[ubuntu] Server side connection closed for sshd.

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http://ubuntuforums.org – I am struggling to figure out why my ubuntu (precise) server always close an connection from the server side at around 24hours. I am using putty in winxp to connect, the server is behind a LAN, but all ports direct to the server by default. And it is not not some restart of router/server, since if i start another ssh session, this one is not closed. Putty message is " Server unexpected close connection". Trying to check which setup option could cause this. I turned on TCPKeepalive. (Hardware)