一、情景

公司刚上几台Linux，现在要把主机之间都能远程ssh免密码登陆。

二、原理
很简单，使用ssh-keygen 在主机A上生成private和public密钥，将生成的public密钥拷贝到远程机器主机B上后,就可以使用ssh命令无需密码登录到另外一台机器主机B上。

三、步骤

主机A：

1、 生成公钥和私钥文件id_rsa和id_rsa.pub （敲3下回车即可）
[root@bogon ~]# ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
67:da:0d:79:e0:d6:2b:cd:7d:22:af:51:7e:9c:75:fe root@bogon
The key's randomart image is:
+--[ RSA 2048]----+
| |
| |
| . |
| . + |
| S B o . o|
| * * = o+|
| . o B +.=|
| . + +.|
| ... E|
+-----------------+

2、ssh-cop-id命令会将指定的公钥文件复制到远程计算机

[root@bogon ~]# ssh-copy-id -i ~/.ssh/id_rsa.pub root@10.1.250.166
The authenticity of host '10.1.250.166 (10.1.250.166)' can't be established.
RSA key fingerprint is c8:9d:6d:92:55:77:3d:3e:af:f5:cb:18:80:5a:47:93.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '10.1.250.166' (RSA) to the list of known hosts.
reverse mapping checking getaddrinfo for bogon [10.1.250.166] failed - POSSIBLE BREAK-IN ATTEMPT!
root@10.1.250.166's password: <主机B的登陆密码>
Now try logging into the machine, with "ssh 'root@10.1.250.166'", and check in:

.ssh/authorized_keys

to make sure we haven't added extra keys that you weren't expecting.

3、然后ssh登陆主机B验证是否需要密码

[root@localhost ~]# ssh 10.1.250.166
reverse mapping checking getaddrinfo for bogon [10.1.250.166] failed - POSSIBLE BREAK-IN ATTEMPT!
Last login: Wed Oct 21 10:05:39 2015 from 10.1.250.141
[root@bogon ~]#

4、登陆成功后，我们需要在主机B也做下以上3步，这样就可以相互免密码ssh登陆。（如果有多台主机，每台主机都做下相同操作，以方便以后管理）。

5、权限问题
建议用其他用户做ssh登陆的话，调整目录文件权限
设置authorized_keys权限

chmod 644 authorized_keys

设置.ssh目录权限

chmod 700 -R .ssh

6、要保证.ssh和authorized_keys都只有用户自己有写权限。否则验证无效。

本文出自 “好大的刀” 博客