Domain Name Service Defined

Domain Name Service Defined

Network naming services were developed to overcome the obstacle of humans having to remember complex computerized addresses. The DNS is a distributed database indexed by domain names . Recall that each domain name exists as a path in a large inverted tree, the domain namespace. The structure of this tree is hierarchical.

All DNS implementations adhere to a specific set of criteria:

• Each node in the tree has a text label that can be up to 63 characters long.

• The root of the tree is labeled with a zero-length name.

• The full name of any node in the tree is the path from the node to the root, using the text labels separated by a dot. When the root node's label is printed, it appears as the name of the node, ending with a dot.

• An absolute domain name is also referred to as a fully qualified domain name (FQDN) .

• DNS specification requires that nodes under the same parent have different labels. This restriction guarantees that a single node is uniquely defined within the tree, regardless of its location in the structure.

How DNS Is Used

DNS is composed of two main components, clients and servers. The servers store information about specific components of the DNS structure and service requests , and the clients issue requests.

Each server contains a partial subset of the entire DNS namespace. These subsets are known as zones . DNS Servers can contain copies of either forward or reverse lookup zones. Forward lookup zones are used to resolve DNS names to IP addresses. For example, the forward lookup zone for microsoft.com resolves www.microsoft.com to its numerical IP address. Reverse lookup zones are responsible for resolving IP addresses to DNS names, or the reverse of the forward lookup zones.

The key to understanding how a DNS client resolves DNS queries is to understand the order in which name resolution occurs. The DNS client follows through these steps when resolving DNS names. If a match is found, the results are returned and no further steps are taken. If all steps have been exhausted, the client receives an error. Initially, the DNS client will attempt to resolve the request using local resources:

• The local cache, which is obtained from previous queries, is searched. The items in this cache remain until the Time-to-Live (TTL) period, which is set on each item, expires . Every time the DNS client is shut down, the cache is cleared.

• The local HOSTS file, which is stored in the %systemroot%\system32\drivers\etc directory is queried. The HOSTS file contains hostname-to-address mappings to enable manual hard-coding of DNS-to-IP addresses. These entries remain static and remain on the system even if it is rebooted.

When the client has exhausted all its locally available options, it sends a query to the DNS server for the record that it is seeking. The DNS server attempts to resolve the client's query the following way:

• If the query result is found in any of the zones for which the DNS server is authoritative, the server responds to the host with an authoritative answer.

• If the result is in the zone entries of the DNS server, the server checks its own local cache for the information.

If the local resources fail to provide an answer to the client's query it attempts to resolve the query by sending the request to other DNS servers in the form of a recursive query. This query is sent to either the server that is listed as a Forwarder, or to the set of servers set up in the DNS server's Root Hints file.

The DNS query is then sent around the Internet until it comes into contact with the DNS servers that are listed as being authoritative for the zone listed in the query. That DNS server then sends back the reply as either affirmative (with the IP address requested ) or negative.

Who Needs DNS?

Not all situations require the use of the DNS. There are other name resolution mechanisms that exist beside DNS, some of which come standard with the operating system that companies deploy. Managing name servers in a domain sometimes is too much overhead. DNS makes life easier, but not all scenarios have the requirement of a complex name resolution structure.

In the past, an organization with a standalone, non- interconnected network could get away with using only host files or Microsoft's Windows Internet Naming Service (WINS) to provide NetBIOS-to-IP address name translation. Some very small environments could also use broadcast protocols such as NetBEUI to provide name resolution. In modern networks, however, DNS becomes a necessity, especially in mixed Windows/Unix environments.

In addition to local name translation, connecting to the Internet makes DNS connectivity a must. The World Wide Web, mail services, file transfer, and remote access services all use DNS services. Simply gaining access to the Internet does not, however, mean that every company or individual connecting to the Internet has to set up its own DNS server. Internet Service Providers (ISPs) can take care of managing DNS services on behalf of the user . A small organization might have a few hosts that access the Internet and might rely on its ISP to host those records. When a company wants to have more control over the domain and the name servers for that domain, it sets up its own DNS servers.