Linux"公私钥"认证的方式来进行ssh登录

假设服务器IP地址为192.168.1.1，机器名：cluster.hpc.org
         客户端IP地址为172.16.16.1，机器名：p470-2.wangrx.sioc.ac.cn
客户端用户yzhao需要使用ssh无密码登录服务器的zhaoy帐户

实现原理
使用一种被称为"公私钥"认证的方式来进行ssh登录. "公私钥"认证方式简单的解释是
首先在客户端上创建一对公私钥 （公钥文件：~/.ssh/id_rsa.pub； 私钥文件：~/.ssh/id_rsa）
然后把公钥放到服务器上（~/.ssh/authorized_keys）, 自己保留好私钥
当ssh登录时,ssh程序会发送私钥去和服务器上的公钥做匹配.如果匹配成功就可以登录了


设置如下：
1、以yzhao用户登录客户机器并在客户端机器上执行"ssh-keygen -t rsa"
(注：每次执行"ssh-keygen -t rsa"产生的私钥文件都会不同)
a）如果文件"~/.ssh/id_rsa"存在，会提示是否覆盖该文件，此时可选择"n"不覆盖该文件而使用已有的id_rsa文件；如果选择"y"则会重新生成"~/.ssh/id_rsa"文件，接下来会提示输入passphrase，回车确定使用空的passphrase，再次回车确认（这里也可以输出passphrase，相当于ssh时登录的密码）。然后会重新生成"~/.ssh/id_rsa"文件和"~/.ssh/id_rsa.pub"文件（结果如下）。
[yzhao@p470-2 ~]$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/disk2/yzhao/.ssh/id_rsa):
/disk2/yzhao/.ssh/id_rsa already exists.
Overwrite (y/n)? y
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /disk2/yzhao/.ssh/id_rsa.
Your public key has been saved in /disk2/yzhao/.ssh/id_rsa.pub.
The key fingerprint is:
6d:a1:17:8a:b6:d2:c0:a1:6c:66:ba:85:0b:7b:9f:0c [e-mail]

b）如果"~/.ssh/id_rsa"文件和"~/.ssh/id_rsa.pub"文件不存在则会自动创建新的"~/.ssh/id_rsa"文件和"~/.ssh/id_rsa.pub"文件，passphrase设置同上。
[yzhao@p470-2 ~]$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/disk2/yzhao/.ssh/id_rsa):
Created directory '/disk2/yzhao/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /disk2/yzhao/.ssh/id_rsa.
Your public key has been saved in /disk2/yzhao/.ssh/id_rsa.pub.
The key fingerprint is:
54:49:ad:33:b3:ff:71:da:6d:db:78:d0:bb:6a:15:bc [e-mail]

2、使用ssh zhaoy@192.168.1.1登录到服务器，编辑服务器上"~/.ssh/authorized_keys"文件，将客户端机器上的"~/.ssh/id_rsa.pub"文件内容追加到"~/.ssh/authorized_keys"文件中。
（注：可以在客户端机器上使用以下命令来实现：
                 cat  ~/.ssh/id_rsa.pub | ssh zhaoy@192.168.1.1 "cat - >> ~/.ssh/authorized_keys"
此时会要求输入zhaoy在服务器上的登录密码，输入后即会将客户端机器上的"~/.ssh/id_rsa.pub"文件内容追加到服务器上的"~/.ssh/authorized_keys"文件中）

如果是首次连接服务器会出现以下的提示，确认连接并输入密码后其他直接回车确定。
[yzhao@p470-2 ~]$ ssh zhaoy@192.168.1.1
The authenticity of host '192.168.1.1 (192.168.1.1)' can't be established.
RSA key fingerprint is 94:91:33:01:6b:e7:10:ae:42:ac:ea:5c:8c:bb:f1:18.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.1.1' (RSA) to the list of known hosts.
zhaoy@192.168.1.1's password:
Last login: Fri Dec 21 17:41:38 2007 from 172.16.16.1
Rocks 4.2.1 (Cydonia)
Profile built 03:58 21-Jun-2007

Kickstarted 12:25 21-Jun-2007
Rocks Frontend Node - Our Cluster Cluster

It doesn't appear that you have set up your ssh key.
This process will make the files:
     /home/zhaoy/.ssh/id_rsa.pub
     /home/zhaoy/.ssh/id_rsa
     /home/zhaoy/.ssh/authorized_keys

Generating public/private rsa key pair.
Enter file in which to save the key (/home/zhaoy/.ssh/id_rsa):
Created directory '/home/zhaoy/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/zhaoy/.ssh/id_rsa.
Your public key has been saved in /home/zhaoy/.ssh/id_rsa.pub.
The key fingerprint is:
7e:f6:ab:b0:79:70:cb:c9:f7:40:37:aa:10:4d:4a:ac [e-mail]

3、如果在第1步中使用了空的passphrase，则可以跳过第4步，此时在客户端即可以使用"ssh zhaoy@192.168.1.1"即可无密码登录到服务器；如果第一步中设置了passphrase，则继续执行以下步骤。

4、如果第1步中设置了passphrase，则此时需要输入该passphrase登录服务器。此时前面我们把输入密码变成了输入passphrase，这没有带来任何方便。但是我们可以通过ssh-agent来帮助我们自动输入passphrase(只是看起来像是自动输入而已)，我们只要在第一次登录时输入一次passphrase, 以后的工作就可以交给ssh-agent。在客户端机器上执行命令ssh-add，这里会提示输入一次passphrase。输入第一步中设置的passphrase之后会修改"~/.ssh/id_rsa"文件。再在客户端执行"ssh zhaoy@192.168.1.1"即可无密码登录到服务器端。

[yzhao@p470-2 ~]$ ssh-add
Enter passphrase for /disk2/yzhao/.ssh/id_rsa:
Identity added: /disk2/yzhao/.ssh/id_rsa (/disk2/yzhao/.ssh/id_rsa)
[yzhao@p470-2 ~]$ ssh zhaoy@192.168.1.1
Last login: Fri Dec 21 17:55:38 2007 from 172.16.16.1
Rocks 4.2.1 (Cydonia)
Profile built 03:58 21-Jun-2007

Kickstarted 12:25 21-Jun-2007
Rocks Frontend Node - Our Cluster Cluster
[zhaoy@cluster ~]$