computer science

Tcpdump

Tcpdump is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Tcpdump rather than just read about it. In short: tcpdump is a data-network packet analyzer computer program that runs under a command line interface. It allows the user to display TCP/IP and other packets being transmitted or received over a network to which the computer is attached.

Key takeaways

Tcpdump belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Tcpdump to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Tcpdump from memory before moving on to harder problems.

Reference excerpt

tcpdump is a data-network packet analyzer computer program that runs under a command line interface. It allows the user to display TCP/IP and other packets being transmitted or received over a network to which the computer is attached. Distributed under the BSD license, tcpdump is free software. Tcpdump works on most Unix-like operating systems: Linux (including Android and embedded distribution such as OpenWrt), Solaris, FreeBSD, DragonFly BSD, NetBSD, OpenBSD, macOS, HP-UX 11i, and AIX. In those systems, tcpdump uses the libpcap library to capture packets. The port of tcpdump for Windows is called WinDump; it uses WinPcap, the Windows version of libpcap.

History tcpdump was originally written in 1988 by Van Jacobson, Sally Floyd, Vern Paxson and Steven McCanne who were, at the time, working in the Lawrence Berkeley Laboratory Network Research Group. By the late 1990s there were numerous versions of tcpdump distributed as part of various operating systems, and numerous patches that were not well coordinated. Michael Richardson (mcr) and Bill Fenner created www.tcpdump.org in 1999.

Common uses tcpdump prints the contents of network packets. It can read packets from a network interface card or from a previously created saved packet file. tcpdump can write packets to standard output or a file. It is also possible to use tcpdump for the specific purpose of intercepting and displaying the communications of another user or computer. A user with the necessary privileges on a system acting as a router or gateway through which unencrypted traffic such as Telnet or HTTP passes can use tcpdump to view login IDs, passwords, the URLs and content of websites being viewed, or any other unencrypted information. The user may optionally apply a BPF-based filter to limit the number of packets seen by tcpdump; this renders the output more usable on networks with a high volume of traffic. Example of available capture interfaces on a Linux system:

Privileges required In some Unix-like operating systems, a user must have superuser privileges to use tcpdump because the packet capturing mechanisms on those systems require elevated privileges. However, the -Z option may be used to drop privileges to a specific unprivileged user after capturing has been set up. In other Unix-like operating systems, the packet capturing mechanism can be configured to allow non-privileged users to use it; if that is done, superuser privileges are not required.

Illustrations

Worked examples

Example 1 — a first encounter with Tcpdump

Start with the simplest possible case. Write down what Tcpdump claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Tcpdump before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Tcpdump ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Tcpdump

In research: Tcpdump appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Tcpdump in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Tcpdump is common in secondary-school and first-year university syllabi. It links to neighbouring topics Command-line software, Cross-platform free software, Free network management software, so understanding it makes those chapters shorter.
In everyday life: Look for Tcpdump outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Tcpdump in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Tcpdump means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Tcpdump out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Tcpdump in simple terms?

Why does Tcpdump matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Tcpdump?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Tcpdump.