A Forgotten Docker Container Had Been Squatting on Port 3000 for Months

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A developer discovered that a Docker container, launched in spring to test Open WebUI and never stopped, had been silently occupying port 3000 on their local machine for months. Each time the Next.js dev server started, it simply shifted to port 3001 without complaint, and the developer moved on without investigating. The mystery was solved using the 'ss -ltnpe' command, which revealed the socket's owning cgroup as docker.service — no root privileges required. The incident highlighted that 'listing open ports' is actually three distinct questions — what is listening, who owns it, and whether it is reachable — each requiring different tools and privilege levels. Commands like ss, lsof, fuser, and nc each answer a different aspect of the problem, and conflating them is the source of much misleading advice online.

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