listening socket

A socket a process has asked the kernel to receive new connections on: it appears in the socket table in state LISTEN, bound to one address and port — 127.0.0.1:5432, 0.0.0.0:80. A port is usually described as a door the system opens or closes. Nothing on the machine looks like that: a port is a 16-bit number in a TCP header, and when a SYN arrives the kernel asks one question — does any socket hold that number? You cannot open a port. You can only start a process that listens on it, which is why no firewall rule makes an absent process answer. ss -ltn lists them; sudo ss -tlnp names the process holding each. No line for your port means nothing listens, and the next hour belongs to that service's journal.