webpointmorpheus Network Info
DNS - Domain Name System

     Overview      Definitions      Hierarchical System      Name Resolution      Top Level Domains
     DNS Request      DNS Configuration      DNS Registration      WINS      Troubleshooting

©2005 - material compiled by Bob Carnaghi, www.webpointmorpheus.com

Overview     Top of Page

DNS is to computer networks as street addresses are to the Post Office. Without DNS, users of the internet would be relinquished to dealing with a bland progression of numbers that have little or no meaning to the image-creative aspect of the human mind. To a person interested in the networking aspect of computers, proper configuration of DNS nameservers is the least that must be considered. There may be as well situations that warrant the setup and configuration of a DNS server that serves the entire namespace for an organization, MX (Mail Exchange) records, Domain Name forwarding, etc. This document attempts to address the middle ground. Included here is enough information to explain the workings of DNS and some basic setup information. The complex task of installing and configuring the BIND DNS server is beyond the scope of this document. For as complex as the DNS system may appear, it can be comprehended if approached incrementally by addressing each of its individual definitions and aspects. For in depth DNS services and information check out www.dyndns.org. There is an introductory DNS explanation by webpointmorpheus here.

If you have a difficult time with the acronymn drenched terms used in this document, check out the Network Definitions Page.

Essential DNS Definitions     Top of Page

Listed below are a few of the key terms that are used in the common parlance of DNS. These terms are specific to DNS, and may be encountered during setup and configuration, troubleshooting, or just during exposure to DNS topics.

1. Forward Lookup Zone: An area within an authoritative DNS server that holds IP addresses and names of systems in a cache for a particular domain or domains. (This server resolves Domain Names to IP addresses.) A Forward Server is like a DNS proxy, that provides information to clients that has already been retrieved and cached, thereby saving time and bandwidth.
2. Reverse Lookup Zone: A DNS service that resolves IP addresses to domain names. Note that this is the exact opposite of the process that takes place through a Forward Lookup Zone.
3. Authoritative DNS Server: The DNS server that holds the actual information entries for a zone or namespace.
4. Cache only DNS server: Servers that store copies of information they get from other DNS servers.
5. HOSTS file: A static text file that resides on a computer to resolve domain names to IP addresses. This file is checked for entries before a DNS request is placed.
6. JEEVES: The first implementation of a DNS server, written by Paul Mockapetris.
7. BIND: Berkley Internet Name Domain. A popular DNS software originally written by Kevin Dunlap. Currently maintained by the Internet Software Consortium.
8. Hierarchical: Based upon an empirical structure, or that of a hierarchy.
9. Distributed Database: A set of files that are dispersed as needed from a central repository to spread the information and relieve the burden of information dispersal.
10. Name Servers: Programs that store information about a domain namespace.
11. Resolvers: Clients that access name servers for name resolution and information about a domain namespace.
12. Stub Resolver: A resolver that only does the lookup function of name resolution.
13. Caching Reslover: A resolver that caches as well as looks up the information for name resolution.
14. Domain Name System: (DNS) The system of assigning names to nodes on a network that map to IP addresses which locate the node.
15. Delegation: The assignment of responsibility for a domain or part of a domain to another organization, typically (an)other name server(s).
16. Zone: A particular realm of hosts that a name server is responsibile (has authority) for, typically by having been delegated.
17. Domain: A grouping of like entities, such as computers, users, etc. In terms of the Internet, a group of computers that share some portion of the domain name.
18. Domain Name: An index into the DNS database that has the intent to connect the name to a specific IP address.
19. Subdomain: A further grouping within a domain.
20. Domain Name Alias: A type of naming shortcut that will direct a certain name to a location distant to the domain on which it resides.
21. Canonical Domain Name: A subdomain, also called a 'cname'. The name portion of the subdivision of a domain into further (sub)domains, such as http://development.webpointmorpheus.com. The 'development' portion is a Canonical Domain Name for the subdomain portion of the webpointmorpheus domain that is 'development'.
22. HOSTS.TXT: A locally stored static text file that matches host names to IP addresses. In the beginning days of computer networking, HOSTS.TXT was the sole means of this mapping. This file is the great-great-grandfather of the DNS system, and is still present on most systems today.
23. LMHOSTS.TXT: A static text file that is the WINS equivalent to the HOSTS.TXT file of DNS.
24. WINS: Windows Naming System. A naming service typically used on peer-to-peer networks, especially Windows NT networks that matches NetBIOS names to IP address.
25. Domain Name Alias: A domain name that points to another domain.
26. FQDN: Fully qualified domain name. A complete domain name from hostname to top-level domain name.
27. Top Level Domain: The domains on the Internet with the highest groupings. The typical portion of the domain name that is last - .com, .org, .edu, etc.
28. Delegation: The decentralization of administration of domains and subdomains by establishing pointers that assign responsibility those entities.
29. Primary Master Name Server: Holds the information for its zone in a file on its host.
30. Secondary Master Name Server: Reads information for its zone from the name server that is authoritative for its zone, the Primary Master Name Server for its zone.
31. Namespace: A set of names available for naming computers, domains, etc.
32. Name Resolution: The process of searching a namespace for data about a domain name.
33. in-addr.arpa: The portion of a namespace that maps IP addresses to domain names.

Hierarchical System     Top of Page

Part of the difficulty in understanding the DNS system is that as it is referenced, all the references are relative to what they reference. If a domain named Domain 1 contains a subdomain named Subdomain 1 that contains subdomains as well, then from the point of view of the nodes within Subdomain 1, it is a domain. One of the keys to understanding the DNS system is to understand the meaning of a hierarchical system. Hierarchical systems are used in many implementations aside from DNS. Corporate, military, government, and many other structures are based upon a hierarchy. In the true essence of the system, at each tier of the hierarchy, any one single entity has a group of entities just below it to carry out tasks of some sort. In a perfect system, any one entity has only to request tasks of the layer below it, and only to answer to the entity above it regarding its tasks. This can be a very organized and efficient system, albeit sometimes cold in terms of human qualities. However, this type of system - once understood - is the same type used for computer operating and file systems, domain/sub-domains, the DNS process, etc. In terms of computers and DNS, the key to finding any one entity in the 'tree' is to find the pathway to that entity.

The Name Resolution Process     Top of Page

DNS is (in its most basic form) a database of relationships between Domain Names and IP addresses. Further, it is a distributed, hierarchical database. The distribution of this database spreads out the load for requests, as well as makes the data redundant: there are several copies of it available in case any one of the sources should go offline. For all the complexity of the DNS process, it's helpful to keep in mind that the ultimate objective is to resolve a Domain Name (human-friendly) to an IP address (computer-friendly). Every computer that wants to negotiate the Internet (or smaller networks as well) must have an IP address, or some way of unique identification.

When a request is made for a web page or resource distant to the client, the location of the host computer must be determined. The first element is to query the top level domain server. Since that server will (most likely) not contain the specific information, it refers the requesting client to the next most likely location that should have the information. This process is repeated and iterated until the final 'authoritative source or a source with the cached information (domain name to IP address) is determined. Typically a resolver initiates the request for resolution.

Top Level Domains     Top of Page

In the world of domain names, there are certain top level domains. The ultimate top level, or root, is considered to be a dot - '.' Just below this are the top level domains are .com, .edu, .net, .gov, .mil, etc. Within each of these top level domains individual domains are registered and listed. There are 13 top level domain servers in the world (as of this writing) that handle all the listings of the domain name requests for their domain. Root servers are queried on the average of thousands of requests per second. Each name server that is queried either gives the querier the answer that it seeks, or information about how to 'get closer' to the answer it's seeking. Please keep in mind that domain names are read from right-to-left in terms of the hierarchy. The '.com' part of the domain name is actually higher up in the system than would be the 'webpointmorpheus' part of the domain name webpointmorpbeus.com.

Top-level domains are a children of the root. First-level domains are children of the top-level domains. Second level domains are children of first level domains. ISO 3166 establishes the official standard for two-letter designations for top-level domains for countries around the world.

A DNS Request     Top of Page

Shown below is a graphic that shows a typical request made on the internet.

DNS Configuration     Top of Page

The typical DNS configuration process must take place from two aspects: the name server-web host configuration, and the client accessing the internet. Listed below is an overview of the process from each perspective.

   Web Server - Web Host DNS Configuration

When setting up a domain name to point to a web server, the name server must be specified. Typically this is done through an administration panel provided for the account that holds the domain name. The nameservers are most often set up by the web hosting company, and their domain name and/or IP addresses are provided.

   Client DNS Configuration

The typical settings here will be for the DNS server as provided from the ISP. On a medium to large network, the installation of a DNS server will speed up and cache DNS requests. There are typically two entries for DNS nameservers - one for the primary, and another for the secondary in case the primary is unreachable.

   Setting Up a DNS Server

The installation and configuration of a DNS server is beyond the scope of this document. Check out these links for excellent help on setting up the BIND or Windows DNS service, or visit the official BIND website.

DNS Registration     Top of Page

Domain Name Registration is a relatively simple and straightforward process. Domain Names must be registered through an accredited registrar, a subsidiary of ICANN (Internet Corporation for Assigned Names and Numbers). DNS records will contain information about many aspects of a domain: Owner, hosting info, MX (Mail Exchange) info, FTP info, newsgroup info, etc. These entries into the DNS record are collectively called its resource record.

WINS     Top of Page

WINS (Windows Internet Naming System) is a native Windows service that resolves NetBIOS names in the NetBEUI protocol suite. This service is common in peer-to-peer networks and Windows NT. It has been superceded in later versions of Windows. WINS has a local static file called the LMHOSTS file. This file can be edited manually if needed.

Troubleshooting     Top of Page

Problems with DNS are almost always in the configuration settings. Listed below are a few commands that can help troubleshoot DNS configuration and settings. There are more troubleshooting and configuration guidelines here.

• nslookup
• ipconfig /displaydns
• ipconfig /flushdns
• ipconfig /release
• ipconfig /renew
• ipconfig /all
• net view
• nbstat
• netstat
• netstat -s
• netstat -nr
• nbstat -c (not on all systems)

   Native Linux items:

• bootp program
• ifconfig eth0 up
• ifconfig eth0 down
• ipchains