DNS

The Domain Name System is really designed as a Network Information Service for internal use by tools rather than directly by users. However, the names it holds appear in location information currently used by many services, and are also the basis for electronic mail routing.

The Domain Name System model is that all objects on the net have a name, and that the name should be that given by the people responsible for the object. However, this name is only part of the full way to specify the object. The fully distinguished name is part of an hierarchy of names. They are written as per postal 'address', for example:

	swan.computer-lab.cambridge.ac.uk

Any owner of a name space must run a server. The owning site then must inform sites at a ``level'' above, where their server is. At the same time, they tell their server where level above is.

Applications (FTP, Mail, TELNET, Mosaic, etc) use a library function to call the resolver. They give the resolver function a name and it sends a request to the local site DNS Server. The DNS server responds with either:

1. The answer.
   1. From its own tables.
   2. From another server it asked on the user's behalf. This is called chaining, and is illustrated for a general directory service in figure 2.2.
2. A site who can answer. This is called referral, and is illustrated in figure 2.3.

 
Figure 2.2:  Chaining in Servers

 
Figure 2.3:  Referral in Servers

The Domain Name System holds general purpose Resource Records.

RRs aren't restricted to describing just Name <->IP address, but can include:

• Aliases
• Mail Relay Addresses
• Pointers to Other DNS Servers
• Start of Authority
• Network users and managers