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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.
