DNS

What Is DNS?
Domain Name System (DNS) is one of the industry-standard suite of protocols that comprise TCP/IP. Microsoft Windows Server 2003. DNS is implemented using two software components: the DNS server and the DNS client (or resolver). Both components are run as background service applications.

Network resources are identified by numeric IP addresses, but these IP addresses are difficult for network users to remember. The DNS database contains records that map user-friendly alphanumeric names for network resources to the IP address used by those resources for communication. In this way, DNS acts as a mnemonic device, making network resources easier to remember for network users.

The Windows Server 2003 DNS Server and Client services use the DNS protocol that is included in the TCP/IP protocol suite. DNS is part of the application layer of the TCP/IP reference model.

DNS in TCP/IP

For more information and to view logical diagrams illustrating how DNS fits with other Windows Server 2003 technologies, see “How DNS Works" in this collection.

By default, Windows Server 2003 DNS is used for all name resolution in a Windows Server 2003 network. In the most typical scenario, when a Windows Server 2003 network user specifies the name of a network host or an internet DNS domain name, the DNS Client service running on the Windows Server 2003 computer of the user contacts a DNS server to resolve the name to an IP address.
Ref: http://technet.microsoft.com/en-us/library/cc787921(WS.10).aspx

How DNS Works?

DNS Domain Names
The Domain Name System is implemented as a hierarchical and distributed database containing various types of data, including host names and domain names. The names in a DNS database form a hierarchical tree structure called the domain namespace. Domain names consist of individual labels separated by dots, for example: mydomain.microsoft.com.

A Fully Qualified Domain Name (FQDN) uniquely identifies the hosts position within the DNS hierarchical tree by specifying a list of names separated by dots in the path from the referenced host to the root. The next figure shows an example of a DNS tree with a host called mydomain within the microsoft.com. domain. The FQDN for the host would be mydomain.microsoft.com.

Understanding the DNS Domain Namespace
The DNS domain namespace, as shown in the following figure, is based on the concept of a tree of named domains. Each level of the tree can represent either a branch or a leaf of the tree. A branch is a level where more than one name is used to identify a collection of named resources. A leaf represents a single name used once at that level to indicate a specific resource.

DNS Domain Name Hierarchy

The previous figure shows how Microsoft is assigned authority by the Internet root servers for its own part of the DNS domain namespace tree on the Internet. DNS clients and servers use queries as the fundamental method of resolving names in the tree to specific types of resource information. This information is provided by DNS servers in query responses to DNS clients, who then extract the information and pass it to a requesting program for resolving the queried name. In the process of resolving a name, keep in mind that DNS servers often function as DNS clients, querying other servers in order to fully resolve a queried name.

How the DNS Domain Namespace Is Organized

Any DNS domain name used in the tree is technically a domain. Most DNS discussions, however, identify names in one of five ways, based on the level and the way a name is commonly used. For example, the DNS domain name registered to Microsoft (microsoft.com.) is known as a second-level domain. This is because the name has two parts (known as labels) that indicate it is located two levels below the root or top of the tree. Most DNS domain names have two or more labels, each of which indicates a new level in the tree. Periods are used in names to separate labels.

DNS Query Types

As shown in the graphic above, a number of queries were used to determine the IP address for www.whitehouse.gov. The query sequence is described below:

1. Recursive query for www.whitehouse.gov (A resource record)
2. Iterative query for www.whitehouse.gov (A resource record)
3. Referral to the .gov name server (NS resource records, for .gov); for simplicity, iterative A queries by the DNS server (on the left) to resolve the IP addresses of the Host names of the name server’s returned by other DNS servers have been omitted.
4. Iterative query for www.whitehouse.gov (A resource record)
5. Referral to the whitehouse.gov name server (NS resource record, for whitehouse.gov)
6. Iterative query for www.whitehouse.gov (A resource record)
7. Answer to the interative query from whitehouse.gov server (www.whitehouse.gov’s IP address)
8. Answer to the original recursive query from local DNS server to Resolver (www.whitehouse.gov’s IP address)

Host (A) resource records
Host (A) resource records are used in a zone to associate DNS domain names of computers (or hosts) to their IP addresses, and can be added manually, Windows clients and servers can also use the DHCP Client service to dynamically register and update their own A resource records in DNS when an IP configuration change occurs. DHCP-enabled client computers running earlier versions of Microsoft operating systems can have their A resource records registered and updated by proxy if they obtain their IP lease from a qualified DHCP server (only the Windows 2000 and Windows Server 2003 DHCP Server service currently supports this feature).

The host (A) resource record is not required for all computers, but is needed by computers that share resources on a network. Any computer that shares resources and needs to be identified by its DNS domain name, needs to use A resource records to provide DNS name resolution to the IP address for the computer.

Most A RRs that are required in a zone can include other workstations or servers that share resources, other DNS servers, mail servers, and Web servers. These resource records comprise the majority of resource records in a zone database.

Alias (CNAME) resource records
Alias (CNAME) resource records are also sometimes called canonical names. These records allow you to use more than one name to point to a single host, making it easy to do such things as host both an FTP server and a Web server on the same computer. For example, the well-known server names (ftp, www) are registered using CNAME RRs that map to the DNS host name, such as “server-1” for the server computer that hosts these services.

Ref: http://technet.microsoft.com/en-us/library/cc772774(WS.10).aspx