Paul Mansfield <paulm@???> writes:
>
> here's an interesting problem. we have a customer who has a sub-domain called,
> say, mail_domain.customer.com
>
> "dig mx mail_domain.customer.com" gives a normal result, answers
> dc_mailhost.customer.com. 1D IN MX 10 ms1.customer.com.
> dc_mailhost.customer.com. 1D IN MX 20 ms2.customer.com.
>
> ms1.customer.com. IN A 1.2.3.4
> ms2.customer.com. IN A 1.2.3.6
>
> now, at first sight the underscore would make it all illegal, however, there's
> some debate about this.
>
> I think its illegal. I believe that an MX record is of the form:
> <domain> IN MX <preference> <domain>
>
> A hostname is really a domain with an A record, and since an underscore
> ("_") is illegal in a hostname it must thus be illegal in a domain name...
> but my colleague disagrees and quotes the RFCs below.
>
> The loser of this debate gets to buy the beer on payday!
I think your argument is wrong, but your conclusion is right!
Domain names in general are not subject to the RFC952/1123 hostname rules.
RFC2181 is very firm on this point. Your claim that because every host name
is a domain name, every domain name is a host name, might have had holes
picked in by Socrates...
However, it's clearly generally believed that the hostname restrictions *do*
apply to the owner field of an MX record. For example, modern versions of
BIND apply such checks (modulo the check-names options in the configuration
file) in that context, and to all-but-the-first-component of domain names
that are to be interprested as e-mail addresses (such as SOA.rname), although
not to domain names in general.
Providing justification for this from the RFCs may not be so easy. However,
we can start off with the observation that RFC821 (although not RFC822)
restricts the components of a mail domain name to the same form as
required by RFC952.
Chris Thompson Cambridge University Computing Service,
Email: cet1@??? New Museums Site, Cambridge CB2 3QG,
Phone: +44 1223 334715 United Kingdom.
--
*** Exim information can be found at
http://www.exim.org/ ***
