[Mimedefang] Domain canonifyin?g and RFCs

Joseph Brennan brennan at columbia.edu
Wed Oct 19 09:19:07 EDT 2011

> RFC 821 section 3.7 states "Whenever domain names are used in SMTP
> only the official names are used, the use of nicknames or aliases is
> not allowed.".  This however changed in RFC 2821 section 3.6 where is
> states that CNAME RRs may be used.
> Currently Sendmail still rewrites the sender domain.  Does this mean
> that the default behaviour of Sendmail is not RFC compliant or are
> there other RFCs that still limits the use of CNAMEs with MX records
> (maybe RFC 1034 section 3.6.2)?

   3.6 Domains

   Only resolvable, fully-qualified, domain names (FQDNs) are permitted
   when domain names are used in SMTP.  In other words, names that can
   be resolved to MX RRs or A RRs (as discussed in section 5) are
   permitted, as are CNAME RRs whose targets can be resolved, in turn,
   to MX or A RRs.

It says a CNAME can be used, if it can be resolved. So sendmail
resolves it. I don't see any problem there.

Under the language of RFC 821, where using a CNAME was not allowed
at all, it was a kind gesture to try resolving it anyway in order
to deal with a non-standards-compliant host. Strictly speaking
maybe using a CNAME could have been grounds for refusing mail. But
we were all more generous in those days.

Joseph Brennan
Columbia University Information Technology

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