Re: [exim] data timeout on connection

On Fri, Oct 18, 2019 at 12:55:17PM +0200, Hardy via Exim-users wrote:
> Hi all,
>
> all of a sudden (after a reboot of the machine, but I cannot see a
> connection to that) exim produces a lot of
>
> data timeout on (message abandoned) on connection from mx.example.com [IP]
> F=<me@???>
>
> in my logs. These are always the same systems, that retry and fail again.
> Other systems don't show probs. By the looks this happens in the rcpt or
> data ACL, as the F= is available in the log.

Symptoms of broken Path MTU Discovery. This is usually a misconfiguration
of network on the sender's side. If so, small packets on the beginning
of SMTP session are passed through, but large (after DATA) are lost,
that's why session is died on timeout.

Run any traffic analyzer to capture packets from/to this [IP],
and study the dump.

> I reinstalled last week's exim.conf to cancel recent changes, but this did
> not help.

For broken Path MTU Disc it should not help. But if so, there is a
workaround: you can reduce announced MSS for this client's IP in order
to lower sender's maximum packet size. It can be done with kernel
packet filter. For example, Linux has a TCPMSS target for iptables,
described in man iptables-extensions.
--
Eugene Berdnikov