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The "UnknownHost" page has been changed by SteveLoughran: https://wiki.apache.org/hadoop/UnknownHost?action=diff&rev1=9&rev2=10 Comment: mention unknown localhost a. The hostname in the configuration files (such as {{{core-site.xml}}}) is misspelled. 1. The hostname in the configuration files (such as {{{core-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. 1. 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. + 1. If it is happening in service startup, it means the hostname of that service (HDFS, YARN, etc) cannot be found in {{{/etc/hosts}}}; the service will fail to start as it cannot determine which network card/address to use. 1. 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). 1. You are running in a cloud infrastructure and the destination machine is no longer there. It may have been deleted from the DNS records, or, due to some race condition, something is trying to talk to a host that hasn't been created yet.
