mjhjmtu

I am running

 sudo tcpdump -i enp0s31f6 -n port 67 and port 68

on one terminal and running

 sudo dhclient -r

on another.

During this, I see nothing on first terminal.

What I am doing wrong?

Both terminals are on the same machine. I wish to sniff DHCP communication on the same machine, which does it.

You are not seeing anything because there is a mistake on your tcpdump boolean logic BPF rules.

It should be:

sudo tcpdump -i enp0s31f6 -n port 67 or port 68

and not as you are doing:

 sudo tcpdump -i enp0s31f6 -n port 67 and port 68

In the first instruction, you are listening to either port 67 OR port 68 (one or the other).

In the second instruction, you are asking for a rule where ports are both 67 AND 68 at the same time, and thus seeing no packets.

You should use the port 68 on the client (lsof -i :68), the command should be :

sudo tcpdump -i enp0s31f6 -n port 68

You can use dhcpdump instead: dhcpdump -i enp0s31f6.

man dhcpdump:

This command parses the output of tcpdump to display the dhcp-packets
for easier checking and debugging.