Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 228
DNS Components

DNS does much more than the name-to-address translation. It also allows for:

The Domain Name Space and resource records. This is the database of grouped names and addresses that are strictly formatted using a tree-structured name space and data associated with the names. The domain system consists of separate sets of local information called zones. The database is divided up into sections called zones, which are distributed among the name servers. While name servers can have several optional functions and sources of data, the essential task of a name server is to answer queries using data in its zones. Conceptually, each node and leaf of the domain name space tree names a set of information, and query operations are attempts to extract specific types of information from a particular set. A query names the domain name of interest and describes the type of resource information that is desired.

Name servers. These are workstations that contain a database of information about hosts in zones. This information can be about well-known services, mail exchanger, or host information. A name server may cache structure or set information about any part of the domain tree, but in general, a particular name server has complete information about a subset of the domain space, and pointers to other name servers that can be used to lead to information from any part of the domain tree. Name servers know the parts of the domain tree for which they have complete information; a name server is said to be an authority for these parts of the name space. Authoritative information is organized into units called zones, and these zones can be automatically distributed to the name servers that provide redundant service for the data in a zone. The name server must periodically refresh its zones from master copies in local files or foreign name servers.

Resolvers. These are programs that generally reside on users’ workstations and send requests over the network to servers on behalf of the users. Resolvers must be able to access at least one name server and use that name server’s information to answer a query directly, or pursue the query using referrals to other name servers.

When a DNS server responds to a resolver, the requester attempts a connection to the host using the IP address and not the name. The preceding example could have used only part of a name: host. This is known as a relative name. It is part of a larger name known as the absolute name. The absolute name for the preceding example could be host.research.Naugle.com. This name would be in the domain name server. Most resolvers will step through a preconfigured list of suffixes (in order of configured input), append it to the name, and attempt a lookup when the full DNS (absolute) name is not specified.