Study of iftop Command

 

Experiment

 Study of iftop Command

🎯 Aim

To analyze real-time network bandwidth usage using the iftop command in Linux.

📘 Objective

By the end of this experiment, students will be able to:

  • Monitor live network traffic
  • Identify bandwidth usage per connection
  • Understand incoming and outgoing data flow
  • Analyze network activity in real time

📖 Theory

The iftop command is a real-time network monitoring tool that displays:

  • Bandwidth usage between hosts
  • Source and destination IP addresses
  • Data transfer rates

👉 It is similar to the top command, but instead of CPU usage, it shows network bandwidth usage.

👉 It listens to network traffic on a selected interface and summarizes usage dynamically.

🧾 Syntax

sudo iftop [options]

Common Options:

  • -i <interface> → Specify network interface (e.g., eth0, wlan0)
  • -n → Disable hostname resolution (faster output)
  • -N → Show port numbers instead of service names
  • -P → Display port numbers
  • -B → Display bandwidth in bytes (default is bits)

🧰 Prerequisites

  • Linux system
  • Root/sudo privileges
  • iftop installed

Installation:

sudo apt install iftop

🔬 Procedure & Commands

1️⃣ Run iftop (Default Interface)

sudo iftop

👉 Displays live bandwidth usage.

2️⃣ Monitor Specific Interface

sudo iftop -i eth0

3️⃣ Disable Hostname Resolution

sudo iftop -n

👉 Speeds up output by showing IP addresses only.

4️⃣ Show Port Numbers

sudo iftop -P

5️⃣ Display in Bytes Instead of Bits

sudo iftop -B

🖥️ Understanding the Output

192.168.1.5  =>  142.250.183.14   1.2Mb   1.0Mb   0.8Mb
              <=                   500Kb   400Kb   300Kb

Explanation:

  • => → Outgoing traffic
  • <= → Incoming traffic
  • Three columns represent:
    • Last 2 seconds
    • Last 10 seconds
    • Last 40 seconds

⌨️ Interactive Keys (During Execution)

KeyFunction
t    Toggle display mode
n    Toggle hostname/IP
p    Toggle port display
P    Pause display
q    Quit

📝 Exercises for Students

  1. Run iftop and observe traffic while browsing websites.
  2. Identify which IP is consuming the most bandwidth.
  3. Compare results with and without -n option.
  4. Monitor traffic on Wi-Fi vs Ethernet interfaces.
  5. Note differences between incoming and outgoing traffic.

❓ Viva Questions

  • What is the purpose of iftop?
  • Difference between iftop and ifconfig?
  • Why is root permission required?
  • What do the three bandwidth columns represent?

⚠️ Precautions

  • Requires root privileges to capture packets
  • May not show encrypted traffic details (only endpoints)
  • Use on authorized networks only

✅ Result

The iftop command is a powerful tool for real-time network bandwidth monitoring, helping users analyze traffic patterns and detect high network usage.

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