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: <5611b12b-d560-cbb8-1d74-d935f60244dd@blackhole.kfki.hu>
Date: Wed, 28 May 2025 15:45:00 +0200 (CEST)
From: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
To: Eric Dumazet <edumazet@...gle.com>
cc: ying chen <yc1082463@...il.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, 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