We have seen above that DNS does not only deal with IP-addresses of hosts, but also exchanges information on name servers. There are in fact a whole bunch of different types of entries the DNS database may have.
A single piece of information from the DNS database is called a resource record, or RR for short. Each record has a type associated with it, describing the sort of data it represents, and a class specifying the type of network it applies to. The latter accommodates the needs of different addressing schemes, like IP-addresses (the IN class), or addresses of Hesiod networks (used at MIT), and a few more. The prototypical resource record type is the A record which associates a fully qualified domain name with an IP-address.
Of course, a host may have more than one name. However, one of these names must be identified as the official, or canonical host name, while the others are simply aliases referring to the former. The difference is that the canonical host name is the one with an A record associated, while the others only have a record of type CNAME which points to the canonical host name.
We will not go through all record types here, but save them for a later chapter, but rather give you a brief example here. Figure- shows a part of the domain database that is loaded into the name servers for the physics.groucho.edu zone.
Apart from 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. This 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 groucho.edu domain. The special name ``@'' used in the SOA record refers to the domain name by itself.
We have seen above 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 associating an address with that name. Since these records are what holds the name space together, they are frequently called the glue records. They are the only instances of records where 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 figure-.