How to Set the Hostname and FQDN on Ubuntu 24.04

The hostname identifies a server in shell prompts, logs and monitoring, and the fully qualified domain name (FQDN) is the full DNS name that other systems use to reach it, such as web01.example.com. Mail servers, Kerberos, some control panels and many clustering tools refuse to work, or misbehave, if the FQDN does not resolve correctly. In this tutorial you will set a persistent hostname and FQDN on Ubuntu 24.04, make sure cloud-init does not overwrite them, and verify forward and reverse DNS.

Prerequisites

To follow this tutorial, you will need:

A server running Ubuntu 24.04 LTS, for example a CubePath VPS. Differences for Rocky Linux 9 are noted where they matter.
A non-root user with sudo privileges.
A domain you control, if you want the FQDN to resolve publicly. This tutorial uses web01 as the hostname and example.com as the domain. Replace them with your own, and replace your_server_ip with the server's public IP address.

Step 1 - Checking the current configuration

hostnamectl shows the hostname that systemd manages:

hostnamectl

 Static hostname: ubuntu-2404
       Icon name: computer-vm
         Chassis: vm
      Machine ID: 3f1c...
         Boot ID: 9a7e...
  Virtualization: kvm
Operating System: Ubuntu 24.04.3 LTS
          Kernel: Linux 6.8.0-84-generic
    Architecture: x86-64

The short hostname and the FQDN are reported by the hostname command:

hostname
hostname -f

ubuntu-2404
ubuntu-2404

hostname -f does not read a setting of its own. It takes the short hostname and resolves it through /etc/hosts and DNS. If it prints only the short name, as above, no FQDN is configured yet.

Some terminology used in the rest of the guide:

Term	Example	Where it is stored
Static hostname	`web01`	`/etc/hostname`
Pretty hostname	`Web server 01`	`/etc/machine-info` (free text, for display only)
FQDN	`web01.example.com`	Resolved from `/etc/hosts` or DNS

Step 2 - Choosing a valid name

A hostname label may contain letters, digits and hyphens, must not start or end with a hyphen, and can be at most 63 characters. The whole FQDN can be up to 253 characters. Use lowercase, and avoid underscores, since they are not valid in hostnames.

Pick a name that describes the role and is unique within your infrastructure, for example web01, db-primary or mail. The FQDN is that name plus a domain you control: web01.example.com. Avoid using the bare domain (example.com) as a server FQDN, since it usually points to your website.

Step 3 - Setting the static hostname

Set the static hostname with hostnamectl. On Ubuntu and Debian the convention is to store the short name here and let /etc/hosts provide the FQDN:

sudo hostnamectl set-hostname web01

This updates /etc/hostname and the running kernel hostname immediately. Verify:

cat /etc/hostname
hostnamectl --static

web01
web01

Optionally set a descriptive pretty hostname, which some tools display:

sudo hostnamectl set-hostname "Web server 01" --pretty

The shell prompt keeps showing the old name until you open a new session. Log out and back in, or start a new shell with exec bash.

Note

On Rocky Linux and other RHEL-based systems the convention is to store the FQDN as the static hostname: sudo hostnamectl set-hostname web01.example.com. Either way, hostname -f must return the FQDN once /etc/hosts is configured in the next step.

Step 4 - Mapping the FQDN in /etc/hosts

The resolver checks /etc/hosts before DNS, so an entry here makes the FQDN resolve even when DNS is not reachable. Open the file:

sudo nano /etc/hosts

Find the line that references the old hostname (on Ubuntu it is usually 127.0.1.1 old-hostname) and replace it. The first name after the address is the canonical name, so the FQDN must come before the short name:

127.0.0.1 localhost
your_server_ip web01.example.com web01

# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

Using the public IP address instead of 127.0.1.1 means services that look up their own hostname (a mail server announcing itself, for example) get an address other hosts can reach. If the IP might change, 127.0.1.1 web01.example.com web01 also works for hostname -f.

Save the file and check the result:

hostname
hostname -f
hostname -d

web01
web01.example.com
example.com

Confirm how the name resolves through the system resolver:

getent hosts web01.example.com

203.0.113.10    web01.example.com web01

Step 5 - Keeping cloud-init from resetting the hostname

Most cloud images, including Ubuntu 24.04 VPS images, run cloud-init at boot. Depending on the provider's configuration, cloud-init can set the hostname and rewrite /etc/hosts from its metadata on every boot, undoing your changes. Check whether cloud-init is present:

cloud-init status

status: done

If the command exists, tell cloud-init to leave the hostname alone. Create a drop-in configuration file:

sudo nano /etc/cloud/cloud.cfg.d/99-preserve-hostname.cfg

Add the following:

preserve_hostname: true
manage_etc_hosts: false

preserve_hostname: true stops cloud-init from changing the hostname, and manage_etc_hosts: false stops it from regenerating /etc/hosts. If /etc/hosts starts with a comment saying it is managed by cloud-init and edits will be lost, this second setting is what makes your edit stick.

Reboot to confirm the configuration persists:

sudo reboot

After reconnecting, run hostname -f again. It should still print web01.example.com.

Step 6 - Creating DNS records

/etc/hosts only affects the server itself. For other systems to resolve the FQDN, create an A record (and an AAAA record if the server has IPv6) in the DNS zone of your domain:

Type	Name	Value
A	`web01`	`your_server_ip`
AAAA	`web01`	`your_server_ipv6`

After the record propagates, check it from the server or your workstation. On Ubuntu, dig is part of the bind9-dnsutils package:

sudo apt install bind9-dnsutils
dig +short web01.example.com A

203.0.113.10

Reverse DNS

Reverse DNS (a PTR record) maps the IP address back to a name. It matters mostly for mail: many receiving servers reject or spam-flag mail from IPs whose PTR does not match the sending hostname. The PTR record is managed by whoever controls the IP address block, which for a VPS is the hosting provider, not your domain's DNS zone. Check the current value:

dig +short -x your_server_ip

web01.example.com.

If it returns a generic provider name, set the PTR to your FQDN through your provider. The forward (A) record for that name must point back to the same IP.

Step 7 - Updating services that cache the hostname

Most services read the hostname when they start. After renaming, restart anything that embeds it, or reboot. Common cases:

Postfix: myhostname in /etc/postfix/main.cf should match the FQDN. Check it with postconf myhostname, set it with sudo postconf -e 'myhostname = web01.example.com', then sudo systemctl restart postfix.
rsyslog and journald: new log lines use the new name after a restart or reboot. Older lines keep the old one.
Monitoring agents: many register the host by hostname, so the server may appear as a new host in your dashboard.

Troubleshooting

sudo: unable to resolve host web01: Name or service not known: the new hostname is not in /etc/hosts. Add it as shown in Step 4.
hostname -f prints only the short name: the FQDN is missing from /etc/hosts, or the short name is listed before it on the line. The FQDN must be the first name after the IP.
The hostname reverts after reboot: cloud-init is setting it. Add the drop-in from Step 5.
hostnamectl fails with an invalid hostname error: the name contains characters such as underscores or spaces. Use only letters, digits, hyphens and dots.

Conclusion

Your server now has a persistent hostname, an FQDN that resolves locally and in DNS, and a matching reverse DNS record. As next steps, you can configure a mail transfer agent that relies on the FQDN, issue a TLS certificate for web01.example.com, and set the server's timezone and locale for consistent logs.

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