Linux Network Administrator Guide, Second Edition

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
}
…
;
; 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
…

Reverse Lookups

Finding the IP address belonging to a host is certainly the most common use for the Domain Name System, but sometimes you'll want to find the canonical hostname corresponding to an address. Finding this hostname is called reverse mapping, and is used by several network services to verify a client's identity. When using a single hosts file, reverse lookups simply involve searching the file for a host that owns the IP address in question. With DNS, an exhaustive search of the namespace is out of the question. Instead, a special domain, in-addr.arpa, has been created that contains the IP addresses of all hosts in a reversed dotted quad notation. For instance, an IP address of 149.76.12.4 corresponds to the name 4.12.76.149.in- addr.arpa. The resource-record type linking these names to their canonical hostnames is PTR.

Creating a zone of authority usually means that its administrators have full control over how they assign addresses to names. Since they usually have one or more IP networks or subnets at their hands, there's a one- to-many mapping between DNS zones and IP networks. The Physics department, for instance, comprises the subnets 149.76.8.0, 149.76.12.0, and 149.76.14.0.

Consequently, new zones in the in-addr.arpa domain have to be created along with the physics zone, and delegated to the network administrators at the department: 8.76.149.in-addr.arpa, 12.76.149.in-addr.arpa, and 14.76.149.in-addr.arpa. Otherwise, installing a new host at the Collider Lab would require them to contact their parent domain to have the new address entered into their in- addr.arpa zone file.

The zone database for subnet 12 is shown in Example 6.6. The corresponding glue records in the database of their parent zone are shown in Example 6.7.

Example 6.6: An Excerpt from the named.rev File for Subnet 12

; the 12.76.149.in-addr.arpa domain.
@ IN SOA niels.physics.groucho.edu. janet.niels.physics.groucho.edu. {
 1999090200 360000 3600 3600000 3600
}
2 IN PTR otto.physics.groucho.edu.
4 IN PTR quark.physics.groucho.edu.
5 IN PTR down.physics.groucho.edu.
6 IN PTR strange.physics.groucho.edu.