从主机A得到id_rsa.pub文件，在主机B创建用户danny加入该文件，实现主机A免密登录主机B

操作步骤：

1.主机A

生成公钥id_rsa.pub文件并导出该文件

root@mytest:~# ssh-keygen 
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:
7e:0f:dc:05:2e:d3:e0:24:c0:5f:fe:cf:e9:7a:f7:49 root@mytest
The key's randomart image is:
+--[ RSA 2048]----+
|     ..          |
|      ..  .      |
|       ..oo .    |
|        .+.+ .   |
|        S +.o .  |
|       . . +..   |
|        . + .o E |
|         . o  * o|
|            o+.oo|
+-----------------+
root@mytest:~# sz /root/.ssh/id_rsa.pub 

这步之后也可以通过以下步骤直接实现公钥认证（但是需要知道主机B用户danny的密码）

# ssh-copy-id -i /root/.ssh/id_rsa.pub danny@12.1.1.2
danny@12.1.1.2's password:      //输入主机B用户danny的登录密码

2.主机B

得到id_rsa.pub文件，创建用户danny，并在该用户家目录创建.ssh文件夹和其文件下的远程认证文件authorized_keys

[root@B danny]# mkdir .ssh
[root@B danny]# cd .ssh/
[root@B .ssh]# touch authorized_keys
[root@B .ssh]# ls
authorized_keys
[root@B .ssh]# cat /home/danny/id_rsa.pub >authorized_keys

主机A登录主机B

root@mytest:/# ssh danny@IP_B