nc

Additional Notes

nc (netcat) reads and writes network connections from the command line. It is a versatile networking tool for port scanning, simple file transfers, ad-hoc chat, and debugging TCP/UDP services.

Multiple implementations exist (traditional, OpenBSD, GNU netcat, Nmap ncat). The OpenBSD version is the most common on Linux and does not support the -e flag for security reasons. Behavior of flags such as -l and -e can differ between builds, so test on your system.

Syntax

nc [options] [host] [port]

Parameters

• options: Flags that change how nc behaves.
• host: Hostname or IP address to connect to.
• port: TCP or UDP port number.

Common Options

• -z: Scan mode; connect without sending data.
• -v: Verbose output; show connection details.
• -l: Listen for an incoming connection (server mode).
• -u: Use UDP instead of TCP.
• -n: Do not resolve hostnames; use numeric addresses.
• -p PORT: Use a specific source port (where supported).
• -w SECONDS: Connection timeout.
• -k: Keep listening after a client disconnects (with -l).
• -4, -6: Force IPv4 or IPv6.

Examples

nc -zv example.com 443

Check whether TCP port 443 is open, with verbose output.

nc -zv example.com 20-30

Scan a small range of TCP ports.

nc -l 9000

Listen on port 9000 and print whatever a client sends.

echo "hello" | nc localhost 9000

Send a single line to a listening nc.

nc -u -l 9000

Listen for UDP packets instead of TCP.

nc -lvnp 9000 > received.bin

Receive a file over a raw TCP connection into received.bin.

nc -v example.com 80 < request.txt

Send the contents of request.txt to a service on port 80.

Practical Notes

• Use nc for quick connectivity tests when curl or telnet is unavailable.
• For reliable file transfers, prefer scp or rsync; nc has no integrity checks.
• OpenBSD nc may require -N to close the connection after EOF.
• Firewalls, NAT, and cloud security groups can block the port you test.
• Only use nc against systems you own or are authorized to test.