Netcat for Mere Mortals

Netcat — nc — is the command line's bare hand on the network: one small tool that opens a connection (or waits for one) and then simply moves bytes between the socket and your terminal or files. It speaks no protocol. It has no opinions about what the bytes mean. Whatever a service says, you see; whatever you type, the service gets. That emptiness is the point: netcat is how you talk to a network service yourself, by hand, when you want to know what it actually says — and how you stand up a listening ear in one line when nothing else is installed.

It is the manual cousin of Nmap for Mere Mortals: nmap knocks on a thousand doors and reports; netcat opens one door and has the conversation. The rule of this guide matches that guide's rule, stated in chapter 1 and repeated until it's reflex: only connect to, and listen on, machines you own or administer. Everything here is practised on a lab that lives entirely on 127.0.0.1 — a banner-and-echo fixture in examples/lab_server.py, a specimen web page served by Python's built-in server, and netcat listening to itself — so the rule costs nothing to keep.

A note on how this guide was verified. The build machine has the OpenBSD netcat (Debian patchlevel 1.226-1ubuntu2), and every captured command in these chapters ran on it, against the lab, on loopback — including the parts that went wrong. Loopback UDP and Unix-domain sockets both work on this machine, so those chapters are captures, not descriptions. Netcat's other dialects (traditional nc, and Nmap's ncat) are not installed here; chapter 12 is honest about which of their differences are documented rather than run. The full ledger — what was captured, what was only described, and the two surprises (a resolver that answers any name, and a caller who exited 0 without ever being heard) — is in criteria.md.

What this guide covers

• What netcat is — sockets in plain words, the permission rule, the lab, and what netcat is not
• Talking to a service — netcat as a client: the banner, the echo, and reading -v's one useful line
• HTTP by hand — typing a web request yourself and reading everything the server says back
• Listening — netcat as the server: the two-terminal chat, -k, and the one-caller-at-a-time truth
• Checking ports — -z and the exit code, refused versus silence, and when to put netcat down and pick up nmap
• Moving files — one file across, in either direction, with the hash to prove it arrived
• Piping and scripts — program output down a socket, and the exit code as the whole interface
• UDP — datagrams: no connection, no promises, and the check that can only shrug
• Timeouts and hangs — why netcat sits there, and -N and -w, the two ways of refusing to
• Unix sockets — the same tool, no network at all
• The double edge — why the admin's pocketknife is also the classic backdoor shape, and the habits that keep it on the admin side
• The netcat family — OpenBSD, traditional, and ncat: whose flags you actually have
• Troubleshooting — the silent failure, the hang, the caller nobody heard, and the resolver that lies
• Glossary — the terms, collected in one place

What it doesn't cover

Real servers. Netcat listens like a champ in a lab and serves one caller at a time with no logging, no access control, and no encryption — the moment a listener matters, you want a real daemon (and the guides for those are elsewhere in this series). It also doesn't cover port scanning beyond the one-port check (that's nmap's book), TLS by hand (ncat's --ssl is named in chapter 12, not taught; use the OpenSSL guide for the by-hand TLS conversation), scripting netcat into a daemon (socat, named in chapter 12, is the tool built for that), or IPv6 beyond a mention — the lab is IPv4 loopback, and -6 is one flag among the variants.

How to use this guide

Read chapter 1, start the lab (python3 examples/lab_server.py, in its own terminal), and keep a second terminal open from chapter 2 on — most of netcat only makes sense with a terminal for each end of the wire. Keep the lab afterwards: a service whose answers you know is a standing rehearsal for the day a real one's behaviour surprises you.

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Part of the series

This guide is one of the For Mere Mortals guides: plain-language guides to the tools programmers and makers use.

Related guides:

• Nmap for Mere Mortals
• tcpdump for Mere Mortals
• curl for Mere Mortals

License

The writing in this guide is licensed under CC BY 4.0. Code examples and lab scripts are licensed under the MIT License.