linux-kernel - Re: [PATCH 5.10 099/167] tcp: disable TFO blackhole logic by default

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Date:   Mon, 31 Jan 2022 11:17:32 -0600
From:   Jesse Hathaway <jesse@...ki-mvuki.org>
To:     ycheng@...gle.com
Cc:     davem@...emloft.net, edumazet@...gle.com,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        ncardwell@...gle.com, pavel@...x.de, sashal@...nel.org,
        soheil@...gle.com, stable@...r.kernel.org, weiwan@...gle.com,
        jesse@...ki-mvuki.org
Subject: Re: [PATCH 5.10 099/167] tcp: disable TFO blackhole logic by default

On Wed, Jul 28, 2021 at 09:32:42 -0700 Yuchung Cheng <ycheng@...gle.com> wrote:

> On the other hand maybe we do not hear middlebox issues because this
> mechanism is working. So I am okay to avoid applying to stable and
> keep in net-next to test this new policy.

This change did indeed break our mail servers at Wikimedia, causing
difficult to diagnose timeout errors on sending outgoing email. I
resorted to bisecting the kernel, which resulted in finding this commit.
I have verified that reverting the sysctl value for
tcp_fastopen_blackhole_timeout_sec to 3600 does resolve the timeouts.

Given that it is not clear how a user would discover that this sysctl
has changed, or know how to fix a middle box somewhere on a path to
their destination, I would love to see this change reverted.

Yours kindly, Jesse Hathaway