mod_headers: changes to support direct HTTP header mapping (+Vary)

[Paul Rhodes <pa...@yahoo.co.uk>]

Hi,

I've been using mod_headers extensively for a number of years to modify 
headers from as they enter and exit the network. We use apache as a 
front end to our appservers and we get requests from multiple sources so 
we often have issues with header translation. This means we have to do 
something like this:

  RewriteCond %{HTTP:INPUTHEADER1}  ([0-9]+)
  RewriteRule (.*) $1 [E=HEADER:%1]

  RewriteCond %{HTTP:INPUTHEADER2}  ([0-9]+)
  RewriteCond %{ENV:HEADER}         (.+)
  RewriteRule (.*) $1 [E=HEADER:%1]

  RewriteCond %{ENV:HEADER}         (.+)
  RewriteRule (.*) $1 [E=HEADER:"default"]


  RequestHeader set OUTPUTHEADER %{HEADER}e env=HEADER

This has worked kind of ok until now, but we are now looking at caching 
issues and wish to ensure that the corresponding Vary is mapped. This 
makes the situation somewhat more complicated and I don't think it's 
really possible to continue doing this via rewrite rules.

I've looked at mod_headers and have started implementing a patch. The 
patch provides the following functionality:

  Header|RequestHeader default <Header> <Value>

  The header <Header> is only set if <Header> does not already exist.


  Header|RequestHeader copy <New-Header> <Original-Header>

  This will copy the value of <Original-Header> to <New-Header> if and 
only if <New-Header> is not already present.


  Header|RequestHeader rename <New-Header> <Original-Header>

  This will copy the value of <Original-Header> to <New-Header> if and 
only if <New-Header> is not already present. In addition to this it will 
remove <Original-Header>.


The combination of the above will allow me to put something like the 
following..

  RequestHeader rename OUTPUTHEADER INPUTHEADER1
  RequestHeader rename OUTPUTHEADER INPUTHEADER2
  RequestHeader default "default"

This is obviously neater, but the main reason to do this is to have an 
explicit link between the headers so that when a response comes back, 
the Vary can be catered for. So if 'Vary: OUTPUTHEADER' is sent back 
from the appserver, apache can convert this to 'Vary: INPUTHEADER1' or 
'Vary: INPUTHEADER2' as appropriate.

Ok. That is a lot of background, but my question is what is the best way 
to track which of the translations took place? I was trying to avoid 
bodging it with setting an environment variable or using notes. Is there 
somewhere within the datastructures where I can keep track of which 
rules fired?

Also, I have made an effort to genericise this implementation so it fits 
logically on top of mod_headers. Is there any possibility that such a 
patch might be commited for mod_headers?

thanks in advance,

Paul