Override Ubuntu 20.04 DNS using systemd-resolved

Question

How do I configure an Ubuntu 20.04 system so it overrides the default DNS?

It seems that by default there is a global and per-link DNS setting.

I tried a couple of things that did not work:

• Edit /etc/systemd/resolved.conf with the DNS Servers
• Created /etc/systemd/network/enp0s3.conf with the DNS servers configured
• Removed all DNS related parameters from the DHCP request by editing /etc/dhcp/dhclient.conf

All these changes (and the combinations) result in the DNS servers being prepended to the list of global DNS servers.

Most 'solutions' are to either install resolvconf or replace the /etc/resolv.conf symbolic link with a file and set the DNS servers there. Both of these seem like a workaround.

I would like to use the existing tooling (systemd-resolved) to override the DNS Servers.

As suggested by @xenoid in the comments: Setting the DNS for the interface through the GUI resulted in a file /etc/NetworkManager/system-connections/enp0s3.nmconnection that contains the correct DNS servers, the output of resolvectl status includes the correct DNS servers, this however is not what I had in mind. I am looking for a solution that does the configuration using systemd-resolved, which is possible from what I can find, but it is unclear how. Since this requires a GUI installation.

Answer 1

1. Update /etc/systemd/resolved.conf and add some DNS servers (in this example, 1.1.1.1 is CloudFlare, and 8.8.8.8 and 8.8.4.4 are Google.)

   [Resolve]
   DNS=1.1.1.1 8.8.8.8
   FallbackDNS=8.8.4.4
   

2. Restart system resolved:

   service systemd-resolved restart
   

3. Run resolvectl status (or systemd-resolve --status in older versions of systemd). The output should look like this:

   Global
            DNS Servers: 1.1.1.1
                         8.8.8.8
   ...
   

Answer 2

This comment from @David R was very helpful:

rm -f /etc/resolv.conf
ln -sv /run/systemd/resolve/resolv.conf /etc/resolv.conf

Answer 3

Let the iface variable contain your main interface name, then you can set your DNS addresses this way :

$ sudo resolvectl dns $iface x.y.z.t1 x.y.z.t2
$ sudo resolvectl domain $iface "myDomain.lan"
$ resolvectl dns $iface
Link 2 (ensX): x.y.z.t1 x.y.z.t2
$ resolvectl status $iface
Link 2 (ensX)
      Current Scopes: DNS
DefaultRoute setting: yes
       LLMNR setting: yes
MulticastDNS setting: no
  DNSOverTLS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
  Current DNS Server: x.y.z.t1
         DNS Servers: x.y.z.t1
                      x.y.z.t2
          DNS Domain: myDomain.lan

Answer 4

you should explicitly specify in this file /etc/systemd/network/enp0s3.conf to ignore DNS from DHCP. I tend to ignore everything from dhcp in most situations, since I prefer to control this via config-management tools. For example, the DHCPv4 section in the above file would look like:

[DHCPv4]
UseHostname=no
UseDNS=no
UseNTP=no
UseDomains=no

Also note that you should be using .network extension for the file. Also the [Match] section can help you so that you dont need to care about what the device name actually is. in my case I dont have any individual device files and instead do overrides when in aws:

section of /etc/systemd/network/01-ec2-overrides.network

[Match]
Driver=ena ixgbevf vif

[DHCPv4]
UseHostname=no
UseDNS=no
UseNTP=no
UseDomains=no

[DHCPv6]
UseHostname=no
UseDNS=no
UseNTP=no