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 -- ## List details at https://lists.exim.org/mailman/listinfo/exim-users ## Exim details at http://www.exim.org/ ## Please use the Wiki with this list - http://wiki.exim.org/
