I recently ran an NMAP scan of my Mac's open ports and discovered port 631 (ipp) was open. Intrigued, I saw it was for the CUPS web interface, and after enabling it I found it had something to do with printing. But what exactly is CUPS?

3 Answers 3


From the CUPS website:

CUPS is the standards-based, open source printing system developed by Apple Inc. for OS X and other UNIX®-like operating systems.

On OS X it provides local printer services as well as shared printer services for other Macs to access your printer via the service if the printer doesn't have its own sharing capabilities. CUPS has become fairly ubiquitous across the Linux/UNIX contingent of OSes as the print server if choice.

That open port you found is because printer sharing is turned on on your machine. You can close port via System Preferences > Sharing and uncheck Printer Sharing.

  • The 631 port is open even with Printer Sharing unchecked. This is on 10.10.4 at least.
    – Blaz
    Commented Jul 22, 2015 at 15:52
  • 1
    I would love it if someone passing by who knew why would tell me why the port is open even though printer sharing is off.
    – nycynik
    Commented Jan 12, 2018 at 13:37
  • see also discussions.apple.com/thread/2078738 includes instructions for disabling cups with launchctl
    – jimmont
    Commented Mar 11, 2019 at 5:48

Actually CUPS (the Common Unix Printing System) was an open source project that Apple bought. Contrary to what is listed at CUPS.org, CUPS existed long before Apple bought it.

I understand that CUPS remains Open Source and that (some?) code contributed by Apple works it's way back into the Open Source version available to and used by other O/S'

Note that CUPS is an excellent (though somewhat complicated) web interface to the printing subsystem on the Mac. In some rare cases it is possible to add and manage printers that do not offer a printer driver for the current version of Mac O/S but do offer one that works with CUPS.


The open port is for internal (localhost) services only. It should not be visible to a remote host, e.g. via the 'nmap' utility.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .