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    SubjectRe: DNS goofups galore...
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    Date
    "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
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