What Is DNS? A Complete Guide to the Domain Name System
This article explains the fundamentals of DNS, covering its purpose, hierarchical structure, key concepts like FQDN and resource records, query mechanisms, server types, ports, and the step‑by‑step process browsers use to resolve domain names into IP addresses.
1. What is DNS
DNS stands for Domain Name System. It translates human‑readable domain names into IP addresses, enabling computers to locate resources on the Internet.
Early networks used the hosts file (/etc/hosts) for static mappings, but this required manual updates. As the number of hosts grew, Berkeley developed BIND, the modern DNS system.
2. Key Concepts
Domain levels : TLD (e.g., .com, .org), second‑level domains, and subdomains. The hierarchy is managed by IANA at the root.
FQDN : Fully Qualified Domain Name, consisting of the host name and domain name (e.g., www.google.com).
Forward lookup : Converting an FQDN to an IP address.
Reverse lookup : Converting an IP address to an FQDN.
Zone : A collection of records for a domain, used in forward or reverse lookups.
3. DNS Resource Records
Common record types include:
SOA : Start of Authority – one per zone.
NS : Name Server – can be multiple.
MX : Mail Exchange – includes priority (0‑99).
A : Maps an FQDN to an IPv4 address (forward lookup).
PTR : Maps an IP address to an FQDN (reverse lookup).
AAAA : Maps an FQDN to an IPv6 address.
CNAME : Canonical name – an alias for another name.
Zone transfer methods: AXFR (full transfer) and IXFR (incremental transfer).
4. DNS Query Process
DNS queries can be recursive or iterative. In a recursive query, the client sends a single request and receives the final answer. In an iterative query, the DNS server contacts other servers step‑by‑step until it finds the answer.
Servers cache responses locally to answer future queries quickly. Authoritative servers provide definitive answers; non‑authoritative responses come from cache and may be outdated.
5. Query Order
Local hosts file
Local DNS cache
Configured DNS server
Iterative queries to higher‑level servers
6. Ports Used by DNS
DNS uses UDP/TCP port 53. Clients typically use UDP; zone transfers between servers use TCP.
7. DNS Server Types
Primary (master) server : Holds the authoritative zone data.
Secondary (slave) server : Receives zone data from the master.
Cache server : Provides non‑authoritative answers from its cache.
Forwarder : Forwards queries it cannot answer to other DNS servers.
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