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## 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
                ...

One important consequence of this is that zones can only be created as supersets of IP-networks, and, even more severe, that these network's netmasks have to be on byte boundaries. All subnets at Groucho Marx University have a netmask of 255.255.255.0, whence an in-addr.arpa zone could be created for each subnet. However, if the netmask was 255.255.255.128 instead, creating zones for the subnet 149.76.12.128 would be impossible, because there's no way to tell DNS that the 12.76.149.in-addr.arpa domain has been split in two zones of authority, with host names ranging from 1 through 127, and 128 through 255, respectively.

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