Messages in this thread |  | | Subject | Re: DNS goofups galore... | From | James Antill <> | Date | 12 Feb 2001 14:19:01 -0500 |
| |
"Henning P. Schmiedehausen" <hps@tanstaafl.de> writes:
> hpa@transmeta.com (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 (mail.foo.org) > and suddently the host reports as mail.bar.org). 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: > > mail.foo.org. IN A 1.2.3.4 > mail.bar.org. IN CNAME mail.foo.org. > bar.org. IN MX 10 mail.bar.org. > > % telnet mail.bar.org smtp > 220 mail.foo.org 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 above. For instance something that is pretty common is...
foo.example.com. IN A 4.4.4.4 foo.example.com. IN MX 10 mail.example.com. foo.example.com. IN MX 20 backup-mx1.example.com.
; This is really mail.example.org. backup-mx1.example.com. IN A 1.2.3.4
...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 -- james@and.org :0: * ^From: .*james@and\.org /dev/null - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://vger.kernel.org/lkml/
|  |