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DNS (Domain Name System)

The Domain Name System (DNS) is often described as the "phonebook of the Internet." Humans access information online through domain names, like google.com or linux.org. Web browsers interact through Internet Protocol (IP) addresses. DNS translates domain names to IP addresses so browsers can load internet resources.

How DNS Works​

When you type www.example.com into your browser, the following lookup process occurs:

  1. Local Cache: Your computer checks its local DNS cache to see if it already knows the IP.
  2. Recursive Resolver: If not found, it asks your ISP's DNS server (or a public one like Google's 8.8.8.8 or Cloudflare's 1.1.1.1).
  3. Root Nameserver: If the resolver doesn't know, it asks the Root Server, which directs it to the Top-Level Domain (TLD) server (like the .com server).
  4. TLD Nameserver: The TLD server points the resolver to the Authoritative Nameserver for example.com.
  5. Authoritative Nameserver: This server holds the actual IP address record and returns it to the resolver, which passes it to your computer.

Common DNS Record Types​

  • A Record: Maps a domain name to an IPv4 address.
  • AAAA Record: Maps a domain name to an IPv6 address.
  • CNAME: Maps one domain name to another (an alias). E.g., www.example.com might point to example.com.
  • MX Record: Directs email to a mail server.
  • TXT Record: Holds text information, often used for verifying domain ownership and email security (SPF, DKIM).

DNS in Linux​

In Linux, the file /etc/resolv.conf defines which DNS resolvers your system uses.

# Example /etc/resolv.conf
nameserver 8.8.8.8
nameserver 1.1.1.1

(Note: On modern systems like Ubuntu, this file is managed by systemd-resolved or NetworkManager, so manual edits might be overwritten).

Troubleshooting DNS​

Linux provides several command-line tools for testing and querying DNS:

  • ping example.com: The simplest way to see if a name resolves.
  • dig example.com: The standard tool for querying DNS. It returns detailed information about records and lookup times.
  • nslookup example.com: An older, but still common, tool for querying DNS.
# Get the MX records for a domain using dig
dig example.com MX

Local Resolution via /etc/hosts​

Before querying DNS servers, Linux checks the local /etc/hosts file. You can manually map IPs to hostnames here, which is very useful for local development without needing a real DNS server.

# /etc/hosts
127.0.0.1   localhost
192.168.1.50 dev-server.local