Linux (Ubuntu and Fedora)

These instructions use systemd-resolved.

1. Open a Terminal.

2. Make sure that systemd-resolved is enabled by running this command:
   sudo systemctl enable systemd-resolved

3. Open the Settings app and go to Network. Click on the settings icon for your connected network. On the IPv4 and IPv6 tabs, turn off Automatic using the radio button next to DNS, and leave the DNS field blank, then click on Apply.  Disable and enable the network using the on/off button to make sure it takes effect.

4. Edit the following file with nano or your favorite text editor:

Fedora: sudo nano /usr/lib/systemd/resolved.conf
Ubuntu: sudo nano /etc/systemd/resolved.conf

Add the following lines in the bottom under [Resolve]. Remove one # from the beginning of a line with the DNS option that you want to use:
#DNS=194.242.2.2#dns.mullvad.net
#DNS=194.242.2.3#adblock.dns.mullvad.net
#DNS=194.242.2.4#base.dns.mullvad.net
#DNS=194.242.2.5#extended.dns.mullvad.net
#DNS=194.242.2.6#family.dns.mullvad.net
#DNS=194.242.2.9#all.dns.mullvad.net
DNSSEC=no
DNSOverTLS=yes
Domains=~.

5. Save the file by pressing Ctrl + O and then Enter, and then Ctrl +X on your keyboard.

6. Create a symbolic link to the file using the following command in the Terminal:
   sudo ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf

7. Restart systemd-resolved by running this command:
   sudo systemctl restart systemd-resolved

8. Restart NetworkManager with this command:
   sudo systemctl restart NetworkManager

9. Verify the DNS settings with:
   resolvectl status

How do I know it’s working?

After you have followed the instructions above, go to https://mullvad.net/check. You should have no DNS leaks. Click on “No DNS leaks” for details; the server that is listed should have “dns” in its name, for example “se-mma-dns-001.mullvad.net”.

Bron: https://mullvad.net/en/help/dns-over-https-and-dns-over-tls#how-to-use