在日常操作練習時，用ssh登錄其他主機時，可能會每次都需要輸入登錄密碼，哪怕用scp複製東西也需要。
爲了更方便快捷的切換主機，ssh免密通信不失爲更好的選擇；
用ssh-keygen生成密鑰，默認保存在本地的/root/.ssh/id_rsa
然後在用ssh-copy-id分發至目標主機之上；登錄用戶@主機名或IP地址
注意：
1.最好是用主機名，在/etc/hosts文件中寫入主機名和對應的IP地址；
2.免密通信需要雙方相互認證，所以也需要在目標主機對本機做一次相同的操作；
[root@www ~]# ssh-keygen -t rsa -P '' Generating public/private rsa key pair. Enter file in which to save the key (/root/.ssh/id_rsa): /root/.ssh/id_rsa already exists. Overwrite (y/n)?
[root@www ~]#ssh-copy-id -i .ssh/id_rsa.pub [email protected] /usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: ".ssh/id_rsa.pub" The authenticity of host '172.16.75.1 (172.16.75.1)' can't be established. ECDSA key fingerprint is SHA256:YXhbTS9MfK2IQC4gtOW4RL8voHvFqC1cAAMJXuF3Wec. ECDSA key fingerprint is MD5:2c:c0:94:e8:3a:e4:74:88:c0:d8:e0:01:20:81:1d:8e. Are you sure you want to continue connecting (yes/no)? yes /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 [email protected]'s password: Number of key(s) added: 1 Now try logging into the machine, with: "ssh '[email protected]'" and check to make sure that only the key(s) you wanted were added.