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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.
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.
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
Configuring multihomed hosts with [Hadoop in Secure Mode](../hadoop-common/SecureMode.html) may require additional configuration.
### Hostname Lookup
Kerberos principals for Hadoop Services are specified using the pattern `ServiceName/_HOST@REALM.TLD` e.g. `nn/_HOST@REALM.TLD`. This allows the same configuration file to be used on all hosts. Services will substitute `_HOST` in the principal with their own hostname looked up at runtime.
When nodes are configured to have multiple hostnames in DNS or in `/etc/hosts` files, a service may lookup a different hostname than what is expected by the server. e.g. intra-cluster traffic between two services may be routed over a private interface but the client service looked up its public hostname. Kerberos authentication will fail since the hostname in the principal does not match the IP address over which the traffic arrived.
The following setting (available starting Apache Hadoop 2.8.0) can be used to control the hostname looked up the service.
<property>
<name>hadoop.security.dns.interface</name>
<description>
The name of the Network Interface from which the service should determine
its host name for Kerberos login. e.g. eth2. In a multi-homed environment,
the setting can be used to affect the _HOST subsitution in the service
Kerberos principal. If this configuration value is not set, the service
