Introduction

• The Domain Name System (DNS) is responsible for translating hostnames into IP addresses.
• It operates as:
  1. A distributed database implemented across a hierarchy of DNS servers.
  2. An application-layer protocol enabling hosts to query this database.

Distributed, Hierarchical Database

• DNS is distributed globally across multiple servers to handle scalability.
• These servers are structured in a hierarchical fashion, ensuring efficient resolution of domain names.

Types of DNS Servers

DNS servers are categorized into three hierarchical classes:

1. Root DNS Servers

   • The top-level servers that direct queries to appropriate TLD servers.
   • There are 13 root server clusters worldwide.

2. Top-Level Domain (TLD) DNS Servers

   • Responsible for handling domain extensions like .com, .org, .net, etc.
   • These servers forward queries to authoritative DNS servers.

3. Authoritative DNS Servers

   • Maintain records for specific domains (e.g., example.com).
   • Provide final IP address resolutions for queries.

Recursive vs. Iterative DNS Queries

• Recursive DNS:

  • The server takes full responsibility for resolving the query.
  • It queries other DNS servers on behalf of the client until an answer is found.

• Iterative DNS:

  • The server provides the best available answer but may refer the client to other DNS servers.
  • The client must follow up on the next query.

DNS Resource Records (RRs)

• DNS servers store Resource Records (RRs), which provide hostname-to-IP mappings.
• A resource record consists of four fields:
  • Name – The domain name (e.g., www.example.com).
  • Value – The corresponding IP address or another DNS name depending on the type.
  • Type – The type of DNS record.
  • TTL (Time-To-Live) – The time the record remains valid before refresh.

Common DNS Record Types

1. A (Address Record): Maps the hostname to it's IP address.
2. NS (Name Server Record): Specifies an authoritative name server for the domain.
3. CNAME (Canonical Name Record): Maps an alias name to a canonical domain name.

Reference Books

1. Kurose, J. F., & Ross, K. W. Computer Networking: A Top-Down Approach. Pearson.
2. Tanenbaum, A. S., & Wetherall, D. J. Computer Networks. Pearson.