作为一名运维人员来说，管理1-5台机器尚有余力，但如果是10台、100台或更多服务器，是不是每次登录输入密码非常繁琐，且费时费力，无法提高工作效率。今天我们通过使用ssh-kengen生成私钥&公钥对，目的：免密码登录SSH。其算法有两种，分别是RSA和DSA。

RSA 是非对称加密算法，可以用来加密和签名。

DSA(Digital Signature Algorithm) 只能用来数字签名的算法。

以下操作适用于OS：Centos 7、Ubuntu 17，其他系统没测，理论上都可以使用。

服务器:

10.10.204.63

10.10.204.64

1.如何生成ssh公钥

登录10.10.204.63服务器生成公私密钥对：

[root@10-10-204-63 ~]# ssh-keygen -b 4096 -t rsa

Generating public/private rsa key pair.
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:qLcoj2nSzq6G9ZpFQZ/OFqFT+oBDf3ousHkt82F1/xM root@10-10-204-63.10.10.204.63
The key's randomart image is:
+---[RSA 4096]----+
|  . . o          |
| . + = o         |
|  o B =          |
|   . X o         |
|  . o B S .      |
|  .= * . . .  E  |
|.oo.B *     .  . |
|oo+*.O o     ..  |
|o*O+o o       .. |
+----[SHA256]-----+

三次回车即可生成 ssh key。

注解：

-b 指定密钥长度。对于RSA密钥，最小要求768位，默认是2048位，最长4096字节。

-t 指定要创建的密钥类型。可以使用：”rsa1″(SSH-1) “rsa”(SSH-2) “dsa”(SSH-2)。

2.查看生成的文件

[root@10-10-204-63 ~]# ll .ssh/
total 8
-rw------- 1 root root 3243 Nov 25 15:58 id_rsa
-rw-r--r-- 1 root root  758 Nov 25 15:58 id_rsa.pub

说明：

id_rsa 私钥

id_rsa.pub 公钥

3.将公钥上传到10.10.204.64

[root@10-10-204-63 ~]# ssh-copy-id -i ~/.ssh/id_rsa.pub root@10.10.204.64
/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/root/.ssh/id_rsa.pub"
The authenticity of host '10.10.204.64 (10.10.204.64)' can't be established.
ECDSA key fingerprint is SHA256:/YI/L4RT1QH7lkfxMCAkKnvniQslyUl15mOUKUo8K3k.
ECDSA key fingerprint is MD5:6d:b6:f3:93:8e:48:53:24:9d:5d:c2:2a:5f:28:f4:d2.
Are you sure you want to continue connecting (yes/no)? yes【输入yes回车】
/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@10.10.204.64's password:【输入服务器密码回车】

Number of key(s) added: 1

Now try logging into the machine, with:   "ssh 'root@10.10.204.64'"
and check to make sure that only the key(s) you wanted were added.

上传成功。

4.修改SSH配置文件

登录10.28.204.64修改，操作如下：

$ vim /etc/ssh/sshd_config

去除以下注释：

RSAAuthentication yes
PubkeyAuthentication yes

5.重启SSH服务

$ systemctl restart sshd

6.测试免密码登录10.10.204.64

[root@10-10-204-63 ~]# ssh 'root@10.10.204.64'
Last failed login: Sat Nov 25 16:09:48 CST 2017 from 83.234.149.66 on ssh:notty
There was 1 failed login attempt since the last successful login.
Last login: Sat Nov 25 15:57:33 2017 from 36.7.69.84
[root@10-10-204-64 ~]#

在不输入密码的情况下成功登录。

登陆成功后，建议在10.10.204.64服务器上也生成ssh公钥，并上传到10.10.204.63服务器，这样以来我们就可以相互免密码SSH登陆。多台服务器亦是如此。

7.查看公钥

[root@10-10-204-64 ~]# ll /root/.ssh/
total 8
-rw-------  1 root root 758 Nov 25 16:08 authorized_keys
-rw-r--r--. 1 root root 175 Aug  9 09:19 known_hosts

authorized_keys是刚上传过来的公钥名称

8.如果公钥丢失，可以使用私钥再次生成公钥，如下：

[root@10-10-204-63 ~]# ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub

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