linux-kernel - Re: linux 5.17.1 disregarding ACK values resulting in stalled TCP connections

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Date:   Fri, 1 Apr 2022 14:09:58 +0200
From:   Florian Westphal <fw@...len.de>
To:     Jaco Kroon <jaco@....co.za>
Cc:     Florian Westphal <fw@...len.de>,
        Eric Dumazet <edumazet@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: linux 5.17.1 disregarding ACK values resulting in stalled TCP
 connections

Jaco Kroon <jaco@....co.za> wrote:
> > Eric Dumazet <edumazet@...gle.com> wrote:
> >> Next step would be to attempt removing _all_ firewalls, especially not
> >> common setups like yours.
> >>
> >> conntrack had a bug preventing TFO deployment for a while, because
> >> many boxes kept buggy kernel versions for years.
> >>
> >> 356d7d88e088687b6578ca64601b0a2c9d145296 netfilter: nf_conntrack: fix
> >> tcp_in_window for Fast Open
> > Jaco could also try with
> > net.netfilter.nf_conntrack_tcp_be_liberal=1
> >
> > and, if that helps, with liberal=0 and
> > sysctl net.netfilter.nf_conntrack_log_invalid=6
> >
> > (check dmesg/syslog/nflog).
> 
> Our core firewalls already had nf_conntrack_tcp_be_liberal for other
> reasons (asymmetric routing combined with conntrackd left-over if I
> recall), so maybe that's why it got through there ... don't exactly want
> to just flip that setting though, is there a way to log if it would have
> dropped anything, without actually dropping it (yet)?

This means conntrack doesn't tag packets as invalid EVEN if it would
consider sequence/ack out-of-window (e.g. due to a bug).

I have a hard time seeing how tcp liberal-mode conntrack would be to
blame here.

Only thing you could also check is if
net.netfilter.nf_conntrack_checksum=0 helps (but i doubt it).