1. ssh远程免密登陆
ssh默认情况下登陆主机需要输入账户密码进行远程登陆，比如下图登陆远程192.168.3.40服务器时，需要输入登陆密码，
有的时候，比如在python程序中，需要在远程服务器上ssh后执行命令的时候，就需要免密登陆,免密登陆就是无需输入密码这个步骤，即可完成登陆
2. 如何实现远程免密登陆
免密登陆借助ssh密钥实现，生成本地主机的一对ssl密钥，将公钥发给远程主机
step1： 检查本地密钥：
luxp@luxpdeMacBook-Pro-2 ~ % ls -al ~/.ssh/id_*.pub
-rw-r--r-- 1 luxp staff 746 7 30 2018 /Users/luxp/.ssh/id_rsa.pub
使用ls -al ~/.ssh/id_*.pub 可以查看到本地是否已经存在密钥，可以使用已经存在的密钥
step2： 也可以生成一对新的ssl密钥：
luxp@luxpdeMacBook-Pro-2 ~ % ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/Users/luxp/.ssh/id_rsa): mykey
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in mykey.
Your public key has been saved in mykey.pub.
The key fingerprint is:
SHA256:ZggxQmT+ImxS2tAgu+UoDBlgU2x3KecIHUvf8r6MD1U luxp@luxpdeMacBook-Pro-2.local
The key's randomart image is:
+---[RSA 3072]----+
|*=B.+o. . |
|oO.=.*o+. |
|= * +.*o . E |
|+X . o oo . |
|B++ . . So |
|+. . oo |
| . . |
| + . |
| ..+ |
+----[SHA256]-----+
luxp@luxpdeMacBook-Pro-2 ~ %
以上操作在当前目录生成：
Your identification has been saved in mykey.
Your public key has been saved in mykey.pub.
step3: 密钥生成后，将公钥传输到远程主机的.ssh配置下
luxp@luxpdeMacBook-Pro-2 ~ % ssh-copy-id -i mykey.pub root@192.168.3.40
/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "mykey.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@192.168.3.40's password:
Number of key(s) added: 1
Now try logging into the machine, with: "ssh 'root@192.168.3.40'"
and check to make sure that only the key(s) you wanted were added.
luxp@luxpdeMacBook-Pro-2 ~ %
以上操作，会将mykey.pub传到192.168.3.40主机
.ssh/authorized_keys
如果没有ssh-copy-id命令，可以
cat .mykey_pub | ssh remote_username@server_ip_address "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"
传输完成后，接口免密登陆
ssh root@192.168.3.40
远程命令执行
ssh root@192.168.3.40 "ls -al"
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