SSH免密登录

使用下例中ssky-keygen和ssh-copy-id，仅需通过3个步骤的简单设置而无需输入密码就能登录远程Linux主机。
ssh-keygen 创建公钥和密钥。
ssh-copy-id 把本地主机的公钥复制到远程主机的authorized_keys文件上。
ssh-copy-id 也会给远程主机的用户主目录（home）和~/.ssh, 和~/.ssh/authorized_keys设置合适的权限 。

步骤1: 用 ssh-key-gen 在本地主机上创建公钥和密钥

root@local-host$ ssh-keygen -t  rsa
Enter file in which to save the key (/home/jsmith/.ssh/id_rsa):[Enter key]
Enter passphrase (empty for no passphrase): [Press enter key]
Enter same passphrase again: [Pess enter key]
Your identification has been saved in /home/jsmith/.ssh/id_rsa.
Your public key has been saved in /home/jsmith/.ssh/id_rsa.pub.
The key fingerprint is: 33:b3:fe:af:95:95:18:11:31:d5:de:96:2f:f2:35:f9
root@local-host

步骤2: 用 ssh-copy-id 把公钥复制到远程主机上

root@local-host$ ssh-copy-id -i ~/.ssh/id_rsa.pub  root@192.168.0.3
root@remote-host‘s password:
Now try logging into the machine, with ―ssh ?remote-host‘‖, and check in:
.ssh/authorized_keys to make sure we haven‘t added extra keys that you weren‘t expecting.

[注: ssh-copy-id 把密钥追加到远程主机的 .ssh/authorized_key 上.]

sshd禁止root用户直接登录
修改/etc/ssh/sshd_config
#PermitRootLogin no => PermitRootLogin yes