hadoop1.2.1文档中这样写：
Now check that you can ssh to the localhost without a passphrase:
$ ssh localhost
If you cannot ssh to localhost without a passphrase, execute the following commands:
$ ssh-keygen -t dsa -P '' -f ~/.ssh/id_dsa
$ cat ~/.ssh/id_dsa.pub >> ~/.ssh/authorized_keys
这里需要修改~/.ssh/authrized_keys权限为600，~/.ssh权限为700，才可以免登陆，
[vincent@localhost ~]$ chmod 600 ~/.ssh/authorized_keys
[vincent@localhost ~]$ ssh localhost
The authenticity of host 'localhost (127.0.0.1)' can't be established.
RSA key fingerprint is 4e:35:30:18:36:08:3e:57:ca:a0:2e:20:aa:ae:44:ea.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'localhost' (RSA) to the list of known hosts.
Last login: Tue Apr 26 00:22:40 2016 from localhost.localdomain