From: Eric Dumazet <eric.dumazet@gmail.com>
To: Marek Majkowski <marek@cloudflare.com>, netdev@vger.kernel.org
Subject: Re: TCP_USER_TIMEOUT, SYN-SENT and tcp_syn_retries
Date: Thu, 26 Sep 2019 09:46:20 -0700 [thread overview]
Message-ID: <61e4c437-cb1e-bcd6-b978-e5317d1e76c3@gmail.com> (raw)
In-Reply-To: <c682fe41-c5ee-83b9-4807-66fcf76388a4@gmail.com>
On 9/26/19 8:05 AM, Eric Dumazet wrote:
>
>
> On 9/25/19 1:46 AM, Marek Majkowski wrote:
>> Hello my favorite mailing list!
>>
>> Recently I've been looking into TCP_USER_TIMEOUT and noticed some
>> strange behaviour on fresh sockets in SYN-SENT state. Full writeup:
>> https://blog.cloudflare.com/when-tcp-sockets-refuse-to-die/
>>
>> Here's a reproducer. It does a simple thing: sets TCP_USER_TIMEOUT and
>> does connect() to a blackholed IP:
>>
>> $ wget https://gist.githubusercontent.com/majek/b4ad53c5795b226d62fad1fa4a87151a/raw/cbb928cb99cd6c5aa9f73ba2d3bc0aef22fbc2bf/user-timeout-and-syn.py
>>
>> $ sudo python3 user-timeout-and-syn.py
>> 00:00.000000 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S]
>> 00:01.007053 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S]
>> 00:03.023051 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S]
>> 00:05.007096 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S]
>> 00:05.015037 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S]
>> 00:05.023020 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S]
>> 00:05.034983 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S]
>>
>> The connect() times out with ETIMEDOUT after 5 seconds - as intended.
>> But Linux (5.3.0-rc3) does something weird on the network - it sends
>> remaining tcp_syn_retries packets aligned to the 5s mark.
>>
>> In other words: with TCP_USER_TIMEOUT we are sending spurious SYN
>> packets on a timeout.
>>
>> For the record, the man page doesn't define what TCP_USER_TIMEOUT does
>> on SYN-SENT state.
>>
>
> Exactly, so far this option has only be used on established flows.
>
> Feel free to send patches if you need to override the stack behavior
> for connection establishment (Same remark for passive side...)
Also please take a look at TCP_SYNCNT, which predates TCP_USER_TIMEOUT
next prev parent reply other threads:[~2019-09-26 16:46 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-25 8:46 TCP_USER_TIMEOUT, SYN-SENT and tcp_syn_retries Marek Majkowski
2019-09-26 15:05 ` Eric Dumazet
2019-09-26 16:46 ` Eric Dumazet [this message]
2019-09-26 16:57 ` Eric Dumazet
2019-09-26 18:03 ` Eric Dumazet
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