# WHATPORT(631)

## NAME

port 631 — IPP / CUPS — The Internet Printing Protocol — a printer, or the CUPS print server on a Mac or Linux box.

Protocol: TCP

## DESCRIPTION

IPP is HTTP with a binary message body: clients POST operations like Print-Job and Get-Printer-Attributes to an ipp:// URL, and RFC 8010 requires printers to support HTTP on the IANA-assigned well-known port 631. CUPS, the print system on macOS and most Linux distributions, listens here for both printing and its web administration interface at http://localhost:631/. Nearly every AirPrint or IPP Everywhere network printer answers on 631 as well, which makes it one of the most common open ports on an office LAN.

## EXPOSURE

**Expose with care.** Fine on a trusted LAN, where it is designed to live. Exposed publicly it leaks the print queue and device details and hands strangers the CUPS admin interface and a print-job submission path — bind CUPS to localhost or the local subnet.

## CHECK

```sh
curl -sI http://TARGET:631/
```

## SEE ALSO

- spec — [RFC 8010 — IPP/1.1: Encoding and Transport (well-known port 631)](https://www.rfc-editor.org/rfc/rfc8010.html) (rfc-editor.org)
- spec — [RFC 8011 — IPP/1.1: Model and Semantics](https://www.rfc-editor.org/rfc/rfc8011.html) (rfc-editor.org)
- source — [OpenPrinting CUPS](https://github.com/OpenPrinting/cups) (github.com)

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