What is using this port?
Shows the program and PID listening on or connected to the port.
lsof -i :Network & port commands
Find what’s using a port and stop it, look up DNS, check your public IP, test connections, inspect HTTP headers and check when an SSL certificate expires.
15 commands
Free a port (kill whatever is on it)
Force-stops every process using the port. Fixes "address already in use".
kill -9 $(lsof -ti :)Force-kills without letting the program save. Check with lsof -i first.
Free a port on Windows
Finds the process listening on the port and stops it.
Stop-Process -Id (Get-NetTCPConnection -LocalPort ).OwningProcess -ForceAll listening ports (macOS)
Every program accepting connections, with numeric addresses and ports.
sudo lsof -iTCP -sTCP:LISTEN -n -PYour public IP address
The IP the internet sees you as.
curl -s https://ifconfig.meLook up a domain’s IP (DNS)
Prints the addresses a domain resolves to. Add a record type like MX or TXT after the domain.
dig +short DNS lookup on Windows
Shows the DNS server used and the addresses for a domain.
nslookup Ping a host
Sends four pings and reports latency and packet loss. (On Windows, just ping host.)
ping -c 4 Trace the route to a host
Lists every hop between you and the destination. On Windows use tracert.
traceroute See a site’s HTTP headers
Fetches only the response headers: status code, caching, redirects, server.
curl -I Download a file
Saves the file with its original name, following redirects.
curl -LO How long does a request take?
Prints the total time for one request.
curl -o /dev/null -s -w "%{time_total}s\n" When does a site’s SSL certificate expire?
Shows the certificate’s valid-from and valid-until dates.
echo | openssl s_client -connect :443 -servername 2>/dev/null | openssl x509 -noout -datesShare a folder over HTTP
Serves the current folder at http://localhost:8000. Great for testing static sites.
python3 -m http.server Who owns a domain
Registrar, creation and expiry dates and name servers.
whois Every command copies as plain, clean text: no curly quotes, no long dashes, no invisible characters.