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: <CAN2Y7hxZdWLfd34LPzhUPZJ-oMksajLMVt5K8B6Gy70e9TXMpw@mail.gmail.com>
Date: Wed, 28 May 2025 21:59:47 +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 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.
>
> Best regards,
> Jozsef
I also suspect it's due to being "out of the window", but I'm not sure why.