新安装的系统，默认情况下禁用Debian Linux上的root登录。

当我们尝试使用root用户身份登录到Debian Jessie Linux服务器时，可以拒绝访问权限：

$ssh root@10.1.1.12
root@10.1.1.12's password: 
Permission denied, please try again.
root@10.1.1.12's password: 
Permission denied, please try again.
root@10.1.1.12's password: 
Permission denied (publickey,password).

在Debian Linux系统上启用root用户的SSH登录，我们需要先配置SSH服务器。

打开/etc/ssh/sshd_config并更改以下行：

FROM:
PermitRootLogin without-password
TO:
PermitRootLogin yes

完成上述更改后，重新启动SSH服务器：

# /etc/init.d/ssh restart
[ ok ] Restarting ssh (via systemctl): ssh.service.

从现在开始，我们将能够作为root登录：

$ssh root@10.1.1.12
root@10.1.1.12's password: 
The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.