Overview

• What you’ll learn: How DNS works, the DNS hierarchy (root, TLD, authoritative), common record types (A, AAAA, CNAME, MX, NS, TXT, SOA, PTR), recursive vs iterative queries, caching, and tools for DNS troubleshooting.
• Prerequisites: Lesson 15 – Network Configuration with Netplan
• Estimated reading time: 22 minutes

Introduction

The Domain Name System (DNS) is often called the “phonebook of the Internet.” It translates human-friendly domain names like www.example.com into the IP addresses that computers use to identify each other on a network. Without DNS, you would have to memorize IP addresses for every website and service you use.

DNS is a distributed, hierarchical database that spans the entire Internet. It is one of the most critical infrastructure services — if DNS fails, virtually all Internet services become inaccessible. Understanding DNS is essential for Linux system administrators because misconfigured DNS is one of the most common causes of network connectivity issues.

In this lesson, you will learn how the DNS hierarchy is structured, what happens during a DNS query, the different record types, and how to use command-line tools to query and troubleshoot DNS on Ubuntu.

The DNS Hierarchy

DNS is organized as an inverted tree structure with the root at the top. Each level in the hierarchy is responsible for a specific portion of the domain name space.

• Root zone (.): The top of the DNS tree. There are 13 root server clusters (a.root-servers.net through m.root-servers.net) distributed globally via anycast.
• Top-Level Domains (TLDs): The first level below root — .com, .org, .net, .tw, .jp, and hundreds more. Managed by registry operators (e.g., Verisign for .com).
• Second-Level Domains: Registered by organizations — example.com, topgiga.tw. Managed by the domain owner.
• Subdomains: Further divisions created by the domain owner — www.example.com, mail.example.com.

# The FQDN (Fully Qualified Domain Name) includes a trailing dot
# representing the root zone:
# www.example.com.
#  |      |     |  |
#  |      |     |  └─ Root zone (.)
#  |      |     └──── TLD (.com)
#  |      └────────── Second-level domain (example)
#  └───────────────── Subdomain (www)

# Query the root servers
$ dig . NS +short
a.root-servers.net.
b.root-servers.net.
c.root-servers.net.
d.root-servers.net.
e.root-servers.net.

# Query the TLD servers for .com
$ dig com. NS +short
a.gtld-servers.net.
b.gtld-servers.net.

How DNS Resolution Works

When your application needs to resolve a domain name, a series of queries takes place. The process involves a stub resolver on your machine, a recursive resolver (usually your ISP or a service like 8.8.8.8), and multiple authoritative name servers.

The recursive resolver does the heavy lifting: it queries the root servers to find the TLD servers, queries the TLD servers to find the authoritative servers, and queries the authoritative servers to get the final answer. Each response may be cached according to its TTL (Time to Live) to reduce future query times.

# Trace the full resolution path for a domain
$ dig +trace www.example.com
;; Received 525 bytes from 127.0.0.53 in 0 ms

.                       518400  IN  NS  a.root-servers.net.
com.                    172800  IN  NS  a.gtld-servers.net.
example.com.            172800  IN  NS  a.iana-servers.net.
www.example.com.        86400   IN  A   93.184.216.34

# Check what DNS server your system is using
$ resolvectl status
Global
    DNS Servers: 192.168.1.1
    DNS Domain: ~.

Link 2 (ens33)
    Current Scopes: DNS
    DNS Servers: 8.8.8.8
                 8.8.4.4

DNS Record Types

DNS stores information in resource records (RRs). Each record has a name, type, class, TTL, and data. The most important record types are:

# A Record — Maps a name to an IPv4 address
$ dig example.com A +short
93.184.216.34

# AAAA Record — Maps a name to an IPv6 address
$ dig example.com AAAA +short
2606:2800:220:1:248:1893:25c8:1946

# CNAME Record — Alias; points one name to another
$ dig www.example.com CNAME +short
example.com.

# MX Record — Mail exchange servers (with priority)
$ dig example.com MX +short
10 mail.example.com.
20 mail2.example.com.

# NS Record — Authoritative name servers for a zone
$ dig example.com NS +short
a.iana-servers.net.
b.iana-servers.net.

# TXT Record — Arbitrary text (used for SPF, DKIM, verification)
$ dig example.com TXT +short
"v=spf1 -all"

# SOA Record — Start of Authority (zone metadata)
$ dig example.com SOA +short
ns.icann.org. noc.dns.icann.org. 2024010101 7200 3600 1209600 3600

# PTR Record — Reverse DNS (IP to name)
$ dig -x 93.184.216.34 +short
93.184.216.34.

DNS Configuration on Ubuntu

Ubuntu uses systemd-resolved as its local DNS stub resolver. It listens on 127.0.0.53 and forwards queries to upstream DNS servers configured via Netplan or DHCP. Understanding how systemd-resolved works is important for troubleshooting DNS issues.

# Check systemd-resolved status
$ resolvectl status
Global
    Protocols: +LLMNR +mDNS -DNSOverTLS DNSSEC=no/unsupported
    DNS Servers: 192.168.1.1

# Test DNS resolution
$ resolvectl query www.example.com
www.example.com: 93.184.216.34
                 2606:2800:220:1:248:1893:25c8:1946

# View the DNS cache statistics
$ resolvectl statistics
DNSSEC supported by current servers: no
Transactions
  Current:   0
  Total:     1234
Cache
  Current Size: 45
  Cache Hits:   890
  Cache Misses: 344

# Flush the DNS cache
$ sudo resolvectl flush-caches

# The /etc/resolv.conf file (symlinked to systemd-resolved)
$ cat /etc/resolv.conf
# This is /run/systemd/resolve/stub-resolv.conf
nameserver 127.0.0.53
options edns0 trust-ad
search localdomain

DNS Troubleshooting Tools

Linux provides several powerful tools for querying and debugging DNS issues. The most commonly used are dig, nslookup, and host.

# dig — the most detailed DNS query tool
$ dig www.example.com
;; ANSWER SECTION:
www.example.com.    86400   IN  A   93.184.216.34

;; Query time: 23 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)

# Query a specific DNS server
$ dig @8.8.8.8 www.example.com

# nslookup — interactive or non-interactive DNS queries
$ nslookup www.example.com
Server:     127.0.0.53
Address:    127.0.0.53#53

Non-authoritative answer:
Name:   www.example.com
Address: 93.184.216.34

# host — simplified DNS lookup
$ host www.example.com
www.example.com has address 93.184.216.34
www.example.com has IPv6 address 2606:2800:220:1:248:1893:25c8:1946

# Check for DNS propagation issues
$ dig +short @a.iana-servers.net example.com A
93.184.216.34
$ dig +short @b.iana-servers.net example.com A
93.184.216.34

Key Takeaways

• DNS is a distributed, hierarchical system that translates domain names to IP addresses; it is one of the most critical Internet infrastructure services.
• The DNS hierarchy consists of root servers, TLD servers, and authoritative name servers; recursive resolvers query these on behalf of clients.
• Common record types include A (IPv4), AAAA (IPv6), CNAME (alias), MX (mail), NS (name server), TXT (text), SOA (zone metadata), and PTR (reverse).
• Ubuntu uses systemd-resolved as its local DNS stub resolver, listening on 127.0.0.53 and forwarding to upstream servers.
• Use dig for detailed DNS queries, resolvectl for managing systemd-resolved, and resolvectl flush-caches to clear the DNS cache.

What’s Next

In Lesson 17, you will learn how to set up a BIND 9 DNS Server — deploying your own authoritative DNS server on Ubuntu to host zones and resolve queries for your domain.