How to use a custom `resolv.conf` file with systemd-resolved in Ubuntu

README.md

How to use a custom resolv.conf file with systemd-resolved in Ubuntu

Stages

• Disable and stop the systemd-resolved service:

  sudo systemctl stop systemd-resolved
  

• Then put the following line in the [main] section of your /etc/NetworkManager/NetworkManager.conf:

     dns=default
     rc-manager=symlink
  

  In this way we are using dns default in conjunction with rc-managed symlink:

  dns=default: NetworkManager will update /etc/resolv.conf to reflect the nameservers provided by currently active connections. Default is used, unless /etc/resolv.conf is a symlink to /run/systemd/resolve/stub-resolv.conf, /run/systemd/resolve/resolv.conf, /lib/systemd/resolv.conf or /usr/lib/systemd/resolv.conf. In that case, systemd-resolved is chosen automatically.

  rc-manager=symlink: If /etc/resolv.conf is a regular file, NetworkManager will replace the file on update. If /etc/resolv.conf is instead a symlink, NetworkManager will leave it alone. Unless the symlink points to the internal file /run/NetworkManager/resolv.conf, in which case the symlink will be updated to emit an inotify notification. This allows the user to conveniently instruct NetworkManager not to manage /etc/resolv.conf by replacing it with a symlink.

• Delete /etc/resolv.conf

  rm /etc/resolv.conf
  

• Create your own /etc/resolv.conf.custom if you need to customize your DNS

  nameserver 8.8.8.8
  nameserver 10.40.1.40
  nameserver 10.40.1.41
  

• Make a symbolic link:

  sudo ln -s /etc/resolv.conf /etc/resolv.conf.custom
  

• Start the systemd-resolved service:

  sudo systemctl start systemd-resolved
  

• Restart network-manager

  sudo service network-manager restart
  

  Or

  sudo systemctl restart NetworkManager.service
  

Sources

• https://manpages.ubuntu.com/manpages/focal/en/man5/NetworkManager.conf.5.html
• https://askubuntu.com/questions/907246/how-to-disable-systemd-resolved-in-ubuntu