iftop command listens to network traffic on a named network interface, or on the first interface it can find which looks like an external interface if none is specified, and displays a table of current bandwidth usage by pairs of hosts. iftop is a perfect tool for remote Linux server over ssh session.

iftop must be run by the root or the user who has sufficient permissions to monitor all network traffic on the network interface.

Install on Ubuntu / Debian

apt-get install iftop

Now to display traffic

iftop

Display traffic of a single interface

iftop -i eth0

However, iftop works best when you use filters. For example, if you want to find out how much bandwidth users are wasting or trying to figure out why the network is slow,

iftop -f icmp

You can display or analyses packet flowing in and out of the 192.168.1.0/24 network

iftop -F 192.168.1.0/24

Disable output for DNS traffic by using filter code such as

iftop -f 'not port domain'

iftop has many options read man page for further information.

See also: Keeping a log of daily network traffic for ADSL or dedicated remote Linux box