MX records and CNAMEs

Jim Reid jim at rfc1035.com
Sun Jun 10 17:16:48 UTC 2001


>>>>> "Wouter" == Wouter Van Crombruggen <woutervc at skynet.be> writes:

    >> The MX records for my domain are currently as follows:
    >> 
    >> domain.com IN MX 0 smtp.domain.com 
    >> domain.com IN MX 10 relay.domain.com
    >> 
    >> With the following CNAME records pointing the specified mail
    >> exchangers to the correct servers
    >> 
    >> smtp.domain.com IN CNAME mail.domain.com 
    >> relay.domain.com IN CNAME mail2.domain.com

    Wouter> It will work, but is not according RFC and might not work
    Wouter> on future releases of BIND (version 9?)

Actually, it *may* work. The targets of an MX record should be
hostnames, NOT CNAMEs. So the above configuration is illegal.
It would be better to fix the MX records to be:
	domain.com. IN MX 0 mail.domain.com.
	domain.com. IN MX 10 mail2.domain.com.
where {mail,mail2}.domain.com were A records.

Sadly, most mail systems have options to follow CNAMEs so they can
deliver mail to broken MX configrations. However it cannot be assumed
that every mail system in the world does this or has this feature
enabled. [In a protocol conformant world, nobody would support this
workaround and the broken setups wouldn't get any mail delivered. That
would be one effective way to solve the problem.] The sensible thing
to do is provide legal MX records from the outset and not rely on
every other mail system in the world accommodating your broken setup.


More information about the bind-users mailing list