How to correctly set hostname and domain name?

Setting your hostname:

• You'll want to edit /etc/hostname with your new hostname.

• Then, run sudo hostname $(cat /etc/hostname).

Setting your domain:

• Then, in /etc/resolvconf/resolv.conf.d/head, you'll add then line domain your.domain.name (not your FQDN, just the domainname).

• Then, run sudo resolvconf -u to update your /etc/resolv.conf (alternatively, just reproduce the previous change into your /etc/resolv.conf).

Both:

Finally, update your /etc/hosts file. There should be at least one line starting with one of your IP (loopback or not), your FQDN and your hostname. grepping out ipv6 addresses, your hosts file could look like this:

127.0.0.1 localhost
1.2.3.4 service.domain.com service

sudo nano /etc/hostname

hostname.domain.com

sudo nano /etc/hosts

127.0.0.1   hostname.domain.com hostname localhost

REBOOT!

MUST HAVE SINGLE HOSTNAME after FQDN in /etc/hosts file. Works fine on Ubuntu 18.04.1 and all other versions. On EC2 and elsewhere.

Didn't mess with resolve file or anything else.

That shows hostname in shell and then has the FQDN when you need it.

Instructions written against Ubuntu 18.04.3 LTS (bionic)

Change the hostname:

sudo hostnamectl set-hostname server1

Check result by running hostnamectl:

root@www:/# hostnamectl
   Static hostname: server1       <-- Check this value
         Icon name: computer-vm
           Chassis: vm
        Machine ID: 202c4264b06d49e48cfe72599781a798
           Boot ID: 43654fe8bdbf4387a0013ab30a155872
    Virtualization: xen
  Operating System: Ubuntu 18.04.3 LTS
            Kernel: Linux 4.15.0-65-generic
      Architecture: x86-64

Change the domain via new network manager, Netplan, by editing /etc/netplan/01-netcfg.yaml and changing the search parameter:

sudoedit /etc/netplan/01-netcfg.yaml

Sample configuration:

# This file describes the network interfaces available on your system
# For more information, see netplan(5).
network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      dhcp4: yes
      nameservers:
          search: [ domain.org ]

Test changes by logging in a second time, and running sudo netplan try in one of the sessions and checking settings in the other:

# netplan try
Do you want to keep these settings?


Press ENTER before the timeout to accept the new configuration


Changes will revert in  97 seconds
Configuration accepted.

# systemd-resolve --status
...
Link 2 (eth0)
      Current Scopes: DNS
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
         DNS Servers: 8.8.8.8
                      8.8.4.4
          DNS Domain: domain.org      <-- Check this value

# cat /etc/resolv.conf
...
nameserver 127.0.0.53
options edns0
search domain.org    <-- Check this value

# hostname -f
server1.domain.org

It all is well, press ENTER at the sudo netplan try prompt to make things permanent.