Install iftop

First, turn on EPEL repo on Linux. See how to install and enable epel repo on CentOS / RHEL / Scientific Linux v4.x/5.x/6.x for more information. Type the following yum command to install iftop on RHEL/CentOS/Fedora Linux:
# yum -y install iftop
Sample outputs:

How do I use iftop command?

The syntax is:

iftop
iftop [options]
iftop -i interface
iftop -i eth0 -f filter_code

Examples

Simply type the following command to monitor traffic on eth0:
# iftop -i eth0
Sample outputs:

Don’t do DNS hostname lookups

By default, iftop will look up the hostnames associated with addresses it finds in packets. Pass the -n option to disable dns lookup:
# iftop -n -i eth0

Hide service name and port numbers

Pass the -N option to hide resolve port number to service names:
# iftop -N -i eth0

Find out packets flowing in and out of 192.168.1.0/24 network

Use the -F option to set net/mask for traffic analysis. In this example, find out packets flowing in and out of the 192.168.1.0/24 network. This is useful to find out if why the network is slow:
# iftop -F 192.168.1.0/255.255.255.0 -i eth0

icmp bandwidth stats

Find out more about icmp ping requests and bandwidth usage per interface by host. Use the -f to set filter:
# iftop -f icmp -i eth0

Hide bar graphs

Pass the -b option to turn off display bar graphs of traffic:
# iftop -b -i eth0

Pass the -B option to display bandwidth rates in bytes/sec rather than bits/sec:
# iftop -B -i eth0

Important keyboard shortcuts

1. Press t control display type. The t cycles through the four line display modes; the default 2-line display, with sent and received traffic on separate lines, and 3 1-line displays, with sent, received, or total traffic shown.
2. Press P to pause the current display.
3. Press o to freeze the current screen order. This has the side effect that traffic between hosts not shown on the screen at the time will not be shown at all, although it will be included in the totals at the bottom of the screen.
4. Press j and k to scroll the display of hosts. This is useful for busy servers.

Recommended readings

• Keeping a Log Of Daily Network Traffic for ADSL or Dedicated Remote Linux Server
• Linux: Check Network Connection Command
• Man pages: tcpdump(8)