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

From: Florian Westphal <[email-masked]>
Date: Fri, 9 Oct 2020 13:03:23 +0200

Jozsef Kadlecsik <[email-masked]> 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.