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DNS
DNS stands for Domain Name System and is an Internet protocol that converts human-readable names to IP addresses, changes IP addresses back to names, and provides easy-to-remember names for many Internet-based services, such as email. At the dawning of the Internet, or as it was known back then, the ARPANET (Advanced Research Projects Agency Network), very few people and machines were actually online. Each computer using the Internet had an IP address, but since there were so few IP addresses, memorizing them wasn’t a big deal. As the number of machines quickly grew, people thought it would be a good idea to use more human-friendly names. Instead of remembering a computer’s IP address, such as 128.171.32.45, ARPANET, users could enter names such as GOPHER-HAWAII. A single text file named HOSTS.TXT served as a name-to-address map.
The Stanford Research Institute (then a part of ARPANET) manually maintained the file, also known as the hosts file, in a single place, and distributed it to ARPANET users. Back then, if you wanted to translate a name to an IP address, you needed to download the latest copy of the hosts file. Likewise, if you wanted to be known by the other parts of ARPANET by name, you needed to contact the maintainer of the hosts file and add yourself to the list. This centralized system quickly proved unscalable. Computer scientist and Internet pioneer Paul Mockapetris began work writing a standards document to define a replacement for host files. He took his proposed standard to the Internet Engineering Task Force (IETF), which still today produces standards documents that define how Internet protocols should operate and interoperate.
In 1983, Mockapetris published the first standards documents in the IETF that would become the basis for the DNS. His proposal called for a decentralized, distributed structure of name servers. More than 30 years later, this same system is still very much in use, making Paul Mockapetris the official Father of DNS. See a timeline that summarizes the evolution of DNS through 1987.
DNS Structure
DNS distributes responsibility for an ever-growing list of network device names. It does this by creating a hierarchy of responsibility.This is often shown with an upside down tree, such as pic 1-2, where the root servers are at the top and the leaves (which represent all the end host nodes on the Internet) are at the bottom. The entire tree represents the namespace of DNS. Each server that is responsible for part of the namespace is called a “name server.” Some name servers just send packets along until they reach an answer.
The root name servers direct DNS queries to name servers for each of the top-level domains, which are the main branches just below it (for example, .com, .net.jp, and .info). Root name servers are authoritative name servers for DNS’s root zone, which is sometimes written as a single dot (.). Being authoritative for a zone means being responsible for that domain, except the parts delegated to different authoritative name servers. Name resolution is the process of following these delegations of responsibility until reaching the name server that has the answer, in other words, the authoritative server for that zone. Root name servers are a critical part of the Internet infrastructure because they represent the first step in name resolution; thus every name server in the world needs to know about them in order to walk down the tree to the end host it is looking for.
The list of root name servers, including their names and addresses, resides on every DNS server in a file known as the root hints file. This allows a company like “Example,” shown in Figure 1-2, to register the domain name “example.com” and manage just the subdomain names within that domain. The rest of the world doesn’t need to know where Example’s name servers are. When an Internet user wants to visit example.com, the user’s device can ask the root name servers, which will send it to .com name servers, which will, in turn, send it to the example.com name servers. The example.com name servers have answers for any subdomain names within example.com.
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