ssh-copy-id是ssh client套件內一个预设的指令，简单的来说他只是一个script，当你在本地主机已经产生了RSA /DSA和authentication，可以通过ssh-copy-id的指令将认证傳送到远端主机。

如何建立RSA /DSA和authentication？

$ ssh-keygen -t dsa 或者ssh-keygen -t rsa

Generating public/private dsa key pair.

Enter file in which to save the key (/root/.ssh/id_dsa) 

RSA与DSA的差别

ssh-keygen can create RSA keys for use by SSH protocol version 1 and RSA  or DSA keys for use by SSH protocol version 2

简单来说，若您只使用SSH protocol version 2，建议使用DSA來建立authentication。

    使用下例中ssk-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
ligh@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 
ligh@local-host

步骤2: 用 ssh-copy-id 把公钥复制到远程主机上（ssh-copy-id -i 认证档案 用户名@主机）
ligh@local-host$ ssh-copy-id -i ~/.ssh/id_rsa.pub  [email protected]
ligh@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.]

步骤3: 直接登录远程主机
ligh@local-host$ ssh remote-host 
Last login: Sun Nov 16 17:22:33 2008 from 192.168.1.2 
[注: SSH 不会询问密码.] 

ligh@remote-host$ 

对22端口的修改问题：

基本上對外服務的主機，ssh listen port都會改掉預設的22 port，如此一來可以減少主機被入侵的機會，更改ssh listen port是最基本的第一道防線，請將設定檔內的 Port 22 進行更改，並且重新啟動ssh的服務。

      但是當您改掉ssh listen port之後，ssh-copy-id這個好用的指令將無法運用，這樣一來不是很可惜嗎？於是我們就可以對ssh-copy-id這個script進行一些修改，讓他可以支援指定不同的service port 。

      更改ssh-copy-id

      $ cp /usr/bin/ssh-copy-id /usr/bin/ssh-copy-id.orig
      $ vi /usr/bin/ssh-copy-id

      #!/bin/sh
       
      # Shell script to install your identity.pub on a remote machine
      # Takes the remote machine name as an argument.
      # Obviously, the remote machine must accept password authentication,
      # or one of the other keys in your ssh-agent, for this to work.
       
      ID_FILE="${HOME}/.ssh/identity.pub"
       
      while getopts ':i:p:P:h' OPTION
      do
          case $OPTION in
              i)
              if [ -n "$OPTARG" ]; then
                  if expr "$OPTARG" : ".*.pub" > /dev/null ; then
                      ID_FILE="$OPTARG"
                  else
                      ID_FILE="$OPTARG.pub"
                  fi
              fi
              ;;
              P|p)
                  PORT=$OPTARG;
              ;;
              h)
                  echo "Usage: $0 [-i [identity_file]] [user@]machine" >&2
                  exit 1
              ;;
          esac;
      done;
       
      shift $(($OPTIND - 1))
       
      if [ $# -lt 1 ] && [ x$SSH_AUTH_SOCK != x ] ; then
         GET_ID="$GET_ID ssh-add -L"
      fi
       
      if [ -z "`eval $GET_ID`" ] && [ -r "${ID_FILE}" ] ; then
        GET_ID="cat ${ID_FILE}"
      fi
       
      if [ -z "`eval $GET_ID`" ]; then
        echo "$0: ERROR: No identities found" >&2
        exit 1
      fi
       
      if [ -z $PORT ]; then
          PORTOPTION=""
      else
          PORTOPTION="-p $PORT "
      fi;
       
      { eval "$GET_ID" ; } | ssh $PORTOPTION $1 "umask 077; test -d .ssh || 
      mkdir .ssh ; cat >> .ssh/authorized_keys" || exit 1
       
      cat <<EOF
      Now try logging into the machine, with "ssh $PORTOPTION'$1'", and check 
in:
       
        .ssh/authorized_keys
       
      to make sure we haven't added extra keys that you weren't expecting.
       
      EOF

      主要是增加了port的設定進去，建議可以直接複製貼上取代即可。
      以上script參考來源：http://blog.vieth.biz/2009/03/23/ssh-copy-id-with-port/

      ssh-copy-id with port的運用

      $ ssh-copy-id -i ~/.ssh/id_dsa.pub -p 1234 boon@tunnel.sdboon
      wawa@remotehost's password: (需要輸入一次密碼)
      Now try logging into the machine, with "ssh -p 22222 '[email protected]'", and check in:
       
        .ssh/authorized_keys
       
      to make sure we haven't added extra keys that you weren't expecting.

      ssh-copy-id with port的認證就完成囉！