On Wed, 21 Jun 2000 18:06:39 CDT, "John R. Andrews" <[log in to unmask]>  said:
> We currently have listserv running on a AIX host, call it machine1.uic.edu.
> We have an DNS "MX" record for "listserv.uic.edu" pointing to this machine,
> but no "A" record. The "A" record is for machine1.uic.edu.
>
> Right now email to [log in to unmask] goes fine and the reply
> address is [log in to unmask] as you'd want. However, (or so I'm
> told) if we add a "C" record for listserv.uic.edu, the replies now say
> [log in to unmask] Apparently, listserv follows the DNS chain to
> the real machine name. Does this make sense? We want to add the "C" name so
> the web server URL can use http://listserv.uic.edu. How can we make
> listserv always use listserv.uic.edu in it's replies?

As has been pointed out, using a CNAME there will cause indigestion.
Use an A record instead.

Incidentally, an MX entry must point at an A record, not a CNAME.
Although it will appear to work most of the time, any given DNS
resolver code Out There is totally within its rights to fail to
get it right.  The worst part is that failures of this are usually
triggered by weird combinations of MTU size, DNS caching, and
other weird stuff (if you understand why a DNS server has both TCP
and UDP port 53s, you're PART of the way there ;).

For Sendmail, you can stop the rewriting by *YOUR* Sendmail
by using 'FEATURE(nocanonify)'.  However, you need to note the
following:

1) Strictly speaking, canonification of hostnames *is required*.
This means that you have to take action to make sure that all hostnames
are canonified at least once.  The trick is identifying exactly when
that once is so you can avoid doing it again.  Beware, here there
be Mean and Nasty Dragons indeed...

2) You'll never get everybody ELSE to use 'nocanonify', so OTHER sites
will keep re-writing the address.

What *we* do is this:

A) On our AIX Listserv box, the hostname is set to 'listserv.vt.edu'.

B) In the DNS:

listserv.vt.edu.        4H IN MX        0 listserv.vt.edu.
listserv.vt.edu.        21m59s IN A     198.82.162.215

215.162.82.198.in-addr.arpa.  23m54s IN PTR  listserv.vt.edu.

We ran into all the hassles you're seeing trying to get a CNAME to
work, and decided it was easier to just rename the machine and use
an A record instead.

And yes, you *DO* want the MX record there, even though it appears
to do nothing.  Putting it there anyhow means that when the remote
end sending you mail does the MX lookup, it will get a hit (and the
A record will be passed along in the 'additional info' field of the
DNS reply).  Without the MX there, a *second* DNS lookup for the A
record will be needed.

                                Valdis Kletnieks
                                Operating Systems Analyst
                                Virginia Tech