DNS system Components

What is DNS System Components

A Domain Name System (DNS) as defined by RFC 1034 includes three parts:

1. Data which describes the domain(s)
   
2. One or more Name Server programs.
   
3. A resolver program or library.
   

A single DNS server may support many domains. The data for each domain describes global properties of the domain and its hosts (or services). This data is defined in the form of textual Resource Records organized in Zone Files. The format of Zone files is defined in RFC 1035 and is supported by most DNS software.
The Name Server program typically does three things:

1. It will read a configuration file which defines the zones for which it is responsible.
   
2. Depending on the Name Servers functionality a configuration file may describe various behaviours, for instance, to cache or not. Some DNS servers are very specialized and do not provide this level of control.
   
3. Respond to questions (queries) from local or remote hosts.

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The resolver program or libraryis located on each host and provides a means of translating a users request for, say, www.inforporch.com into one or more queries to DNS servers using UDP (or TCP) protocols.
Note: The resolver on all Windows systems and the majority of *nix systems is actually a stub resolver - a minimal resolver that can only work with a DNS that supports recursive queries. The caching resolver on MS Windows 2K and XP is a stub resolver with a cache to speed up responses and reduce network usage.
While BIND is the best known of the DNS servers and much of this guide documents BIND features, it is by no means the only solution or for that matter the only Open Source solution.

Zones and Zone Files

Zone files contain Resource Records that describe a domain or sub-domain. The format of zone files is an IETF standard defined by RFC 1035. Almost any sensible DNS software should be able to read zone files. A zone file will consist of the following types of data:

1. Data that indicates the top of the zone and some of its general properties (a SOA Record.
   
2. Authoritative data for all nodes or hosts within the zone (typically A (IPv4) or AAA (IPv6) Records).
   
3. Data that describes global information for the zone (including mail MX Records and Name Server NS Records).
   
4. In the case of sub-domain delegation the name servers responsible for this sub-domain (one or more NS Records).
   
5. In the case of sub-domain delegation one or more glue records that allows a name server to reach the sub-domain (typically one or more A or AAA Records) for the sub-domain name servers.
   

The individual Resource Records are described and numerous sample configuration files are provided and documented.