Re: DNS goofups galore...

From: James Antill (
Date: Mon Feb 12 2001 - 14:19:01 EST

"Henning P. Schmiedehausen" <> writes:

> (H. Peter Anvin) writes:
> >> In other words, you do a lookup, you start with a primary lookup
> >> and then possibly a second lookup to resolve an MX or CNAME. It's only
> >> the MX that points to a CNAME that results in yet another lookup. An
> >> MX pointing to a CNAME is almost (almost, but not quite) as bad as a
> >> CNAME pointing to a CNAME.
> >>
> >There is no reducibility problem for MX -> CNAME, unlike the CNAME ->
> >CNAME case.
> >Please explain how there is any different between an CNAME or MX pointing
> >to an A record in a different SOA versus an MX pointing to a CNAME
> >pointing to an A record where at least one pair is local (same SOA).
> CNAME is the "canonical name" of a host. Not an alias. There is good
> decriptions for the problem with this in the bat book. Basically it
> breaks if your mailer expects one host on the other side (
> and suddently the host reports as The sender is
> allowed to assume that the name reported after the "220" greeting
> matches the name in the MX. This is impossible with a CNAME:
> IN A
> IN MX 10
> % telnet smtp
> 220 ESMTP ready
> ^^^^^^^^^^^^
> This kills loop detection. Yes, it is done this way =%-) and it breaks
> if done wrong.

 This is humour, yeh ?

 I would be supprised if even sendmail assumed braindamage like the
 For instance something that is pretty common is... IN A IN MX 10 IN MX 20

; This is really IN A

...another is to have "farms" of mail servers (the A record for the MX
has multiple entries).
 If it "broke" as you said, then a lot of mail wouldn't be being routed.

# James Antill --
* ^From: .*james@and\.org
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

This archive was generated by hypermail 2b29 : Thu Feb 15 2001 - 21:00:19 EST