[Hadoop Wiki] Update of "UnknownHost" by SteveLoughran

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The "UnknownHost" page has been changed by SteveLoughran:
https://wiki.apache.org/hadoop/UnknownHost?action=diff&rev1=2&rev2=3

Comment:
move typos and wrong hostname for role up to first causes

  
  
  Some possible causes (not an exclusive list):
+  * 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
-  * There's some error in the configuration files and the hostname is actually wrong.
-  * 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 expect, so it isn't resolvable.
+  * 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.
   * The host's IP address has changed but a long-lived JVM is caching the old value. This is a known problem with JVMs (search for "java negative DNS caching" for the details and solutions). The quick solution: restart the JVMs
@@ -25, +26 @@

   1. As well as {{{nslookup}}}, the {{{dig}}} command is invaluable for tracking down DNS problems, though it does assume you understand DNS records. Now is a good time to learn.
   1. Restart the JVMs to see if that makes it go away.
   1. Restart the servers to see if that makes it go away.
+  1. Reboot the network switches.
  
  Remember, unless the route cause has been identified, the problem may return.