Updated: 28 July 2022
Server ports
Check if a server port is accessible. Could be blocked by any of:
- Local workstation
- Local network router
- VPN or proxy
- DNS Resolver
- Internet service provider (ISP) (e.g. Verizon)
- Web application firewall (WAF) (e.g. Sucuri)
- Content delivery network (CDN) (e.g. Cloudflare)
- Web server
Attempt to connect to port 21 on a domain but timeout after 15 seconds if no response (-z check without sending data, -v verbose, -w timeout)
netcat -vzw 15 domain.com 21
Credit https://www.inmotionhosting.com
Local ports
Local port 8080 is free if output is 1
sudo nc localhost 8080 < /dev/null; echo $?