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On the server both the application and netstat show a connection open. The client however lists no such connection, and the client software is definitely stopped.

I'd already lowered the TCP keepalive settings:

cat /proc/sys/net/ipv4/tcp_keepalive_time
cat /proc/sys/net/ipv4/tcp_keepalive_intvl
cat /proc/sys/net/ipv4/tcp_keepalive_probes

What's causing these connections to remain, even though the client has most definitely disconnected?

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What's under the State heading in netstat? –  Alan Curry Aug 15 '12 at 19:28
@AlanCurry ESTABLISHED –  Alec Aug 15 '12 at 19:31
Can you do a packet capture? You'll see the traffic and responses and be able to tell which side is having the issue. –  MaQleod Aug 15 '12 at 19:40
How did you stop the client software? I hope it wasn't a kill -9 .... –  ott-- Aug 15 '12 at 19:44
FIN should be sent by the server even after kill -9. Possibly there is another process on the client that inherited the socket fd and hasn't died yet? –  Alan Curry Aug 15 '12 at 19:47

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