Re: seeking: Ian Jackson
martin f krafft <madduck@debian.org> writes:
> iwj@d.o expands to a greenend.org.uk address, and the mx for that
> domain refuses to accept my mail.
>
> <ianXXXXXXXX.greenend.org.uk>: host
> mx-relay.chiark.greenend.org.uk[212.13.197.229] said: 550
> invalid MAIL-FROM: Error during DNS MX lookup for
> lapse.madduck.net: DNS alias found where canonical name wanted
> [Irritated] (in reply to RCPT TO command)
>
> Yes, lapse.madduck.net is a CNAME (*c*anonical *name*) to an MX RR,
> and that's RFC-compliant ttbomk. If it is not, I would appreciate if
> someone shoved the relevant sections into my face.
RFC 1034 section 3.6.2:
Domain names in RRs which point at another name should always point at
the primary name and not the alias. This avoids extra indirections in
accessing information.
See also RFC 1912 section 2.4:
Don't use CNAMEs in combination with RRs which point to other names
like MX, CNAME, PTR and NS. (PTR is an exception if you want to
implement classless in-addr delegation.) For example, this is
strongly discouraged:
podunk.xx. IN MX mailhost
mailhost IN CNAME mary
mary IN A 1.2.3.4
[RFC 1034] in section 3.6.2 says this should not be done, and [RFC
974] explicitly states that MX records shall not point to an alias
defined by a CNAME. This results in unnecessary indirection in
accessing the data, and DNS resolvers and servers need to work more
to get the answer. If you really want to do this, you can accomplish
the same thing by using a preprocessor such as m4 on your host files.
There is some disagreement though:
http://www.mengwong.com/misc/rfc1912-is-wrong.html
/Simon
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