The Berkeley line printer daemon — a print queue, usually on a printer or an old Unix print server.
LPD, specified in RFC 1179, is the original Unix network printing protocol: a client connects to TCP 515 and sends a one-byte command code plus a queue name to submit a job, list the queue, or remove jobs. The spec also expects the client to bind a source port in the range 721–731, which is its entire notion of authentication — a privileged source port means a trusted host. Most network printers still answer here for backward compatibility even though IPP on 631 is what modern print systems use, and CUPS can serve LPD clients through its cups-lpd helper.
Do not expose
No real authentication — trust is based on source port and hostname — and job submission plus queue control is available to anyone who can reach the port. Keep print queues on the LAN.
$ nc -vz -w 3 TARGET 515Seen 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.
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