[CLUE-Tech] Sending mail using an IP address?

[Jed S. Baer]

On Tue, 19 Mar 2002 13:35:45 -0700 (MST)
Jim Ockers <ockers at ockers.net> wrote:

> So consider how the DNS might work for an IP address.  Let's say that
> you wanted to send an e-mail to person at 2.3.4.5 .  Would "5" be the top-
> level domain?  How is your mailer supposed to walk the DNS tree to
> find the MX for a subdomain in the "5" domain?  (AFAIK single numbers
> are not valid top-level domains; you can use .COM .NET .ORG .US etc. but
> not numbers.)

Speaking from total ignorance of the relevant RFCs, I can only assert that
a dotted-quad shouldn't require a walk down the DNS lane, or up it's tree,
as the case might be. I mean, traceroute happily notices that I've given
it an IP address, instead of a server name, and scoots right along. We'd
be in a world of hurt, wouldn't we, if we couldn't tell whether
207.217.96.29 was an IP address or a domain name?

> Remember that in the DNS, IP addresses are actually represented in the
> inverse notation - ie.e, "5.4.3.2.in-addr.arpa" would be the domain.
> It is possible to put MX records in the inverse zone files for the DNS
> for inverse domains, i.e., the "5" record in that inverse zonefile could
> have an MX record pointing to itself.  Then you could send an e-mail to
> person at 5.4.3.2.in-addr.arpa if you wanted to.  (Assuming that the mail
> program on that system was configured to recognize that domain name as
> well.)
> 
> In practice it is very unusual to find MX records in the inverse zones
> since everybody uses the domain name.
> 
> The integration of sendmail (and internet mail in general) with the DNS
> is by design.  It was not just an arbitrary decision on the part of some
> software developers - that's actually how it is supposed to work.
> 
> If you don't want to use the DNS you can just telnet to port 25 on the
> server and send some SMTP commands.

The application design of most (all?) MTAs to use DNS to find the MX is
completely understandable, and sensible. What a mess we'd have if we
needed to know the server name where Joe-Bob receives e-mail. That used to
be the case, and I fondly ;-) [if vaguely] remember some of the strange
routing notations necessary for getting mail through various protocols and
gateways. I think some of that syntax is probably still valid, if one
knows how to assemble the proper incantations, although perhaps not using
purely SMTP. There were exclamation points and percent signs, IIRC.

However, to have internet e-mail wholly reliant on DNS seems a poor
decision as well, since DNS problems are possible. As Charlie and Kevin
have pointed out, one can get around the DNS lookup. In my case, I didn't
feel it was enough of _my_ problem to warrant calling somebody in
Berkeley, CA. But being able to send mail to postmaster@[n.n.n.n] to say,
"Hey, e-mail to this domain is broken because DNS doesn't resolve.", seems
entirely reasonable.

I don't really understand the MX records in inverse zones thing, and I'm
not at the point of wanting too. But then, I'm still stuck on the notion
of being able to use the actual mailserver name or IPA, when necessary, so
I wouldn't want to use a DNS lookup for an MX record in that case.

jed
-- 
"Those who expect to reap the blessings of freedom must, like men,
 undergo the fatigue of supporting it."
 - Thomas Paine