netdev - Re: [PATCH v4 net-next] tcp: avoid reusing FIN_WAIT2 when trying to find port in connect() process

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Message-ID: <CANn89iKkg6FtrcugffcWTixmbiXcZRJyvM9JcBjF46mXjkGbvA@mail.gmail.com>
Date: Fri, 23 Aug 2024 08:31:18 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, 
	dsahern@...nel.org, kuniyu@...zon.com, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>, Jade Dong <jadedong@...cent.com>
Subject: Re: [PATCH v4 net-next] tcp: avoid reusing FIN_WAIT2 when trying to
 find port in connect() process

On Fri, Aug 23, 2024 at 2:12 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.com>
>
> We found that one close-wait socket was reset by the other side
> due to a new connection reusing the same port which is beyond our
> expectation, so we have to investigate the underlying reason.
>
> The following experiment is conducted in the test environment. We
> limit the port range from 40000 to 40010 and delay the time to close()
> after receiving a fin from the active close side, which can help us
> easily reproduce like what happened in production.
>
> Here are three connections captured by tcpdump:
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965525191
> 127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 2769915070
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [F.], seq 1, ack 1
> // a few seconds later, within 60 seconds
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730
> 127.0.0.1.9999 > 127.0.0.1.40002: Flags [.], ack 2
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [R], seq 2965525193
> // later, very quickly
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730
> 127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 3120990805
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1
>
> As we can see, the first flow is reset because:
> 1) client starts a new connection, I mean, the second one
> 2) client tries to find a suitable port which is a timewait socket
>    (its state is timewait, substate is fin_wait2)
> 3) client occupies that timewait port to send a SYN
> 4) server finds a corresponding close-wait socket in ehash table,
>    then replies with a challenge ack
> 5) client sends an RST to terminate this old close-wait socket.
>
> I don't think the port selection algo can choose a FIN_WAIT2 socket
> when we turn on tcp_tw_reuse because on the server side there
> remain unread data. In some cases, if one side haven't call close() yet,
> we should not consider it as expendable and treat it at will.
>
> Even though, sometimes, the server isn't able to call close() as soon
> as possible like what we expect, it can not be terminated easily,
> especially due to a second unrelated connection happening.
>
> After this patch, we can see the expected failure if we start a
> connection when all the ports are occupied in fin_wait2 state:
> "Ncat: Cannot assign requested address."
>
> Reported-by: Jade Dong <jadedong@...cent.com>
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---

Reviewed-by: Eric Dumazet <edumazet@...gle.com>