bind address

The address a process passes to the kernel as it opens its listening socket, deciding who can reach it: 127.0.0.1 for this machine's processes alone, 0.0.0.0 for every IPv4 address it has. It looks like network or kernel state, so people go hunting for it in ip a, in routes, in firewall rules. It is none of those: it is an argument, chosen once, and the value lives in the service's own configuration — HOST= in /etc/example-api.env, listen_addresses for PostgreSQL, a listen directive for nginx. Two consequences. A running process never re-reads it, so the edit is worth nothing until sudo systemctl restart. And curl http://127.0.0.1:3000/ cannot verify the fix, because that test passed before it. Read sudo ss -tlnp again, then test on the interface address.