Finding which process owns a port.
WHY THIS COMES UP CONSTANTLY
A service will not start because something already holds its port.
WHAT TO RUN
ss with options for listening, processes, and both protocols.
WHAT THE OUTPUT SHOWS
The address and port The process name and identifier
WHAT THE ADDRESS COLUMN MEANS
An address of all zeroes means every interface A loopback address means local only A specific address means that interface only
WHY THAT DISTINCTION MATTERS
A service listening only locally is unreachable from outside, whatever the firewall says.
WHAT TO DO WHEN A PORT IS IN USE
Identify the process, and decide whether it should be running.
WHAT COMMONLY HOLDS A PORT UNEXPECTEDLY
A previous instance that did not exit A different service configured for the same port
WHAT TO DO ABOUT A STALE PROCESS
Stop it properly before starting the new one.
WHAT lsof PROVIDES HERE
The same answer, with more detail about the process.
WHAT TO CHECK WHEN A SERVICE RUNS BUT IS UNREACHABLE
That it is listening On the expected address That the firewall permits it That nothing upstream blocks it
WHY IN THAT ORDER
It moves outward from the machine.
