SSH配置

            1)检查是否安装了ssh相关软件包(openssh-server + openssh-clients + openssh)
                $yum list installed | grep ssh

             没有安装ssh，安装ssh

            yum install openssh-server

            2)检查是否启动了sshd进程
                $>ps -Af | grep sshd
            
            3)在client侧生成公私秘钥对。(以下命令在centos7.5上会提示参数过长)
                $>ssh-keygen -t rsa -P '' -f ~/.ssh/id_rsa

              修改后：

             $>ssh-keygen -t rsa -f ~/.ssh/id_rsa

             一路回车到底
            
            4)生成~/.ssh文件夹，里面有id_rsa(私钥) + id_rsa.pub(公钥)

            5)追加公钥到~/.ssh/authorized_keys文件中(文件名、位置固定)
                $>cd ~/.ssh
                $>cat id_rsa.pub >> authorized_keys
            
            6)修改authorized_keys的权限为644.( 注：权限不为644，每次ssh登陆都要输入密码)
                $>chmod 644 authorized_keys
            
            7)测试
                $>ssh localhost