我们ssh连接到某台linux主机时，需要输入账号密码。为了免密码登录，我们通常使用SSH Key认证的方式。原理大概如下：
我们要从A主机ssh登录到B主机，通常我们在A主机上生成密钥对，然后将A主机的公钥传到B主机的某个账户家目录下的.ssh/authorized_keys文件中。比如我们需要从A主机上的root账户下用B主机的root账号来登录B主机，则需要将A主机上root账号的公钥传到B主机的root账户下的authorized_keys文件中。
实验环境：
主机A：172.16.206.132
主机B：172.16.206.131
1、首先在A主机上生成密钥对：
[email protected]:~/.ssh# 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: 82:6d:c4:a8:84:d4:f9:5f:05:42:c9:e5:64:0a:cb:c9 [email protected] The key‘s randomart image is: +--[ RSA 2048]----+ | .. ..ooo=. | |.. ooo+o* . | |. . oEo. .. | | . . = . | | . . = S | | . o | | | | | | | +-----------------+
查看生成的文件，注意文件权限
[email protected]:~/.ssh# ll total 16 -rw-r--r-- 1 root root 392 Feb 17 01:02 authorized_keys -rw------- 1 root root 1675 Jun 14 21:36 id_rsa -rw-r--r-- 1 root root 396 Jun 14 21:36 id_rsa.pub -rw-r--r-- 1 root root 1976 Jun 14 21:19 known_hosts
id_rsa：私钥文件
id_rsa.pub：公钥文件
authorized_keys：存放其他主机上的公钥文件，实现SSH 认证的
known_hosts：ssh登录过的主机
2、将A主机的公钥传到B主机的root账户下
此操作需要输入B主机上root用户的密码
ssh-copy-id -i ~/.ssh/id_rsa.pub [email protected] The authenticity of host ‘172.16.206.131 (172.16.206.131)‘ can‘t be established. RSA key fingerprint is 74:d8:e6:2a:9e:86:22:a7:1e:82:01:f5:79:0d:35:5a. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added ‘172.16.206.131‘ (RSA) to the list of known hosts. [email protected]‘s password: Now try logging into the machine, with "ssh ‘[email protected]‘", and check in: .ssh/authorized_keys to make sure we haven‘t added extra keys that you weren‘t expecting.
注意：上面的方法等效于直接在B主机的root账户的/root/.ssh/目录下创建authorized_keys文件，然后将A主机的公钥复制到这个文件中。注意文件的权限。
3、登录B主机查看authorized_keys文件，可以看到A主机的root账户的公钥
[[email protected] .ssh]# cat authorized_keys ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAsZOYbiGl5zu1BNETCDNJQryHIwz1rVgE/Bhp/dWpJBHTUkTASWBdAe9AP85ADe5rXtjt9atFkXZ/v4O8e2C8fPyY1qXaGcTq2qaB1x62SVrsYvL0eE3TEr0T7eIIC3I1540t/NNe4Tc6ao5/kC+NOFJPfFNqQ+UOdZTdkuHWsCZqd0aFLrXkgLaMe1ZFhBeG54ncmZ6heAbmbtpmc19qrPY9C8HY5FsV851MMjRvMohRiAMvB0JuHrneJwRKQ3OJLsgm4gDs3rfDj1cjveLzFSihsikVB9XKGmPquoY5BfZ2zQo6Mu3OwJ+5lhbe9ObSCoC9bkZnTJd9Rwz2sUlEiQ== [email protected]
4、从A主机ssh登录B主机
[email protected]:~# ssh [email protected] Last login: Thu Jun 16 11:05:21 2016 from 172.16.206.1
A主机登录B主机无需输入密码，直接可以登录。
扩展：从上面的案例我们可以知道谁想要SSH连接到其他主机，则需要在本机上生成密钥对，然后将公钥传到对方账户下的authorized_keys文件中即可。但是如果假设有这样一种情况，现在有很多个主机，都想要SSH登录A主机，但是不希望在每一台主机上都创建密钥对，怎么办呢？
可以将A主机的私钥文件拷贝到每台主机上面，然后A主机的公钥拷贝到A主机的authorized_keys文件中，这样A主机拿公钥，其他主机拿私钥，就可以实现SSH key认证了。
总结：SSH Key认证很灵活，只要密钥对是匹配的，就可以实现认证，不管这个密钥对是由谁生成的。