Ports

A one-stop Linux troubleshooting reference organized by process/load, port/network, disk/memory, and logs/services, helping you quickly spot performance bottlenecks, connection anomalies, and resource hogs during live incidents.

When a port refuses to start or a deleted file still pins disk space, lsof answers "who opened what": by process, port, user, or file. This reference covers the multi-axis lookups (listener + process, file-to-user, connections) and practical flags like +L1 for freeing files marked deleted-but-still-held.

For checking connection states, confirming what is listening, and counting per-protocol connections, netstat is the veteran Linux command, complementary to ss. This reference covers listing connections, filtering by TCP/UDP, listening ports, the routing table, and interface stats, and flags -p to pin down the owning process.

netcat (`nc`) is a Swiss-army knife for network checks: test whether a port responds, stand up a throwaway listener, relay between sockets, or move a file — without installing a server. This reference covers connect/scan, listen-and-echo, HTTP debugging, forwarding, and UDP mode, for quick connectivity checks under pressure.