Pointing MX Records at CNAMES

Jim Reid jim at rfc1035.com
Tue Aug 15 20:32:08 UTC 2000


>>>>> "Martin" == Martin McCormick <martin at dc.cis.okstate.edu> writes:

    Martin> 	Is it or is it not legal to point a MX record at a
    Martin> name which is actually an alias for another system?

It's illegal. See RFC1035. I quote from section 3.3.9 MX RDATA format:

    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                  PREFERENCE                   |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   EXCHANGE                    /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

where:

PREFERENCE      A 16 bit integer which specifies the preference given to
                this RR among others at the same owner.  Lower values
                are preferred.

EXCHANGE        A <domain-name> which specifies a host willing to act as
                a mail exchange for the owner name.

MX records cause type A additional section processing for the host
specified by EXCHANGE.  The use of MX RRs is explained in detail in
[RFC-974].


So the EXCHANGE domain name should be a host name that has an A
record. RFC974 has some information on the problems that can occur
when the target of an MX record is a CNAME.

However it's depressingly common for people to have CNAMEs as the
targets of MX records. So common that sendmail has an option to
determine whether it should follow CNAMEs or not. Mind you, sendmail's
got options for just about anything....



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