CUPS

CUPS (formerly an acronym for Common UNIX Printing System) is a modular printing system for Unix-like computer operating systems which allows a computer to act as a print server. A computer running CUPS is a host that can accept print jobs from client computers, process them, and send them to the appropriate printer.

CUPS consists of a print spooler and scheduler, a filter system that converts the print data to a format that the printer will understand, and a backend system that sends this data to the print device. CUPS uses the Internet Printing Protocol (IPP) as the basis for managing print jobs and queues. It also provides the traditional command line interfaces for the System V and Berkeley print systems, and provides support for the Berkeley print system's Line Printer Daemon protocol and limited support for the Server Message Block (SMB) protocol. System administrators can configure the device drivers which CUPS supplies by editing text files in Adobe's PostScript Printer Description (PPD) format. There are a number of user interfaces for different platforms that can configure CUPS, and it has a built-in web-based interface. CUPS is free software, provided under the Apache License.

Michael Sweet, who owned Easy Software Products, started developing CUPS in 1997 and the first public betas appeared in 1999. The original design of CUPS used the Line Printer Daemon protocol (LPD), but due to limitations in LPD and vendor incompatibilities, the Internet Printing Protocol (IPP) was chosen instead. CUPS was initially called "The Common UNIX Printing System". This name was shortened to just "CUPS" beginning with CUPS 1.4 due to legal concerns with the UNIX trademark. CUPS was quickly adopted as the default printing system for most Linux distributions. In March 2002, Apple Inc. adopted CUPS as the printing system for Mac OS X 10.2. In February 2007, Apple Inc. hired chief developer Michael Sweet and purchased the CUPS source code. On December 20, 2019, Michael Sweet announced on his blog that he had left Apple. In 2020, the OpenPrinting organization forked the project, with Michael Sweet continuing work on it. Apple retained the builds for macOS, iOS, and iPadOS with latest release of Apple CUPS being version 2.3.6 on May 25, 2022. In 2024, a critical vulnerability involving remote code execution in CUPS was found impacting all GNU/Linux systems.

CUPS provides a mechanism that allows print jobs to be sent to printers in a standard fashion. The print data goes to a scheduler which sends jobs to a filter system that converts the print job into a format the printer will understand. The filter system then passes the data on to a backend—a special filter that sends print data to a device or network connection. The system makes extensive use of PostScript and rasterization of data to convert the data into a format suitable for the destination printer.

CUPS offers a standard and modularised printing system that can process numerous data formats on the print server. Before CUPS, it was difficult to find a standard printer management system that would accommodate the very wide variety of printers on the market using their own printer languages and formats. For instance, the System V and Berkeley printing systems were largely incompatible with each other, and they required complicated scripts and workarounds to convert the program's data format to a printable format. They often could not detect the file format that was being sent to the printer and thus could not automatically and correctly convert the data stream. Additionally, data conversion was performed on individual workstations rather than a central server.

CUPS allows printer manufacturers and printer-driver developers to create drivers more easily that work natively on the print server. Processing occurs on the server, allowing for easier network-based printing than with other Unix printing systems. With Samba installed, users can address printers on remote Windows computers, and generic PostScript drivers can be used for printing across the network.

The CUPS scheduler implements Internet Printing Protocol (IPP) over HTTP/1.1. A helper application (cups-lpd) converts Line Printer Daemon protocol (LPD) requests to IPP. The scheduler also provides a web-based interface for managing print jobs, the configuration of the server, and for documentation about CUPS itself.

An authorization module controls which IPP and HTTP messages can pass through the system. Once the IPP/HTTP packets are authorized they are sent to the client module, which listens for and processes incoming connections. The client module is also responsible for executing external CGI programs as needed to support web-based printers, classes, and job status monitoring and administration. Once this module has processed its requests, it sends them to the IPP module which performs Uniform Resource Identifier (URI) validation to prevent a client from sidestepping any access controls or authentication on the HTTP server. The URI is a text string that indicates a name or address that can be used to refer to an abstract or physical resource on a network.