linux-kernel - [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: <CAN2Y7hxscai7JuC0fPE8DZ3QOPzO_KsE_AMCuyeTYRQQW_mA2w@mail.gmail.com>
Date: Wed, 28 May 2025 20:52:09 +0800
From: ying chen <yc1082463@...il.com>
To: pablo@...filter.org, kadlec@...filter.org, fw@...len.de, 
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, 
	netfilter-devel@...r.kernel.org, coreteam@...filter.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [bug report, linux 6.15-rc4] A large number of connections in the
 SYN_SENT state caused the nf_conntrack table to be full.

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