On Wed, Oct 21, 1998 at 03:29:57PM +0100, Paul Mansfield wrote:
>
> 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.
I would say you are right. It is illegal.
Why?
Snips from RFC-1035:
Snip 1:
The labels must follow the rules for ARPANET host names. They must
start with a letter, end with a letter or digit, and have as
interior characters only letters, digits, and hyphen. There are
also some restrictions on the length. Labels must be 63 characters
or less.
Snip 2:
<domain-name> is a domain name represented as a series of labels,
and terminated by a label with zero length.
Snip 3:
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.
So. a MX exchange is a <domain-name>, which is made up of labels, which
cannot contain underscores.
David
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