netdev - Re: TCP connection will hang in FIN_WAIT1 after closing if zero window is advertised

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Date:	Tue, 16 Sep 2014 08:11:46 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andrey Dmitrov <andrey.dmitrov@...etlabs.ru>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	netdev <netdev@...r.kernel.org>,
	"Alexandra N. Kossovsky" <Alexandra.Kossovsky@...etlabs.ru>,
	Konstantin Ushakov <kostik@...etlabs.ru>
Subject: Re: TCP connection will hang in FIN_WAIT1 after closing if zero
 window is advertised

On Tue, Sep 16, 2014 at 6:09 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2014-09-16 at 16:47 +0400, Andrey Dmitrov wrote:
>> On 16/09/14 03:15, Hannes Frederic Sowa wrote:
>> > Also thanks for the report.
>> >
>> > Do you see any tcp window repair messages in dmesg? Can you send some
>> > output of ss -oemit state FIN-WAIT-1 from the target host?
>> Hannes,
>> no, there aren't any messages in dmesg until net.ipv4.tcp_max_orphans is
>> achieved.
>
>
> Andrey, you should take a look at Labrea Tarpit,
>
> http://www.sans.org/reading-room/whitepapers/casestudies/smart-ids-hybrid-labrea-tarpit-33254
>
> What happens is the following :
>
> A normal TCP session is established, traffic is sent from server to
> client.
>
> Client sends a zero window.
>
> 1) This can be normal, because application reading client queue no
> longer can. (For example its a ssh session, and output to the terminal
> is blocked by CTRL S). There are valid cases when you block this for
> many hours.
>
> 2) This can be faked by malicious peer, willing to make server enter
> this mode (inability to send more data, data stack in output queue, one
> probe sent every RTO). This is a very well known way to let servers
> consume a lot of kernel memory and eventually OOM.
>
>
> Then server sends a probe every RTO, and client responds with a ACK with
> win=0
>
> TCP specs say : This can last forever, even if socket is eventually
> closed by the server (because he gave up) and enters FIN_WAIT
>
> Supposedly, if a server is about to give up, it might tell the TCP
> stack : Oh, do not bother absolutely sending the remaining bytes you
> have in output queue (I, the application, already waited for a very
> reasonable time)
>
> Normally SO_LINGER could be used, or TCP_USER_TIMEOUT. This requires a
> system call before doing the close().
>
> 1) TCP_USER_TIMEOUT would be the fit for this, but its current
> implementation do not take care of the probes sent, even in FIN_WAIT
> state when in this zero window mode. A patch would be needed.
Yes that's what I meant. I am proposing we should patch
TCP_USER_TIMEOUT to do this.


>
> 2) SO_LINGER, timeout=0 might work.
>
>
>
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