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    SubjectRe: DNS goofups galore...
    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.

    Regards
    Henning
    --
    Dipl.-Inf. (Univ.) Henning P. Schmiedehausen -- Geschaeftsfuehrer
    INTERMETA - Gesellschaft fuer Mehrwertdienste mbH hps@intermeta.de

    Am Schwabachgrund 22 Fon.: 09131 / 50654-0 info@intermeta.de
    D-91054 Buckenhof Fax.: 09131 / 50654-20
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