Chapter 25 Domain Name System (DNS)
Chapter 25 of Data Communications and Networking by Behrouz A. Forouzan focuses on the Domain Name System (DNS), a fundamental internet service that translates domain names to IP addresses. Here are the detailed notes:
25.1 Name Space
To uniquely identify each host on the internet, DNS uses a hierarchical name space. There are two types of name spaces:
Flat Name Space: Names are assigned to addresses without structure, making them unsuitable for large networks like the internet.
Hierarchical Name Space: Names are structured and divided into several parts. This model enables decentralized management.
25.2 Domain Name Space
DNS organizes its name space into a hierarchical, inverted tree structure where each node has a label. The root node is at the top, and below it are domain names organized by levels. These include:
Fully Qualified Domain Names (FQDN): Domain names that include all labels, from the specific host to the root, ending in a dot (e.g.,
www.example.com.).Partially Qualified Domain Names (PQDN): Domain names that are not fully specified, usually relying on local resolvers to complete the name.
25.3 Distribution of Name Space
Because DNS data is massive, it's distributed across different servers to improve efficiency and reliability. DNS uses zones to divide the tree into smaller parts. Each zone is managed by a DNS server, which may have authority over multiple zones.
25.4 DNS in the Internet
The internet's DNS is divided into three sections:
Generic Domains: These include common top-level domains (TLDs) like
.com,.edu, and.org.Country Domains: These use two-letter country codes like
.us(United States) or.jp(Japan).Inverse Domain: Used for reverse DNS lookups, which map IP addresses back to domain names.
25.5 Resolution
Resolution is the process of mapping a name to an address or vice versa. It involves:
Resolvers: DNS clients that send queries to DNS servers.
Recursive Resolution: The DNS server processes the entire request and provides a complete answer.
Iterative Resolution: The server provides the address of another server that might have the answer, requiring the client to query multiple servers.
25.6 DNS Messages
DNS messages come in two types:
Query: Sent by the client to request information.
Response: Sent by the server, containing the requested information.
Both message types include a header and sections for questions, answers, authority records, and additional information.
25.7 Types of Records
DNS uses resource records to store name-to-address mappings and other data:
A Record: Maps a domain name to an IPv4 address.
AAAA Record: Maps a domain name to an IPv6 address.
PTR Record: Used in reverse DNS lookups.
MX Record: Identifies mail servers for a domain.
25.8 Registrars
Registrars are accredited entities that manage domain name registrations. They ensure that each domain name is unique and update the DNS database accordingly.
25.9 Dynamic Domain Name System (DDNS)
To cope with the rapid growth of the internet, DDNS allows DNS records to be updated dynamically. When changes occur, such as adding new hosts or changing IP addresses, DDNS updates the DNS records in real time without manual intervention. It often works in conjunction with DHCP.
25.10 Encapsulation
DNS can use both UDP and TCP protocols. UDP is used for small queries and responses (less than 512 bytes), while TCP is used for larger messages or zone transfers between servers.
25.13 Summary
DNS is a client/server application that provides name-to-IP address mapping.
It uses a hierarchical structure to organize domain names.
DNS resolution can be recursive or iterative, and messages are exchanged using well-defined formats.
DNS uses resource records to store various types of information, and DDNS supports dynamic updates to DNS databases.
This chapter highlights the critical role of DNS in internet functionality, ensuring that user-friendly domain names are translated into IP addresses that computers can use for communication .
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