16.2 Wildcards

Something else we haven't covered yet is DNS wildcards. At times 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 be used in a DNS datafile as a wildcard name. It will match any number of labels in a name, as long as that name isn't an exact match with a name already in the DNS 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 would relay 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 like 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 your internal zone data. Remember that some mailers apply the search list when looking up MX records:

C:\>nslookup 
Default Server:  wormhole.movie.edu 
Address:  192.249.249.1 

> set type=mx             Look up MX records 
> cujo.movie.edu          for cujo.movie.edu. 
Server:  wormhole.movie.edu 
Address:  192.249.249.1 

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 names for which there is already data. Suppose you did use wildcards within your zone data:

*.movie.edu.   IN  MX  10 mail-hub.movie.edu. 
et.movie.edu.  IN  MX  10 et.movie.edu. 
jaws.movie.edu IN  A   192.253.253.113

Mail to terminator.movie.edu will be sent to mail-hub, but mail to et.movie.edu will be sent directly to et. An MX lookup of jaws.movie.edu would result in a response that says there is no MX data for that name. The wildcard doesn't apply because an A record exists. Can you use wildcards safely within your zone data? Yes. We'll cover that case a little later in this chapter.