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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.
Next: Configuring the Networking Hardware Up: The Domain Name System Previous: The DNS Database Andrew Anderson
Thu Mar 7 23:22:06 EST 1996