ring to (Web site design and hosting) the official hostname. The difference is
ring to the official hostname. The difference is that the canonical hostname is the one with an associated A record, while the others only have a record of type CNAME that points to the canonical hostname. We will not go through all record types here, but we will give you a brief example. Example 6.4 shows a part of the domain database that is loaded into the name servers for the physics.groucho.edu zone. Example 6.4: An Excerpt from the named.hosts File for the Physics Department ; Authoritative Information on physics.groucho.edu. @ IN SOA niels.physics.groucho.edu. janet.niels.physics.groucho.edu. { 1999090200 ; serial no 360000 ; refresh 3600 ; retry 3600000 ; expire 3600 ; default ttl } ; ; Name servers IN NS niels IN NS gauss.maths.groucho.edu. gauss.maths.groucho.edu. IN A 149.76.4.23 ; ; Theoretical Physics (subnet 12) niels IN A 149.76.12.1 IN A 149.76.1.12 name server IN CNAME niels otto IN A 149.76.12.2 quark IN A 149.76.12.4 down IN A 149.76.12.5 strange IN A 149.76.12.6 … ; Collider Lab. (subnet 14) boson IN A 149.76.14.1 muon IN A 149.76.14.7 bogon IN A 149.76.14.12 … Apart from the A and CNAME records, you can see a special record at the top of the file, stretching several lines. This is the SOA resource record signaling the Start of Authority, which holds general information on the zone the server is authoritative for. The SOA record comprises, for instance, the default time to live for all records. Note that all names in the sample file that do not end with a dot should be interpreted relative to the physics. groucho.edu domain. The special name (@) used in the SOA record refers to the domain name by itself. We have seen earlier that the name servers for the groucho.edu domain somehow have to know about the physics zone so that they can point queries to their name servers. This is usually achieved by a pair of records: the NS record that gives the server’s FQDN, and an A record that associates an address with that name. Since these records are what holds the namespace together, they are frequently called glue records. They are the only instances of records in which a parent zone actually holds information on hosts in the subordinate zone. The glue records pointing to the name servers for physics.groucho.edu are shown in Example 6.5. Example 6.5: An Excerpt from the named.hosts File for GMU ; Zone data for the groucho.edu zone. @ IN SOA vax12.gcc.groucho.edu. joe.vax12.gcc.groucho.edu. { 1999070100 ; serial no 360000 ; refresh 3600 ; retry 3600000 ; expire 3600 ; default ttl } …. ;