DNS Fundamentals and Name Resolution Mechanisms

What is a DNS Service?

A DNS service is a distributed database that stores hierarchical information associated with domain names on a network.

Primary Uses of DNS

• Name Resolution: Given the full name of a host, the service retrieves its IP address.
• Reverse Address Resolution: This is the inverse of the previous mechanism. Given an IP address, it retrieves the name associated with it.
• Mail Server Resolution: Given a domain name, it identifies the server through which email delivery should be handled.

Core Components of the DNS Service

The main components include:

• The DNS Client: A program that runs on the user's computer and generates DNS resolution requests for names to be sent to a DNS server.
• DNS Servers: These answer customer requests. Recursive servers are able to forward the request to another server if they do not have the requested address.
• Areas of Authority: These are portions of the domain name space that store data. Each area of authority includes at least one domain and potentially its subdomains.

Essential DNS Terminology

• Host Name: The name of a host, typically consisting of a single word.
• FQHN (Fully Qualified Host Name): The "full name" of a host. It consists of the hostname followed by a period and the corresponding domain name.
• Domain Name: A sequence of names concatenated by periods.
• TLD (Top-Level Domains): These are domains that do not belong to another domain.

Understanding Internet Domains

An Internet domain is a network identification associated with a group of appliances and equipment connected to the internet.

DNS Protocols and Ports

The DNS system works mostly with the UDP protocol (port 53) but also utilizes TCP (also on port 53).

The DNS Name Resolution Process

The resolution of names is performed transparently by client applications. When you make a request that requires a DNS lookup, the request is sent to the local operating system's DNS resolver. Before establishing any external connection, the operating system checks if the answer is found in the cache. In the event that it is not, the request is then sent to one or more DNS servers.