Ubuntu 18.04 Server - how to check DNS IP server setting being used [closed] - ubuntu-server

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Working with Ubuntu 18.04 Server LTS. I am trying to find a way to check the DNS IP(s) that is actually being used when set via DHCP. The resolv.conf only will point to 127.0.0.53 now. /etc/systemd/network folder is empty. The NSLOOKUP output also always references the internal 127.0.0.53 IP. Seems all the old tricks aren't working.
The Ubuntu documentation is not updated yet as it still looks like the help for 16.04 referencing eth0, ifup and ifdown which are all deprecated on 18: https://help.ubuntu.com/lts/serverguide/network-configuration.html
I've tried setting a static adapter setup with Netplan via a custom .yaml. The settings work fine but I can't seem to find the DNS IP that I set anywhere. I expect it will be consistent between DHCP and static settings but I'm not sure where to look now.
I would settle for either a C library call or a bash CLI method at this point. Does anyone have a way to check this on 18.04 Server?

Answer 1

I found the following showed my the DNS servers by adapter towards the end of the output:
systemd-resolve --status
It contains a list under 'DNS Servers' organized by Link. I think this has changed from previous versions with Ubuntu. It will take a little text parsing work but this gives me what I'm after.

Answer 2

Another way is:
cat /run/systemd/resolve/resolv.conf
That file is dynamically generated by systemd-resolved, but contains the actual DNS servers instead of 127.0.0.53.
In fact, if you want make that the default for /etc/resolv.conf, you simply create symlink for it. (/etc/resolv.conf is a symlink that points to /run/systemd/resolve/stub-resolv.conf by default):
sudo mv /etc/resolv.conf /etc/resolv.conf.orig
sudo ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
Sample /run/systemd/resolve/resolv.conf :
# This file is managed by man:systemd-resolved(8). Do not edit.
#
# This is a dynamic resolv.conf file for connecting local clients directly to
# all known uplink DNS servers. This file lists all configured search domains.
#
# Third party programs must not access this file directly, but only through the
# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,
# replace this symlink by a static file or a different symlink.
#
# See man:systemd-resolved.service(8) for details about the supported modes of
# operation for /etc/resolv.conf.
nameserver 10.1.2.3
search host.domain.com
For more info:
http://manpages.ubuntu.com/manpages/bionic/man8/systemd-resolved.service.8.html

Answer 3

You can also use:
nmcli dev show eth0
Replace eth0 with your network interface to see how it was configured. If you got the DNS address over DHCP, you should see it here.

Answer 4

I'm running ubuntu 22.04.
sudo nmcli dev show <interface> didn't have any dns listings, but sudo resolvectl status <interface> showed me the current dns server.

Answer 5

I am suprised at this too. I am running Ubuntu 16.04 LTS and see something similar.
If you issue a
dig www.google.ca +trace | grep 'Received'
The first host in the response is the one doing the lookups for your server.
$ dig www.google.ca +trace | grep 'Received'
;; Received 525 bytes from 127.0.1.1#53(127.0.1.1) in 19 ms
;; Received 42 bytes from 199.7.91.13#53(d.root-servers.net) in 32 ms
This means that this box is doing it's own dns lookups directly to the root servers and following the path recursively to get my dns lookups.
Found the issue is that dnsmasq is enabled. To disable it do as follows.
edit the NetworkManager.conf file
sudo gedit /etc/NetworkManager/NetworkManager.conf
comment out the dnsmasq line
#dns=dnsmasq
restart the network-manager service
sudo /etc/init.d/network-manager restart
then verify that it is using the dns-server assigned via dhcp
$ dig www.google.ca +trace | grep 'Received'
;; Received 525 bytes from 192.168.30.1#53(192.168.30.1) in 18 ms
;; Received 42 bytes from 198.41.0.4#53(a.root-servers.net) in 32 ms