Reverse Lookups

Figure 5. An excerpt from the named.hosts file for the
Physics Department.

;
; 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
...

Beside looking up the IP-address belonging to a host, it is sometimes desirable to find out the canonical host name corresponding to an address. This 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 name space is out of the question, of course. Instead, a special domain, in-addr.arpa, has been created which contains the IP-addresses of all hosts in a reverted 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 host names is PTR.

Figure 6.   An excerpt from the named.hosts file for GMU.
;
; 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
...

Creating a zone of authority usually means that its administrators are given 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.

As a consequence, 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 figure-. the corresponding glue records in the database of their parent zone is shown in figure-.

Figure 7.   An excerpt from the named.rev file for subnet  12.
 
;
; the 12.76.149.in-addr.arpa domain.
@                IN     SOA   {
                 niels.physics.groucho.edu.
                 hostmaster.niels.physics.groucho.edu.
                 233 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.

Figure 8.    An  excerpt  from  the named.rev file for network
149.76.

;
; the 76.149.in-addr.arpa domain.
@                   IN       SOA          {
                  vax12.gcc.groucho.edu.
                  hostmaster.vax12.gcc.groucho.edu.
                  233 360000 3600 3600000 3600
                 }
...
; subnet 4: Mathematics Dept.
1.4              IN     PTR      sophus.maths.groucho.edu.
17.4             IN     PTR      erdos.maths.groucho.edu.
23.4             IN     PTR      gauss.maths.groucho.edu.
...
; subnet 12: Physics Dept, separate zone
12               IN     NS       niels.physics.groucho.edu.
                   IN     NS       gauss.maths.groucho.edu.
niels.physics.groucho.edu. IN  A 149.76.12.1
gauss.maths.groucho.edu. IN  A   149.76.4.23
...