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: <CANn89iLG4mgzHteS7ARwafw-5KscNv7vBD3zM9J6yZwDq+RbcQ@mail.gmail.com>
Date: Wed, 28 May 2025 06:41:45 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: ying chen <yc1082463@...il.com>
Cc: 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 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.