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 accomodates 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 canocical 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.
;
; Authoritative Information on physics.groucho.edu
@ IN SOA {
niels.physics.groucho.edu.
hostmaster.niels.physics.groucho.edu.
1034 ; 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
nameserver 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
...
Figure:
An excerpt from the named.hosts file for the
Physics Department.
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, signalling 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 .
;
; Zone data for the groucho.edu zone.
@ IN SOA {
vax12.gcc.groucho.edu.
hostmaster.vax12.gcc.groucho.edu.
233 ; serial no
360000 ; refresh
3600 ; retry
3600000 ; expire
3600 ; default ttl
}
....
;
; Glue records for the physics.groucho.edu zone
physics IN NS niels.physics.groucho.edu.
IN NS gauss.maths.groucho.edu.
niels.physics IN A 149.76.12.1
gauss.maths IN A 149.76.4.23
...
Figure:
An excerpt from the named.hosts file for GMU.