问题
I have installed CDH4 on CentOS 6.3 64-bit in Pseudo Distributed mode using the following instructions. Everything is set to localhost in the Hadoop configuration files. But, still when I format the name node the below exception appears. When I add an 192.168.1.101 CentOSHost entry to the /etc/hosts file the exception goes away and I am able to run format/start HDFS and run MR jobs.
I want to run MR jobs even when I am not connected to the network without adding an entry to the /etc/hosts file. How to get this done?
12/08/27 22:17:15 WARN net.DNS: Unable to determine address of the host-falling back to "localhost" address java.net.UnknownHostException: CentOSHost: CentOSHost
at java.net.InetAddress.getLocalHost(InetAddress.java:1360)
at org.apache.hadoop.net.DNS.resolveLocalHostIPAddress(DNS.java:283)
at org.apache.hadoop.net.DNS.(DNS.java:59) at org.apache.hadoop.hdfs.server.namenode.NNStorage.newBlockPoolID(NNStorage.java:1017)
at org.apache.hadoop.hdfs.server.namenode.NNStorage.newNamespaceInfo(NNStorage.java:565)
at org.apache.hadoop.hdfs.server.namenode.FSImage.format(FSImage.java:145)
at org.apache.hadoop.hdfs.server.namenode.NameNode.format(NameNode.java:724)
at org.apache.hadoop.hdfs.server.namenode.NameNode.createNameNode(NameNode.java:1095)
at org.apache.hadoop.hdfs.server.namenode.NameNode.main(NameNode.java:1193)
回答1:
It looks like some where the configuration is returning/ using the hostname as CentOSHost.
What does hostname --fqdn returns to you?
For Hadoop, it is important that name look-up and reverse look-up work successfully. You should be able to resolve the ip-address and resolve hostname from the ip-address (Reverse resolution). This can be tested using the above command.
The entry to /etc/hosts is required for the reverse resolution to work. Unless the entry and the configuration are pointing to localhost. Even in that case the hostname --fqdn should return as localhost.
来源：https://stackoverflow.com/questions/12146439/unknownhostexception-while-formatting-hdfs