netdev - Re: Verify ACK packets in handshake in kernel module (Access TCP state table)

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Message-ID: <20190921031529.GG1889@1wt.eu>
Date:   Sat, 21 Sep 2019 05:15:29 +0200
From:   Willy Tarreau <w@....eu>
To:     Swarm <thesw4rm@...me>
Cc:     netdev@...r.kernel.org
Subject: Re: Verify ACK packets in handshake in kernel module (Access TCP
 state table)

On Fri, Sep 20, 2019 at 11:43:50PM +0000, Swarm wrote:
> First time emailing to this mailing list so please let me know if I made a
> mistake in how I sent it. I'm trying to receive a notification from the
> kernel once it verifies an ACK packet in a handshake. Problem is, there is no
> API or kernel resource I've seen that supports this feature for both
> syncookies and normal handshakes. Where exactly in the kernel does the ACK
> get verified? If there isn't a way to be notified of it, where should I start
> adding that feature into the kernel?

Just searching for TCP_ESTABLISHED immediately brought me to tcp_input.c
(tcp_rcv_state_process() to be precise), so I'm not sure you've searched
that much. As you've noticed there's nothing specifically called in this
case, but in practice a caller of accept() on a listening socket will be
woken up.

Hoping this helps,
Willy