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24. Traffic Sniffing and Analysis

24.2 tcpdump: Command-Line Capture

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tcpdump captures packets from the terminal – perfect on a server with no GUI (like your EC2 instances).

sudo tcpdump -D                         # list interfaces
sudo tcpdump -i eth0                     # capture on an interface
sudo tcpdump -i eth0 -n                  # do not resolve names (faster, clearer)
sudo tcpdump -i eth0 -c 100              # stop after 100 packets
sudo tcpdump -i eth0 port 80             # only HTTP traffic
sudo tcpdump -i eth0 host 192.168.56.20  # only traffic to/from one host
sudo tcpdump -i eth0 'tcp port 80 and host 192.168.56.20'   # combine
sudo tcpdump -i eth0 -A port 80          # show packet contents as text (see HTTP)
sudo tcpdump -i eth0 -w capture.pcap     # save to a file for Wireshark
sudo tcpdump -r capture.pcap             # read a saved file

Filter keywords: host, net, port, src, dst, and and/or/not. -w writes a .pcap you open later in Wireshark.

Ravindra Bagale's Tip

Without -n, tcpdump tries to look up a name for every IP, which makes it slow and the output hard to read. Always use -n. And instead of capturing all traffic, filter with port or host – otherwise you won't find what you need among thousands of lines.

Lab

On Metasploitable 2 (or your own server), run sudo tcpdump -i eth0 -n -c 50 port 80 -w web.pcap while you browse its website from Kali. Then read it with tcpdump -r web.pcap and copy the file to open in Wireshark next.

Extended teaching notes

Why this lesson matters

Use this lesson to strengthen systems you own: understand the risk, practise safely in a lab, then write and apply the fix.

Apply the fix across stacks

Same class of bug, same idea in every language.

Java / Spring Boot

Parameterise queries, encode output, CSRF tokens, limited Actuator, dependency scanning.

ASP.NET Core

EF Core / parameterised SQL, Razor encoding, HTTPS, Data Protection.

Node.js / Express

helmet, parameterised SQL, npm audit, avoid eval and shell concat.

Python / Django / Flask / FastAPI

ORM, autoescape, pip-audit, Bandit; never shell=True with user input.

PHP / Laravel

PDO/Eloquent, Blade escaping, composer audit.

Go / Ruby

database/sql placeholders / ActiveRecord; html/template / ERB escaping; govulncheck / Brakeman.

nginx / Apache / Tomcat

Hide versions, TLS, security headers, least-open ports.

MySQL / PostgreSQL / MongoDB / Redis

Private networks, auth on, least-privilege users, no 0.0.0.0/0.

AWS / Azure / GCP

Roles not long-lived keys, private data stores, cloud audit logs on.

Mini case study

Suppose we are on a purple-team exercise for an Indian mid-size company (sample data). Red notes a technique from this lesson against a lab clone. Blue must show the control that stops it, the log that would have fired, and the ticket that hardens production. That is the standard this course trains you for.

Defender's worksheet (do this today)

  1. Write the lesson title, date and the exact lab target you are allowed to use.
  2. List three owned systems (VMs, apps, or cloud accounts) where this lesson applies.
  3. Run one safe check from the lesson on a lab system only. Paste the command and output.
  4. Write a two-sentence fix you would give a teammate.
  5. Name one related public advisory (CERT-In, CISA, NVD, or a vendor bulletin) with its date.

Indian workplace (sample data)

Suppose we are a junior analyst at Infosys, TCS, Wipro or a fintech SOC in Bengaluru/Pune. Turn this lesson into one checklist item with an owner, a due date and a place to store evidence (ticket + screenshot).

Global practice

Compare your notes with OWASP, CIS Benchmarks, NIST CSF/IR guidance or a cloud Well-Architected security question. Write five lines on what overlaps.

Everyday analogy

Explain the risk and the fix to a family member without jargon. If they can repeat the warning, you understand it well enough to teach juniors.

Key terms

  • Control: a safeguard (patch, config, process, detection).
  • Finding: a weakness backed by evidence.
  • Remediation: the fix plus how you verified it.
  • Scope: systems you have written permission to touch.
  • Least privilege: only the access the task needs.
  • Defence in depth: several layers so one failure does not end the story.

Common mistakes

  • Skipping written permission "just for a quick scan".
  • Practising on hostel, office or public targets without authorisation.
  • Collecting tool output without writing the fix.
  • Copying attack steps onto production to "verify". Defenders verify with versions, configs and patches.
  • Publishing screenshots that contain secrets, cookies or internal hostnames.

Recap

  • Evidence, fix, owner, date.
  • Own lab only.
  • Fix > spectacle.