[Mimedefang] Domain canonifyin​g and RFCs

[Joseph Brennan]

Marcus Schopen <lists at localguru.de> wrote:

>> P.S.  I know the setting confDONT_EXPAND_CNAMES will disable this
>> behaviour, but I would prefer not to implement this.  Also I cannot
>> modify the DNS records since it is an external DNS server.
>> They insist their DNS settings are RFC compliant.
>>
>> Their DNS records looks like this:
>> c.domain.com      IN      A       1.1.1.1
>> a.b.domain.com.  IN      CNAME   c.domain.com.
>> c.domain.com.    IN      MX      10   mail.domain2.com
>> c.domain.com.    IN      MX      20   mail2.domain2.com
>>
>> user at a.b.domain.com will be rewitten to user at c.domain.com


What harm results from this? The CNAME record defines that the canonical 
name of a.b.domain.com is c.domain.com. The above structure looks compliant 
to me too. I don't understand the question.

The address user at a.b.domain.com is undeliverable otherwise, since there is 
no MX or A record for a.b.domain.com.

RFC 2821 says:

   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.

So a CNAME is allowed if it can be resolved to a MX or A RR. The next 
update, RFC 5321 says the same thing, adding IPv6 AAAA records:

   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 address (i.e., A or AAAA) RRs (as discussed
   in Section 5) are permitted, as are CNAME RRs whose targets can be
   resolved, in turn, to MX or address RRs.

So the only reason a.b.domain.com can be used (successfully) in an address 
is that it resolves to c.domain.com. It has to be rewritten.


Joseph Brennan
Columbia University