linux-kernel - Re: [bug report, linux 6.15-rc4] A large number of connections in the SYN_SENT state caused the nf

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Message-ID: <CAN2Y7hy=DRkm9zpNbHfkqjHeGm6UAWeUweXzPH2+Nzf=O9i7-Q@mail.gmail.com>
Date: Wed, 28 May 2025 22:51:52 +0800
From: ying chen <yc1082463@...il.com>
To: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
Cc: Eric Dumazet <edumazet@...gle.com>, Florian Westphal <fw@...len.de>, pablo@...filter.org, 
	kadlec@...filter.org, davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, 
	netfilter-devel@...r.kernel.org, coreteam@...filter.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [bug report, linux 6.15-rc4] A large number of connections in the
 SYN_SENT state caused the nf_conntrack table to be full.

On Wed, May 28, 2025 at 10:18 PM Jozsef Kadlecsik
<kadlec@...ckhole.kfki.hu> wrote:
>
> On Wed, 28 May 2025, ying chen wrote:
>
> > On Wed, May 28, 2025 at 9:45 PM Jozsef Kadlecsik
> > <kadlec@...ckhole.kfki.hu> wrote:
> >>
> >> On Wed, 28 May 2025, Eric Dumazet wrote:
> >>
> >>> On Wed, May 28, 2025 at 6:26 AM ying chen <yc1082463@...il.com> wrote:
> >>>>
> >>>> On Wed, May 28, 2025 at 9:10 PM Florian Westphal <fw@...len.de> wrote:
> >>>>>
> >>>>> ying chen <yc1082463@...il.com> wrote:
> >>>>>> Hello all,
> >>>>>>
> >>>>>> I encountered an "nf_conntrack: table full" warning on Linux 6.15-rc4.
> >>>>>> Running cat /proc/net/nf_conntrack showed a large number of
> >>>>>> connections in the SYN_SENT state.
> >>>>>> As is well known, if we attempt to connect to a non-existent port, the
> >>>>>> system will respond with an RST and then delete the conntrack entry.
> >>>>>> However, when we frequently connect to non-existent ports, the
> >>>>>> conntrack entries are not deleted, eventually causing the nf_conntrack
> >>>>>> table to fill up.
> >>>>>
> >>>>> Yes, what do you expect to happen?
> >>>> I understand that the conntrack entry should be deleted immediately
> >>>> after receiving the RST reply.
> >>>
> >>> Then it probably hints that you do not receive RST for all your SYN
> >>> packets.
> >>
> >> And Eric has got right: because the states are in SYN_SENT then either the
> >> RST packets were not received or out of the window or invalid from other
> >> reasons.
> > I also suspect it's due to being "out of the window", but I'm not sure why.
>
> tcpdump of the traffic from the targeted machine with both the SYN and RST
> packets could help (raw pcap or at least the output with absolute seqs).
>
> Best regards,
> Jozsef

Using bpftrace, I found that the RST is under the lower bound and
printed the values of the following variables:
receiver->td_maxwin = 1
sender->td_end = 0
receiver->td_maxwin =1