Domain Name System (DNS)

The Domain Name System (DNS) is one of the most important client-server applications in the Internet. It acts as the telephone directory of the Internet by translating human-readable domain names into IP addresses that computers use for communication.

Definition

DNS (Domain Name System) is a distributed hierarchical naming system that maps domain names (such as www.google.com) to IP addresses (such as 142.250.190.78).

Without DNS, users would have to remember numerical IP addresses instead of easy-to-remember names.

Need for DNS

TCP/IP protocols communicate using IP addresses, but users prefer domain names.

For example,

User remembers: www.google.com
Computer needs: 142.250.x.x

DNS performs this conversion automatically.

Analogy

Just like a telephone directory maps a person's name to a telephone number, DNS maps a domain name to an IP address.

Purpose of DNS

DNS provides:

Name-to-IP address mapping
IP-to-name mapping (reverse lookup)
Distributed database management
Faster communication through caching
Scalable naming system for the Internet

Working of DNS

Suppose a user wants to access


www.example.com

The following steps occur:

User enters the domain name.
The application sends the domain name to the DNS client (resolver).
The DNS client contacts the nearest DNS server.
The DNS server searches for the corresponding IP address.
The DNS server returns the IP address.
The application uses the IP address to communicate with the destination server.


User
   │
Enter Domain Name
   │
Application
   │
DNS Resolver (Client)
   │
DNS Server
   │
Returns IP Address
   │
Application connects to destination

Thus, before connecting to a web server, DNS first translates the host name into an IP address.

Name Space in DNS

A name space is a collection of all valid names.

DNS supports two types of name spaces.

1. Flat Name Space

Every host has a unique name.
No hierarchical structure.
Requires central management.
Suitable only for small networks.
Not suitable for the Internet.

Example:


computer1
computer2
computer3

Limitations

Difficult to manage
Name duplication possible
Not scalable

2. Hierarchical Name Space

DNS uses a hierarchical name space.

Each name consists of several parts separated by dots.

Example


www.mec.ac.in

where

in → Country
ac → Academic institution
mec → Organization
www → Host

Advantages

Easy management
No duplicate names
Distributed administration
Highly scalable

Domain Name Space

The DNS name space is organized as an inverted tree.


                    Root (.)
                     |
        ----------------------------
       |            |              |
      com          edu            org
       |
   google
       |
      www

Characteristics

Root is at the top.
Maximum depth is 128 levels.
Every node has a label.
Labels are separated by dots.

Label

A label is one component of a domain name.

Example


www.google.com

Labels are

www
google
com

Characteristics

Maximum length = 63 characters
Children of the same node must have different labels.

Domain Name

A domain name is a sequence of labels separated by dots.

Example


www.google.com.

A fully qualified domain name always ends with a dot representing the root.

Fully Qualified Domain Name (FQDN)

An FQDN contains the complete path from the host to the root.

Example


www.google.com.

Features

Complete domain name
Ends with "."
Globally unique

Partially Qualified Domain Name (PQDN)

A PQDN does not include the complete path.

Example


www.google

The missing suffix is supplied automatically by the resolver.

Domain

A domain is a subtree of the DNS hierarchy.

Example

com

contains


google.com
amazon.com
microsoft.com

A domain can contain many subdomains.

Distribution of Name Space

The Internet is too large for one DNS server.

Therefore, the DNS database is distributed among many DNS servers.

Advantages

Faster response
Reliability
Scalability
Load sharing

Hierarchy of DNS Servers


                  Root Server
                      |
          -------------------------
          |                       |
      .com Server             .edu Server
          |
    google.com Server
          |
     Local DNS Server

Each server is responsible for a specific portion of the DNS tree.

Zone

A zone is the portion of the DNS database managed by one DNS server.

If a server manages an entire domain,


Zone = Domain

If authority is delegated,


Domain ≠ Zone

The server stores all zone information in a Zone File.

Types of DNS Servers

1. Root Server

Highest-level DNS server.
Knows addresses of top-level domain servers.
Does not usually store host IP addresses.
Distributed worldwide.

2. Primary DNS Server

Stores the original zone database.
Creates and updates zone files.
Authoritative server.

3. Secondary DNS Server

Keeps a copy of the primary server database.
Obtains updates from the primary server.
Provides redundancy.
Also authoritative.

Generic Domains (Top-Level Domains)

These classify organizations.

Domain	Purpose
.com	Commercial organizations
.edu	Educational institutions
.gov	Government
.org	Non-profit organizations
.net	Network organizations
.mil	Military
.biz	Business
.info	Information services
.name	Personal names
.museum	Museums
.coop	Cooperatives
.pro	Professionals
.aero	Aerospace
.int	International organizations

Country Domains (ccTLD)

Represent countries.

Examples

Domain	Country
.in	India
.us	United States
.uk	United Kingdom
.jp	Japan
.au	Australia
.fr	France

Example


mec.ac.in

in → India
ac → Academic
mec → Institution

Name Resolution

Name Resolution is the process of converting a domain name into an IP address.

DNS supports two methods.

1. Recursive Resolution

The local DNS server performs all the work.


Client
   │
Local DNS
   │
Root DNS
   │
TLD DNS
   │
Authoritative DNS
   │
Returns IP

Characteristics

Client sends only one request.
DNS servers communicate with each other.
Easier for clients.

2. Iterative Resolution

Each DNS server returns the address of the next DNS server.


Client
   │
Root Server
   │
Returns TLD Server
   │
TLD Server
   │
Returns Authoritative Server
   │
Authoritative Server
   │
Returns IP Address

Characteristics

Client or local DNS repeatedly queries servers.
Less workload on DNS servers.

DNS Caching

To improve performance, DNS stores recently resolved addresses.

Advantages

Faster response
Less network traffic
Reduced server load

Cached entries are temporary.

Time To Live (TTL)

Each DNS record contains a TTL (Time To Live).

TTL specifies how long a cached entry remains valid.

After TTL expires,

Cache entry is removed.
Fresh lookup is performed.

Resource Records (RR)

DNS stores information as Resource Records.

General format


(Domain Name, Type, Class, TTL, Value)

Common Resource Record Types

Type	Purpose
A	Maps hostname to IPv4 address
AAAA	Maps hostname to IPv6 address
NS	Authoritative name server
MX	Mail server
CNAME	Alias (Canonical Name)
SOA	Start of Authority

Example


google.com    A      142.250.190.78

DNS Message Format

DNS uses two messages:

Query
Response

DNS Query Message

Contains

Header
Question Section

DNS Response Message

Contains

Header
Question Section
Answer Section
Authoritative Section
Additional Information Section


+------------------+
| Header           |
+------------------+
| Question         |
+------------------+
| Answer           |
+------------------+
| Authority        |
+------------------+
| Additional Info  |
+------------------+

DNS Transport Protocol

DNS uses Port 53.

It can use both UDP and TCP.

UDP

Used when

Response size ≤ 512 bytes
Faster
Default method

TCP

Used when

Response size > 512 bytes
Zone transfers
Large DNS responses

Domain Registration

New domains are registered through Registrars accredited by ICANN.

During registration, the organization provides:

Domain name
IP address
DNS server details

Example


Domain : wonderful.com
Host    : ws.wonderful.com
IP      : 200.200.200.5

Dynamic DNS (DDNS)

Traditional DNS required manual updates.

DDNS automatically updates DNS records whenever:

IP address changes
New host is added
Host is removed

Usually works with DHCP.

Benefits

Automatic updates
Less manual administration
Suitable for dynamic IP addresses

DNS Security (DNSSEC)

DNS is vulnerable to attacks such as:

Fake DNS responses
DNS spoofing
Cache poisoning
DNS hijacking
Denial-of-Service (DoS)

To overcome these, DNSSEC provides:

Message authentication
Data integrity
Digital signatures

However, DNSSEC does not provide confidentiality (encryption) of DNS messages.

Advantages of DNS

Easy-to-remember names
Distributed architecture
Highly scalable
Fault tolerant
Faster through caching
Supports load distribution
Easy administration

Limitations of DNS

Initial lookup adds slight delay.
Cached records may become outdated until TTL expires.
Vulnerable to spoofing and cache-poisoning attacks without DNSSEC.
Large DNS responses may require TCP instead of UDP.

Summary

DNS (Domain Name System) is a distributed, hierarchical naming system that maps domain names to IP addresses.
It uses a client-server architecture, where a DNS resolver queries DNS servers to obtain the IP address of a requested host.
DNS organizes names in a hierarchical domain name space, supports generic and country domains, and distributes information among root, primary, and secondary DNS servers.
Name resolution can be recursive or iterative, and caching with TTL improves lookup performance.
DNS stores information using resource records (A, AAAA, MX, NS, CNAME, SOA, etc.), typically uses UDP port 53 for normal queries and TCP port 53 for large responses and zone transfers.
Dynamic DNS (DDNS) automates updates to DNS records, while DNSSEC enhances security by providing authentication and integrity for DNS data