[sf-lug] EDNS: Re: Mail problems (or Firefox, or systemd,...)

[Michael Paoli]

> From: "Ken Shaffer" <kenshaffer80 at gmail.com>
> Subject: Re: [sf-lug] Mail problems (or Firefox, or systemd,...)
> Date: Tue, 18 Dec 2018 17:29:42 -0800

> Odd indeed. See launchpad bug 1804487.  Looks like
> a combination of an NXDOMAIN error resulting in a reduced function set UDP
> trigger, and mail.comcast.net 's big (greater than 512 bytes) causing
> a problem. (It's time dependent too, with the UDP fallback getting reset
> after a grace period.)

Uhm, looks/sounds to me that it's not
"mail.comcast.net 's big (greater than 512 bytes) causing
a problem", but rather something downstream of that
(network/firewall(s)/resolver/systemd-let's_reinvent_the_wheel_but_poorly)
screwing up and having a problem with it.
EDNS - and with UDP packets over 512 bytes don't "just happen", the client
needs tell server it's okay with that (and even more particular/precise
than that as to what capabilities thereof), and only then does the server
send such.  So sounds like somewhere between authoritative DNS servers,
and your resolver (or configuration? or systemd-whatever) there is (or at
least was?) a problem.
https://en.wikipedia.org/wiki/Extension_mechanisms_for_DNS

Some tools such as dig, delv, etc., can be quite useful for determining
when there's an EDNS related issue.  E.g. the dig(1) man page has 17
lines that make at least some mention of EDNS.  (And, alas, delv has
zero ... so guess I won't be giving up dig all that soon?)