Subject: Wildcards and SRV records

From: Andrew Draper <[email-masked]>
Date: Mon, 7 Sep 1998 16:21:10 +0100

The current DNS specs allow for the use of wildcard records to "fill in" host 
names which do not exist, for example:

*.madge.com.            IN  MX 50 mailserver.madge.com.

If I were to try and do this using the SRV record then I suppose that I would
end up with a record such as:

_smtp._tcp.*.madge.com. IN SRV 50 0 25 mailserver.madge.com.

By my reading of RFC1034 this is illegal (wildcards can only appear as the
first label in an name).

While this does not seem useful in the case of SMTP (the owner of a domain
could just use MX records instead) there are probably other protocols where
wildcard records will be needed.  One example is h.323, where a lookup for
SRV records is being proposed as the method of resolving an email-address
into the address of a server which can be used to contact the person
specified.

To implement this I suggest that we should define a new type of wildcard
record - for the sake of description call this a non-underscore wildcard and
represent it as "*_" within zone files (although better names and
representations are probably possible).

The non-underscore wildcard is allowed to be present as the leftmost label
of a name, or following one or more labels which start with underscores, but
not following labels which don't start with underscores.  Only one
non-underscore wildcard is allowed in a name.

The non-underscore wildcard matches one or more labels which don't start
with an underscore, providing that the labels to the left and right of the
wildcard all match.

Obviously, implementation of the non-underscore wildcard would be optional
for a server, and transparent for a client.  It requires no changes to bits
on the wire.

It does however mean that it must be possible for a server to tell whether a
label is part of a standard domain label or is the service or protocol value
for a SRV lookup.  I would thus prefer that the underscores remain in the
definition of the SRV record.

Is the non-underscore wildcard a good idea?

Regards,

    Andy.

-- 
Andrew Draper - Principal Development Engineer - WAVE Software
Madge Networks, Wexham Springs, Framewood Road, Wexham, Berks.
mailto:[email-masked]  phone:+44 1753 661329
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