How to make Ubuntu server 18.04 use dns-nameservers from /etc/network/interfaces?

Question

On Ubuntu server 18.04 I've disabled netplan and using /etc/network/interfaces.

sudo apt-get -y install ifupdown

cat <<EOM  | sudo bash -c 'cat > /etc/network/interfaces'
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 192.168.5.5
netmask 255.255.255.0
gateway 192.168.5.254
dns-nameservers 192.168.5.53 192.168.5.5
EOM

sudo ifdown --force eth0 lo && sudo ifup -a

sudo systemctl stop networkd-dispatcher
sudo systemctl disable networkd-dispatcher
sudo systemctl mask networkd-dispatcher
sudo apt-get -y purge nplan netplan.io

But apparently the systemd-resolve doesn't take into account the dns-nameservers field.

In the /etc/resolv.conf it puts only this:

nameserver 127.0.0.53

Output of ls -la /etc/resolv.conf is:

lrwxrwxrwx 1 root root 39 Oct  2 15:28 /etc/resolv.conf -> ../run/systemd/resolve/stub-resolv.conf

Output of the sudo systemd-resolve --status | grep -A3 Server is empty.

I can define DNS servers in /etc/systemd/resolved.conf like this:

[Resolve]
DNS=192.168.5.53 192.168.5.5

Then the output of sudo systemd-resolve --status | grep -A3 Server is:

     DNS Servers: 192.168.5.53
                  192.168.5.5
      DNSSEC NTA: 10.in-addr.arpa
                  16.172.in-addr.arpa

And they are used correctly and appear in /etc/resolv.conf.

How to make the dns-nameservers config line work?

Answer 1

You need the package resolvconf, too.

So, to get rid of netplan completely:

1. Configure /etc/network/interfaces
2. Install ifupdown and friends: $ sudo apt install ifupdown resolvconf
3. Remove netplan: $ sudo apt remove nplan netplan.io

Answer 2

How to make the dns-nameservers config line work?

As you can see by the status command, the line is working perfectly:

DNS Servers: 192.168.5.53
              192.168.5.5

The symbolic link /etc/resolv.conf shows:

nameserver 127.0.0.53

That indicates that dnsmasq is running here: https://help.ubuntu.com/community/Dnsmasq

In short, this means that domain name lookups will first query a local cache to see if the site has been visited before. 127.0.0.xx is 'local' in this context. If so, the local cache will provide the IP address. If the site has not been visited before and no reference is available in the local cache, only then will the listed DNS nameservers; 192.168.5.53 and 192.168.5.5, in your case, be used.

The readings you’ve gven above are entirely normal and expected.

Answer 3

See the answer here.

You need to edit /etc/systemd/resolved.conf file to change DNS on Ubuntu 18.04.

Answer 4

network interfaces files have been changed a lot on ubuntu 18. I suggest you to check this website or another one to correctly configure your network adapter

how to configure network ubuntu 18

Anyways you can also (and that's the easiest way) configure your ip and nameservers from the network manager on settings.