computer science

Netcat

Netcat 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 Netcat rather than just read about it. In short: netcat (often abbreviated to nc) is a computer networking utility for reading from and writing to network connections using TCP or UDP. The command is designed to be a dependable back-end that can be used directly or easily driven by other programs and scripts.

Key takeaways

Netcat 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 Netcat to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Netcat from memory before moving on to harder problems.

Reference excerpt

netcat (often abbreviated to nc) is a computer networking utility for reading from and writing to network connections using TCP or UDP. The command is designed to be a dependable back-end that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and investigation tool, since it can produce almost any kind of connection its user could need and has a number of built-in capabilities. It is able to perform port scanning, file transferring and port listening.

Features The original netcat's features include:

Outbound or inbound connections, TCP or UDP, to or from any ports Full DNS forward/reverse checking, with appropriate warnings Ability to use any local source port Ability to use any locally configured network source address Built-in port-scanning capabilities, with randomization Built-in loose source-routing capability Can read command line arguments from standard input Slow-send mode, one line every N seconds Hex dump of transmitted and received data Optional ability to let another program service establish connections Optional telnet-options responder Rewrites like GNU's and OpenBSD's support additional features. For example, OpenBSD's nc supports TLS, and GNU netcat natively supports a tunneling mode supporting UDP and TCP (optionally allowing one to be tunneled over the other) in a single command, where other versions may require piping data from one netcat instance to another.

Ports and reimplementations The original version of netcat was a Unix program. The last version (1.10) was released in March 1996. There are several implementations on POSIX systems, including rewrites from scratch like GNU netcat or OpenBSD netcat, the latter of which supports IPv6 and TLS. The OpenBSD version has been ported to the FreeBSD base, Windows/Cygwin, and Linux. Mac OS X comes with netcat installed as of OSX 10.13 or users can use MacPorts to install a variant. A DOS version of netcat called NTOOL is included in the FreeDOS Package group Networking. It is based on the WatTCP stack and licensed under the European Union Public Licence Version 1.1. Known ports for embedded systems includes versions for Windows CE (named "Netcat 4 wince") or for the iPhone. BusyBox includes by default a lightweight version of netcat named mini-netcat. Another version with more features, named bloaty nc, is also available. Solaris 11 includes netcat implementation based on OpenBSD netcat. Socat is a more complex variant of netcat. It is larger and more flexible and has more options that must be configured for a given task. On February 1, 2016, Santiago Zanella-Beguelin and Microsoft Vulnerability Research issued a security advisory regarding a composite Diffie-Hellman parameter which had been hard-coded into the OpenSSL implementation of socat. The implausibility that a composite might have been unintentionally introduced where a prime number is required has led to the suspicion of sabotage to introduce a backdoor software vulnerability. This socat bug affected version 1.7.3.0 and 2.0.0-b8 it was corrected in following releases from 1.7.3.1 and 2.0.0-b9. Cryptcat is a version of netcat with integrated transport encryption capabilities. In the middle of 2005, Nmap announced another netcat incarnation called Ncat. It features new possibilities such as "Connection Brokering", TCP/UDP Redirection, SOCKS4 client and server support, ability to "Chain" Ncat processes, HTTP CONNECT proxying (and proxy chaining), SSL connect/listen support and IP address/connection filtering. Like Nmap, Ncat is cross-platform. On some systems, modified versions or similar netcat utilities go by the command name(s) nc, ncat, pnetcat, socat, sock, socket, sbd.

ncat Ncat is a similar tool to netcat provided by Nmap suite. "While Ncat isn't built on any code from the “traditional” Netcat (or any other implementation), Ncat is most definitely based on Netcat in spirit and functionality." Ncat features includes: ability to chain Ncats together, redirect both TCP and UDP ports to other sites, SSL support, and proxy connections via SOCKS4 or HTTP (CONNECT method) proxies (with optional proxy authentication as well).

Illustrations

Worked examples

Example 1 — a first encounter with Netcat

Start with the simplest possible case. Write down what Netcat 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 Netcat 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 Netcat 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 Netcat

In research: Netcat 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 Netcat 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: Netcat is common in secondary-school and first-year university syllabi. It links to neighbouring topics Free network management software, Free software programmed in C, Network analyzers, so understanding it makes those chapters shorter.
In everyday life: Look for Netcat 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 Netcat in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Netcat 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 Netcat out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Netcat in simple terms?

Why does Netcat 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 Netcat?

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 Netcat.