Hadoop hostname: Unknown host

本来下想在一台虚拟机上，搭建一个hadoop的测试hadoop，用于调试和阅读hadoop源代码，发现在虚拟机上执行：

$hostname -i

hostname: Unknown host

这个是因为没有设置/etc/hosts文件导致的，如果linux的环境是这样的，那么将会在启动datanode和tasktracker的时候失败，

datanode和namenode将会抛出如下的异常：

2015-01-28 12:36:36,506 INFO org.apache.hadoop.metrics2.impl.MetricsConfig: loaded properties from hadoop-metrics2.properties

2015-01-28 12:36:36,535 INFO org.apache.hadoop.metrics2.impl.MetricsSourceAdapter: MBean for source MetricsSystem,sub=Stats registered.

2015-01-28 12:36:36,540 ERROR org.apache.hadoop.metrics2.impl.MetricsSystemImpl: Error getting localhost name. Using 'localhost'...

java.net.UnknownHostException: PC_PAT1: PC_PAT1: Name or service not known

        at java.net.InetAddress.getLocalHost(InetAddress.java:1473)

        at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.getHostname(MetricsSystemImpl.java:463)

        at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.configureSystem(MetricsSystemImpl.java:394)

        at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.configure(MetricsSystemImpl.java:390)

        at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.start(MetricsSystemImpl.java:152)

        at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.init(MetricsSystemImpl.java:133)

        at org.apache.hadoop.metrics2.lib.DefaultMetricsSystem.init(DefaultMetricsSystem.java:40)

        at org.apache.hadoop.metrics2.lib.DefaultMetricsSystem.initialize(DefaultMetricsSystem.java:50)

        at org.apache.hadoop.hdfs.server.datanode.DataNode.instantiateDataNode(DataNode.java:1520)

        at org.apache.hadoop.hdfs.server.datanode.DataNode.createDataNode(DataNode.java:1539)

        at org.apache.hadoop.hdfs.server.datanode.DataNode.secureMain(DataNode.java:1665)

        at org.apache.hadoop.hdfs.server.datanode.DataNode.main(DataNode.java:1682)

Caused by: java.net.UnknownHostException: PC_PAT1: Name or service not known

        at java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)

        at java.net.InetAddress$1.lookupAllHostAddr(InetAddress.java:901)

        at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1293)

        at java.net.InetAddress.getLocalHost(InetAddress.java:1469)

        ... 11 more

解决方案是在/etc/hosts文件中添加如下红色的内容，即172.18.140.24 PC_PAT1

hadoop@PC_PAT1:~/hadoop-1.0.3> less /etc/hosts

#

# hosts         This file describes a number of hostname-to-address

#               mappings for the TCP/IP subsystem.  It is mostly

#               used at boot time, when no name servers are running.

#               On small systems, this file can be used instead of a

#               "named" name server.

# Syntax:

#

# IP-Address  Full-Qualified-Hostname  Short-Hostname

#

#127.0.0.1      localhost

#127.0.0.1      localhost.localdomain localhost cpyftest-2

127.0.0.1       localhost localhost.localdomain localhost4 localhost4.localdomain4

172.18.140.24   PC_PAT1

# special IPv6 addresses

::1             localhost ipv6-localhost ipv6-loopback

fe00::0         ipv6-localnet

ff00::0         ipv6-mcastprefix

ff02::1         ipv6-allnodes

ff02::2         ipv6-allrouters

ff02::3         ipv6-allhosts

添加后，hostname –i 将会得到正确的结果：

将hostname unkown的问题解决后，重启hadoop集群，可以发现，datanode/tasktracker , namenode/jobtracker都启动了：

ERROR org.apache.hadoop.hdfs.server.namenode.NameNode: java.io.IOException: Incomplete HDFS URI, no host: hdfs://data_181.uc:9000

着实郁闷到了，原来在Hadoop中，主机名不要包含下划线“_”，但可以有横线“-”。

Incomplete HDFS URI

巴特西

Hadoop hostname: Unknown host

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