Re: [PATCH nf v2] netfilter: conntrack: connection timeout after re-register

Florian Westphal
2020-10-09T11:03:33Z

Jozsef Kadlecsik <kadlec@netfilter.org> wrote:
> > Any comments?
> > Here is a simple reproducer. The idea is to show that keepalive packets
> > in an idle tcp connection will be dropped (and the connection will time
> > out) if conntrack hooks are de-registered and then re-registered. The
> > reproducer has two files. client_server.py creates both ends of a tcp
> > connection, bounces a few packets back and forth, and then blocks on a
> > recv on the client side. The client's keepalive is configured to time
> > out in 20 seconds. This connection should not time out. test is a bash
> > script that creates a net namespace where it sets iptables rules for the
> > connection, starts client_server.py, and then clears and restores the
> > iptables rules (which causes conntrack hooks to be de-registered and
> > re-registered).
>
> In my opinion an iptables restore should not cause conntrack hooks to be
> de-registered and re-registered, because important TCP initialization
> parameters cannot be "restored" later from the packets. Therefore the
> proper fix would be to prevent it to happen. Otherwise your patch looks OK
> to handle the case when conntrack is intentionally restarted.

The repro clears all rules, waits 4 seconds, then restores the ruleset.
using iptables-restore < FOO; sleep 4; iptables-restore < FOO will
not result in any unregister ops.

We could make kernel defer unregister via some work queue but i don't
see what this would help/accomplish (and its questionable of how long it
should wait).

We could disallow unregister, but that seems silly (forces reboot...).

I think the patch is fine.