The OpenPrinting CUPS Snap

Complete CUPS printing stack in a Snap

CUPS in the Snap Store

All from OpenPrinting in the Snap Store

Snapping applications which print

Introduction

This is a complete printing stack in a Snap. It contains not only CUPS but also cups-filters, Ghostscript, and Poppler (the two latter as PostScript and PDF interpreters). This is everything (except printer-model-specific drivers) which is needed for printing.

This Snap is designed for the following three use cases:

1. Providing a printing stack for a purely Snap-based operating system, like Ubuntu Core.
2. Providing a printing stack for a classic Linux system, installing the Snap instead of the system's usual printing packages. It is planned that Ubuntu Desktop switches over to use CUPS from this Snap.
3. Being a proxy/firewall between application Snaps which print and the system's classically installed (DEB, RPM, ...) CUPS, to block administrative CUPS requests.

Note that this Snap is still under development and therefore there are probably still many bugs.

Installation and Usage

The CUPS Snap works on both classic systems (standard Linux distributions like Ubuntu Desktop) and purely snap-based systems (like Ubuntu Core).

If on your classic system there is already CUPS running, the CUPS Snap's cups-daemon works as a proxy to protect the system's CUPS daemon against adminstrative requests from applications from the Snap Store (proxy mode). For this the CUPS Snap will get automatically installed as soon as a Snap which prints is installed from the Snap Store. Then it does its job fully automatically without any configuration (like creating queues) needed by the user.

For developemnt purposes the CUPS Snap's CUPS daemon can also be run as an independent second daemon in parallel to the system's CUPS, on an alternative port and domain socket (parallel mode). Note that running two independent CUPS instances on one system is not recommended on production systems. It is error-prone and confusing for users. This mode is also not well tested. Disable or remove your system's CUPS if you want to use the Snap's CUPS as your standard CUPS.

Usually if you decide to manually install this Snap, you want to use it as the standard CUPS for your system. Therefore we tell here how to disable the system's original, classically installed CUPS and run the CUPS Snap as standard CUPS (stand-alone mode).

Stand-alone mode

NOTE: The CUPS Snap does not support classic printer drivers (consisting of filters and PPD files, usually installed as DEB or RPM packages), but only Printer Applications (see below, links to Printer Applications: "Printer Applications" under "Discussion and Links")

If the Snap's CUPS runs alone, the standard resources, port 631 and (/var)/run/cups/cups.sock are used to assure maximum compatibility with both snapped and classically installed client applications.

To assure that the CUPS Snap will be the only CUPS running on your system and you have a systemd-based system (like Ubuntu), run

sudo systemctl mask cups-browsed
sudo systemctl stop cups-browsed
sudo systemctl mask cups
sudo systemctl mask cups.socket
sudo systemctl stop cups

and either

sudo mv /etc/cups /etc/cups.old

or

sudo touch /var/snap/cups/common/no-proxy

before downloading and installing the CUPS Snap.

The stop commands stop the daemons immediately, the mask commands exclude them from being started during boot (use unmask to restore). The mv command makes the system's CUPS completely invisible to the CUPS Snap, to prevent the Snap from entering proxy mode. The alternative touch command creates a file to make the CUPS Snap suppress its proxy mode altogether (remove the file to re-enable proxy mode), meaning that, as before, we get stand-alone mode if no classic cupsd is running and parallel mode otherwise.

The Snap is available in the Edge channel of the Snap Store and from there one can install it via

snap install --edge cups

If you want to install from source, you have to build it via

snapcraft snap

or if this does not work on your system with

snapcraft cleanbuild

and install it with the command

sudo snap install --dangerous <file>.snap

with <file>.snap being the name of the snap file.

For maximum compatibility with most snapped and unsnapped applications this Snap's CUPS will be accessible through the usual socket /run/cups/cups.sock and port 631.

To use use the snap's command line utilities acting on the snap's CUPS, preceed the commands with cups.:

cups.lpstat -H
cups.lpstat -v
cups.cupsctl
cups.lpadmin -p printer -E -v file:/dev/null

You can run administrative commands without sudo and without getting asked for a password if you are member of the "lpadmin" group if it exists on your system, otherwise if you are member of the "adm" group (this is the case for the first user created on a classic Ubuntu system). This works on classic systems (you can also add a user to the "lpadmin" or "adm" group) but not on snap-based systems (the standard user is not in the "adm" group and you cannot add users to the "adm" group). You can always run administrative programs as root (for example running them with sudo).

The snap's command line utilities can only access files in the calling user's home directory if they are not hidden (name begins with a dot '.'). So you can usually print with a command like

cups.lp -d <printer> <file>

For hidden files you have to pipe the file into the command, like with

cat <file> | cups.lp -d <printer>

or copy or rename the file into a standard file.

The web interface can be accessed under

http://localhost:631/

To make administrative tasks working, you have to enter user name and password of a user in the "lpadmin" or "adm" group, or "root" and the root password if your system is configured appropriately.

You can also use the standard CUPS utilities installed on your system when the CUPS Snap is in stand-alone mode as then its CUPS daemon listens on the standard domain and port.

Proxy mode

Only if there is already a CUPS instance running on your system (like one installed via classic Debian or RPM packages), the Snap's CUPS will run in a proxy mode which makes the CUPS Snap's CUPS daemon run in parallel to the system's classic CUPS daemon to serve as a proxy for snapped applications to prevent those applications from performing administrative tasks (like modifying print queues) on the system's CUPS.

If you have an application which prints in a Snap and such a Snap is installed from the Snap Store, the Snap's "cups" plug is auto-connected to the appropriate slot of the CUPS Snap, and if the CUPS Snap is not installed, it gets installed automatically, going into stand-alone mode if the system has no CUPS installed and into proxy mode otherwise, fully automatically without need of user intervention. The snapped application can only print to the snapped CUPS, never to a classically installed system CUPS, and the snapped CUPS only allows it to list queues, jobs, and printer options, and to print, not to do admninistrative tasks as creating or modifying queues.

If there is a classic CUPS installed and therefore the snapped CUPS is in proxy mode, the snapped CUPS mirrors all print queues of the classic CUPS and so the snapped application sees the same printers as an unsnapped application which directly accesses the system's CUPS. The mirrored queues have exactly the same printer options and jobs are passed unfiltered to the system's CUPS where the user's drivers, especially classic and proprietary drivers which are not available as Printer Applications, convert the data to the printer's language and send the jobs off to the physical printers.

Even discovered IPP printers for which the system's CUPS creates temporary queues on-demand are mirrored, but as permanent queues on the snapped CUPS.

The configuration of the system's CUPS does not need to be changed by the user for that, nor is it changed by the CUPS Snap (the CUPS Snap does no administrative action on the system's classic CUPS at all). The printers of the system's CUPS do not even need to get shared for the proxy to work. If unsnapped applications on the local machine can print, the proxy works.

All this goes fully automatically. The user does not need to do anything with the CUPS Snap in proxy mode. All printing administration he performs on the system's CUPS.

Parallel mode

NOTE: Using this mode is not reconmmended on production systems. It is not well tested and error-prone. Two independent CUPS daemons on one system can also easily confuse users.

You can also run the CUPS daemon of the CUPS Snap as an independent second CUPS daemon in parallel to the system's classically installed CUPS daemon. This is for development purposes to test the interaction of the two CUPS daemons and their attached cups-browsed instances.

This mode does not get invoked automatically. To activate it, you have to create a file to tell the Snap to use this mode instead of proxy mode if there is already a classically installed CUPS. Run

sudo touch /var/snap/cups/common/no-proxy

and then restart the Snap with

sudo snap stop cups
sudo snap start cups

to switch into parallel mode. Remove the file and restart the CUPS Snap again to get back to proxy mode. Note that proxy mode removes all print queues you have created in parallel mode.

In this mode the Snap's CUPS will run on port 10631 (instead of port 631) and it will use the domain socket /var/snap/cups/common/run/cups.sock (instead of the standard (/var)/run/cups/cups.sock).

The Snap's utilities (names prefixed with cups.) will access the Snap's CUPS, the system's utilities (no prefixes) will access the system's CUPS.

The web interface is available under

http://localhost:10631/

You can also access the snap's CUPS with the system's utilities by specifying the server (example if the snap's CUPS runs in parallel with a system's one, on port 10631 and /var/snap/cups/common/run/cups.sock):

lpstat -h localhost:10631 -v
lpstat -h /var/snap/cups/common/run/cups.sock -v

For the very rare case that you want to do administrative tasks on your system's classically installed CUPS using the utilities which come with the CUPS Snap, you need to manually connect the utilities' "cups-internal" interface:

sudo snap connect cups:cups-internal cups:cups-control

cups-browsed

cups-browsed is automatically started together with CUPS in both stand-alone and parallel mode. So queues to discovered IPP printers and shared queues on remote CUPS servers get automatically created.

Configuration files and logs

Independent in which mode the CUPS Snap is running, the configuration files are in the

/var/snap/cups/common/etc/cups

directory. They can be edited. To make sure you chnges do not get overwritten by CUPS and to get your changes activated, stop the CUPS Snap before editing

sudo snap stop cups

and after editing start it again:

sudo snap start cups

Note that some items in the configuration files cannot be changed and get overwritten by the CUPS Snap when it is started. These settings are required for CUPS being able to run in the confinements of a Snap.

Log files you find in

/var/snap/cups/current/var/log

The CUPS Snap is set to debug mode by default, so you have verbose logs for CUPS (error_log), cups-browsed (cups-browsed_log, stand-alone and parallel mode), and cups-proxyd (cups-proxyd_log, proxy mode).

What is planned/still missing?

• Spin out cups-browsed into a separate Snap
• Add a script for easy migration from a classically installed CUPS to the CUPS Snap

Printer drivers deprecated -> Printer Applications

Printer drivers (printer-model-specific software and/or data) in the form of filters and PPD files added to CUPS are deprecated. They get replaced by Printer Applications, simple daemons which emulate a driverless IPP printer on localhost and do the filtering of the incoming jobs and connection to the printer. These daemons will also be packaged in Snaps, to make the driver packages distribution-independent.

Therefore we will not add a printer driver interface as it is not needed any more.

Note that this Snap DOES NOT support classic printer drivers!

Nearly all free software printer drivers (at least the ones for which there are packages in the Debian Linux distribution) are available in Printer Application Snaps now. See the section "Printer Applications" under "Discussion and Links" below.

There is a Printer Application for PostScript, HPLIP, and Gutenprint. All the other drivers are joined together in the GhostScript Printer Application.

See also the overview in the Snap Store.

In addition, there is also the Legacy Printer Application contained in pappl-retrofit which makes all drivers classically installed for the system's classically installed CUPS available in a Printer Application and this way for the CUPS Snap. It is especially helpful for drivers which are not (yet) available as Printer Application. Note that this Printer Application cannot be provided as a Snap as it otherwise had no access to the drivers.

If your printer is a driverless IPP printer (AirPrint, Mopria, IPP Everywhere, Wi-Fi Direct Print) you do not need a Printer Application. If you connect such a printer via USB, you need IPP-over-USB support, also available as Snap.

Default paper size

The CUPS in this Snap uses libpaper which allows to configure a system-wide default paper size by simply dropping the name of the desired size into the configuration file named papersize, usually "a4" or "letter". CUPS is then supposed to use this paper size as the default for newly created print queues.

This actually works for creating queues via the CUPS web interface but not with the lpadmin command line utility or with some printer setup tools, so it is of restricted use.

But if you are user of the web interface and get notorically the wrong paper size as default, create a file named /var/snap/cups/common/etc/papersize and drop the name of your desired default page size in it, in a single line, without spaces, commands, ..., simply the page size name, usually "a4" or "letter". Then restart the CUPS Snap for the change to take effect.

Discussion and Links

Call for testing:

• Call for testing on Snapcraft forum
• Call for testing on Discourse

Printing with Snaps:

• New interface: “cups” for all Snaps which print (How to use, how it exactly works)
• Printing and managing printers from your Snap

Documentation requests:

• The "cups" interface, by Graham Morrison
• My original documentation request for the "cups" interface

The development of this Snap is discussed on the Snapcraft forum:

• “cups” interface merged into snapd - Additional steps to complete (solved)
• Handling of the “cups” plug by snapd, especially auto-connection (How we came to the proxy mode of the CUPS Snap, solved)
• CUPS Snap: Needs fontconfig for text filter, get “Fontconfig error: Cannot load default config file”
• General development
• Developer sprint Sep 17th, 2018
• Interface request: “cups-control” on CUPS snap and including D-Bus
• Snapping CUPS Printing Stack: Providing cups-control interface
• Snapping CUPS drivers as plugins (DEPRECATED)

Related topics on the forum:

• How to add or workaround a udev rule
• No mdns support in snaps (should core have a modified nsswitch.conf ?)
• Multiple users and groups in snaps
• Snapped daemon running as root cannot create file in directory with odd ownerships/permissions
• Hardware-associated snaps - Snap Store search by hardware signature
• User authentication in snapd (pam mediation)

Getting the Snap into the store:

• Call for testing: OpenPrinting’s printing-stack-snap (Printing in a Snap) (DEPRECATED)
• Post a snap on behalf of OpenPrinting

Printer Applications

• PostScript Printer Application (Snap Store): Printer Application Snap for PostScript printers which are supported by the manufacturer's PPD files. User can add PPD files if the needed one is not included or outdated.
• Ghostscript Printer Application (Snap Store): Printer Application with Ghostscript and many other drivers, for practically all Linux-supported printers which are not PostScript and not supported by HPLIP or Gutenprint.
• HPLIP Printer Application (Snap Store): HPLIP in a Printer Application Snap. Supports nearly every HP printer ever made. Installing HP's proprietary plugin (needed for a few printers) into the Snap is supported and easily done with the web interface.
• Gutenprint Printer Application (Snap Store): High quality output and a lot of knobs to adjust, especially for Epson and Canon inkjets but also for many other printers, in a Printer Application Snap.
• Legacy Printer Application (not available as Snap): It is a part of the pappl-retrofit package and it makes drivers classically installed for the system's classically installed CUPS available in a Printer Application and this way for the CUPS Snap. It is especially helpful for drivers which are not (yet) available as Printer Application.
• PAPPL: Base infrastructure for all the Printer Applications linked above.
• PAPPL CUPS driver retro-fit library: Retro-fit layer to integrate CUPS drivers consisting of PPD files, CUPS filters, and CUPS backends into Printer Applications.
• Printer Applications 2020 (PDF)
• Printer Applications 2021 (PDF)
• CUPS 2018 (PDF, pages 28-29)
• CUPS 2019 (PDF, pages 30-35)
• cups-filters 2018 (PDF, page 11)
• cups-filters 2019 (PDF, pages 16-17)
• cups-filters 2020 (PDF)
• cups-filters 2021 (PDF)

Snapping of ipp-usb

• [DONE] ipp-usb Snap in the Snap Store
• ippusbxd got discontinued and replaced by ipp-usb, so it will not get snapped
• Feature request: Support for systemd templates (Not needed for the ipp-usb Snap)

Requests for auto-connection to interfaces

• Request: CUPS Snap auto connection to "cups-control" interface (accepted)
• Request: CUPS Snap to use the cups-socket-directory attribute and auto-connection of any Snap’s “cups” plug to the “cups” slot of the CUPS Snap (accepted)
• Request: CUPS Snap (“cups”) auto connection to of cups:cups-control to cups:cups-control and also of the network-manager-observe interface (accepted)
• Request: CUPS Snap (“cups”) auto connection to avahi-control, raw-usb, cups-control, and system-files interfaces (accepted)
• Request: Printing Stack Snap auto connection to avahi-control, raw-usb, and home interfaces (DEPRECATED)

Links on other platforms:

• Pull request on cupsd to fix CUPS D-Bus service access via "cups-control" interface
• Pull request on snapd to fix detection if cups interface is connected (merged)
• Pull request on snapd for adding the cups interface printing always through CUPS Snap (merged)
• Pull request on snapd for adding a new exit code for snapctl --is-connected for cases when the peer is from the same snap (solved differently)
• Pull request on snapd for making the cups interface implicit on classic (dropped)
• Pull Request on snapd for adding the client Snap checking API (merged)
• Pull request on snapd-glib for adding support for exit codes returned by snapctl commands issued via library function (merged)
• Pull request on CUPS to add Snap mediation (merged)
• Trello card about adding API to check client Snaps whether they plug a certain interface