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: <a752bbbf-08c0-3885-65ba-79577a1ad5a8@blackhole.kfki.hu>
Date: Wed, 28 May 2025 15:41:48 +0200 (CEST)
From: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
To: ying chen <yc1082463@...il.com>
cc: Florian Westphal <fw@...len.de>, pablo@...filter.org, kadlec@...filter.org, 
    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: 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, 28 May 2025, ying chen 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.

No, the conntrack entry will be in the CLOSE state with the timeout value 
of /proc/sys/net/netfilter/nf_conntrack_tcp_timeout_close

Best regards,
Jozsef