Re: [exim] av_scanner is broken suddenly?

On 30/12/2020 13:10, Victor Sudakov via Exim-users wrote:
> You must be a wise man Evgeniy! On the exim host, I set sysctl
> net.inet.tcp.fastopen.client_enable=0 and the problem seemed to go away.

That's good to know; thanks.
It may be worth raising a bug against FreeBSD.

> Now if you care :-) please explain what it was and what I've lost by
> disabling fastopen client.

The fastopen implementation in the FreeBSD kernel is broken
or has changed in some way that breaks previously-reasonable
use of it.

For this use (a very low-latency path, to a sibling VM) it's
a very minor efficiency measure. Disabling it won't lose you
anything measurable. For the more-general SMTP use of it, over
global-latency paths - you still won't lose much at this time
because I've not managed yet to find an implementation of TFO
which can place data on the SYN,ACK packet. Because SMTP is a
server-first protocol that's the one that matters (apart from
for TLS-on-connect, usable by MSA connections [commonly port 465],
where there's potential for getting the TLS Client Hello in the
SYN). But (at least) Linux has only support for data-on-SYN TFO
and not for data-on-SYN,ACK. I live in hope.
--
Cheers,
Jeremy