Wildcards (DNS and BIND, 4th Edition)

16.2. Wildcards

Something else we haven't covered in detail yet is DNS wildcards. There are times when you want a single resource record to cover any possible name, rather than creating zillions of resource records that are all the same except for the domain name to which they apply. DNS reserves a special character, the asterisk (*), to use in zone data files as a wildcard name. It will match any number of labels in a name as long as there isn't an exact match with a name already in the name server's database.

Most often, you'd use wildcards to forward mail to non-Internet-connected networks. Suppose our site weren't connected to the Internet, but we had a host that relayed mail between the Internet and our network. We could add a wildcard MX record to the movie.edu zone for Internet consumption that points all our mail to the relay. Here is an example:

*.movie.edu.  IN  MX  10 movie-relay.nea.gov.

Since the wildcard matches one or more labels, this resource record would apply to names such as terminator.movie.edu, empire.fx.movie.edu, or casablanca.bogart.classics.movie.edu. The danger with wildcards is that they clash with search lists. This wildcard also matches cujo.movie.edu.movie.edu, making wildcards dangerous to use in our internal zone data. Remember that some versions of sendmail apply the search list when looking up MX records:

% nslookup
Default Server:  wormhole
Address:  0.0.0.0

> set type=mx                                    -- Look up MX records
> cujo.movie.edu                                -- for cujo
Server:  wormhole
Address:  0.0.0.0

cujo.movie.edu.movie.edu     -- This isn't a real host's name!
        preference = 10, mail exchanger = movie-relay.nea.gov

What are the limitations of wildcards? Wildcards do not match domain names for which there is already data. Suppose we did use wildcards within our zone data, as in these partial contents of db.movie.edu:

*     IN  MX  10 mail-hub.movie.edu.
et    IN  MX  10 et.movie.edu.
jaws  IN  A   192.253.253.113
fx    IN  NS  bladerunner.fx.movie.edu.
fx    IN  NS  outland.fx.movie.edu.

Mail to terminator.movie.edu is sent to mail-hub.movie.edu, but mail to et.movie.edu is sent directly to et.movie.edu. An MX lookup of jaws.movie.eduwould result in a response saying there was no MX data for that domain name. The wildcard doesn't apply because an A record exists. The wildcard also doesn't apply to domain names in fx.movie.edu because wildcards don't apply across delegation. Nor does the wildcard apply to the domain name movie.edu, because the wildcard amounts to zero or more labels followed by a dot, followed by movie.edu.


16. Miscellaneous		16.3. A Limitation of MX Records