DNS

The Domain Name System is the name resolution system used by the Internet. It is a hierarchical naming system based on the concept of domains. At the top level, there is the domain . (the root domain), below it are domains such as com, edu, or ie, which act as a first partition of the name space. Individual organizations have their own domains below these domains. Below com you find companies in the US (for example, sun.com), below edu are American educational institutions (for example, stanford.edu), and below ie you find institutions in Ireland (for example, tcd.ie). The individual organizations often divide these domains into subdomains.

DNS works by delegation. Each domain is served by one or more DNS servers, which has a database of the hosts in the domain. In addition, the DNS servers also have a list of other DNS servers to query in case they cannot resolve a name locally. This list typically consists of a set of DNS servers called root servers at the top of the DNS hierarchy, which in turn know what DNS servers hold data about the different top-level domains.

Individual hosts use the Domain Name System to resolve name queries by becoming DNS clients. To configure a DNS client, you specify the IP address of the DNS server that you want to respond to the queries for you. You have to do this even if the individual host is a DNS server, in which case you typically set it to answer the queries itself.

The hostname of a computer together with its full domain name (ending in the top level domain) makes up its complete DNS name. If the host stimpy resides in a domain called comedy, which is a subdomain of cartoon under the top level domain net, then stimpy.comedy.cartoon.net is the complete name for stimpy.

An important distinction used in the text below is that between a fully qualified name or a partially qualified name. When referring to the fully qualified name of a host of a domain, it means the complete DNS name ending in a trailing period. The fully qualified name for stimpy is stimpy.comedy.cartoon.net..

A partially qualified name is a name that does not specify the domain branch all the way up to the top. Partially qualified names are used as a shorthand when the name resolution software can attach the rest of the domain name. If you are in the domain comedy.cartoon.net and use stimpy to mean stimpy.comedy.cartoon.net, then you are using a partially qualified name. If a DNS name does not end with a trailing period, it is treated as partially qualified.

The distinction between fully and partially qualified names is important in many of the DNS configuration tasks - if you experience any problems, please refer to the help pages, which always tell you which one to use.

DNS Server Options

There are several different types of a DNS server. At the most basic, the server does not hold any permanent data about any domains itself, but simply forwards queries to other servers (a cache-only server) and stores the result. A DNS primary server has a master database for a domain. A DNS secondary server provides a local copy of master database for a domain that it copies from a primary server. DNS server can be both a primary and a secondary domain server at the same time.

All DNS servers store the results of successful queries (whether it resolved the query itself or forwarded it to another DNS server). If the server receives another query for the same name, it replies with the stored answer. This is called caching. A server that only does this (a cache-only server) can be useful to shorten the response time compared with contacting a more remote DNS server.

There are two different ways in which a DNS server can provide data for a domain; it can be a primary, or a secondary server for the domain. For the primary server, the DNS administrator maintains the master database for the hosts in the domain on the server. For the secondary server, the server keeps a local copy of the master database for the domain that it retrieves from the primary server. It periodically compares its database to the one on the primary server and requests a new copy if a difference is detected.

The difference between normal caching and being a secondary server for a domain is that normal caching only stores the results from previous queries. A secondary server actively retrieves the information in anticipation of future requests. This reduces the load on the primary server, and also makes it a backup in case the primary server cannot be contacted. If your server is a primary server for a domain, you should have a secondary as a backup.

There are two different types of DNS server configuration that you use depending on what the structure of your local domain is. The first type, which is referred to as Basic DNS Server, involves specifying a list of other DNS servers to query if a name cannot be resolved locally. The second type is called a DNS Internal Root Server. It is used on Intranets without an Internet connection (in other words, without access to any other DNS servers), and also on large Intranets with several subdomains, where there is a need for special DNS servers for the internal hierarchy.

Finally, a DNS Primary Server may need to provide reverse maps (IP address to name) as well as forward maps (name to IP address). Please consult with your ISP to find out whether this responsibility is delegated to you or not.