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Network Security & Firewall CLI

Linux nc Command Reference (Netcat): Usage, Examples, and

4 min read

linux nc command (netcat) reads and writes data over TCP or UDP sockets; it functions as a simple client or server for network debugging and automation.

nc [options] [hostname] [port]

On Ubuntu/Debian, the OpenBSD netcat variant is installed by default; other distributions may ship traditional or NMAP nc, which differ in flag support. Common uses: verify firewall rules, test service availability, transfer files, scan ports, or serve raw HTTP responses.

Installation

If nc is missing, install the appropriate package:

sudo apt-get install netcat    # Debian/Ubuntu
sudo yum install nc             # RHEL/CentOS 7
sudo dnf install nc             # Fedora/RHEL 8+

Netcat Command Cheat Sheet

Action CLI Command Key Flag Description
Connect to a TCP port nc host 80 None Send stdin to port and display response
Listen for connections nc -l -p 4444 -l -p TCP server on port 4444
Port scan a range nc -zv host 20-80 -z -v Scan ports 20-80 with verbose output
Send a file (sender) nc -l -p 9999 < file.txt -l -p Listen; receiver connects to get file
Receive a file (receiver) nc host 9999 > file.txt None Connect to sender and save stream
UDP test nc -u -l -p 1234 -u Listen for UDP datagrams
HTTP request printf "GET / HTTP/1.1rnHost: example.comrnrn" | nc example.com 80 None Fetch page via raw HTTP
See also  pathping Command Reference: Troubleshoot Latency & Packet Loss

Common Netcat Options

Flag Type Default Description
-l Mode None Listen mode – wait for inbound connections
-p Port None Local port to listen on (used with -l)
-v Verbosity Minimal Verbose output – show connection details
-u Protocol TCP Use UDP instead of TCP
-z Mode None Zero-I/O port scan – no data transfer
-w Timeout None Connect/read timeout in seconds
-k Flag Off Keep listening after client disconnects
-n DNS Disabled Suppress DNS resolution (not in all variants)
-s Source Any Bind to a specific source IP

Note: Flags like -R, -W, -cf, -g are available in OpenBSD nc but less common. Check man nc.

Usage Examples

Port Scanning

nc -zv 192.168.1.1 22-80

Scans ports 22 through 80 on the target. The -z flag suppresses data transfer; -v prints open ports. UDP scanning (-u) is less reliable because UDP is stateless.

File Transfer

Sender (server):

nc -l -p 8888 < backup.tar.gz

Receiver (client):

nc sender-ip 8888 > backup.tar.gz

Transfers a file over a plain TCP connection. The listener waits for a single client; use -k for multiple connections. No encryption — pipe through openssl enc for sensitive data.

Performing an HTTP Request

printf "GET / HTTP/1.1rnHost: example.comrnConnection: closernrn" | nc example.com 80

Constructs a raw HTTP GET. Useful for debugging web servers or testing response headers.

Troubleshooting & Common Errors

Error Message / Code Root Cause Resolution Command
nc: Address already in use Port occupied by another process sudo lsof -i:port or fuser -k port/tcp
nc: Connection refused No service listening on target port Verify with ss -tlnp or start listener
nc: invalid option -- 'X' Flag not supported by nc variant Check man nc; use netcat-openbsd or nmap-ncat accordingly
Timeout with -w No response within timeout Increase timeout (-w 10) or check firewall
See also  SPAN CLI Reference: Troubleshooting Switch Port Mirroring

Performance Considerations

nc is single-threaded and does not expose buffer-size flags. Tune OS socket buffers with sysctl: sysctl -w net.core.rmem_default=262144 and wmem_default=262144. Use -w for idle timeout and -i to insert delay (seconds) between lines. Example with tuning:

cat data.txt | nc -w 5 -i 0.5 target_host 5000

For parallel transfers, run multiple nc processes via shell loop or xargs -P.

Frequently Asked Questions

What is the difference between nc -l and nc -lp?

Answer: In newer OpenBSD nc, -l listens and the port is given as a positional argument; -lp is deprecated. Use nc -lv 4444. Verify with nc -h | grep -E '^-l|-p'.

When should I use the -z flag with nc?

Answer: Use -z for zero-I/O port scanning — check if a TCP port is open without sending data. Add -v for verbose: nc -zv 192.168.1.1 22 80 443. Combine with -w for timeout: nc -zv -w 2 10.0.0.5 3306.

How do I fix “nc: getaddrinfo: Name or service not known” error?

Answer: DNS resolution failure. Check DNS with nslookup example.com. Use numeric IP: nc 93.184.216.34 80. For IPv6, wrap address in brackets: nc [::1] 22.

Does nc work on AWS EC2, Azure VM, and GCP Compute Engine?

Answer: Yes, netcat is pre-installed or easily installed on all major Linux distributions used in those clouds. On Amazon Linux 2 / RHEL 8+: sudo yum install nmap-ncat -y. On Ubuntu/Debian: sudo apt install netcat-openbsd -y.

What is the fastest way to transfer a binary file between two Linux hosts using nc?

Answer: Receiver: nc -l -p 9000 > file.bin. Sender: cat file.bin | nc receiver-ip 9000. For WAN, add compression: cat file.bin | gzip -c | nc -w 3 receiver-ip 9000; receiver uses gunzip.