DNS and hostnames

Table of Contents

Understanding Hostnames on Linux

On a Linux system, the hostname is the human‑readable name that identifies a machine on a network. It’s used by DNS, monitoring systems, SSH, prompts, and logs.

There are three closely related concepts:

Short hostname: e.g. web01
Domain name: e.g. example.com
Fully Qualified Domain Name (FQDN): e.g. web01.example.com

On a typical Linux system:

hostname (without options) usually shows the short hostname
hostname -f or hostnamectl --static may show the FQDN, depending on configuration

Viewing the Current Hostname

Key commands:

Show current hostname:

  hostname

Show FQDN (if configured and resolvable):

  hostname -f

Show details with systemd:

  hostnamectl

If hostname -f fails or returns something unexpected, it often means the system’s hostname or name resolution configuration is incomplete.

Setting the Hostname (systemd-based systems)

Most modern distributions use systemd, so the recommended tool is hostnamectl.

Set a static hostname (survives reboot):

  sudo hostnamectl set-hostname web01.example.com

This sets the static hostname to web01.example.com. The short hostname will be web01.

Optionally set “pretty” and “transient” hostnames:

  sudo hostnamectl set-hostname "Web Server 01" --pretty
  sudo hostnamectl set-hostname temp-host --transient

static: persistent, used by most tools
pretty: purely cosmetic, may include spaces
transient: lost on reboot (e.g. provided by DHCP)

On older non‑systemd systems, hostnames may be set in files such as /etc/hostname or /etc/sysconfig/network, but the exact details are distribution‑specific and usually covered in distro documentation.

Hostnames and `/etc/hostname`

On many distributions, the static hostname is stored in /etc/hostname as a single line:

web01.example.com

To change it manually:

Edit the file:

   sudo nano /etc/hostname

Put the desired hostname (short or FQDN) on a single line.
Apply without reboot:

   sudo hostname "$(cat /etc/hostname)"

or simply reboot.

Whenever possible, prefer hostnamectl instead of editing files directly, as it updates all relevant locations.

Name Resolution on Linux

When a program needs to translate a hostname (like web01.example.com) to an IP address, Linux uses the name service switch configuration in /etc/nsswitch.conf. A typical line:

hosts: files dns

This means:

Look in local files (e.g. /etc/hosts)
If not found, query DNS

Other sources (like mdns, ldap, wins) can be added, but in many setups files and dns are enough.

The `/etc/hosts` File

/etc/hosts provides a simple, static way to map hostnames to IP addresses without using DNS.

Example content:

127.0.0.1   localhost
127.0.1.1   web01 web01.example.com
192.0.2.10  db01 db01.example.com
203.0.113.5 backup backup.example.com

Key points:

Multiple names can map to the same IP on one line.
Common pattern on desktops/servers:

127.0.0.1 localhost
A second line mapping the system’s hostname/FQDN to 127.0.1.1 (Debian/Ubuntu style).

/etc/hosts is checked before DNS when hosts: files dns is configured, so entries here override DNS responses.

Use /etc/hosts for:

Small static networks
Temporary overrides (testing new DNS mappings, migration, etc.)
Ensuring local services can resolve the machine’s own hostname even without network

DNS Configuration Files

For DNS lookups, most systems use a resolver library which reads configuration from:

/etc/resolv.conf (traditional)
Or a local stub resolver (like systemd-resolved) that is referenced by /etc/resolv.conf

`/etc/resolv.conf` Basics

A minimal resolv.conf might look like:

nameserver 1.1.1.1
nameserver 9.9.9.9
search example.com internal.example.com

Directives:

nameserver IP_ADDRESS

DNS servers the system should query
Up to 3 listed, order matters

search domain1 domain2 ...

Domains appended when you query a short hostname
If you run ping web01, the resolver tries:

web01.example.com
web01.internal.example.com

Stops when it finds a match

Many systems do not want you to edit /etc/resolv.conf directly, because it’s managed by:

NetworkManager
systemd-resolved
dhclient or other DHCP clients

Edits may be overwritten when the network restarts. The correct way to set DNS servers often depends on your network manager (GUI, nmcli, netplan, or distro-specific tools).

systemd-resolved and Stub Resolvers

On distributions using systemd-resolved, /etc/resolv.conf may be a symlink to something like /run/systemd/resolve/stub-resolv.conf. In that case, configure DNS via:

systemd-resolve --status
sudo resolvectl status          # newer syntax

And change settings via NetworkManager, netplan, or resolved configuration, instead of directly editing resolv.conf.

How DNS Names Map to IPs

DNS provides a hierarchical, distributed database mapping names to IP addresses and other information.

Common record types:

A — IPv4 address
AAAA — IPv6 address
CNAME — canonical name (alias)
MX — mail exchange
NS — name server
PTR — reverse lookup (IP → hostname)

On a client system, you mainly care about:

Resolving hostnames to IPs (A, AAAA)
Occasionally seeing CNAME chains
Understanding reverse DNS (PTR) for tools like ssh or logging

Forward vs Reverse DNS

Forward lookup: hostname → IP
web01.example.com → 203.0.113.10 (A record)
Reverse lookup: IP → hostname
203.0.113.10 → web01.example.com (PTR record)

Reverse DNS is useful for:

Logging (show hostnames instead of raw IPs)
Some security checks and access controls
Mail servers (many reject mail from IPs without proper reverse DNS)

Note that forward and reverse DNS are separate; having one doesn’t guarantee the other is configured.