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I've been investigating a special issue on our docker host server (17.09.0-ce) and found that every 3-5 minutes, many connections disappear from /proc/net/nf_conntrack causing the client side of the connection to time out.

I'm on CentOS 6.

I didn't see any special packets that may cause this behavior (I used tshark).

I don't see any errors in /var/log/messages or in dmesg.

Also, it looks like the sockets are not closed though as they seem to be remain in the output of netstat -atp.

I'm kind of stuck here at the moment and I have no other idea how to find the root cause of this. How should I debug this issue further?

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It turned out it was Rancher killing those connections. When this dockerhost was registered into our Rancher installation, several rancher docker containers were started on the host. The container rancher/network-manager was the faulty part. Unregistered this host from our Rancher installation and the issue was gone.

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