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: <CAN2Y7hxi+CoMhPO7QzaqzHOcg2ksP9ixCzxHazAV73P5bbeFFQ@mail.gmail.com>
Date: Wed, 28 May 2025 21:27:48 +0800
From: ying chen <yc1082463@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: pablo@...filter.org, kadlec@...filter.org, fw@...len.de, 
	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 8:59 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, May 28, 2025 at 5:52 AM 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.
> >
> > The problem can be reproduced using the following command:
> > hping3 -S -V -p 9007 --flood xx.x.xxx.xxx
> >
> > ~$ cat /proc/net/nf_conntrack
> > ipv4     2 tcp      6 112 SYN_SENT src=xx.x.xxx.xxx dst=xx.xx.xx.xx
> > sport=2642 dport=9007 src=xx.xx.xx.xx dst=xx.x.xxx.xxx sport=9007
> > dport=2642 mark=0 zone=0 use=2
> > ipv4     2 tcp      6 111 SYN_SENT src=xx.x.xxx.xxx dst=xx.xx.xx.xx
> > sport=11510 dport=9007 src=xx.xx.xx.xx dst=xx.x.xxx.xxx sport=9007
> > dport=11510 mark=0 zone=0 use=2
> > ipv4     2 tcp      6 111 SYN_SENT src=xx.x.xxx.xxx dst=xx.xx.xx.xx
> > sport=28611 dport=9007 src=xx.xx.xx.xx dst=xx.x.xxx.xxx sport=9007
> > dport=28611 mark=0 zone=0 use=2
> > ipv4     2 tcp      6 112 SYN_SENT src=xx.x.xxx.xxx dst=xx.xx.xx.xx
> > sport=62849 dport=9007 src=xx.xx.xx.xx dst=xx.x.xxx.xxx sport=9007
> > dport=62849 mark=0 zone=0 use=2
> > ipv4     2 tcp      6 111 SYN_SENT src=xx.x.xxx.xxx dst=xx.xx.xx.xx
> > sport=3410 dport=9007 src=xx.xx.xx.xx dst=xx.x.xxx.xxx sport=9007
> > dport=3410 mark=0 zone=0 use=2
> > ipv4     2 tcp      6 111 SYN_SENT src=xx.x.xxx.xxx dst=xx.xx.xx.xx
> > sport=44185 dport=9007 [UNREPLIED] src=xx.xx.xx.xx dst=xx.x.xxx.xxx
> > sport=9007 dport=44185 mark=0 zone=0 use=2
> > ipv4     2 tcp      6 111 SYN_SENT src=xx.x.xxx.xxx dst=xx.xx.xx.xx
> > sport=51099 dport=9007 src=xx.xx.xx.xx dst=xx.x.xxx.xxx sport=9007
> > dport=51099 mark=0 zone=0 use=2
> > ipv4     2 tcp      6 112 SYN_SENT src=xx.x.xxx.xxx dst=xx.xx.xx.xx
> > sport=23609 dport=9007 src=xx.xx.xx.xx dst=xx.x.xxx.xxx sport=9007
> > dport=23609 mark=0 zone=0 use=2
>
> The default timeout is 120 seconds.
>
> /proc/sys/net/netfilter/nf_conntrack_tcp_timeout_syn_sent
Yes，The  timeout is 120 seconds.