当我们在SSH到某个机器的时候，如果有问题的时候，最好的发现问题所在的开始是
ssh verbose具体的信息，然后去找解决方法。
例如今天所遇到的问题如下：
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$ ssh -v -l $USER $REMOTE_MACHINE
OpenSSH_5.1p1, OpenSSL 0.9.8i 15 Sep 2008
debug1: Connecting to $REMOTE_MACHINE port 22.
debug1: Connection established.
debug1: identity file ~/.ssh/identity type -1
debug1: identity file ~/.ssh/id_rsa type 1
debug1: identity file ~/.ssh/id_dsa type -1
debug1: ssh_exchange_identification:
debug1: ssh_exchange_identification:
debug1: ssh_exchange_identification: Error: Must authenticate before using this service.
ssh_exchange_identification: Connection closed by remote host
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should pass the intranet firewall, and after I passed the firewall, ssh worked.