实验：实现openssh key验证

实验前准备：
　　 两台已经安装了openssh的Centos，主机A的IP地址为172.22.52.16，主机B的IP地址为172.22.52.17。

步骤一：首先在A机器生成密钥对，命令：ssh-keygen，生成的秘钥默认会保存在家目录的.ssh/下id_rsa和id_rsa.pub)，设置密钥对密码，这里不设置了全部回车跳过

[19:16:09root@localhost data]#ssh-keygen
Enter file in which to save the key (/root/.ssh/id_rsa):
Created directory '/root/.ssh'.
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:
SHA256:esARTVf53owszvSNjetiqWF4qcrzSMISX/TB+J0gsAg root@localhost.localdomain
The key's randomart image is:
[19:16:58root@localhost data]#ls -a ~/.ssh
.  ..  id_rsa  id_rsa.pub

步骤二：将客户端的公钥（id_rsa.pub）拷贝到服务端(B主机)上root的家目录（因为我要登录的是root用户）并命名为authorized_keys,方法有好几种，我就使用最方便的一个：ssh-copy-id，这个命令会自己在相应的目录生成authorized_keys文件，当然也可以用scp等等拷贝过去。

[19:56:12root@localhost ~]#ssh-copy-id -i id\_rsa.pub root@172.22.52.17
/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/root/.ssh/id_rsa.pub"
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
root@172.22.52.17's password:  #输入主机B登录密码以完成公钥拷贝Number of key(s) added: 1Now try logging into the machine, with:   "ssh 'root@172.22.52.17'"
and check to make sure that only the key(s) you wanted were added.

步骤三：当客户端再次发送一个连接请求，包括ip、用户名；服务端得到客户端的请求后，会到authorized_keys中查找，如果有响应的IP和用户，就会随机生成一个字符串，服务端将使用客户端拷贝过来的公钥进行加密，然后发送给客户端，得到服务端发来的消息后，客户端会使用私钥进行解密，然后将解密后的字符串发送给服务端，服务端接受到客户端发来的字符串后，跟之前的字符串进行对比，如果一致，就允许免密码登录了

[19:57:02root@localhost ~]#ssh root@172.22.52.17
Last login: Tue Apr 16 13:03:58 2019 from 172.22.52.16
[18:04:36root@m9l ~]#exit
logout
Connection to 172.22.52.17 closed.

Ps：第二步如果出现下面这个错误是因为id_rsa.pub命令只在当前工作目录寻找公钥文件，进入.ssh/目录或指定公钥绝对路径即可解决

[19:54:29root@localhost ~]#ssh-copy-id -i id_rsa.pub root@172.22.52.17/usr/bin/ssh-copy-id: ERROR: failed to open ID file 'id_rsa.pub': No such file or directory