Sockets and Ports

What is listening, what is connected, and which process owns each socket. ss is the modern tool (replaces netstat); lsof -i answers the inverse; given a port, which process opened it.

        sequenceDiagram
    participant SP as Server process
    participant K as Kernel socket table
    participant CP as Client process
    SP->>K: socket, bind(ip port), listen
    Note over K: LISTEN entry (ip, port, fd, pid)
    CP->>K: socket, connect(ip port)
    K->>K: 3-way handshake (SYN, SYN-ACK, ACK)
    Note over K: ESTABLISHED entry added
    SP->>K: accept
    K-->>SP: returns connection fd
    CP->>SP: write / send bytes
    SP->>CP: read / recv bytes
    Note over SP,CP: ss -tulpn or lsof -i read this table for you
    

operator@system:~$ ss -tulpn
operator@system:~$ ss -ant
operator@system:~$ ss -s
operator@system:~$ lsof -i :8080