这里对方的80端口是开启并允许通信的。当对端端口没有开启时：
[root@centos7 ~]# telnet 10.0.1.251 81 Trying 10.0.1.251... telnet: connect to address 10.0.1.251: No route to host
当对端拒绝连接时：
[root@centos7 ~]# telnet 10.0.1.251 8085 Trying 10.0.1.251... telnet: connect to address 10.0.1.251: Connection refused
10、ssh 远程登录程序
ssh [OPTIONS]... [user@]hostname [command]
ssh的全称是Secure Shell，在不安全的网络主机间提供安全加密的通信，旨在代替其他远程登录协议。
[root@centos7 ~]# ssh 10.0.1.253 The authenticity of host '10.0.1.253 (10.0.1.253)' can't be established. ECDSA key fingerprint is 96:bd:a3:a7:87:09:1b:53:44:4c:9b:b9:5f:b2:97:89. Are you sure you want to continue connecting (yes/no)? yes #这里输入yes Warning: Permanently added '10.0.1.253' (ECDSA) to the list of known hosts. root@10.0.1.253's password: #这里输入密码 Last login: Fri Nov 11 09:04:01 2016 from 192.168.78.137 [root@idc-v-71253 ~]# #已登录
当命令ssh后直接跟主机IP时表示使用默认用户root登录，如果是首次登录，需要确认添加该主机的认证key，当输入yes后，即会在本机/root/.ssh/known_hosts中增加一条该主机的记录，下一次登录时就不用再次确认了。然后需要输入用户密码，通过验证之后，我们就获得了目的主机的一个shell，我们就可以在这个shell中执行命令了。
在新shell中输入exit即可退回到原shell。
如果需要频繁登录某主机，但不想每次都输入密码，可以设置免密码登录：
[root@centos7 ~]# ssh-keygen -t rsa Generating public/private rsa key pair. Enter file in which to save the key (/root/.ssh/id_rsa): #回车 Enter passphrase (empty for no passphrase): #回车 Enter same passphrase again: #回车 Your identification has been saved in /root/.ssh/id_rsa. #私钥 Your public key has been saved in /root/.ssh/id_rsa.pub. #公钥 The key fingerprint is: be:c3:d0:02:50:35:35:fe:60:d6:2f:26:96:f0:e1:e6 root@centos7 The key's randomart image is: +--[ RSA 2048]----+ | ...o.o | | . o o | | . . * . | | . * = . | | . .S + . | | o=.o . | | +E | | o. | | .. | +-----------------+ [root@centos7 ~]# [root@centos7 ~]# ssh-copy-id 10.0.1.253 /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@10.0.1.253's password: Number of key(s) added: 1 Now try logging into the machine, with: "ssh '10.0.1.253'" and check to make sure that only the key(s) you wanted were added. [root@centos7 ~]#
其中命令ssh-keygen用来生成公钥私钥，选项-t指明密钥类型。之后使用命令ssh-copy-id将公钥发送至目标主机，这里需要输入目标主机用户密码。然后就可以免密码登录了：
