Registered as commplex-link, but in practice Synology DSM over HTTPS or an iperf 2 server.
The IANA name for 5001 is commplex-link, a registration with no public protocol behind it, so what you actually meet on this port is a squatter. On a home or small-office network it is usually a Synology NAS: DSM's web interface answers HTTP on 5000 and HTTPS on 5001 by default. On a lab or transit-test host it is more often an iperf 2 server, whose default listen port is 5001 (iperf3 moved to 5201).
Expose with care
A DSM login page on the public internet is a brute-force magnet — put it behind a VPN or reverse proxy with MFA. An iperf server accepts traffic from anyone who finds it and should never be left running on a public address.
$ curl -sk -o /dev/null -w '%{http_code}\n' https://TARGET:5001/Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
Get Beige Box →also available as JSON · Markdown
Corrections or a missing port? Reply @rimrocksystems.