Introduction to the Domain Name System (DNS)

The Internet, and many larger private Internet Protocol (IP) networks, rely on the Domain Name System (DNS) to help direct traffic. The DNS maintains a distributed database of network names and addresses, and it provides methods for computers to remotely query the database. Some people call DNS the "phone book of the Internet."

DNS and the World Wide Web

All public Web sites run on servers connected to the Internet with public IP addresses. The Web servers at About.com, for example, have addresses like 207.241.148.80. Although people can type address information like http://207.241.148.80/ into their Web browser to visit sites,

The Internet utilizes DNS as a worldwide name resolution service for public Web sites. When someone types a site's name into their browser, DNS looks up the corresponding IP address for that site, the data required to make the desired network connections between Web browsers and Web servers.

DNS Servers and Name Hierarchy

DNS uses a client/server network architecture. DNS servers are the computers designated to store DNS database records (names and addresses), while clients of the DNS include PCs, phones and other devices of end users. DNS servers also interface with each other, acting as clients to each other when needed.

The DNS organizes its servers into a hierarchy. For the Internet, so-called root name servers reside at the top of the DNS hierarchy. The Internet root name servers manage DNS server information for the Web's top-level domains (TLD) (like ".com" and ".uk"), specifically the names and IP addresses of the original (called authoritative) DNS servers responsible for answering queries about each TLD individually. Servers at the next lower level of the DNS hierarchy track second-level domain names and addresses (like "about.com") , and additional levels manage Web domains (like "compnetworking.about.com").

DNS servers are installed and maintained by private businesses and Internet governing bodies around the world. For the Internet, 13 root name servers (actually redundant pools of machines around the world) support the hundreds of Internet top-level domains, while About.com provides authoritative DNS server information for the sites within its network. Organizations can similarly deploy DNS on their private networks separately, on the smaller scale.

Configuring Networks for DNS

DNS clients (called resolvers) wanting to use DNS must have it configured on their network. Resolvers query the DNS using fixed (static) IP addresses of one or more DNS servers. On a home network, DNS server addresses can be configured once on a broadband router and automatically picked up by client devices, or the addresses can be configured on each client individually. Home network administrators can get valid DNS server addresses from either their Internet service provider or third-party Internet DNS providers like Google Public DNS and OpenDNS.

Types of DNS Lookups

DNS is most commonly used by Web browsers automatically converting Internet domain names to IP addresses. Beside these forward lookups, the DNS also is used for:

    finding the correct servers to deliver Internet email

    reverse lookups that convert an IP address back to a domain name

The network requests supporting DNS lookups run over TCP and UDP, port 53 by default.

DNS Caches

To better process high volumes of requests, the DNS utilizes caching. DNS caches store local copies of recently-accessed DNS records while the originals continue to be maintained on their designated servers. Having local copies of DNS records avoids having to generate network traffic up and through the DNS server hierarchy. However, if a DNS cache becomes outdated, network connectivity issues can result. DNS caches have also been prone to attack by network hackers. Network administrators can flush a DNS cache if needed using ipconfig and similar utilities.

Dynamic DNS

Standard DNS requires all IP address information stored in the database to be fixed. This works fine for supporting typical Web sites but not for devices using dynamic IP addresses such as Internet Web cams or home Web servers. Dynamic DNS (DDNS) adds network protocol extensions to DNS to enable name resolution service for dynamic clients.

Various third-party providers offer dynamic DNS packages designed for those wanting to remotely access their home network via the Internet. Setting up an Internet DDNS environment requires signing up with the chosen provider and installing additional software on the local network. The DDNS provider remotely monitors subscribed devices and makes the required DNS name server updates.

Alternatives to DNS

The Microsoft Windows Internet Naming Service (WINS) supports name resolution similar to DNS but works only on Windows computers and using a different name space. WINS is used on some private networks of Windows PCs.

Dot-BIT is an open source project based based on BitCoin technology that is working to add support for a ".bit" top-level domain to the Internet DNS.