browser
sends a HTTP Referer header.</p>
<div class="example"><pre>
RewriteCond %{HTTP_REFERER} <strong>!^$</strong>
RewriteCond %{HTTP_REFERER} !^http://www.quux-corp.de/~quux/.*$ [NC]
RewriteRule <strong>.*\.gif$</strong> - [F]
</pre></div>
<div class="example"><pre>
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !.*/foo-with-gif\.html$
RewriteRule <strong>^inlined-in-foo\.gif$</strong> - [F]
</pre></div>
</dd>
</dl>
<h3>Host Deny</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we forbid a list of externally configured hosts
from using our server?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>For Apache >= 1.3b6:</p>
<div class="example"><pre>
RewriteEngine on
RewriteMap hosts-deny txt:/path/to/hosts.deny
RewriteCond ${hosts-deny:%{REMOTE_HOST}|NOT-FOUND} !=NOT-FOUND [OR]
RewriteCond ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND} !=NOT-FOUND
RewriteRule ^/.* - [F]
</pre></div>
<p>For Apache <= 1.3b6:</p>
<div class="example"><pre>
RewriteEngine on
RewriteMap hosts-deny txt:/path/to/hosts.deny
RewriteRule ^/(.*)$ ${hosts-deny:%{REMOTE_HOST}|NOT-FOUND}/$1
RewriteRule !^NOT-FOUND/.* - [F]
RewriteRule ^NOT-FOUND/(.*)$ ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND}/$1
RewriteRule !^NOT-FOUND/.* - [F]
RewriteRule ^NOT-FOUND/(.*)$ /$1
</pre></div>
<div class="example"><pre>
##
## hosts.deny
##
## ATTENTION! This is a map, not a list, even when we treat it as such.
## mod_rewrite parses it for key/value pairs, so at least a
## dummy value "-" must be present for each entry.
##
193.102.180.41 -
bsdti1.sdm.de -
192.76.162.40 -
</pre></div>
</dd>
</dl>
<h3>Proxy Deny</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we forbid a certain host or even a user of a
special host from using the Apache proxy?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We first have to make sure <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
is below(!) <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code> in the Configuration
file when compiling the Apache webserver. This way it gets
called <em>before</em> <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code>. Then we
configure the following for a host-dependent deny...</p>
<div class="example"><pre>
RewriteCond %{REMOTE_HOST} <strong>^badhost\.mydomain\.com$</strong>
RewriteRule !^http://[^/.]\.mydomain.com.* - [F]
</pre></div>
<p>...and this one for a user@host-dependent deny:</p>
<div class="example"><pre>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>^badguy@badhost\.mydomain\.com$</strong>
RewriteRule !^http://[^/.]\.mydomain.com.* - [F]
</pre></div>
</dd>
</dl>
<h3>Special Authentication Variant</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Sometimes a very special authentication is needed, for
instance a authentication which checks for a set of
explicitly configured users. Only these should receive
access and without explicit prompting (which would occur
when using the Basic Auth via <code class="module"><a href="../mod/mod_access.html">mod_access</a></code>).</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We use a list of rewrite conditions to exclude all except
our friends:</p>
<div class="example"><pre>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>!^friend1@client1.quux-corp\.com$</strong>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>!^friend2</strong>@client2.quux-corp\.com$
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>!^friend3</strong>@client3.quux-corp\.com$
RewriteRule ^/~quux/only-for-friends/ - [F]
</pre></div>
</dd>
</dl>
<h3>Referer-based Deflector</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we program a flexible URL Deflector which acts
on the "Referer" HTTP header and can be configured with as
many referring pages as we like?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>Use the following really tricky ruleset...</p>
<div class="example"><pre>
RewriteMap deflector txt:/path/to/deflector.map
RewriteCond %{HTTP_REFERER} !=""
RewriteCond ${deflector:%{HTTP_REFERER}} ^-$
RewriteRule ^.* %{HTTP_REFERER} [R,L]
RewriteCond %{HTTP_REFERER} !=""
RewriteCond ${deflector:%{HTTP_REFERER}|NOT-FOUND} !=NOT-FOUND
RewriteRule ^.* ${deflector:%{HTTP_REFERER}} [R,L]
</pre></div>
<p>... in conjunction with a corresponding rewrite
map:</p>
<div class="example"><pre>
##
## deflector.map
##
http://www.badguys.com/bad/index.html -
http://www.badguys.com/bad/index2.html -
http://www.badguys.com/bad/index3.html http://somewhere.com/
</pre></div>
<p>This automatically redirects the request back to the
referring page (when "<code>-</code>" is used as the value
in the map) or to a specific URL (when an URL is specified
in the map as the second argument).</p>
</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2><a name="other" id="other">Other</a></h2>
<h3>External Rewriting Engine</h3>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>A FAQ: How can we solve the FOO/BAR/QUUX/etc.
problem? There seems no solution by the use of
<code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>...</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>Use an external <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>, i.e. a program which acts
like a <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>. It is run once on startup of Apache
receives the requested URLs on <code>STDIN</code> and has
to put the resulting (usually rewritten) URL on
<code>STDOUT</code> (same order!).</p>
<div class="example"><pre>
RewriteEngine on
RewriteMap quux-map <strong>prg:</strong>/path/to/map.quux.pl
RewriteRule ^/~quux/(.*)$ /~quux/<strong>${quux-map:$1}</strong>
</pre></div>
<div class="example"><pre>
#!/path/to/perl
# disable buffered I/O which would lead
# to deadloops for the Apache server
$| = 1;
# read URLs one per line from stdin and
# generate substitution URL on stdout
while (<>) {
s|^foo/|bar/|;
print $_;
}
</pre></div>
<p>This is a demonstration-only example and just rewrites
all URLs <code>/~quux/foo/...</code> to
<code>/~quux/bar/...</code>. Actually you can program
whatever you like. But notice that while such maps can be
<strong>used</strong> also by an average user, only the
system administrator can <strong>define</strong> it.</p>
</dd>
</dl>
</div></div><div id="footer"><p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div></body></html>
1.1 httpd-2.0/docs/manual/misc/index.xml
Index: index.xml
===================================================================
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
<manualpage>
<relativepath href=".." />
<title>Apache Miscellaneous Documentation</title>
<summary>
<p>Below is a list of additional documentation pages that apply
to the Apache web server development project.</p>
<dl>
<dt><a href="custom_errordocs.html">How to use XSSI and
Negotiation for custom ErrorDocuments</a></dt>
<dd>
<p>Describes a solution which uses XSSI and negotiation to
custom-tailor the Apache ErrorDocuments to taste, adding the
advantage of returning internationalized versions of the
error messages depending on the client's language
preferences.</p>
</dd>
<dt><a href="descriptors.html">File Descriptor use in
Apache</a></dt>
<dd>
<p>Describes how Apache uses file descriptors and talks
about various limits imposed on the number of descriptors
available by various operating systems.</p>
</dd>
<dt><a
href="fin_wait_2.html"><code>FIN_WAIT_2</code></a></dt>
<dd>
<p>A description of the causes of Apache processes going
into the <code>FIN_WAIT_2</code> state, and what you can do
about it.</p>
</dd>
<dt><a href="known_client_problems.html">Known Client
Problems</a></dt>
<dd>
<p>A list of problems in HTTP clients which can be mitigated
by Apache.</p>
</dd>
<dt><a href="perf-tuning.html">Performance Notes - Apache
Tuning</a></dt>
<dd>
<p>Notes about how to (run-time and compile-time) configure
Apache for highest performance. Notes explaining why Apache
does some things, and why it doesn't do other things (which
make it slower/faster).</p>
<note type="warning"><strong>Warning:</strong>
This document has not been fully updated
to take into account changes made in the 2.0 version of the
Apache HTTP Server. Some of the information may still be
relevant, but please use it with care.</note>
</dd>
<dt><a href="security_tips.html">Security Tips</a></dt>
<dd>
<p>Some "do"s - and "don't"s - for keeping your Apache web
site secure.</p>
</dd>
</dl>
</summary>
</manualpage>
1.1 httpd-2.0/docs/manual/misc/perf-tuning.xml
Index: perf-tuning.xml
===================================================================
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
<manualpage>
<relativepath href=".." />
<title>Apache Performance Notes</title>
<summary>
<note type="warning"><strong>Warning:</strong>
This document has not been fully updated
to take into account changes made in the 2.0 version of the
Apache HTTP Server. Some of the information may still be
relevant, but please use it with care.</note>
<p>Orignally written by Dean Gaudet.</p>
<p>Apache 2.0 is a general-purpose webserver, designed to
provide a balance of flexibility, portability, and performance.
Although it has not been designed specifically to set benchmark
records, Apache 2.0 is capable of high performance in many
real-world situations.</p>
<p>Compared to Apache 1.3, release 2.0 contains many additional
optimizations to increase throughput and scalability. Most of
these improvements are enabled by default. However, there are
compile-time and run-time configuration choices that can
significantly affect performance. This document describes the
options that a server administrator can configure to tune the
performance of an Apache 2.0 installation. Some of these
configuration options enable the httpd to better take advantage
of the capabilities of the hardware and OS, while others allow
the administrator to trade functionality for speed.</p>
</summary>
<section id="hardware">
<title>Hardware and Operating System Issues</title>
<p>The single biggest hardware issue affecting webserver
performance is RAM. A webserver should never ever have to swap,
swapping increases the latency of each request beyond a point
that users consider "fast enough". This causes users to hit
stop and reload, further increasing the load. You can, and
should, control the <directive module="mpm_common"
>MaxClients</directive> setting so that your server
does not spawn so many children it starts swapping.</p>
<p>Beyond that the rest is mundane: get a fast enough CPU, a
fast enough network card, and fast enough disks, where "fast
enough" is something that needs to be determined by
experimentation.</p>
<p>Operating system choice is largely a matter of local
concerns. But some guidelines that have proven generally
useful are:</p>
<ul>
<li>
<p>Run the latest stable release and patchlevel of the
operating system that you choose. Many OS suppliers have
introduced significant performance improvements to their
TCP stacks and thread libraries in recent years.</p>
</li>
<li>
<p>If your OS supports a <code>sendfile(2)</code> system
call, make sure you install the release and/or patches
needed to enable it. (With Linux, for example, this means
using Linux 2.4 or later. For early releases of Solaris 8,
you may need to apply a patch.) On systems where it is
available, <code>sendfile</code> enables Apache 2 to deliver
static content faster and with lower CPU utilization.</p>
</li>
</ul>
</section>
<section id="runtime">
<title>Run-Time Configuration Issues</title>
<related>
<modulelist>
<module>mod_dir</module>
<module>mpm_common</module>
<module>mod_status</module>
</modulelist>
<directivelist>
<directive module="core">AllowOverride</directive>
<directive module="mod_dir">DirectoryIndex</directive>
<directive module="core">HostnameLookups</directive>
<directive module="core">EnableMMAP</directive>
<directive module="core">KeepAliveTimeout</directive>
<directive module="prefork">MaxSpareServers</directive>
<directive module="prefork">MinSpareServers</directive>
<directive module="core">Options</directive>
<directive module="mpm_common">StartServers</directive>
</directivelist>
</related>
<section>
<title><code>HostnameLookups</code></title>
<p>Prior to Apache 1.3, <directive module="core"
>HostnameLookups</directive> defaulted to <code>On</code>.
This adds latency to every request because it requires a
DNS lookup to complete before the request is finished. In
Apache 1.3 this setting defaults to <code>Off</code>.
However (1.3 or later), if you use any <code>Allow from domain</code>
or <code>Deny from domain</code> directives then you will pay for
a double reverse DNS lookup (a reverse, followed by a forward
to make sure that the reverse is not being spoofed). So for the
highest performance avoid using these directives (it's fine to
use IP addresses rather than domain names).</p>
<p>Note that it's possible to scope the directives, such as
within a <code><Location /server-status></code> section.
In this case the DNS lookups are only performed on requests
matching the criteria. Here's an example which disables lookups
except for <code>.html</code> and <code>.cgi</code> files:</p>
<example><pre>
HostnameLookups off
<Files ~ "\.(html|cgi)$">
HostnameLookups on
</Files>
</pre></example>
<p>But even still, if you just need DNS names in some CGIs you
could consider doing the <code>gethostbyname</code> call in the
specific CGIs that need it.</p>
<p>Similarly, if you need to have hostname information in your
server logs in order to generate reports of this information,
you can postprocess your log file with <a
href="../programs/logresolve.html"><code>logresolve</code></a>,
so that these lookups can be done without making the client wait.
It is recommended that you do this postprocessing, and any other
statistical analysis of the log file, somewhere other than your
production web server machine, in order that this activity does
not adversely affect server performance.</p>
</section>
<section>
<title><code>FollowSymLinks</code> and <code>SymLinksIfOwnerMatch</code></title>
<p>Wherever in your URL-space you do not have an <code>Options
FollowSymLinks</code>, or you do have an <code>Options
SymLinksIfOwnerMatch</code> Apache will have to issue extra
system calls to check up on symlinks. One extra call per
filename component. For example, if you had:</p>
<example><pre>
DocumentRoot /www/htdocs
<Directory />
Options SymLinksIfOwnerMatch
</Directory>
</pre></example>
<p>and a request is made for the URI <code>/index.html</code>.
Then Apache will perform <code>lstat(2)</code> on
<code>/www</code>, <code>/www/htdocs</code>, and
<code>/www/htdocs/index.html</code>. The results of these
<code>lstats</code> are never cached, so they will occur on
every single request. If you really desire the symlinks
security checking you can do something like this:</p>
<example><pre>
DocumentRoot /www/htdocs
<Directory />
Options FollowSymLinks
</Directory>
<Directory /www/htdocs>
Options -FollowSymLinks +SymLinksIfOwnerMatch
</Directory>
</pre></example>
<p>This at least avoids the extra checks for the
<directive module="core">DocumentRoot</directive> path.
Note that you'll need to add similar sections if you
have any <directive module="mod_alias">Alias</directive> or
<directive module="mod_rewrite">RewriteRule</directive> paths
outside of your document root. For highest performance,
and no symlink protection, set <code>FollowSymLinks</code>
everywhere, and never set <code>SymLinksIfOwnerMatch</code>.</p>
</section>
<section>
<title><code>AllowOverride</code></title>
<p>Wherever in your URL-space you allow overrides (typically
<code>.htaccess</code> files) Apache will attempt to open
<code>.htaccess</code> for each filename component. For
example,</p>
<example><pre>
DocumentRoot /www/htdocs
<Directory />
AllowOverride all
</Directory>
</pre></example>
<p>and a request is made for the URI <code>/index.html</code>.
Then Apache will attempt to open <code>/.htaccess</code>,
<code>/www/.htaccess</code>, and
<code>/www/htdocs/.htaccess</code>. The solutions are similar
to the previous case of <code>Options FollowSymLinks</code>.
For highest performance use <code>AllowOverride None</code>
everywhere in your filesystem.</p>
</section>
<section>
<title>Negotiation</title>
<p>If at all possible, avoid content-negotiation if you're
really interested in every last ounce of performance. In
practice the benefits of negotiation outweigh the performance
penalties. There's one case where you can speed up the server.
Instead of using a wildcard such as:</p>
<example><pre>
DirectoryIndex index
</pre></example>
<p>Use a complete list of options:</p>
<example><pre>
DirectoryIndex index.cgi index.pl index.shtml index.html
</pre></example>
<p>where you list the most common choice first.</p>
<p>Also note that explicitly creating a <code>type-map</code>
file provides better performance than using
<code>MultiViews</code>, as the necessary information can be
determined by reading this single file, rather than having to
scan the directory for files.</p>
</section>
<section>
<title>Memory-mapping</title>
<p>In situations where Apache 2.0 needs to look at the contents
of a file being delivered--for example, when doing server-side-include
processing--it normally memory-maps the file if the OS supports
some form of <code>mmap(2)</code>.</p>
<p>On some platforms, this memory-mapping improves performance.
However, there are cases where memory-mapping can hurt the performance
or even the stability of the httpd:</p>
<ul>
<li>
<p>On some operating systems, <code>mmap</code> does not scale
as well as <code>read(2)</code> when the number of CPUs increases.
On multiprocessor Solaris servers, for example, Apache 2.0 sometimes
delivers server-parsed files faster when <code>mmap</code> is disabled.</p>
</li>
<li>
<p>If you memory-map a file located on an NFS-mounted filesystem
and a process on another NFS client machine deletes or truncates
the file, your process may get a bus error the next time it tries
to access the mapped file content.</p>
</li>
</ul>
<p>For installations where either of these factors applies, you
should use <code>EnableMMAP off</code> to disable the memory-mapping
of delivered files. (Note: This directive can be overridden on
a per-directory basis.)</p>
</section>
<section>
<title>Process Creation</title>
<p>Prior to Apache 1.3 the <directive module="prefork"
>MinSpareServers</directive>, <directive module="prefork"
>MaxSpareServers</directive>, and <directive module="mpm_common"
>StartServers</directive> settings all had drastic effects on
benchmark results. In particular, Apache required a "ramp-up"
period in order to reach a number of children sufficient to serve
the load being applied. After the initial spawning of
<directive module="mpm_common">StartServers</directive> children,
only one child per second would be created to satisfy the
<directive module="prefork">MinSpareServers</directive>
setting. So a server being accessed by 100 simultaneous
clients, using the default <directive module="mpm_common"
>StartServers</directive> of <code>5</code> would take on
the order 95 seconds to spawn enough children to handle
the load. This works fine in practice on real-life servers,
because they aren't restarted frequently. But does really
poorly on benchmarks which might only run for ten minutes.</p>
<p>The one-per-second rule was implemented in an effort to
avoid swamping the machine with the startup of new children. If
the machine is busy spawning children it can't service
requests. But it has such a drastic effect on the perceived
performance of Apache that it had to be replaced. As of Apache
1.3, the code will relax the one-per-second rule. It will spawn
one, wait a second, then spawn two, wait a second, then spawn
four, and it will continue exponentially until it is spawning
32 children per second. It will stop whenever it satisfies the
<directive module="prefork">MinSpareServers</directive>
setting.</p>
<p>This appears to be responsive enough that it's almost
unnecessary to twiddle the <directive module="prefork"
>MinSpareServers</directive>, <directive module="prefork"
>MaxSpareServers</directive> and <directive module="mpm_common"
>StartServers</directive> knobs. When more than 4 children are
spawned per second, a message will be emitted to the
<directive module="core">ErrorLog</directive>. If you
see a lot of these errors then consider tuning these settings.
Use the <module>mod_status</module> output as a guide.</p>
<p>Related to process creation is process death induced by the
<directive module="mpm_common">MaxRequestsPerChild</directive>
setting. By default this is <code>0</code>,
which means that there is no limit to the number of requests
handled per child. If your configuration currently has this set
to some very low number, such as <code>30</code>, you may want to bump this
up significantly. If you are running SunOS or an old version of
Solaris, limit this to <code>10000</code> or so because of memory leaks.</p>
<p>When keep-alives are in use, children will be kept busy
doing nothing waiting for more requests on the already open
connection. The default <directive module="core"
>KeepAliveTimeout</directive> of <code>15</code>
seconds attempts to minimize this effect. The tradeoff here is
between network bandwidth and server resources. In no event
should you raise this above about <code>60</code> seconds, as <a
href="http://www.research.digital.com/wrl/techreports/abstracts/95.4.html">
most of the benefits are lost</a>.</p>
</section>
</section>
<section id="compiletime">
<title>Compile-Time Configuration Issues</title>
<section>
<title>mod_status and ExtendedStatus On</title>
<p>If you include <module>mod_status</module> and you also set
<code>ExtendedStatus On</code> when building and running
Apache, then on every request Apache will perform two calls to
<code>gettimeofday(2)</code> (or <code>times(2)</code>
depending on your operating system), and (pre-1.3) several
extra calls to <code>time(2)</code>. This is all done so that
the status report contains timing indications. For highest
performance, set <code>ExtendedStatus off</code> (which is the
default).</p>
</section>
<section>
<title>accept Serialization - multiple sockets</title>
<p>This discusses a shortcoming in the Unix socket API. Suppose
your web server uses multiple <directive module="mpm_common"
>Listen</directive> statements to listen on either multiple
ports or multiple addresses. In order to test each socket
to see if a connection is ready Apache uses
<code>select(2)</code>. <code>select(2)</code> indicates that a
socket has <em>zero</em> or <em>at least one</em> connection
waiting on it. Apache's model includes multiple children, and
all the idle ones test for new connections at the same time. A
naive implementation looks something like this (these examples
do not match the code, they're contrived for pedagogical
purposes):</p>
<example><pre>
for (;;) {
for (;;) {
fd_set accept_fds;
FD_ZERO (&accept_fds);
for (i = first_socket; i <= last_socket; ++i) {
FD_SET (i, &accept_fds);
}
rc = select (last_socket+1, &accept_fds, NULL, NULL, NULL);
if (rc < 1) continue;
new_connection = -1;
for (i = first_socket; i <= last_socket; ++i) {
if (FD_ISSET (i, &accept_fds)) {
new_connection = accept (i, NULL, NULL);
if (new_connection != -1) break;
}
}
if (new_connection != -1) break;
}
process the new_connection;
}
</pre></example>
<p>But this naive implementation has a serious starvation problem.
Recall that multiple children execute this loop at the same
time, and so multiple children will block at
<code>select</code> when they are in between requests. All
those blocked children will awaken and return from
<code>select</code> when a single request appears on any socket
(the number of children which awaken varies depending on the
operating system and timing issues). They will all then fall
down into the loop and try to <code>accept</code> the
connection. But only one will succeed (assuming there's still
only one connection ready), the rest will be <em>blocked</em>
in <code>accept</code>. This effectively locks those children
into serving requests from that one socket and no other
sockets, and they'll be stuck there until enough new requests
appear on that socket to wake them all up. This starvation
problem was first documented in <a
href="http://bugs.apache.org/index/full/467">PR#467</a>. There
are at least two solutions.</p>
<p>One solution is to make the sockets non-blocking. In this
case the <code>accept</code> won't block the children, and they
will be allowed to continue immediately. But this wastes CPU
time. Suppose you have ten idle children in
<code>select</code>, and one connection arrives. Then nine of
those children will wake up, try to <code>accept</code> the
connection, fail, and loop back into <code>select</code>,
accomplishing nothing. Meanwhile none of those children are
servicing requests that occurred on other sockets until they
get back up to the <code>select</code> again. Overall this
solution does not seem very fruitful unless you have as many
idle CPUs (in a multiprocessor box) as you have idle children,
not a very likely situation.</p>
<p>Another solution, the one used by Apache, is to serialize
entry into the inner loop. The loop looks like this
(differences highlighted):</p>
<example><pre>
for (;;) {
<strong>accept_mutex_on ();</strong>
for (;;) {
fd_set accept_fds;
FD_ZERO (&accept_fds);
for (i = first_socket; i <= last_socket; ++i) {
FD_SET (i, &accept_fds);
}
rc = select (last_socket+1, &accept_fds, NULL, NULL, NULL);
if (rc < 1) continue;
new_connection = -1;
for (i = first_socket; i <= last_socket; ++i) {
if (FD_ISSET (i, &accept_fds)) {
new_connection = accept (i, NULL, NULL);
if (new_connection != -1) break;
}
}
if (new_connection != -1) break;
}
<strong>accept_mutex_off ();</strong>
process the new_connection;
}
</pre></example>
<p><a id="serialize" name="serialize">The functions</a>
<code>accept_mutex_on</code> and <code>accept_mutex_off</code>
implement a mutual exclusion semaphore. Only one child can have
the mutex at any time. There are several choices for
implementing these mutexes. The choice is defined in
<code>src/conf.h</code> (pre-1.3) or
<code>src/include/ap_config.h</code> (1.3 or later). Some
architectures do not have any locking choice made, on these
architectures it is unsafe to use multiple
<directive module="mpm_common">Listen</directive>
directives.</p>
<dl>
<dt><code>USE_FLOCK_SERIALIZED_ACCEPT</code></dt>
<dd>
<p>This method uses the <code>flock(2)</code> system call to
lock a lock file (located by the <directive module="mpm_common"
>LockFile</directive> directive).</p>
</dd>
<dt><code>USE_FCNTL_SERIALIZED_ACCEPT</code></dt>
<dd>
<p>This method uses the <code>fcntl(2)</code> system call to
lock a lock file (located by the <directive module="mpm_common"
>LockFile</directive> directive).</p>
</dd>
<dt><code>USE_SYSVSEM_SERIALIZED_ACCEPT</code></dt>
<dd>
<p>(1.3 or later) This method uses SysV-style semaphores to
implement the mutex. Unfortunately SysV-style semaphores have
some bad side-effects. One is that it's possible Apache will
die without cleaning up the semaphore (see the
<code>ipcs(8)</code> man page). The other is that the
semaphore API allows for a denial of service attack by any
CGIs running under the same uid as the webserver
(<em>i.e.</em>, all CGIs, unless you use something like
<code>suexec</code> or <code>cgiwrapper</code>). For these
reasons this method is not used on any architecture except
IRIX (where the previous two are prohibitively expensive
on most IRIX boxes).</p>
</dd>
<dt><code>USE_USLOCK_SERIALIZED_ACCEPT</code></dt>
<dd>
<p>(1.3 or later) This method is only available on IRIX, and
uses <code>usconfig(2)</code> to create a mutex. While this
method avoids the hassles of SysV-style semaphores, it is not
the default for IRIX. This is because on single processor
IRIX boxes (5.3 or 6.2) the uslock code is two orders of
magnitude slower than the SysV-semaphore code. On
multi-processor IRIX boxes the uslock code is an order of
magnitude faster than the SysV-semaphore code. Kind of a
messed up situation. So if you're using a multiprocessor IRIX
box then you should rebuild your webserver with
<code>-DUSE_USLOCK_SERIALIZED_ACCEPT</code> on the
<code>EXTRA_CFLAGS</code>.</p>
</dd>
<dt><code>USE_PTHREAD_SERIALIZED_ACCEPT</code></dt>
<dd>
<p>(1.3 or later) This method uses POSIX mutexes and should
work on any architecture implementing the full POSIX threads
specification, however appears to only work on Solaris (2.5
or later), and even then only in certain configurations. If
you experiment with this you should watch out for your server
hanging and not responding. Static content only servers may
work just fine.</p>
</dd>
</dl>
<p>If your system has another method of serialization which
isn't in the above list then it may be worthwhile adding code
for it (and submitting a patch back to Apache).</p>
<p>Another solution that has been considered but never
implemented is to partially serialize the loop -- that is, let
in a certain number of processes. This would only be of
interest on multiprocessor boxes where it's possible multiple
children could run simultaneously, and the serialization
actually doesn't take advantage of the full bandwidth. This is
a possible area of future investigation, but priority remains
low because highly parallel web servers are not the norm.</p>
<p>Ideally you should run servers without multiple
<directive module="mpm_common">Listen</directive>
statements if you want the highest performance.
But read on.</p>
</section>
<section>
<title>accept Serialization - single socket</title>
<p>The above is fine and dandy for multiple socket servers, but
what about single socket servers? In theory they shouldn't
experience any of these same problems because all children can
just block in <code>accept(2)</code> until a connection
arrives, and no starvation results. In practice this hides
almost the same "spinning" behaviour discussed above in the
non-blocking solution. The way that most TCP stacks are
implemented, the kernel actually wakes up all processes blocked
in <code>accept</code> when a single connection arrives. One of
those processes gets the connection and returns to user-space,
the rest spin in the kernel and go back to sleep when they
discover there's no connection for them. This spinning is
hidden from the user-land code, but it's there nonetheless.
This can result in the same load-spiking wasteful behaviour
that a non-blocking solution to the multiple sockets case
can.</p>
<p>For this reason we have found that many architectures behave
more "nicely" if we serialize even the single socket case. So
this is actually the default in almost all cases. Crude
experiments under Linux (2.0.30 on a dual Pentium pro 166
w/128Mb RAM) have shown that the serialization of the single
socket case causes less than a 3% decrease in requests per
second over unserialized single-socket. But unserialized
single-socket showed an extra 100ms latency on each request.
This latency is probably a wash on long haul lines, and only an
issue on LANs. If you want to override the single socket
serialization you can define
<code>SINGLE_LISTEN_UNSERIALIZED_ACCEPT</code> and then
single-socket servers will not serialize at all.</p>
</section>
<section>
<title>Lingering Close</title>
<p>As discussed in <a
href="http://www.ics.uci.edu/pub/ietf/http/draft-ietf-http-connection-00.txt">
draft-ietf-http-connection-00.txt</a> section 8, in order for
an HTTP server to <strong>reliably</strong> implement the
protocol it needs to shutdown each direction of the
communication independently (recall that a TCP connection is
bi-directional, each half is independent of the other). This
fact is often overlooked by other servers, but is correctly
implemented in Apache as of 1.2.</p>
<p>When this feature was added to Apache it caused a flurry of
problems on various versions of Unix because of a
shortsightedness. The TCP specification does not state that the
<code>FIN_WAIT_2</code> state has a timeout, but it doesn't prohibit it.
On systems without the timeout, Apache 1.2 induces many sockets
stuck forever in the <code>FIN_WAIT_2</code> state. In many cases this
can be avoided by simply upgrading to the latest TCP/IP patches
supplied by the vendor. In cases where the vendor has never
released patches (<em>i.e.</em>, SunOS4 -- although folks with
a source license can patch it themselves) we have decided to
disable this feature.</p>
<p>There are two ways of accomplishing this. One is the socket
option <code>SO_LINGER</code>. But as fate would have it, this
has never been implemented properly in most TCP/IP stacks. Even
on those stacks with a proper implementation (<em>i.e.</em>,
Linux 2.0.31) this method proves to be more expensive (cputime)
than the next solution.</p>
<p>For the most part, Apache implements this in a function
called <code>lingering_close</code> (in
<code>http_main.c</code>). The function looks roughly like
this:</p>
<example><pre>
void lingering_close (int s)
{
char junk_buffer[2048];
/* shutdown the sending side */
shutdown (s, 1);
signal (SIGALRM, lingering_death);
alarm (30);
for (;;) {
select (s for reading, 2 second timeout);
if (error) break;
if (s is ready for reading) {
if (read (s, junk_buffer, sizeof (junk_buffer)) <= 0) {
break;
}
/* just toss away whatever is here */
}
}
close (s);
}
</pre></example>
<p>This naturally adds some expense at the end of a connection,
but it is required for a reliable implementation. As HTTP/1.1
becomes more prevalent, and all connections are persistent,
this expense will be amortized over more requests. If you want
to play with fire and disable this feature you can define
<code>NO_LINGCLOSE</code>, but this is not recommended at all.
In particular, as HTTP/1.1 pipelined persistent connections
come into use <code>lingering_close</code> is an absolute
necessity (and <a
href="http://www.w3.org/Protocols/HTTP/Performance/Pipeline.html">
pipelined connections are faster</a>, so you want to support
them).</p>
</section>
<section>
<title>Scoreboard File</title>
<p>Apache's parent and children communicate with each other
through something called the scoreboard. Ideally this should be
implemented in shared memory. For those operating systems that
we either have access to, or have been given detailed ports
for, it typically is implemented using shared memory. The rest
default to using an on-disk file. The on-disk file is not only
slow, but it is unreliable (and less featured). Peruse the
<code>src/main/conf.h</code> file for your architecture and
look for either <code>USE_MMAP_SCOREBOARD</code> or
<code>USE_SHMGET_SCOREBOARD</code>. Defining one of those two
(as well as their companions <code>HAVE_MMAP</code> and
<code>HAVE_SHMGET</code> respectively) enables the supplied
shared memory code. If your system has another type of shared
memory, edit the file <code>src/main/http_main.c</code> and add
the hooks necessary to use it in Apache. (Send us back a patch
too please.)</p>
<note>Historical note: The Linux port of Apache didn't start to
use shared memory until version 1.2 of Apache. This oversight
resulted in really poor and unreliable behaviour of earlier
versions of Apache on Linux.</note>
</section>
<section>
<title><code>DYNAMIC_MODULE_LIMIT</code></title>
<p>If you have no intention of using dynamically loaded modules
(you probably don't if you're reading this and tuning your
server for every last ounce of performance) then you should add
<code>-DDYNAMIC_MODULE_LIMIT=0</code> when building your
server. This will save RAM that's allocated only for supporting
dynamically loaded modules.</p>
</section>
</section>
<section id="trace">
<title>Appendix: Detailed Analysis of a Trace</title>
<p>Here is a system call trace of Apache 2.0.38 with the worker MPM
on Solaris 8. This trace was collected using:</p>
<example>
truss -l -p <em>httpd_child_pid</em>.
</example>
<p>The <code>-l</code> option tells truss to log the ID of the
LWP (lightweight process--Solaris's form of kernel-level thread)
that invokes each system call.</p>
<p>Other systems may have different system call tracing utilities
such as <code>strace</code>, <code>ktrace</code>, or <code>par</code>.
They all produce similar output.</p>
<p>In this trace, a client has requested a 10KB static file
from the httpd. Traces of non-static requests or requests
with content negotiation look wildly different (and quite ugly
in some cases).</p>
<example><pre>
/67: accept(3, 0x00200BEC, 0x00200C0C, 1) (sleeping...)
/67: accept(3, 0x00200BEC, 0x00200C0C, 1) = 9
</pre></example>
<p>In this trace, the listener thread is running within LWP #67.</p>
<note>Note the lack of <code>accept(2)</code> serialization. On this
particular platform, the worker MPM uses an unserialized accept by
default unless it is listening on multiple ports.</note>
<example><pre>
/65: lwp_park(0x00000000, 0) = 0
/67: lwp_unpark(65, 1) = 0
</pre></example>
<p>Upon accepting the connection, the listener thread wakes up
a worker thread to do the request processing. In this trace,
the worker thread that handles the request is mapped to LWP #65.</p>
<example><pre>
/65: getsockname(9, 0x00200BA4, 0x00200BC4, 1) = 0
</pre></example>
<p>In order to implement virtual hosts, Apache needs to know
the local socket address used to accept the connection. It
is possible to eliminate this call in many situations (such
as when there are no virtual hosts, or when
<directive module="mpm_common">Listen</directive> directives
are used which do not have wildcard addresses). But
no effort has yet been made to do these optimizations. </p>
<example><pre>
/65: brk(0x002170E8) = 0
/65: brk(0x002190E8) = 0
</pre></example>
<p>The <code>brk(2)</code> calls allocate memory from the heap.
It is rare to see these in a system call trace, because the httpd
uses custom memory allocators (<code>apr_pool</code> and
<code>apr_bucket_alloc</code>) for most request processing.
In this trace, the httpd has just been started, so it must
call <code>malloc(3)</code> to get the blocks of raw memory
with which to create the custom memory allocators.</p>
<example><pre>
/65: fcntl(9, F_GETFL, 0x00000000) = 2
/65: fstat64(9, 0xFAF7B818) = 0
/65: getsockopt(9, 65535, 8192, 0xFAF7B918, 0xFAF7B910, 2190656) = 0
/65: fstat64(9, 0xFAF7B818) = 0
/65: getsockopt(9, 65535, 8192, 0xFAF7B918, 0xFAF7B914, 2190656) = 0
/65: setsockopt(9, 65535, 8192, 0xFAF7B918, 4, 2190656) = 0
/65: fcntl(9, F_SETFL, 0x00000082) = 0
</pre></example>
<p>Next, the worker thread puts the connection to the client (file
descriptor 9) in non-blocking mode. The <code>setsockopt(2)</code>
and <code>getsockopt(2)</code> calls are a side-effect of how
Solaris's libc handles <code>fcntl(2)</code> on sockets.</p>
<example><pre>
/65: read(9, " G E T / 1 0 k . h t m".., 8000) = 97
</pre></example>
<p>The worker thread reads the request from the client.</p>
<example><pre>
/65: stat("/var/httpd/apache/httpd-8999/htdocs/10k.html", 0xFAF7B978) = 0
/65: open("/var/httpd/apache/httpd-8999/htdocs/10k.html", O_RDONLY) = 10
</pre></example>
<p>This httpd has been configured with <code>Options FollowSymLinks</code>
and <code>AllowOverride None</code>. Thus it doesn't need to
<code>lstat(2)</code> each directory in the path leading up to the
requested file, nor check for <code>.htaccess</code> files.
It simply calls <code>stat(2)</code> to verify that the file:
1) exists, and 2) is a regular file, not a directory.</p>
<example><pre>
/65: sendfilev(0, 9, 0x00200F90, 2, 0xFAF7B53C) = 10269
</pre></example>
<p>In this example, the httpd is able to send the HTTP response
header and the requested file with a single <code>sendfilev(2)</code>
system call. Sendfile semantics vary among operating systems. On some other
systems, it is necessary to do a <code>write(2)</code> or
<code>writev(2)</code> call to send the headers before calling
<code>sendfile(2)</code>.</p>
<example><pre>
/65: write(4, " 1 2 7 . 0 . 0 . 1 - ".., 78) = 78
</pre></example>
<p>This <code>write(2)</code> call records the request in the
access log. Note that one thing missing from this trace is a
<code>time(2)</code> call. Unlike Apache 1.3, Apache 2.0 uses
<code>gettimeofday(3)</code> to look up the time. On some operating
systems, like Linux or Solaris, <code>gettimeofday</code> has an
optimized implementation that doesn't require as much overhead
as a typical system call.</p>
<example><pre>
/65: shutdown(9, 1, 1) = 0
/65: poll(0xFAF7B980, 1, 2000) = 1
/65: read(9, 0xFAF7BC20, 512) = 0
/65: close(9) = 0
</pre></example>
<p>The worker thread does a lingering close of the connection.</p>
<example><pre>
/65: close(10) = 0
/65: lwp_park(0x00000000, 0) (sleeping...)
</pre></example>
<p>Finally the worker thread closes the file that it has just delivered
and blocks until the listener assigns it another connection.</p>
<example><pre>
/67: accept(3, 0x001FEB74, 0x001FEB94, 1) (sleeping...)
</pre></example>
<p>Meanwhile, the listener thread is able to accept another connection
as soon as it has dispatched this connection to a worker thread (subject
to some flow-control logic in the worker MPM that throttles the listener
if all the available workers are busy). Though it isn't apparent from
this trace, the next <code>accept(2)</code> can (and usually does, under
high load conditions) occur in parallel with the worker thread's handling
of the just-accepted connection.</p>
</section>
</manualpage>
1.1 httpd-2.0/docs/manual/misc/rewriteguide.xml
Index: rewriteguide.xml
===================================================================
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
<manualpage>
<relativepath href=".." />
<title>URL Rewriting Guide</title>
<summary>
<note>
<address>Originally written by<br />
Ralf S. Engelschall <rse@apache.org><br />
December 1997</address>
</note>
<p>This document supplements the <module>mod_rewrite</module>
<a href="../mod/mod_rewrite.html">reference documentation</a>.
It describes how one can use Apache's <module>mod_rewrite</module>
to solve typical URL-based problems webmasters are usually confronted
with in practice. I give detailed descriptions on how to
solve each problem by configuring URL rewriting rulesets.</p>
</summary>
<section id="ToC1">
<title>Introduction to <code>mod_rewrite</code></title>
<p>The Apache module <module>mod_rewrite</module> is a killer
one, i.e. it is a really sophisticated module which provides
a powerful way to do URL manipulations. With it you can nearly
do all types of URL manipulations you ever dreamed about.
The price you have to pay is to accept complexity, because
<module>mod_rewrite</module>'s major drawback is that it is
not easy to understand and use for the beginner. And even
Apache experts sometimes discover new aspects where
<module>mod_rewrite</module> can help.</p>
<p>In other words: With <module>mod_rewrite</module> you either
shoot yourself in the foot the first time and never use it again
or love it for the rest of your life because of its power.
This paper tries to give you a few initial success events to
avoid the first case by presenting already invented solutions
to you.</p>
</section>
<section id="ToC2">
<title>Practical Solutions</title>
<p>Here come a lot of practical solutions I've either invented
myself or collected from other peoples solutions in the past.
Feel free to learn the black magic of URL rewriting from
these examples.</p>
<note type="warning">ATTENTION: Depending on your server-configuration
it can be necessary to slightly change the examples for your
situation, e.g. adding the <code>[PT]</code> flag when
additionally using <module>mod_alias</module> and
<module>mod_userdir</module>, etc. Or rewriting a ruleset
to fit in <code>.htaccess</code> context instead
of per-server context. Always try to understand what a
particular ruleset really does before you use it. It
avoid problems.</note>
</section>
<section id="url">
<title>URL Layout</title>
<section>
<title>Canonical URLs</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>On some webservers there are more than one URL for a
resource. Usually there are canonical URLs (which should be
actually used and distributed) and those which are just
shortcuts, internal ones, etc. Independent of which URL the
user supplied with the request he should finally see the
canonical one only.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We do an external HTTP redirect for all non-canonical
URLs to fix them in the location view of the Browser and
for all subsequent requests. In the example ruleset below
we replace <code>/~user</code> by the canonical
<code>/u/user</code> and fix a missing trailing slash for
<code>/u/user</code>.</p>
<example><pre>
RewriteRule ^/<strong>~</strong>([^/]+)/?(.*) /<strong>u</strong>/$1/$2 [<strong>R</strong>]
RewriteRule ^/([uge])/(<strong>[^/]+</strong>)$ /$1/$2<strong>/</strong> [<strong>R</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Canonical Hostnames</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>...</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<example><pre>
RewriteCond %{HTTP_HOST} !^fully\.qualified\.domain\.name [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteCond %{SERVER_PORT} !^80$
RewriteRule ^/(.*) http://fully.qualified.domain.name:%{SERVER_PORT}/$1 [L,R]
RewriteCond %{HTTP_HOST} !^fully\.qualified\.domain\.name [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteRule ^/(.*) http://fully.qualified.domain.name/$1 [L,R]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Moved <code>DocumentRoot</code></title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Usually the <directive module="core">DocumentRoot</directive>
of the webserver directly relates to the URL "<code>/</code>".
But often this data is not really of top-level priority, it is
perhaps just one entity of a lot of data pools. For instance at
our Intranet sites there are <code>/e/www/</code>
(the homepage for WWW), <code>/e/sww/</code> (the homepage for
the Intranet) etc. Now because the data of the <directive module="core"
>DocumentRoot</directive> stays at <code>/e/www/</code> we had
to make sure that all inlined images and other stuff inside this
data pool work for subsequent requests.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We just redirect the URL <code>/</code> to
<code>/e/www/</code>. While is seems trivial it is
actually trivial with <module>mod_rewrite</module>, only.
Because the typical old mechanisms of URL <em>Aliases</em>
(as provides by <module>mod_alias</module> and friends)
only used <em>prefix</em> matching. With this you cannot
do such a redirection because the <directive module="core"
>DocumentRoot</directive> is a prefix of all URLs. With
<module>mod_rewrite</module> it is really trivial:</p>
<example><pre>
RewriteEngine on
RewriteRule <strong>^/$</strong> /e/www/ [<strong>R</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Trailing Slash Problem</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Every webmaster can sing a song about the problem of
the trailing slash on URLs referencing directories. If they
are missing, the server dumps an error, because if you say
<code>/~quux/foo</code> instead of <code>/~quux/foo/</code>
then the server searches for a <em>file</em> named
<code>foo</code>. And because this file is a directory it
complains. Actually it tries to fix it itself in most of
the cases, but sometimes this mechanism need to be emulated
by you. For instance after you have done a lot of
complicated URL rewritings to CGI scripts etc.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>The solution to this subtle problem is to let the server
add the trailing slash automatically. To do this
correctly we have to use an external redirect, so the
browser correctly requests subsequent images etc. If we
only did a internal rewrite, this would only work for the
directory page, but would go wrong when any images are
included into this page with relative URLs, because the
browser would request an in-lined object. For instance, a
request for <code>image.gif</code> in
<code>/~quux/foo/index.html</code> would become
<code>/~quux/image.gif</code> without the external
redirect!</p>
<p>So, to do this trick we write:</p>
<example><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^foo<strong>$</strong> foo<strong>/</strong> [<strong>R</strong>]
</pre></example>
<p>The crazy and lazy can even do the following in the
top-level <code>.htaccess</code> file of their homedir.
But notice that this creates some processing
overhead.</p>
<example><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteCond %{REQUEST_FILENAME} <strong>-d</strong>
RewriteRule ^(.+<strong>[^/]</strong>)$ $1<strong>/</strong> [R]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Webcluster through Homogeneous URL Layout</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>We want to create a homogeneous and consistent URL
layout over all WWW servers on a Intranet webcluster, i.e.
all URLs (per definition server local and thus server
dependent!) become actually server <em>independent</em>!
What we want is to give the WWW namespace a consistent
server-independent layout: no URL should have to include
any physically correct target server. The cluster itself
should drive us automatically to the physical target
host.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>First, the knowledge of the target servers come from
(distributed) external maps which contain information
where our users, groups and entities stay. The have the
form</p>
<example><pre>
user1 server_of_user1
user2 server_of_user2
: :
</pre></example>
<p>We put them into files <code>map.xxx-to-host</code>.
Second we need to instruct all servers to redirect URLs
of the forms</p>
<example><pre>
/u/user/anypath
/g/group/anypath
/e/entity/anypath
</pre></example>
<p>to</p>
<example><pre>
http://physical-host/u/user/anypath
http://physical-host/g/group/anypath
http://physical-host/e/entity/anypath
</pre></example>
<p>when the URL is not locally valid to a server. The
following ruleset does this for us by the help of the map
files (assuming that server0 is a default server which
will be used if a user has no entry in the map):</p>
<example><pre>
RewriteEngine on
RewriteMap user-to-host txt:/path/to/map.user-to-host
RewriteMap group-to-host txt:/path/to/map.group-to-host
RewriteMap entity-to-host txt:/path/to/map.entity-to-host
RewriteRule ^/u/<strong>([^/]+)</strong>/?(.*) http://<strong>${user-to-host:$1|server0}</strong>/u/$1/$2
RewriteRule ^/g/<strong>([^/]+)</strong>/?(.*) http://<strong>${group-to-host:$1|server0}</strong>/g/$1/$2
RewriteRule ^/e/<strong>([^/]+)</strong>/?(.*) http://<strong>${entity-to-host:$1|server0}</strong>/e/$1/$2
RewriteRule ^/([uge])/([^/]+)/?$ /$1/$2/.www/
RewriteRule ^/([uge])/([^/]+)/([^.]+.+) /$1/$2/.www/$3\
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Move Homedirs to Different Webserver</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Many webmasters have asked for a solution to the
following situation: They wanted to redirect just all
homedirs on a webserver to another webserver. They usually
need such things when establishing a newer webserver which
will replace the old one over time.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>The solution is trivial with <module>mod_rewrite</module>.
On the old webserver we just redirect all
<code>/~user/anypath</code> URLs to
<code>http://newserver/~user/anypath</code>.</p>
<example><pre>
RewriteEngine on
RewriteRule ^/~(.+) http://<strong>newserver</strong>/~$1 [R,L]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Structured Homedirs</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Some sites with thousands of users usually use a
structured homedir layout, i.e. each homedir is in a
subdirectory which begins for instance with the first
character of the username. So, <code>/~foo/anypath</code>
is <code>/home/<strong>f</strong>/foo/.www/anypath</code>
while <code>/~bar/anypath</code> is
<code>/home/<strong>b</strong>/bar/.www/anypath</code>.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We use the following ruleset to expand the tilde URLs
into exactly the above layout.</p>
<example><pre>
RewriteEngine on
RewriteRule ^/~(<strong>([a-z])</strong>[a-z0-9]+)(.*) /home/<strong>$2</strong>/$1/.www$3
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Filesystem Reorganization</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>This really is a hardcore example: a killer application
which heavily uses per-directory
<code>RewriteRules</code> to get a smooth look and feel
on the Web while its data structure is never touched or
adjusted. Background: <strong><em>net.sw</em></strong> is
my archive of freely available Unix software packages,
which I started to collect in 1992. It is both my hobby
and job to to this, because while I'm studying computer
science I have also worked for many years as a system and
network administrator in my spare time. Every week I need
some sort of software so I created a deep hierarchy of
directories where I stored the packages:</p>
<example><pre>
drwxrwxr-x 2 netsw users 512 Aug 3 18:39 Audio/
drwxrwxr-x 2 netsw users 512 Jul 9 14:37 Benchmark/
drwxrwxr-x 12 netsw users 512 Jul 9 00:34 Crypto/
drwxrwxr-x 5 netsw users 512 Jul 9 00:41 Database/
drwxrwxr-x 4 netsw users 512 Jul 30 19:25 Dicts/
drwxrwxr-x 10 netsw users 512 Jul 9 01:54 Graphic/
drwxrwxr-x 5 netsw users 512 Jul 9 01:58 Hackers/
drwxrwxr-x 8 netsw users 512 Jul 9 03:19 InfoSys/
drwxrwxr-x 3 netsw users 512 Jul 9 03:21 Math/
drwxrwxr-x 3 netsw users 512 Jul 9 03:24 Misc/
drwxrwxr-x 9 netsw users 512 Aug 1 16:33 Network/
drwxrwxr-x 2 netsw users 512 Jul 9 05:53 Office/
drwxrwxr-x 7 netsw users 512 Jul 9 09:24 SoftEng/
drwxrwxr-x 7 netsw users 512 Jul 9 12:17 System/
drwxrwxr-x 12 netsw users 512 Aug 3 20:15 Typesetting/
drwxrwxr-x 10 netsw users 512 Jul 9 14:08 X11/
</pre></example>
<p>In July 1996 I decided to make this archive public to
the world via a nice Web interface. "Nice" means that I
wanted to offer an interface where you can browse
directly through the archive hierarchy. And "nice" means
that I didn't wanted to change anything inside this
hierarchy - not even by putting some CGI scripts at the
top of it. Why? Because the above structure should be
later accessible via FTP as well, and I didn't want any
Web or CGI stuff to be there.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>The solution has two parts: The first is a set of CGI
scripts which create all the pages at all directory
levels on-the-fly. I put them under
<code>/e/netsw/.www/</code> as follows:</p>
<example><pre>
-rw-r--r-- 1 netsw users 1318 Aug 1 18:10 .wwwacl
drwxr-xr-x 18 netsw users 512 Aug 5 15:51 DATA/
-rw-rw-rw- 1 netsw users 372982 Aug 5 16:35 LOGFILE
-rw-r--r-- 1 netsw users 659 Aug 4 09:27 TODO
-rw-r--r-- 1 netsw users 5697 Aug 1 18:01 netsw-about.html
-rwxr-xr-x 1 netsw users 579 Aug 2 10:33 netsw-access.pl
-rwxr-xr-x 1 netsw users 1532 Aug 1 17:35 netsw-changes.cgi
-rwxr-xr-x 1 netsw users 2866 Aug 5 14:49 netsw-home.cgi
drwxr-xr-x 2 netsw users 512 Jul 8 23:47 netsw-img/
-rwxr-xr-x 1 netsw users 24050 Aug 5 15:49 netsw-lsdir.cgi
-rwxr-xr-x 1 netsw users 1589 Aug 3 18:43 netsw-search.cgi
-rwxr-xr-x 1 netsw users 1885 Aug 1 17:41 netsw-tree.cgi
-rw-r--r-- 1 netsw users 234 Jul 30 16:35 netsw-unlimit.lst
</pre></example>
<p>The <code>DATA/</code> subdirectory holds the above
directory structure, i.e. the real
<strong><em>net.sw</em></strong> stuff and gets
automatically updated via <code>rdist</code> from time to
time. The second part of the problem remains: how to link
these two structures together into one smooth-looking URL
tree? We want to hide the <code>DATA/</code> directory
from the user while running the appropriate CGI scripts
for the various URLs. Here is the solution: first I put
the following into the per-directory configuration file
in the <directive module="core">DocumentRoot</directive>
of the server to rewrite the announced URL
<code>/net.sw/</code> to the internal path
<code>/e/netsw</code>:</p>
<example><pre>
RewriteRule ^net.sw$ net.sw/ [R]
RewriteRule ^net.sw/(.*)$ e/netsw/$1
</pre></example>
<p>The first rule is for requests which miss the trailing
slash! The second rule does the real thing. And then
comes the killer configuration which stays in the
per-directory config file
<code>/e/netsw/.www/.wwwacl</code>:</p>
<example><pre>
Options ExecCGI FollowSymLinks Includes MultiViews
RewriteEngine on
# we are reached via /net.sw/ prefix
RewriteBase /net.sw/
# first we rewrite the root dir to
# the handling cgi script
RewriteRule ^$ netsw-home.cgi [L]
RewriteRule ^index\.html$ netsw-home.cgi [L]
# strip out the subdirs when
# the browser requests us from perdir pages
RewriteRule ^.+/(netsw-[^/]+/.+)$ $1 [L]
# and now break the rewriting for local files
RewriteRule ^netsw-home\.cgi.* - [L]
RewriteRule ^netsw-changes\.cgi.* - [L]
RewriteRule ^netsw-search\.cgi.* - [L]
RewriteRule ^netsw-tree\.cgi$ - [L]
RewriteRule ^netsw-about\.html$ - [L]
RewriteRule ^netsw-img/.*$ - [L]
# anything else is a subdir which gets handled
# by another cgi script
RewriteRule !^netsw-lsdir\.cgi.* - [C]
RewriteRule (.*) netsw-lsdir.cgi/$1
</pre></example>
<p>Some hints for interpretation:</p>
<ol>
<li>Notice the <code>L</code> (last) flag and no
substitution field ('<code>-</code>') in the forth part</li>
<li>Notice the <code>!</code> (not) character and
the <code>C</code> (chain) flag at the first rule
in the last part</li>
<li>Notice the catch-all pattern in the last rule</li>
</ol>
</dd>
</dl>
</section>
<section>
<title>NCSA imagemap to Apache <code>mod_imap</code></title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>When switching from the NCSA webserver to the more
modern Apache webserver a lot of people want a smooth
transition. So they want pages which use their old NCSA
<code>imagemap</code> program to work under Apache with the
modern <module>mod_imap</module>. The problem is that there
are a lot of hyperlinks around which reference the
<code>imagemap</code> program via
<code>/cgi-bin/imagemap/path/to/page.map</code>. Under
Apache this has to read just
<code>/path/to/page.map</code>.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We use a global rule to remove the prefix on-the-fly for
all requests:</p>
<example><pre>
RewriteEngine on
RewriteRule ^/cgi-bin/imagemap(.*) $1 [PT]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Search pages in more than one directory</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Sometimes it is necessary to let the webserver search
for pages in more than one directory. Here MultiViews or
other techniques cannot help.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We program a explicit ruleset which searches for the
files in the directories.</p>
<example><pre>
RewriteEngine on
# first try to find it in custom/...
# ...and if found stop and be happy:
RewriteCond /your/docroot/<strong>dir1</strong>/%{REQUEST_FILENAME} -f
RewriteRule ^(.+) /your/docroot/<strong>dir1</strong>/$1 [L]
# second try to find it in pub/...
# ...and if found stop and be happy:
RewriteCond /your/docroot/<strong>dir2</strong>/%{REQUEST_FILENAME} -f
RewriteRule ^(.+) /your/docroot/<strong>dir2</strong>/$1 [L]
# else go on for other Alias or ScriptAlias directives,
# etc.
RewriteRule ^(.+) - [PT]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Set Environment Variables According To URL Parts</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Perhaps you want to keep status information between
requests and use the URL to encode it. But you don't want
to use a CGI wrapper for all pages just to strip out this
information.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We use a rewrite rule to strip out the status information
and remember it via an environment variable which can be
later dereferenced from within XSSI or CGI. This way a
URL <code>/foo/S=java/bar/</code> gets translated to
<code>/foo/bar/</code> and the environment variable named
<code>STATUS</code> is set to the value "java".</p>
<example><pre>
RewriteEngine on
RewriteRule ^(.*)/<strong>S=([^/]+)</strong>/(.*) $1/$3 [E=<strong>STATUS:$2</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Virtual User Hosts</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Assume that you want to provide
<code>www.<strong>username</strong>.host.domain.com</code>
for the homepage of username via just DNS A records to the
same machine and without any virtualhosts on this
machine.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>For HTTP/1.0 requests there is no solution, but for
HTTP/1.1 requests which contain a Host: HTTP header we
can use the following ruleset to rewrite
<code>http://www.username.host.com/anypath</code>
internally to <code>/home/username/anypath</code>:</p>
<example><pre>
RewriteEngine on
RewriteCond %{<strong>HTTP_HOST</strong>} ^www\.<strong>[^.]+</strong>\.host\.com$
RewriteRule ^(.+) %{HTTP_HOST}$1 [C]
RewriteRule ^www\.<strong>([^.]+)</strong>\.host\.com(.*) /home/<strong>$1</strong>$2
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Redirect Homedirs For Foreigners</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>We want to redirect homedir URLs to another webserver
<code>www.somewhere.com</code> when the requesting user
does not stay in the local domain
<code>ourdomain.com</code>. This is sometimes used in
virtual host contexts.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>Just a rewrite condition:</p>
<example><pre>
RewriteEngine on
RewriteCond %{REMOTE_HOST} <strong>!^.+\.ourdomain\.com$</strong>
RewriteRule ^(/~.+) http://www.somewhere.com/$1 [R,L]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Redirect Failing URLs To Other Webserver</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>A typical FAQ about URL rewriting is how to redirect
failing requests on webserver A to webserver B. Usually
this is done via <directive module="core"
>ErrorDocument</directive> CGI-scripts in Perl, but
there is also a <module>mod_rewrite</module> solution.
But notice that this performs more poorly than using an
<directive module="core">ErrorDocument</directive>
CGI-script!</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>The first solution has the best performance but less
flexibility, and is less error safe:</p>
<example><pre>
RewriteEngine on
RewriteCond /your/docroot/%{REQUEST_FILENAME} <strong>!-f</strong>
RewriteRule ^(.+) http://<strong>webserverB</strong>.dom/$1
</pre></example>
<p>The problem here is that this will only work for pages
inside the <directive module="core">DocumentRoot</directive>. While you can add more
Conditions (for instance to also handle homedirs, etc.)
there is better variant:</p>
<example><pre>
RewriteEngine on
RewriteCond %{REQUEST_URI} <strong>!-U</strong>
RewriteRule ^(.+) http://<strong>webserverB</strong>.dom/$1
</pre></example>
<p>This uses the URL look-ahead feature of <module>mod_rewrite</module>.
The result is that this will work for all types of URLs
and is a safe way. But it does a performance impact on
the webserver, because for every request there is one
more internal subrequest. So, if your webserver runs on a
powerful CPU, use this one. If it is a slow machine, use
the first approach or better a <directive module="core"
>ErrorDocument</directive> CGI-script.</p>
</dd>
</dl>
</section>
<section>
<title>Extended Redirection</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Sometimes we need more control (concerning the
character escaping mechanism) of URLs on redirects.
Usually the Apache kernels URL escape function also
escapes anchors, i.e. URLs like "<code>url#anchor</code>".
You cannot use this directly on redirects with
<module>mod_rewrite</module> because the
<code>uri_escape()</code> function of Apache
would also escape the hash character.
How can we redirect to such a URL?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We have to use a kludge by the use of a NPH-CGI script
which does the redirect itself. Because here no escaping
is done (NPH=non-parseable headers). First we introduce a
new URL scheme <code>xredirect:</code> by the following
per-server config-line (should be one of the last rewrite
rules):</p>
<example><pre>
RewriteRule ^xredirect:(.+) /path/to/nph-xredirect.cgi/$1 \
[T=application/x-httpd-cgi,L]
</pre></example>
<p>This forces all URLs prefixed with
<code>xredirect:</code> to be piped through the
<code>nph-xredirect.cgi</code> program. And this program
just looks like:</p>
<example><pre>
#!/path/to/perl
##
## nph-xredirect.cgi -- NPH/CGI script for extended redirects
## Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved.
##
$| = 1;
$url = $ENV{'PATH_INFO'};
print "HTTP/1.0 302 Moved Temporarily\n";
print "Server: $ENV{'SERVER_SOFTWARE'}\n";
print "Location: $url\n";
print "Content-type: text/html\n";
print "\n";
print "<html>\n";
print "<head>\n";
print "<title>302 Moved Temporarily (EXTENDED)</title>\n";
print "</head>\n";
print "<body>\n";
print "<h1>Moved Temporarily (EXTENDED)</h1>\n";
print "The document has moved <a HREF=\"$url\">here</a>.<p>\n";
print "</body>\n";
print "</html>\n";
##EOF##
</pre></example>
<p>This provides you with the functionality to do
redirects to all URL schemes, i.e. including the one
which are not directly accepted by <module>mod_rewrite</module>.
For instance you can now also redirect to
<code>news:newsgroup</code> via</p>
<example><pre>
RewriteRule ^anyurl xredirect:news:newsgroup
</pre></example>
<note>Notice: You have not to put <code>[R]</code> or
<code>[R,L]</code> to the above rule because the
<code>xredirect:</code> need to be expanded later
by our special "pipe through" rule above.</note>
</dd>
</dl>
</section>
<section>
<title>Archive Access Multiplexer</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Do you know the great CPAN (Comprehensive Perl Archive
Network) under <a href="http://www.perl.com/CPAN"
>http://www.perl.com/CPAN</a>?
This does a redirect to one of several FTP servers around
the world which carry a CPAN mirror and is approximately
near the location of the requesting client. Actually this
can be called an FTP access multiplexing service. While
CPAN runs via CGI scripts, how can a similar approach
implemented via <module>mod_rewrite</module>?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>First we notice that from version 3.0.0
<module>mod_rewrite</module> can
also use the "<code>ftp:</code>" scheme on redirects.
And second, the location approximation can be done by a
<directive module="mod_rewrite">RewriteMap</directive>
over the top-level domain of the client.
With a tricky chained ruleset we can use this top-level
domain as a key to our multiplexing map.</p>
<example><pre>
RewriteEngine on
RewriteMap multiplex txt:/path/to/map.cxan
RewriteRule ^/CxAN/(.*) %{REMOTE_HOST}::$1 [C]
RewriteRule ^.+\.<strong>([a-zA-Z]+)</strong>::(.*)$ ${multiplex:<strong>$1</strong>|ftp.default.dom}$2 [R,L]
</pre></example>
<example><pre>
##
## map.cxan -- Multiplexing Map for CxAN
##
de ftp://ftp.cxan.de/CxAN/
uk ftp://ftp.cxan.uk/CxAN/
com ftp://ftp.cxan.com/CxAN/
:
##EOF##
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Time-Dependent Rewriting</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>When tricks like time-dependent content should happen a
lot of webmasters still use CGI scripts which do for
instance redirects to specialized pages. How can it be done
via <module>mod_rewrite</module>?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>There are a lot of variables named <code>TIME_xxx</code>
for rewrite conditions. In conjunction with the special
lexicographic comparison patterns <code><STRING</code>,
<code>>STRING</code> and <code>=STRING</code> we can
do time-dependent redirects:</p>
<example><pre>
RewriteEngine on
RewriteCond %{TIME_HOUR}%{TIME_MIN} >0700
RewriteCond %{TIME_HOUR}%{TIME_MIN} <1900
RewriteRule ^foo\.html$ foo.day.html
RewriteRule ^foo\.html$ foo.night.html
</pre></example>
<p>This provides the content of <code>foo.day.html</code>
under the URL <code>foo.html</code> from
<code>07:00-19:00</code> and at the remaining time the
contents of <code>foo.night.html</code>. Just a nice
feature for a homepage...</p>
</dd>
</dl>
</section>
<section>
<title>Backward Compatibility for YYYY to XXXX migration</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we make URLs backward compatible (still
existing virtually) after migrating <code>document.YYYY</code>
to <code>document.XXXX</code>, e.g. after translating a
bunch of <code>.html</code> files to <code>.phtml</code>?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We just rewrite the name to its basename and test for
existence of the new extension. If it exists, we take
that name, else we rewrite the URL to its original state.</p>
<example><pre>
# backward compatibility ruleset for
# rewriting document.html to document.phtml
# when and only when document.phtml exists
# but no longer document.html
RewriteEngine on
RewriteBase /~quux/
# parse out basename, but remember the fact
RewriteRule ^(.*)\.html$ $1 [C,E=WasHTML:yes]
# rewrite to document.phtml if exists
RewriteCond %{REQUEST_FILENAME}.phtml -f
RewriteRule ^(.*)$ $1.phtml [S=1]
# else reverse the previous basename cutout
RewriteCond %{ENV:WasHTML} ^yes$
RewriteRule ^(.*)$ $1.html
</pre></example>
</dd>
</dl>
</section>
</section>
<section id="content">
<title>Content Handling</title>
<section>
<title>From Old to New (intern)</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Assume we have recently renamed the page
<code>bar.html</code> to <code>foo.html</code> and now want
to provide the old URL for backward compatibility. Actually
we want that users of the old URL even not recognize that
the pages was renamed.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We rewrite the old URL to the new one internally via the
following rule:</p>
<example><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>foo</strong>\.html$ <strong>bar</strong>.html
</pre></example>
</dd>
</dl>
</section>
<section>
<title>From Old to New (extern)</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Assume again that we have recently renamed the page
<code>bar.html</code> to <code>foo.html</code> and now want
to provide the old URL for backward compatibility. But this
time we want that the users of the old URL get hinted to
the new one, i.e. their browsers Location field should
change, too.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We force a HTTP redirect to the new URL which leads to a
change of the browsers and thus the users view:</p>
<example><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>foo</strong>\.html$ <strong>bar</strong>.html [<strong>R</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Browser Dependent Content</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>At least for important top-level pages it is sometimes
necessary to provide the optimum of browser dependent
content, i.e. one has to provide a maximum version for the
latest Netscape variants, a minimum version for the Lynx
browsers and a average feature version for all others.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We cannot use content negotiation because the browsers do
not provide their type in that form. Instead we have to
act on the HTTP header "User-Agent". The following condig
does the following: If the HTTP header "User-Agent"
begins with "Mozilla/3", the page <code>foo.html</code>
is rewritten to <code>foo.NS.html</code> and and the
rewriting stops. If the browser is "Lynx" or "Mozilla" of
version 1 or 2 the URL becomes <code>foo.20.html</code>.
All other browsers receive page <code>foo.32.html</code>.
This is done by the following ruleset:</p>
<example><pre>
RewriteCond %{HTTP_USER_AGENT} ^<strong>Mozilla/3</strong>.*
RewriteRule ^foo\.html$ foo.<strong>NS</strong>.html [<strong>L</strong>]
RewriteCond %{HTTP_USER_AGENT} ^<strong>Lynx/</strong>.* [OR]
RewriteCond %{HTTP_USER_AGENT} ^<strong>Mozilla/[12]</strong>.*
RewriteRule ^foo\.html$ foo.<strong>20</strong>.html [<strong>L</strong>]
RewriteRule ^foo\.html$ foo.<strong>32</strong>.html [<strong>L</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Dynamic Mirror</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Assume there are nice webpages on remote hosts we want
to bring into our namespace. For FTP servers we would use
the <code>mirror</code> program which actually maintains an
explicit up-to-date copy of the remote data on the local
machine. For a webserver we could use the program
<code>webcopy</code> which acts similar via HTTP. But both
techniques have one major drawback: The local copy is
always just as up-to-date as often we run the program. It
would be much better if the mirror is not a static one we
have to establish explicitly. Instead we want a dynamic
mirror with data which gets updated automatically when
there is need (updated data on the remote host).</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>To provide this feature we map the remote webpage or even
the complete remote webarea to our namespace by the use
of the <i>Proxy Throughput</i> feature
(flag <code>[P]</code>):</p>
<example><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>hotsheet/</strong>(.*)$ <strong>http://www.tstimpreso.com/hotsheet/</strong>$1 [<strong>P</strong>]
</pre></example>
<example><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>usa-news\.html</strong>$ <strong>http://www.quux-corp.com/news/index.html</strong> [<strong>P</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Reverse Dynamic Mirror</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>...</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<example><pre>
RewriteEngine on
RewriteCond /mirror/of/remotesite/$1 -U
RewriteRule ^http://www\.remotesite\.com/(.*)$ /mirror/of/remotesite/$1
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Retrieve Missing Data from Intranet</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>This is a tricky way of virtually running a corporate
(external) Internet webserver
(<code>www.quux-corp.dom</code>), while actually keeping
and maintaining its data on a (internal) Intranet webserver
(<code>www2.quux-corp.dom</code>) which is protected by a
firewall. The trick is that on the external webserver we
retrieve the requested data on-the-fly from the internal
one.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>First, we have to make sure that our firewall still
protects the internal webserver and that only the
external webserver is allowed to retrieve data from it.
For a packet-filtering firewall we could for instance
configure a firewall ruleset like the following:</p>
<example><pre>
<strong>ALLOW</strong> Host www.quux-corp.dom Port >1024 --> Host www2.quux-corp.dom Port <strong>80</strong>
<strong>DENY</strong> Host * Port * --> Host www2.quux-corp.dom Port <strong>80</strong>
</pre></example>
<p>Just adjust it to your actual configuration syntax.
Now we can establish the <module>mod_rewrite</module>
rules which request the missing data in the background
through the proxy throughput feature:</p>
<example><pre>
RewriteRule ^/~([^/]+)/?(.*) /home/$1/.www/$2
RewriteCond %{REQUEST_FILENAME} <strong>!-f</strong>
RewriteCond %{REQUEST_FILENAME} <strong>!-d</strong>
RewriteRule ^/home/([^/]+)/.www/?(.*) http://<strong>www2</strong>.quux-corp.dom/~$1/pub/$2 [<strong>P</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Load Balancing</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Suppose we want to load balance the traffic to
<code>www.foo.com</code> over <code>www[0-5].foo.com</code>
(a total of 6 servers). How can this be done?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>There are a lot of possible solutions for this problem.
We will discuss first a commonly known DNS-based variant
and then the special one with <module>mod_rewrite</module>:</p>
<ol>
<li>
<strong>DNS Round-Robin</strong>
<p>The simplest method for load-balancing is to use
the DNS round-robin feature of <code>BIND</code>.
Here you just configure <code>www[0-9].foo.com</code>
as usual in your DNS with A(address) records, e.g.</p>
<example><pre>
www0 IN A 1.2.3.1
www1 IN A 1.2.3.2
www2 IN A 1.2.3.3
www3 IN A 1.2.3.4
www4 IN A 1.2.3.5
www5 IN A 1.2.3.6
</pre></example>
<p>Then you additionally add the following entry:</p>
<example><pre>
www IN CNAME www0.foo.com.
IN CNAME www1.foo.com.
IN CNAME www2.foo.com.
IN CNAME www3.foo.com.
IN CNAME www4.foo.com.
IN CNAME www5.foo.com.
IN CNAME www6.foo.com.
</pre></example>
<p>Notice that this seems wrong, but is actually an
intended feature of <code>BIND</code> and can be used
in this way. However, now when <code>www.foo.com</code> gets
resolved, <code>BIND</code> gives out <code>www0-www6</code>
- but in a slightly permutated/rotated order every time.
This way the clients are spread over the various
servers. But notice that this not a perfect load
balancing scheme, because DNS resolve information
gets cached by the other nameservers on the net, so
once a client has resolved <code>www.foo.com</code>
to a particular <code>wwwN.foo.com</code>, all
subsequent requests also go to this particular name
<code>wwwN.foo.com</code>. But the final result is
ok, because the total sum of the requests are really
spread over the various webservers.</p>
</li>
<li>
<strong>DNS Load-Balancing</strong>
<p>A sophisticated DNS-based method for
load-balancing is to use the program
<code>lbnamed</code> which can be found at <a
href="http://www.stanford.edu/~schemers/docs/lbnamed/lbnamed.html">
http://www.stanford.edu/~schemers/docs/lbnamed/lbnamed.html</a>.
It is a Perl 5 program in conjunction with auxilliary
tools which provides a real load-balancing for
DNS.</p>
</li>
<li>
<strong>Proxy Throughput Round-Robin</strong>
<p>In this variant we use <module>mod_rewrite</module>
and its proxy throughput feature. First we dedicate
<code>www0.foo.com</code> to be actually
<code>www.foo.com</code> by using a single</p>
<example><pre>
www IN CNAME www0.foo.com.
</pre></example>
<p>entry in the DNS. Then we convert
<code>www0.foo.com</code> to a proxy-only server,
i.e. we configure this machine so all arriving URLs
are just pushed through the internal proxy to one of
the 5 other servers (<code>www1-www5</code>). To
accomplish this we first establish a ruleset which
contacts a load balancing script <code>lb.pl</code>
for all URLs.</p>
<example><pre>
RewriteEngine on
RewriteMap lb prg:/path/to/lb.pl
RewriteRule ^/(.+)$ ${lb:$1} [P,L]
</pre></example>
<p>Then we write <code>lb.pl</code>:</p>
<example><pre>
#!/path/to/perl
##
## lb.pl -- load balancing script
##
$| = 1;
$name = "www"; # the hostname base
$first = 1; # the first server (not 0 here, because 0 is myself)
$last = 5; # the last server in the round-robin
$domain = "foo.dom"; # the domainname
$cnt = 0;
while (<STDIN>) {
$cnt = (($cnt+1) % ($last+1-$first));
$server = sprintf("%s%d.%s", $name, $cnt+$first, $domain);
print "http://$server/$_";
}
##EOF##
</pre></example>
<note>A last notice: Why is this useful? Seems like
<code>www0.foo.com</code> still is overloaded? The
answer is yes, it is overloaded, but with plain proxy
throughput requests, only! All SSI, CGI, ePerl, etc.
processing is completely done on the other machines.
This is the essential point.</note>
</li>
<li>
<strong>Hardware/TCP Round-Robin</strong>
<p>There is a hardware solution available, too. Cisco
has a beast called LocalDirector which does a load
balancing at the TCP/IP level. Actually this is some
sort of a circuit level gateway in front of a
webcluster. If you have enough money and really need
a solution with high performance, use this one.</p>
</li>
</ol>
</dd>
</dl>
</section>
<section>
<title>Reverse Proxy</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>...</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<example><pre>
##
## apache-rproxy.conf -- Apache configuration for Reverse Proxy Usage
##
# server type
ServerType standalone
Listen 8000
MinSpareServers 16
StartServers 16
MaxSpareServers 16
MaxClients 16
MaxRequestsPerChild 100
# server operation parameters
KeepAlive on
MaxKeepAliveRequests 100
KeepAliveTimeout 15
Timeout 400
IdentityCheck off
HostnameLookups off
# paths to runtime files
PidFile /path/to/apache-rproxy.pid
LockFile /path/to/apache-rproxy.lock
ErrorLog /path/to/apache-rproxy.elog
CustomLog /path/to/apache-rproxy.dlog "%{%v/%T}t %h -> %{SERVER}e URL: %U"
# unused paths
ServerRoot /tmp
DocumentRoot /tmp
CacheRoot /tmp
RewriteLog /dev/null
TransferLog /dev/null
TypesConfig /dev/null
AccessConfig /dev/null
ResourceConfig /dev/null
# speed up and secure processing
<Directory />
Options -FollowSymLinks -SymLinksIfOwnerMatch
AllowOverride None
</Directory>
# the status page for monitoring the reverse proxy
<Location /apache-rproxy-status>
SetHandler server-status
</Location>
# enable the URL rewriting engine
RewriteEngine on
RewriteLogLevel 0
# define a rewriting map with value-lists where
# mod_rewrite randomly chooses a particular value
RewriteMap server rnd:/path/to/apache-rproxy.conf-servers
# make sure the status page is handled locally
# and make sure no one uses our proxy except ourself
RewriteRule ^/apache-rproxy-status.* - [L]
RewriteRule ^(http|ftp)://.* - [F]
# now choose the possible servers for particular URL types
RewriteRule ^/(.*\.(cgi|shtml))$ to://${server:dynamic}/$1 [S=1]
RewriteRule ^/(.*)$ to://${server:static}/$1
# and delegate the generated URL by passing it
# through the proxy module
RewriteRule ^to://([^/]+)/(.*) http://$1/$2 [E=SERVER:$1,P,L]
# and make really sure all other stuff is forbidden
# when it should survive the above rules...
RewriteRule .* - [F]
# enable the Proxy module without caching
ProxyRequests on
NoCache *
# setup URL reverse mapping for redirect reponses
ProxyPassReverse / http://www1.foo.dom/
ProxyPassReverse / http://www2.foo.dom/
ProxyPassReverse / http://www3.foo.dom/
ProxyPassReverse / http://www4.foo.dom/
ProxyPassReverse / http://www5.foo.dom/
ProxyPassReverse / http://www6.foo.dom/
</pre></example>
<example><pre>
##
## apache-rproxy.conf-servers -- Apache/mod_rewrite selection table
##
# list of backend servers which serve static
# pages (HTML files and Images, etc.)
static www1.foo.dom|www2.foo.dom|www3.foo.dom|www4.foo.dom
# list of backend servers which serve dynamically
# generated page (CGI programs or mod_perl scripts)
dynamic www5.foo.dom|www6.foo.dom
</pre></example>
</dd>
</dl>
</section>
<section>
<title>New MIME-type, New Service</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>On the net there are a lot of nifty CGI programs. But
their usage is usually boring, so a lot of webmaster
don't use them. Even Apache's Action handler feature for
MIME-types is only appropriate when the CGI programs
don't need special URLs (actually <code>PATH_INFO</code>
and <code>QUERY_STRINGS</code>) as their input. First,
let us configure a new file type with extension
<code>.scgi</code> (for secure CGI) which will be processed
by the popular <code>cgiwrap</code> program. The problem
here is that for instance we use a Homogeneous URL Layout
(see above) a file inside the user homedirs has the URL
<code>/u/user/foo/bar.scgi</code>. But
<code>cgiwrap</code> needs the URL in the form
<code>/~user/foo/bar.scgi/</code>. The following rule
solves the problem:</p>
<example><pre>
RewriteRule ^/[uge]/<strong>([^/]+)</strong>/\.www/(.+)\.scgi(.*) ...
... /internal/cgi/user/cgiwrap/~<strong>$1</strong>/$2.scgi$3 [NS,<strong>T=application/x-http-cgi</strong>]
</pre></example>
<p>Or assume we have some more nifty programs:
<code>wwwlog</code> (which displays the
<code>access.log</code> for a URL subtree and
<code>wwwidx</code> (which runs Glimpse on a URL
subtree). We have to provide the URL area to these
programs so they know on which area they have to act on.
But usually this ugly, because they are all the times
still requested from that areas, i.e. typically we would
run the <code>swwidx</code> program from within
<code>/u/user/foo/</code> via hyperlink to</p>
<example><pre>
/internal/cgi/user/swwidx?i=/u/user/foo/
</pre></example>
<p>which is ugly. Because we have to hard-code
<strong>both</strong> the location of the area
<strong>and</strong> the location of the CGI inside the
hyperlink. When we have to reorganize the area, we spend a
lot of time changing the various hyperlinks.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>The solution here is to provide a special new URL format
which automatically leads to the proper CGI invocation.
We configure the following:</p>
<example><pre>
RewriteRule ^/([uge])/([^/]+)(/?.*)/\* /internal/cgi/user/wwwidx?i=/$1/$2$3/
RewriteRule ^/([uge])/([^/]+)(/?.*):log /internal/cgi/user/wwwlog?f=/$1/$2$3
</pre></example>
<p>Now the hyperlink to search at
<code>/u/user/foo/</code> reads only</p>
<example><pre>
HREF="*"
</pre></example>
<p>which internally gets automatically transformed to</p>
<example><pre>
/internal/cgi/user/wwwidx?i=/u/user/foo/
</pre></example>
<p>The same approach leads to an invocation for the
access log CGI program when the hyperlink
<code>:log</code> gets used.</p>
</dd>
</dl>
</section>
<section>
<title>From Static to Dynamic</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we transform a static page
<code>foo.html</code> into a dynamic variant
<code>foo.cgi</code> in a seamless way, i.e. without notice
by the browser/user.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We just rewrite the URL to the CGI-script and force the
correct MIME-type so it gets really run as a CGI-script.
This way a request to <code>/~quux/foo.html</code>
internally leads to the invocation of
<code>/~quux/foo.cgi</code>.</p>
<example><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^foo\.<strong>html</strong>$ foo.<strong>cgi</strong> [T=<strong>application/x-httpd-cgi</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>On-the-fly Content-Regeneration</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Here comes a really esoteric feature: Dynamically
generated but statically served pages, i.e. pages should be
delivered as pure static pages (read from the filesystem
and just passed through), but they have to be generated
dynamically by the webserver if missing. This way you can
have CGI-generated pages which are statically served unless
one (or a cronjob) removes the static contents. Then the
contents gets refreshed.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
This is done via the following ruleset:
<example><pre>
RewriteCond %{REQUEST_FILENAME} <strong>!-s</strong>
RewriteRule ^page\.<strong>html</strong>$ page.<strong>cgi</strong> [T=application/x-httpd-cgi,L]
</pre></example>
<p>Here a request to <code>page.html</code> leads to a
internal run of a corresponding <code>page.cgi</code> if
<code>page.html</code> is still missing or has filesize
null. The trick here is that <code>page.cgi</code> is a
usual CGI script which (additionally to its <code>STDOUT</code>)
writes its output to the file <code>page.html</code>.
Once it was run, the server sends out the data of
<code>page.html</code>. When the webmaster wants to force
a refresh the contents, he just removes
<code>page.html</code> (usually done by a cronjob).</p>
</dd>
</dl>
</section>
<section>
<title>Document With Autorefresh</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Wouldn't it be nice while creating a complex webpage if
the webbrowser would automatically refresh the page every
time we write a new version from within our editor?
Impossible?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>No! We just combine the MIME multipart feature, the
webserver NPH feature and the URL manipulation power of
<module>mod_rewrite</module>. First, we establish a new
URL feature: Adding just <code>:refresh</code> to any
URL causes this to be refreshed every time it gets
updated on the filesystem.</p>
<example><pre>
RewriteRule ^(/[uge]/[^/]+/?.*):refresh /internal/cgi/apache/nph-refresh?f=$1
</pre></example>
<p>Now when we reference the URL</p>
<example><pre>
/u/foo/bar/page.html:refresh
</pre></example>
<p>this leads to the internal invocation of the URL</p>
<example><pre>
/internal/cgi/apache/nph-refresh?f=/u/foo/bar/page.html
</pre></example>
<p>The only missing part is the NPH-CGI script. Although
one would usually say "left as an exercise to the reader"
;-) I will provide this, too.</p>
<example><pre>
#!/sw/bin/perl
##
## nph-refresh -- NPH/CGI script for auto refreshing pages
## Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved.
##
$| = 1;
# split the QUERY_STRING variable
@pairs = split(/&/, $ENV{'QUERY_STRING'});
foreach $pair (@pairs) {
($name, $value) = split(/=/, $pair);
$name =~ tr/A-Z/a-z/;
$name = 'QS_' . $name;
$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
eval "\$$name = \"$value\"";
}
$QS_s = 1 if ($QS_s eq '');
$QS_n = 3600 if ($QS_n eq '');
if ($QS_f eq '') {
print "HTTP/1.0 200 OK\n";
print "Content-type: text/html\n\n";
print "&lt;b&gt;ERROR&lt;/b&gt;: No file given\n";
exit(0);
}
if (! -f $QS_f) {
print "HTTP/1.0 200 OK\n";
print "Content-type: text/html\n\n";
print "&lt;b&gt;ERROR&lt;/b&gt;: File $QS_f not found\n";
exit(0);
}
sub print_http_headers_multipart_begin {
print "HTTP/1.0 200 OK\n";
$bound = "ThisRandomString12345";
print "Content-type: multipart/x-mixed-replace;boundary=$bound\n";
&print_http_headers_multipart_next;
}
sub print_http_headers_multipart_next {
print "\n--$bound\n";
}
sub print_http_headers_multipart_end {
print "\n--$bound--\n";
}
sub displayhtml {
local($buffer) = @_;
$len = length($buffer);
print "Content-type: text/html\n";
print "Content-length: $len\n\n";
print $buffer;
}
sub readfile {
local($file) = @_;
local(*FP, $size, $buffer, $bytes);
($x, $x, $x, $x, $x, $x, $x, $size) = stat($file);
$size = sprintf("%d", $size);
open(FP, "&lt;$file");
$bytes = sysread(FP, $buffer, $size);
close(FP);
return $buffer;
}
$buffer = &readfile($QS_f);
&print_http_headers_multipart_begin;
&displayhtml($buffer);
sub mystat {
local($file) = $_[0];
local($time);
($x, $x, $x, $x, $x, $x, $x, $x, $x, $mtime) = stat($file);
return $mtime;
}
$mtimeL = &mystat($QS_f);
$mtime = $mtime;
for ($n = 0; $n &lt; $QS_n; $n++) {
while (1) {
$mtime = &mystat($QS_f);
if ($mtime ne $mtimeL) {
$mtimeL = $mtime;
sleep(2);
$buffer = &readfile($QS_f);
&print_http_headers_multipart_next;
&displayhtml($buffer);
sleep(5);
$mtimeL = &mystat($QS_f);
last;
}
sleep($QS_s);
}
}
&print_http_headers_multipart_end;
exit(0);
##EOF##
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Mass Virtual Hosting</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>The <directive type="section" module="core"
>VirtualHost</directive> feature of Apache is nice
and works great when you just have a few dozens
virtual hosts. But when you are an ISP and have hundreds of
virtual hosts to provide this feature is not the best
choice.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>To provide this feature we map the remote webpage or even
the complete remote webarea to our namespace by the use
of the <i>Proxy Throughput</i> feature (flag <code>[P]</code>):</p>
<example><pre>
##
## vhost.map
##
www.vhost1.dom:80 /path/to/docroot/vhost1
www.vhost2.dom:80 /path/to/docroot/vhost2
:
www.vhostN.dom:80 /path/to/docroot/vhostN
</pre></example>
<example><pre>
##
## httpd.conf
##
:
# use the canonical hostname on redirects, etc.
UseCanonicalName on
:
# add the virtual host in front of the CLF-format
CustomLog /path/to/access_log "%{VHOST}e %h %l %u %t \"%r\" %>s %b"
:
# enable the rewriting engine in the main server
RewriteEngine on
# define two maps: one for fixing the URL and one which defines
# the available virtual hosts with their corresponding
# DocumentRoot.
RewriteMap lowercase int:tolower
RewriteMap vhost txt:/path/to/vhost.map
# Now do the actual virtual host mapping
# via a huge and complicated single rule:
#
# 1. make sure we don't map for common locations
RewriteCond %{REQUEST_URL} !^/commonurl1/.*
RewriteCond %{REQUEST_URL} !^/commonurl2/.*
:
RewriteCond %{REQUEST_URL} !^/commonurlN/.*
#
# 2. make sure we have a Host header, because
# currently our approach only supports
# virtual hosting through this header
RewriteCond %{HTTP_HOST} !^$
#
# 3. lowercase the hostname
RewriteCond ${lowercase:%{HTTP_HOST}|NONE} ^(.+)$
#
# 4. lookup this hostname in vhost.map and
# remember it only when it is a path
# (and not "NONE" from above)
RewriteCond ${vhost:%1} ^(/.*)$
#
# 5. finally we can map the URL to its docroot location
# and remember the virtual host for logging puposes
RewriteRule ^/(.*)$ %1/$1 [E=VHOST:${lowercase:%{HTTP_HOST}}]
:
</pre></example>
</dd>
</dl>
</section>
</section>
<section id="access">
<title>Access Restriction</title>
<section>
<title>Blocking of Robots</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we block a really annoying robot from
retrieving pages of a specific webarea? A
<code>/robots.txt</code> file containing entries of the
"Robot Exclusion Protocol" is typically not enough to get
rid of such a robot.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We use a ruleset which forbids the URLs of the webarea
<code>/~quux/foo/arc/</code> (perhaps a very deep
directory indexed area where the robot traversal would
create big server load). We have to make sure that we
forbid access only to the particular robot, i.e. just
forbidding the host where the robot runs is not enough.
This would block users from this host, too. We accomplish
this by also matching the User-Agent HTTP header
information.</p>
<example><pre>
RewriteCond %{HTTP_USER_AGENT} ^<strong>NameOfBadRobot</strong>.*
RewriteCond %{REMOTE_ADDR} ^<strong>123\.45\.67\.[8-9]</strong>$
RewriteRule ^<strong>/~quux/foo/arc/</strong>.+ - [<strong>F</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Blocked Inline-Images</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Assume we have under <code>http://www.quux-corp.de/~quux/</code>
some pages with inlined GIF graphics. These graphics are
nice, so others directly incorporate them via hyperlinks to
their pages. We don't like this practice because it adds
useless traffic to our server.</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>While we cannot 100% protect the images from inclusion,
we can at least restrict the cases where the browser
sends a HTTP Referer header.</p>
<example><pre>
RewriteCond %{HTTP_REFERER} <strong>!^$</strong>
RewriteCond %{HTTP_REFERER} !^http://www.quux-corp.de/~quux/.*$ [NC]
RewriteRule <strong>.*\.gif$</strong> - [F]
</pre></example>
<example><pre>
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !.*/foo-with-gif\.html$
RewriteRule <strong>^inlined-in-foo\.gif$</strong> - [F]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Host Deny</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we forbid a list of externally configured hosts
from using our server?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>For Apache >= 1.3b6:</p>
<example><pre>
RewriteEngine on
RewriteMap hosts-deny txt:/path/to/hosts.deny
RewriteCond ${hosts-deny:%{REMOTE_HOST}|NOT-FOUND} !=NOT-FOUND [OR]
RewriteCond ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND} !=NOT-FOUND
RewriteRule ^/.* - [F]
</pre></example>
<p>For Apache <= 1.3b6:</p>
<example><pre>
RewriteEngine on
RewriteMap hosts-deny txt:/path/to/hosts.deny
RewriteRule ^/(.*)$ ${hosts-deny:%{REMOTE_HOST}|NOT-FOUND}/$1
RewriteRule !^NOT-FOUND/.* - [F]
RewriteRule ^NOT-FOUND/(.*)$ ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND}/$1
RewriteRule !^NOT-FOUND/.* - [F]
RewriteRule ^NOT-FOUND/(.*)$ /$1
</pre></example>
<example><pre>
##
## hosts.deny
##
## ATTENTION! This is a map, not a list, even when we treat it as such.
## mod_rewrite parses it for key/value pairs, so at least a
## dummy value "-" must be present for each entry.
##
193.102.180.41 -
bsdti1.sdm.de -
192.76.162.40 -
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Proxy Deny</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we forbid a certain host or even a user of a
special host from using the Apache proxy?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We first have to make sure <module>mod_rewrite</module>
is below(!) <module>mod_proxy</module> in the Configuration
file when compiling the Apache webserver. This way it gets
called <em>before</em> <module>mod_proxy</module>. Then we
configure the following for a host-dependent deny...</p>
<example><pre>
RewriteCond %{REMOTE_HOST} <strong>^badhost\.mydomain\.com$</strong>
RewriteRule !^http://[^/.]\.mydomain.com.* - [F]
</pre></example>
<p>...and this one for a user@host-dependent deny:</p>
<example><pre>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>^badguy@badhost\.mydomain\.com$</strong>
RewriteRule !^http://[^/.]\.mydomain.com.* - [F]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Special Authentication Variant</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>Sometimes a very special authentication is needed, for
instance a authentication which checks for a set of
explicitly configured users. Only these should receive
access and without explicit prompting (which would occur
when using the Basic Auth via <module>mod_access</module>).</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>We use a list of rewrite conditions to exclude all except
our friends:</p>
<example><pre>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>!^friend1@client1.quux-corp\.com$</strong>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>!^friend2</strong>@client2.quux-corp\.com$
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>!^friend3</strong>@client3.quux-corp\.com$
RewriteRule ^/~quux/only-for-friends/ - [F]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Referer-based Deflector</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>How can we program a flexible URL Deflector which acts
on the "Referer" HTTP header and can be configured with as
many referring pages as we like?</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>Use the following really tricky ruleset...</p>
<example><pre>
RewriteMap deflector txt:/path/to/deflector.map
RewriteCond %{HTTP_REFERER} !=""
RewriteCond ${deflector:%{HTTP_REFERER}} ^-$
RewriteRule ^.* %{HTTP_REFERER} [R,L]
RewriteCond %{HTTP_REFERER} !=""
RewriteCond ${deflector:%{HTTP_REFERER}|NOT-FOUND} !=NOT-FOUND
RewriteRule ^.* ${deflector:%{HTTP_REFERER}} [R,L]
</pre></example>
<p>... in conjunction with a corresponding rewrite
map:</p>
<example><pre>
##
## deflector.map
##
http://www.badguys.com/bad/index.html -
http://www.badguys.com/bad/index2.html -
http://www.badguys.com/bad/index3.html http://somewhere.com/
</pre></example>
<p>This automatically redirects the request back to the
referring page (when "<code>-</code>" is used as the value
in the map) or to a specific URL (when an URL is specified
in the map as the second argument).</p>
</dd>
</dl>
</section>
</section>
<section id="other">
<title>Other</title>
<section>
<title>External Rewriting Engine</title>
<dl>
<dt><strong>Description:</strong></dt>
<dd>
<p>A FAQ: How can we solve the FOO/BAR/QUUX/etc.
problem? There seems no solution by the use of
<module>mod_rewrite</module>...</p>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<p>Use an external <directive module="mod_rewrite"
>RewriteMap</directive>, i.e. a program which acts
like a <directive module="mod_rewrite"
>RewriteMap</directive>. It is run once on startup of Apache
receives the requested URLs on <code>STDIN</code> and has
to put the resulting (usually rewritten) URL on
<code>STDOUT</code> (same order!).</p>
<example><pre>
RewriteEngine on
RewriteMap quux-map <strong>prg:</strong>/path/to/map.quux.pl
RewriteRule ^/~quux/(.*)$ /~quux/<strong>${quux-map:$1}</strong>
</pre></example>
<example><pre>
#!/path/to/perl
# disable buffered I/O which would lead
# to deadloops for the Apache server
$| = 1;
# read URLs one per line from stdin and
# generate substitution URL on stdout
while (<>) {
s|^foo/|bar/|;
print $_;
}
</pre></example>
<p>This is a demonstration-only example and just rewrites
all URLs <code>/~quux/foo/...</code> to
<code>/~quux/bar/...</code>. Actually you can program
whatever you like. But notice that while such maps can be
<strong>used</strong> also by an average user, only the
system administrator can <strong>define</strong> it.</p>
</dd>
</dl>
</section>
</section>
</manualpage>