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  ---
  Hadoop Distributed File System-${project.version} - Support for Multi-Homed Networks
  ---
  ---
  ${maven.build.timestamp}

HDFS Support for Multihomed Networks

  This document is targetted to cluster administrators deploying <<<HDFS>>> in
  multihomed networks. Similar support for <<<YARN>>>/<<<MapReduce>>> is
  work in progress and will be documented when available.

%{toc|section=1|fromDepth=0}

* Multihoming Background

  In multihomed networks the cluster nodes are connected to more than one
  network interface. There could be multiple reasons for doing so.

  [[1]] <<Security>>: Security requirements may dictate that intra-cluster
  traffic be confined to a different network than the network used to
  transfer data in and out of the cluster.

  [[2]] <<Performance>>: Intra-cluster traffic may use one or more high bandwidth
  interconnects like Fiber Channel, Infiniband or 10GbE.

  [[3]] <<Failover/Redundancy>>: The nodes may have multiple network adapters
  connected to a single network to handle network adapter failure.


  Note that NIC Bonding (also known as NIC Teaming or Link
  Aggregation) is a related but separate topic. The following settings
  are usually not applicable to a NIC bonding configuration which handles
  multiplexing and failover transparently while presenting a single 'logical
  network' to applications.

* Fixing Hadoop Issues In Multihomed Environments

** Ensuring HDFS Daemons Bind All Interfaces

  By default <<<HDFS>>> endpoints are specified as either hostnames or IP addresses.
  In either case <<<HDFS>>> daemons will bind to a single IP address making
  the daemons unreachable from other networks.

  The solution is to have separate setting for server endpoints to force binding
  the wildcard IP address <<<INADDR_ANY>>> i.e. <<<0.0.0.0>>>. Do NOT supply a port
  number with any of these settings.

----
<property>
  <name>dfs.namenode.rpc-bind-host</name>
  <value>0.0.0.0</value>
  <description>
    The actual address the RPC server will bind to. If this optional address is
    set, it overrides only the hostname portion of dfs.namenode.rpc-address.
    It can also be specified per name node or name service for HA/Federation.
    This is useful for making the name node listen on all interfaces by
    setting it to 0.0.0.0.
  </description>
</property>

<property>
  <name>dfs.namenode.servicerpc-bind-host</name>
  <value>0.0.0.0</value>
  <description>
    The actual address the service RPC server will bind to. If this optional address is
    set, it overrides only the hostname portion of dfs.namenode.servicerpc-address.
    It can also be specified per name node or name service for HA/Federation.
    This is useful for making the name node listen on all interfaces by
    setting it to 0.0.0.0.
  </description>
</property>

<property>
  <name>dfs.namenode.http-bind-host</name>
  <value>0.0.0.0</value>
  <description>
    The actual adress the HTTP server will bind to. If this optional address
    is set, it overrides only the hostname portion of dfs.namenode.http-address.
    It can also be specified per name node or name service for HA/Federation.
    This is useful for making the name node HTTP server listen on all
    interfaces by setting it to 0.0.0.0.
  </description>
</property>

<property>
  <name>dfs.namenode.https-bind-host</name>
  <value>0.0.0.0</value>
  <description>
    The actual adress the HTTPS server will bind to. If this optional address
    is set, it overrides only the hostname portion of dfs.namenode.https-address.
    It can also be specified per name node or name service for HA/Federation.
    This is useful for making the name node HTTPS server listen on all
    interfaces by setting it to 0.0.0.0.
  </description>
</property>
----

** Clients use Hostnames when connecting to DataNodes

  By default <<<HDFS>>> clients connect to DataNodes using the IP address
  provided by the NameNode. Depending on the network configuration this
  IP address may be unreachable by the clients. The fix is letting clients perform
  their own DNS resolution of the DataNode hostname. The following setting
  enables this behavior.

----
<property>
  <name>dfs.client.use.datanode.hostname</name>
  <value>true</value>
  <description>Whether clients should use datanode hostnames when
    connecting to datanodes.
  </description>
</property>
----

** DataNodes use HostNames when connecting to other DataNodes

  Rarely, the NameNode-resolved IP address for a DataNode may be unreachable
  from other DataNodes. The fix is to force DataNodes to perform their own
  DNS resolution for inter-DataNode connections. The following setting enables
  this behavior.

----
<property>
  <name>dfs.datanode.use.datanode.hostname</name>
  <value>true</value>
  <description>Whether datanodes should use datanode hostnames when
    connecting to other datanodes for data transfer.
  </description>
</property>
----