How to know what DNS am I using in Ubuntu from 14.04 onwards

(A follow up to similar question for 12.04.)

Prior to Ubuntu 12.04, you may see the active DNS in /etc/resolv.conf. In Ubuntu 12.04, NetworkManager no longer works with the file. You have to directly consult the command line tool nm-tool.

Interestingly, nm-tool is no longer installed by default in 14.04 and later. Although you may still install through apt-get install, you can't assume all Ubuntu to have that out of the box.

So the question remains. How do you know, by default installation, the DNS you're using by command line?

Quick Answer

A new NetworkManager tool nmcli is installed by default now. The command line tool is very powerful but a bit harder to learn. Stick to our question, the short answer is:

nmcli dev show | grep DNS

or, to have cleaner output

nmcli dev show | grep DNS | sed 's/\s\s*/\t/g' | cut -f 2

Explain

If you have time, I can explain the above jumbo-mumble:

1. nmcli dev show

   Works a bit like the old nm-tool command. It elaborate the current networking info.

   You may also learn the setting of a certain interface by adding the interface name. For example, to learn the information of eth0, you may use nmcli dev show eth0.

2. grep DNS

   Obviously grep only the lines with the text "DNS" in it.

3. sed 's/\s\s*/\t/g' | cut -f 2

   This is only to clean up the output. The cut may select the output by column, but it takes only 1 character as separator (while nmcli uses MANY SPACE). The sed turns the spaces, in original output, into TAB.

Packet analysis would be an alternative method that works regardless of NetworkManager or other network connection tool that you use. Basic idea is to send a dns query with nslookup and in a second terminal check where the packets go.

For that we'd need to connect to the network for the first time, so that there is nothing cluttering the connections, and run the following command:

sudo tcpdump -vv -i wlan0 -W 1200 | grep google.com  

In alternative terminal run:

nslookup google.com 

Once you get packets listing from the tcpdump , check where do they go from your IP address.

For example,

$ sudo tcpdump -vv -i wlan0 -W 1200 | grep google.com                            
tcpdump: listening on wlan0, link-type EN10MB (Ethernet), capture size 65535 bytes
    eagle.29862 > b.resolvers.Level3.net.domain: [udp sum ok] 64057+ [1au] A? google.com. ar: . OPT UDPsize=4096 (39)
    b.resolvers.Level3.net.domain > eagle.29862: [udp sum ok] 64057 q: A? google.com. 11/0/0 google.com. A 173.194.115.64, google.com. A 173.194.115.65, google.com. A 173.194.115.72, google.com. A 173.194.115.66, google.com. A 173.194.115.69, google.com. A 173.194.115.78, google.com. A 173.194.115.70, google.com. A 173.194.115.71, google.com. A 173.194.115.68, google.com. A 173.194.115.67, google.com. A 173.194.115.73 (204)
    eagle.16429 > b.resolvers.Level3.net.domain: [udp sum ok] 38822+ A? google.com. (28)

As you can see , my laptop,eagle, sends packets to my university's dns , b.resolvers.Level3.net.domain. If you want to see the IP address, you can use the -n flag with tcpdump.

For example:

$ sudo tcpdump -n -vv -i wlan0 -W 1200 | grep google.com                         
tcpdump: listening on wlan0, link-type EN10MB (Ethernet), capture size 65535 bytes
    10.10.87.145.56474 > 4.2.2.2.53: [udp sum ok] 15606+ A? google.com. (28)

If somebody has the same question as me for Ubuntu 18.04LTS:

List all network devices managed through network-manager:

networkctl list

Show configuration of specific device:

networkctl status eth0

Instead of eth0 you have to enter the name of your network device shown in the list before. If there no DNS-entry, your card has no configured nameserver

check your network connections :

ls /etc/NetworkManager/system-connections/

and choose the connection you want to configure.

 sudo cat /etc/NetworkManager/system-connections/Internet | grep dns

Replace "Internet" without your connection name

Use can still use nm-tool:

nm-tool | grep DNS

Install it for U14.04 and later using

sudo apt-get install nm-tool

example:

nm-tool | grep DNS
    DNS:             192.168.1.1
    DNS:             192.168.10.1
    DNS:             192.168.11.1

It is still available by default on version 14.04 as it is bundled with network-manager. It has since been dropped from network-manager (version 15.04 and later) and is not even available through apt-get.

For now, on version 15.04, you can download and extract nm-tool from the old package manually. Run the following commands.

First, create a temp directory to work in:

cd
mkdir APTGET;cd APTGET

Then, download the old version and extract the files:

wget 'http://us.archive.ubuntu.com/ubuntu/pool/main/n/network-manager/network-manager_0.9.8.8-0ubuntu7.1_amd64.deb'
ar xvf *
tar xvf dat*

Make a new directory:

mkdir ~/bin

(if it says file already exists, just ignore the message and proceed).

Copy the file to the new directory:

cp ./usr/bin/nm-tool ~/bin

Return to the home directory and delete the temp directory:

cd ..
rm -R APTGET

Now, set an alias for nm-tool:

cp ~/.bashrc ~/.bashback
echo 'alias nm-tool="~/bin/nm-tool"' | tee -a ~/.bashrc;. ~/.bashrc

The current user should now be able to run nm-tool from the terminal.

Additionally, this may still not accurately provide all the actual DNS resolvers you are using.

You can go to DNSleaktest.com to get a full report. Click on Extended Test to get a full report.