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TCP connections freezing unexplicably

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I have a problem where TCP connections to a machine of mine tend to
freeze in a way I can't explain.

Here are the symptoms:
- I ssh into the machine, and right in the middle of some output the
  connection just freezes.  If I have other simultaneous SSH connections
  multiplexed over the same underlying TCP connection, they freeze
  as well.
- I connect via imap-over-ssl to that same machine, and similarly the
  connection randomly hangs.
- As mentioned, those hangs seem to happen when there is activity,
  rather than when the connection is idle.
- One TCP connection freezing does not prevent other TCP connections
  from working just fine.  E.g. if I have two SSH connections (not
  multiplexed over the same TCP connection), one might freeze while the
  other keeps working fine.
- This happens when connecting from various different places (e.g. from
  home behind a NAT router, from a server with no NAT in sight, ...).
- I can somewhat reproduce the freeze at will, by connecting over ssh
  and then doing "du /home" (which generates a fair bit of output).
  Maybe I'll have to do "du /home" a second or a third time, but I've
  never needed more to reproduce the problem.
- There's nothing magical about "du", the freeze also happens in many
  other cases.
- "dmesg" is desperately silent.

Some more info about the context:

This machine is connected to the Internet via its eth0 with a fixed
routable IPv4 address.  It is running Debian stable over Linux-3.4
(well, the linux-sunxi variant of it, but that shouldn't make much
difference here, hopefully).

Other than its eth0 interface it has two more network interfaces:
- an ethernet USB dongle (name "eth-usb").
- a tun0 handled by OpenVPN.
Each of those uses a local non-routable (192.168.x.x) network, so
it uses NAT to provide Internet access to those machines connected over
one of those two interfaces.
The iptables setup is very simple, see below.

The SSH and IMAP-over-SSL connections I talk about above all come from
the Internet (i.e. over the eth0 interface).

Any idea where the problem might be coming from?  How could I try to
track it down?


        Stefan


# iptables -t nat -vL
Chain PREROUTING (policy ACCEPT 605K packets, 65M bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain INPUT (policy ACCEPT 595K packets, 65M bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain OUTPUT (policy ACCEPT 2104 packets, 147K bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain POSTROUTING (policy ACCEPT 11 packets, 874 bytes)
 pkts bytes target     prot opt in     out     source               destination         
 2296  161K MASQUERADE  all  --  any    eth0    anywhere             anywhere            
# iptables -vL       
Chain INPUT (policy ACCEPT 1109K packets, 189M bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain FORWARD (policy ACCEPT 1925 packets, 417K bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain OUTPUT (policy ACCEPT 97455 packets, 29M bytes)
 pkts bytes target     prot opt in     out     source               destination         
# 

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Thread

TCP connections freezing unexplicably Stefan Monnier <monnier@iro.umontreal.ca> - 2014-05-11 22:52 -0400
  Re: TCP connections freezing unexplicably Jorgen Grahn <grahn+nntp@snipabacken.se> - 2014-05-12 10:21 +0000
    Re: TCP connections freezing unexplicably Richard Kettlewell <rjk@greenend.org.uk> - 2014-05-12 20:35 +0100
      Re: TCP connections freezing unexplicably Stefan Monnier <monnier@iro.umontreal.ca> - 2014-05-13 09:26 -0400

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