A network printer: send bytes, they print — no protocol, no login.
Registered with IANA as pdl-datastream / hp-pdl-datastr, this is the AppSocket protocol, better known as JetDirect after the HP print servers that popularised it. There is no handshake and no authentication: the client opens the socket, writes a page description stream such as PostScript, PCL, or ZPL, and closes it. CUPS drives it with the socket backend (socket://host:9100), and it is one of the most commonly open ports on any office network. Label printers, MFPs, and print server appliances all sit here.
Do not expose
Anyone who can reach the port can print, waste consumables, and on many devices push firmware or PJL commands that change settings — there is no authentication in the protocol at all.
$ nc -vz -w 3 TARGET 9100Seen 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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Corrections or a missing port? Reply @rimrocksystems.