I'm reading “feel free to introduce yourself,” so I'm Léo Gaspard,
nickname Ekleog (also lurking on IRC though usually reading only
highlights on channels I'm not really active in). I'm running OpenSMTPD
in production on NixOS, and that's sometimes fun (most often in the
“good” meaning of the word).

So the question I was wondering about that caused my subscription to
this ML is whether OpenSMTPD does the A lookup fallback in case there is
no MX record, as per RFC5321 §5.1. I can't see anywhere in the code
where that'd be done (grepping for T_A gives an answer in dns.c, but it
doesn't seem to be called from anywhere).

The reason I'm asking this is because I'm trying to setup some automated
test that could be integrated in NixOS, so that an OpenSMTPD workflow
could not be unknowingly broken by an update somewhere. Case in mind,
the recent dovecot 2.3.1 update that made its LDA stricter wrt.
Return-Path parsing and triggered a thread on the Dovecot ML and the two
subsequent equivalent PRs, [1] and [2].

In these tests, the easy way of testing is by just adding the hosts to
/etc/hosts, and be done with it. Otherwise, there's a need to spawn a
DNS daemon in one of the VMs, and things immediately start being more

So do you know whether there's an easy way to do what I'm attempting? Or
should there be (if OpenSMTPD doesn't implement the MX → A)? Or is it
another issue, as /etc/hosts wouldn't generate “an empty list of MXs is
returned,” and should I just go the “embed a DNS server in the VMs” way?

Thanks for your feedback and especially for the work on OpenSMTPD!

[1] https://github.com/OpenSMTPD/OpenSMTPD/pull/847

[2] https://github.com/OpenSMTPD/OpenSMTPD/pull/849

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