nc (netcat) on Linux: Test Open Ports and Simple TCP Debugging

nc (netcat) is the quick port-check tool when you need yes/no on TCP connectivity — faster than configuring a scanner for one port. Searches spike during deploys (“is the load balancer reaching my app on 8080?”).

See: ss,
wait-for-port script,
UFW basics.

Test a remote port

Zero-I/O mode -z scans without sending data; -v is verbose.

nc -zv -w 3 203.0.113.10 443
nc -zv -w 3 127.0.0.1 3306

Listen locally

Useful to confirm a firewall rule reaches the host before the real app is ready.

nc -l -p 9999 -v

Send minimal HTTP

Manual HTTP helps when curl fails mysteriously — you see raw TCP behavior.

printf 'GET / HTTP/1.0\r\nHost: example.com\r\n\r\n' | nc -w 3 example.com 80 | head

Interpret failures

Connection refused = reachable host, nothing listening; timeout = firewall or wrong IP/route.

Quick Reference

• Scan: nc -zv host PORT
• Listen: nc -l -p PORT
• Refused vs timed out tells you a lot

Related tutorials

• Port monitor script
• firewalld
• SSH refused

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