🚀 go-pugleaf

msir@uhura.cc.rochester.edu (Mark Sirota) writes:
   What exactly is an MX Record?  I understand the functionality, but how are
   they implemented?  I assume it's not just a line in the sendmail.cf which
   aliases one name to another...

MX records are resource records (RRs) in your nameserver.  They
identify the host to which mail for a given domain entity should be
given.  They are particularly useful for, e.g., non-IP sites (notably,
UUCP sites) which wish to be domain-addressable.

Syntax in a nameserver zone file is

<entity>	IN	MX	<metric> <MX hostname>

as in

$ORIGIN rochester.edu.
some-uucp-host-in-your-domain	IN	MX	10 your-gateway.rochester.edu.

The <metric> provided is used by your mailer to determine preference
order in the case of multiple MX's.  There can be an arbitrarily large
number of MX records for any given <entity>, though I can't recall
having ever seen more than about 5.  The <metric>s provided tell the
mailer which to try first; and if it doesn't answer the phone, as it
were, which to try second, and so on.

Sendmail as of (about) 5.51 (5.54?  I have long since lost track)
learned to put MX records to use automatically.  When the mailer
chosen in S0 is P=[IPC], then MX records are requested from the
nameservers, and delivery is actually attempted to the hosts so
identified.  If no MX records exist, then sendmail expects to find an
A (address) record for the indicated destination, so that it can
deliver directly.

Of course, the host which is advertised as being the MX host for some
given <entity> had better have some sort of schema for detecting those
hosts for which it does MX'ing, and must not resolve in S0 to P=[IPC].
I do it with a set of classes identifying those domains for which I
MX, and hand them to a mailer P=/bin/smail, with a paths file which
knows the mapping to real UUCP hosts for those domains.

--Karl