google 之，有很多類似的錯誤和解答
有一種可能是
http://lists.freebsd.org/pipermail/freebsd-questions/2005-August/097491.html
DNS. Sshd is not finding you in DNS before it gives up. Change "UseDNS"
in /etc/ssh/sshd_config to "no" from "yes", restart sshd, and my guess
is your problem will disappear.
http://www.google.com/search?hl=en&q=ssh+%2B+server+unexpectedly+closed+network+connection
※ 引述《PowerSec (0day)》之銘言：
: 當我用 putty 連到機器一輸入完帳號密碼登入後
: 就秀出 server unexpectedly closed network connection 立即斷線
: 請問這問題該怎麼解決呢
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