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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.

Serge Fonville
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  • What is your end goal here? Do you want to point the machine to your own DNS server on your local network? Are you on a home network you control or in an enterprise environment? – Martin Konrad May 24 '20 at 16:06
  • I want this system to have different DNS servers from those that are provided over DHCP. While still using the default tooling that is used for configuring DNS. – Serge Fonville May 24 '20 at 16:47
  • *I want this system to have different DNS servers from those that are provided over DHCP* In the network, config dialogs you can configure your system to ignore the DHCP-provided DNS and use specific ones. – xenoid May 24 '20 at 17:25
  • I disable and mask `systemd-resolved` and then hand code the `/etc/resolv.conf` to point to my local `unbound` . You have to delete the resolv.conf link to create a static file. I have had no problems. – Stephen Boston May 19 '22 at 15:51

5 Answers5

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  1. Update /etc/systemd/resolved.conf
[Resolve]
DNS=1.1.1.1 8.8.8.8
FallbackDNS=8.8.4.4
  1. Restart system resolved: service systemd-resolved restart
  2. Run systemd-resolve --status

the output should look like this:

Global
         DNS Servers: 1.1.1.1
                      8.8.8.8
...
Tombart
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    I did that, and that did not change the per-link DNS settings – Serge Fonville May 26 '20 at 09:06
  • @SergeFonville Is `systemd-networkd` active? What is the output `networkctl status enp0s3` – Tombart May 27 '20 at 10:35
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    `systemd-networkd` is not active and `networkctl status enp0s3` outputs the settings of the interface, I noticed specifically `State: n/a (unmanaged)`. Do you want to know something specific? – Serge Fonville May 28 '20 at 22:59
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    You need to change the file the `/etc/resolv.conf` link points to. Point it at `/run/systemd/resolve/resolv.conf`. See this bug: https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1774632 – David R. Dec 20 '20 at 19:01
  • This did the trick for me. Thanks. – DribblzAroundU82 Apr 07 '21 at 18:12
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    Saved my day. Always the same problems with ubuntu, year after year. It remains a good distribution – Kiruahxh Apr 13 '21 at 14:36
3

This comment from @David R was very helpful:

rm -f /etc/resolv.conf
ln -sv /run/systemd/resolve/resolv.conf /etc/resolv.conf
leodotcloud
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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
$ 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
SebMa
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  • All right (whew) that certainly worked but... will it be made persistent? And, if no, where/what has to be changed to make the change persistent — short of having a script manually setting it up on boot? – Gwyneth Llewelyn Sep 20 '22 at 20:16
  • Note: I can confirm that things were changed in `/run/systemd/resolve/resolv.conf` but... `/run` is a `tmpfs` in memory... so where does it get stored? Or, rather, where does the non-global configuration come from? – Gwyneth Llewelyn Sep 20 '22 at 20:26
1

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
gbolo
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0

Remember to run systemctl enable systemd-resolved to ensure it is running after every reboot. Many posts on the web talk about disabling systemd-resolved.

Absolutely awesome! I have had this problem on a zimbra mail server on ubuntu for a long time and always used a manual workaround on reboot. Had the zimbra experts looking for a cure as well, but couldn't find it.. This post was the answer.

Fco Javier Balón
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Scott
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