NC(1)

📦 version 4.3.1, in alpine 3.24

🌸 NC(1) — the nc girlie's guide 🌸

okay so you opened the manual page for nc — short for netcat — and bestie, you're in the right place, this thing is iconic. It lives in the General Commands Manual section and it is genuinely one of the most versatile little tools in the whole toolbox. Let's get into it 💅

✨ NAME ✨

nc — arbitrary TCP and UDP connections and listens. that's it. that's the vibe. 🦋

🎀 SYNOPSIS 🎀

nc [-46cDdFhklNnrStUuvz] [-C certfile] [-e name] [-H hash] [-I length]
   [-i interval] [-K keyfile] [-M ttl] [-m minttl] [-O length]
   [-o staplefile] [-P proxy_username] [-p source_port] [-R CAfile]
   [-s sourceaddr] [-T keyword] [-V rtable] [-W recvlimit] [-w timeout]
   [-X proxy_protocol] [-x proxy_address[:port]] [-Z peercertfile]
   [destination] [port]

💖 DESCRIPTION 💖

So. The nc (or netcat) utility is used for just about anything under the sun involving TCP, UDP, or Unix-domain sockets. Like, literally anything. It can open TCP connections, send UDP packets, listen on arbitrary TCP and UDP ports, do port scanning, and deal with both IPv4 and IPv6. no cap, it does all of that 🫶

And here's the thing that makes it so much better than telnet(1): nc scripts nicely, and it separates error messages onto standard error instead of dumping them into standard output, which telnet(1) does with some. so your pipes stay clean, so your scripts stay slaying. periodt 💅

Common uses include:

  • simple TCP proxies
  • shell-script based HTTP clients and servers
  • network daemon testing
  • a SOCKS or HTTP ProxyCommand for ssh(1)
  • and much, much more

The options are as follows — buckle up, there's a lot, but we're getting through all of them together 💜

  • -4 — Use IPv4 addresses only.
  • -6 — Use IPv6 addresses only.
  • -C certfile — Load the public key part of the TLS peer certificate from certfile, in PEM format. Requires -c.
  • -c — Use TLS to connect or listen. Cannot be used together with any of the options -FuU.
  • -D — Enable debugging on the socket.
  • -d — Do not attempt to read from stdin.
  • -e name — Only accept the TLS peer certificate if it contains the name. Requires -c. If not specified, destination is used.
  • -F — Pass the first connected socket using sendmsg(2) to stdout and exit. This is useful in conjunction with -X to have nc perform connection setup with a proxy but then leave the rest of the connection to another program (e.g. ssh(1) using the ssh_config(5) ProxyUseFdpass option). Cannot be used with -c or -U.
  • -H hash — Only accept the TLS peer certificate if its hash returned from tls_peer_cert_hash(3) matches hash. Requires -c and cannot be used with -T noverify.
  • -h — Print out the nc help text and exit.
  • -I length — Specify the size of the TCP receive buffer.
  • -i interval — Sleep for interval seconds between lines of text sent and received. Also causes a delay time between connections to multiple ports.
  • -K keyfile — Load the TLS private key from keyfile, in PEM format. Requires -c.
  • -k — When a connection is completed, listen for another one. Requires -l. When used together with the -u option, the server socket is not connected and it can receive UDP datagrams from multiple hosts.
  • -l — Listen for an incoming connection rather than initiating a connection to a remote host. Cannot be used together with any of the options -psxz. Additionally, any timeouts specified with the -w option are ignored.
  • -M ttl — Set the TTL / hop limit of outgoing packets.
  • -m minttl — Ask the kernel to drop incoming packets whose TTL / hop limit is under minttl.
  • -N — shutdown(2) the network socket after EOF on the input. Some servers require this to finish their work.
  • -n — Do not perform domain name resolution. If a name cannot be resolved without DNS, an error will be reported.
  • -O length — Specify the size of the TCP send buffer.
  • -o staplefile — During the TLS handshake, load data to be stapled from staplefile, which is expected to contain an OCSP response from an OCSP server in DER format. Requires -c and -C.
  • -P proxy_username — Specifies a username to present to a proxy server that requires authentication. If no username is specified then authentication will not be attempted. Proxy authentication is only supported for HTTP CONNECT proxies at present.
  • -p source_port — Specify the source port nc should use, subject to privilege restrictions and availability. Cannot be used together with -l.
  • -R CAfile — Load the root CA bundle for TLS certificate verification from CAfile, in PEM format, instead of /etc/ssl/cert.pem. Requires -c.
  • -r — Choose source and/or destination ports randomly instead of sequentially within a range or in the order that the system assigns them.
  • -S — Enable the RFC 2385 TCP MD5 signature option.
  • -s sourceaddr — Set the source address to send packets from, which is useful on machines with multiple interfaces. For Unix-domain datagram sockets, specifies the local temporary socket file to create and use so that datagrams can be received. Cannot be used together with -l or -x.
  • -T keyword — Change the IPv4 TOS/IPv6 traffic class value or the TLS options.

For TLS options, keyword may be one of: noverify, which disables certificate verification; noname, which disables certificate name checking; clientcert, which requires a client certificate on incoming connections; or muststaple, which requires the peer to provide a valid stapled OCSP response with the handshake. The following TLS options specify a value in the form of a key=value pair: alpn, which allows the TLS ALPN to be specified (see tls_config_set_alpn(3) for further details); ciphers, which allows the supported TLS ciphers to be specified (see tls_config_set_ciphers(3) for further details); protocols, which allows the supported TLS protocols to be specified (see tls_config_parse_protocols(3) for further details). Specifying TLS options requires -c.

For the IPv4 TOS/IPv6 traffic class value, keyword may be one of critical, inetcontrol, lowdelay, netcontrol, throughput, reliability, or one of the DiffServ Code Points: ef, af11 ... af43, cs0 ... cs7; or a number in either hex or decimal.

  • -t — Send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests. This makes it possible to use nc to script telnet sessions.
  • -U — Use Unix-domain sockets. Cannot be used together with any of the options -cFx.
  • -u — Use UDP instead of TCP. Cannot be used together with -c or -x. For Unix-domain sockets, use a datagram socket instead of a stream socket. If a Unix-domain socket is used, a temporary receiving socket is created in /tmp unless the -s flag is given.
  • -V rtable — Set the routing table to be used.
  • -v — Produce more verbose output.
  • -W recvlimit — Terminate after receiving recvlimit packets from the network.
  • -w timeout — Connections which cannot be established or are idle timeout after timeout seconds. The -w flag has no effect on the -l option, i.e. nc will listen forever for a connection, with or without the -w flag. The default is no timeout.
  • -X proxy_protocol — Use proxy_protocol when talking to the proxy server. Supported protocols are 4 (SOCKS v.4), 4A (SOCKS v.4A), 5 (SOCKS v.5) and connect (HTTPS proxy). If the protocol is not specified, SOCKS version 5 is used. Note that the SOCKS v.4 protocol is very limited and can only be used when the destination host can be resolved to an IPv4 address. The other protocols pass the destination as a string to be interpreted by the remote proxy and do not have this limitation.
  • -x proxy_address[:port] — Connect to destination using a proxy at proxy_address and port. If port is not specified, the well-known port for the proxy protocol is used (1080 for SOCKS, 3128 for HTTPS). An IPv6 address can be specified unambiguously by enclosing proxy_address in square brackets. A proxy cannot be used with any of the options -lsuU.
  • -Z peercertfile — Save the peer certificates to peercertfile, in PEM format. Requires -c.
  • -z — Only scan for listening daemons, without sending any data to them. Cannot be used together with -l.

kween, quick word about the two positional args 💫

destination can be a numerical IP address or a symbolic hostname (unless the -n option is given). In general, a destination must be specified, unless the -l option is given (in which case the local host is used). For Unix-domain sockets, a destination is required and is the socket path to connect to (or listen on if the -l option is given).

port can be specified as a numeric port number or as a service name. Port ranges may be specified as numeric port numbers of the form nn-mm. In general, a destination port must be specified, unless the -U option is given. For some options, the value 0 requests that the system choose a port number.

🦋 CLIENT/SERVER MODEL 🦋

okay this is the part where it gets cute — building a very basic client/server model with nc is quite simple. Like, genuinely. Watch this 💖

On one console, start nc listening on a specific port for a connection. For example:

$ nc -l 1234

nc is now listening on port 1234 for a connection. Obsessed. Now on a second console (or a second machine), connect to the machine and port being listened on:

$ nc -N 127.0.0.1 1234

There should now be a connection between the ports ✨ Anything typed at the second console will be concatenated to the first, and vice-versa. And here's the slay part: after the connection has been set up, nc does not really care which side is being used as a ‘server’ and which side is being used as a ‘client’. main character energy on both ends, honestly. The connection may be terminated using an EOF (‘^D’), as the -N flag was given.

🍓 DATA TRANSFER 🍓

The example in the previous section can be expanded to build a basic data transfer model. Any information input into one end of the connection will be output to the other end, and input and output can be easily captured in order to emulate file transfer. lowkey genius.

Start by using nc to listen on a specific port, with output captured into a file:

$ nc -l 1234 > filename.out

Using a second machine, connect to the listening nc process, feeding it the file which is to be transferred:

$ nc -N host.example.com 1234 < filename.in

After the file has been transferred, the connection will close automatically. no cleanup, no drama, it just ends 💅✨

💬 TALKING TO SERVERS 💬

Okay so hear me out — sometimes you wanna talk to a server by hand instead of going through some fancy UI. And honestly? It's lowkey the move for troubleshooting, like when you gotta verify exactly what data a server is sending back in response to the commands the client flexes at it. Detective mode: activated 🔍

For example, say you wanna grab the home page of a web site:

$ printf "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80

Heads up bestie: this also shows you the headers the web server sent back. If they're giving you clutter vibes, you can filter 'em out with a tool like sed(1). 💅

And when you actually know the format of requests the server wants? That's when you can build up way more complicated examples. Like, you can submit an email to an SMTP server with:

$ nc localhost 25 << EOF
HELO host.example.com
MAIL FROM:<user@host.example.com>
RCPT TO:<user2@host.example.com>
DATA
Body of email.
.
QUIT
EOF

Iconic. No notes. ✨

🔎 PORT SCANNING 🔎

Knowing which ports are open and which services are running on a target machine is genuinely so useful. The -z flag is the whole trick — it tells nc to report open ports instead of actually initiating a connection. Observe:

$ nc -z host.example.com 20-30
Connection to host.example.com 22 port [tcp/ssh] succeeded!
Connection to host.example.com 25 port [tcp/smtp] succeeded!

You see that port range? That's there to limit the search to ports 20 - 30. Keeping it focused, periodt. 🎯

Alternatively — and this is the fun one — you might wanna know which server software is running, and which versions. That info is usually chilling right inside the greeting banners. To grab 'em, you gotta first make a connection, and then break the connection once the banner's been retrieved. Two ways to do that: specify a small timeout with the -w flag, or just issue a "QUIT" command to the server and let it end things itself:

$ echo "QUIT" | nc host.example.com 20-30
SSH-1.99-OpenSSH_3.6.1p2
Protocol mismatch.
220 host.example.com IMS SMTP Receiver Version 0.84 Ready

Look at that — versions, right there in the wild. 🦋

🌈 EXAMPLES 🌈

Bestie, this is the good part. Real ones, ready to copy. 🫶

Open a TCP connection to port 42 of host.example.com, using port 31337 as the source port, with a timeout of 5 seconds:

$ nc -p 31337 -w 5 host.example.com 42

Open a TCP connection to port 443 of www.example.com, and negotiate TLS with any supported TLS protocol version and "compat" ciphers:

$ nc -cv -T protocols=all -T ciphers=compat www.example.com 443

Open a TCP connection to port 443 of www.google.ca, and negotiate TLS. Check for a different name in the certificate for validation:

$ nc -cv -e adsf.au.doubleclick.net www.google.ca 443

Open a UDP connection to port 53 of host.example.com:

$ nc -u host.example.com 53

Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the IP for the local end of the connection:

$ nc -s 10.1.2.3 host.example.com 42

Create and listen on a Unix-domain stream socket:

$ nc -lU /var/tmp/dsocket

Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4, port 8080. Fun fact: this example could also be used by ssh(1) — peep the ProxyCommand directive in ssh_config(5) for more info. 👀

$ nc -x10.2.3.4:8080 -Xconnect host.example.com 42

The same example again, but this time enabling proxy authentication with username “ruser” in case the proxy requires it:

$ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42

That's the whole lineup. Ate and left no crumbs. 🔥

👑 SEE ALSO 👑

cat(1), ssh(1)

💖 AUTHORS 💖

Original implementation by Hobbit <hobbit@avian.org>. Rewritten with IPv6 support by Eric Jackson <ericj@monkey.org>.

Shoutout to the legends fr fr. 🙌

⚠️ CAVEATS ⚠️

Okay so this is the important bit, don't skip it bestie. 💀

UDP port scans using the -uz combination of flags will always report success, no matter what state the target machine is actually in. Like, it's not lying to you on purpose, it just literally cannot tell. So don't take that "success" as gospel, okay?

However — if you pair it up with a traffic sniffer, either on the target machine or on some intermediary device, the -uz combination could actually be useful for communications diagnostics. Slay for that specific use case.

Also note: the amount of UDP traffic generated may be limited, either due to hardware resources and/or configuration settings. So don't expect it to go infinite on you. 🙈

That's the whole page, bestie — go forth and nc responsibly. 💜

This page is from Debian, dated June 24, 2025, and it's NC(1) — catch you in the next one! 🎀

📦 where this page comes from