How can it be determined which ports are currently open and being listened to by the server?

To identify open and listening ports on a server, you can use several commands and tools:

• netstat: a classic tool for viewing network connections and listening ports.

  netstat -tuln
  

  where:

  • -t — TCP,
  • -u — UDP,
  • -l — listening ports,
  • -n — output in numeric format (without attempting to resolve names).

• ss: a modern alternative to netstat, faster and more informative.

  ss -tuln
  

• lsof: shows open files, including network sockets.

  lsof -i -P -n | grep LISTEN
  

• nmap: an external port scanner that can be used to check open ports locally or remotely.

  nmap -sT -O localhost
  

These tools will help determine which ports are currently being listened to by the server.