nc command in Linux: checking ports and transferring data, examples

Q: "Is this port reachable from here?"

nc -zvw3 host port. refused is not the same as timed out: the first means the service isn't running/it's the wrong port (the network is fine); the second means it never got there (firewall/routing). This is the key distinction.

Q: The port is open locally, but not from outside

On the server, nc -zv 127.0.0.1 8080 checks whether it's listening. If yes, but there's a timeout from outside, the issue is the bind address (0.0.0.0 vs 127.0.0.1) or the firewall (see ss, firewall).

Q: You need a quick L4 connectivity check between hosts

nc -l 8080 on one side, nc -zv host 8080 on the other — does TCP/UDP get through on that exact port (useful when debugging a firewall).

Q: nc "hangs"

Without -w, a connection can wait forever. Add a timeout (-w3), and -z for a clean check with no data exchange.

Overview

nc (netcat) is the TCP/UDP "Swiss Army knife": check whether a port is open, send raw data, spin up a temporary listener, move a file between machines.

bash

nc -zv host 22                    # check whether port 22 is open (z=scan, v=verbose)
nc -zv host 80 443                # several ports at once

On Ubuntu this is usually netcat-openbsd (options as below). There's also a ncat variant (from nmap) with similar syntax.

Options

8

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Option	Meaning
`-z`	scan mode: only check, don't send data
`-v`	verbose output (refused / open / timed out)
`-u`	UDP instead of TCP
`-l`	listen (server mode)
`-p N`	the local port
`-w N`	connection timeout, in seconds
`-n`	skip DNS resolution (faster)
`-k`	(with `-l`) don't close after one client

Option	What it does	Example
`-z`	Scan mode — only check reachability, do not send data.	`nc -zv host 22`
`-v`	Verbose output — refused, open, timed out.	`nc -zv host 80`
`-l`	Listen for an incoming connection (server mode).	`nc -l 9000`
`-w` N	Connection timeout in seconds — keeps the command from hanging forever.	`nc -zvw3 host 22`
`-u`	Use UDP instead of TCP.	`nc -zvu host 53`
`-n`	Do not resolve DNS names — faster and more reliable for checks.	`nc -zvn 10.0.0.5 22`
`-p` N	Use a specific local port.	`nc -p 5000 host 9000`
`-k`	With -l — keep listening after one client disconnects.	`nc -lk 9000`

Nothing found. Try a shorter query.

Recipes

2

Checking ports

bash

nc -zv 10.0.0.5 5432             # is the postgres port reachable from this machine
nc -zvw3 host 22                 # with a 3s timeout (don't hang)
nc -zvu host 53                  # a UDP port (DNS)
nc -zv host 20-25                # a range of ports
for p in 80 443 5432; do nc -zvw2 host $p; done   # a list of ports

Distinguish the responses: succeeded/open — the port is listening; Connection refused — reached the host, but nothing is listening; timed out — never reached it (firewall/routing).

Data and transfer

bash

nc -l 9000                        # listen on port 9000 (a simple test server)
echo "hello" | nc host 9000       # send a string
printf 'GET / HTTP/1.0\r\n\r\n' | nc host 80   # a raw HTTP request
nc -lk 9000                       # keep listening after a client disconnects

# transfer a file (receiver, then sender):
nc -l 9000 > out.bin              # on the receiver
nc host 9000 < file.bin           # on the sender

# a simple "chat"/two-way channel to test connectivity:
nc -l 9000        # server   |   nc host 9000   # client

Troubleshooting

4

"Is this port reachable from here?"

nc -zvw3 host port. refused is not the same as timed out: the first means the service isn't running/it's the wrong port (the network is fine); the second means it never got there (firewall/routing). This is the key distinction.

The port is open locally, but not from outside

On the server, nc -zv 127.0.0.1 8080 checks whether it's listening. If yes, but there's a timeout from outside, the issue is the bind address (0.0.0.0 vs 127.0.0.1) or the firewall (see ss, firewall).

You need a quick L4 connectivity check between hosts

nc -l 8080 on one side, nc -zv host 8080 on the other — does TCP/UDP get through on that exact port (useful when debugging a firewall).

nc "hangs"

Without -w, a connection can wait forever. Add a timeout (-w3), and -z for a clean check with no data exchange.

nc — checking ports and transferring data

Cheat sheet

Overview

Options

Recipes

Checking ports

Data and transfer

Troubleshooting

"Is this port reachable from here?"

The port is open locally, but not from outside

You need a quick L4 connectivity check between hosts

nc "hangs"

See also