[Hadoop Wiki] Update of "UnknownHost" by ArpitAgarwal

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The "UnknownHost" page has been changed by ArpitAgarwal:
https://wiki.apache.org/hadoop/UnknownHost?action=diff&rev1=5&rev2=6

   * The hostname in the configuration files (such as {{{hadoop-site.xml}}}) is misspelled.
   * The hostname in the configuration files (such as {{{hadoop-site.xml}}}) is confused with the hostname of another service. For example, you are using the hostname of the YARN Resource Manager in the {{{fs.defaultFS}}} configuration option to define the namenode.
   * The site's DNS server does not have an entry for the node. Test: do an {{{nslookup <hostname>}}} from the client machine.
-  * The calling machine's host table {{{/etc/hosts}}} lacks an entry for the host, and DNS isn't helping out
+  * The calling machine's host table {{{/etc/hosts}}} lacks an entry for the host, and DNS isn't helping out. When using {{{/etc/hosts}}} based lookups the hostname entry must contain a trailing dot. See the [[#UsingHostsFiles|Using Hosts files]] section below for more.
   * A worker node thinks it has a given name -which it reports to the NameNode and JobTracker, but that isn't the name that the network team gave it, so it isn't resolvable.
   * The calling machine is on a different subnet from the target machine, and short names are being used instead of fully qualified domain names (FQDNs).
   * The client's network card is playing up (network timeouts, etc), the network is overloaded, or even the switch is dropping DNS packets.
@@ -30, +30 @@

  
  Unless the route cause has been identified, the problem may return.
  
+ <<Anchor(UsingHostsFiles)>>
+ == Using hosts files ==
+ If you are using {{{/etc/hosts}}} files instead of DNS-based lookups and your hosts files have FQDNs, you must ensure that the FQDN includes a trailing dot. Thus a correct hosts file entry for {{{foo.example.com}}} may look like this.
+ {{{
+ 1.2.3.4  foo.example.com foo.example.com. foo
+ }}}
+ The Hadoop host resolver ensures hostnames are terminated with a trailing {{{.}}} prior to lookup to avoid the security issue described in [[https://ietf.org/rfc/rfc1535.txt|RFC 1535]].
  
  == Unknown Host Exception in HA HDFS ==