Handling of the "cups" plug by snapd, especially auto-connection

Can you detail why this is too complex? I think that this would be a great solution, is to have the cups snap operate in two modes, one where there is no other cupsd on the system in which case the cupsd from the cups snap does everything, and two where cupsd from the cups snap detects another cups on the system and instead runs itself in the cupsd firewall/proxy mode, forwarding all requests to the classic cupsd the user has installed.

This setup would ensure that we can have the cups snap always be installed even on other distros where there is a classic cupsd and optionally classic drivers as well but still have a good user experience, and also would let us safely make it so that installing a snap which plugs the cups interface ends up also installing the cups snap.

This is not enough, since any kind of action the snapcraft-runner script to prevent access to cups is done after the confinement has been setup, so someone could just manually craft a snap which sidesteps the snapcraft-runner and thus is able to escalate privileges to lpadmin. Any kind of group dropping operation would need to be done by snap-confine inside snapd, and I don’t think we have any precedence for always dropping a group from the currently executing process, it feels a bit user-hostile to do that IMHO…

Edit: actually I see you came to that realization in your followup post…

Proposal for a new direction

I don’t want to disregard all the existing work that has been done on this, but I do want to propose what I view as a fundamentally better setup than what we have today, albeit a slightly more complicated one, that has IMHO the best user experience and security properties. Here is my proposal:

  1. The cups snap exposes a content interface slot which provides the cupsd socket from somewhere in $SNAP_DATA, etc. and not the global location - this content interface slot can be made to auto-connect globally to any snap wishing to connect a plug to it via store assertions

  2. Application snaps wishing to print can plug a content interface which auto-connects to the above content interface and uses default-provider to ensure that the cups snap always gets installed and can always provide the cupsd socket

  3. A snapcraft generated wrapper script from one of the extensions sets the CUPS_SERVER env var to the location of the cupsd socket that is shared over the content interface, and the extension also ensures that the content interface plug is added to any snap that is desktopy. Or maybe all this cups stuff becomes it’s own separate, orthogonal extension (that might be better since there are probably a lot of desktop snaps that don’t need to print and thus don’t need to have the the cups snap installed on the users system).

  4. The cups snap auto-detects whether there is already a global cups socket from a classically installed cupsd, and if that exists, then it operates in the firewall/proxy mode. If there is no such socket already being listened on, then cupsd from the cups snap either listens on the global cups socket in addition to the one in $SNAP_DATA (I know that this kind of thing can be done via Go to listen on multiple unix sockets at the same time, I dunno how easy it is to do in cupsd source code), or it creates a symlink at the global location pointing to the one in $SNAP_DATA, etc. (then unconfined apps not run as snaps just end up talking directly to the socket in $SNAP_DATA by following the symlink). This ensures that when there is a classic cupsd, classic apps talk to it without need to care that the other side is a snap, and snap apps always talk to cupsd over the content interface shared from the cups snap and thus are always mediated.

  5. We could then simplify the cupsd snap implementation to one that checks if the client process connecting to it’s socket is a snap or not, if it is a snap, then we always provide it permission to print, and we deny it ability to do admin tasks if the client process does not have the cups-control interface plugged.

  6. All of the above means we can actually just get rid of the cups interface as it exists, and simply make the claim that snaps can by default always print if they are doing so via the cups snap content interface which is sharing the socket (and a user wanting to deny access to printing can disconnect the content interface, then the app snap does not have access to the socket at all).

  7. Due to 6, we can simplify the policy in snapd to only allow an application slotting cups-control to read/write to the global socket in /run, and this global socket access is not allowed anywhere else. The cups interface can go away entirely, and the plug side of the cups-control interface also can just be empty, effectively only existing for the mediating version of cupsd to inspect and grant admin access to a snap plugging cups-control. If we really wanted we could even introduce a new interface cups-support which allows doing all the things that cupsd needs to do from inside a snap, but that’s probably unnecessary at this point and we can just put all of those accesses to the permanent slot side of cups-control.

The above setup has the following nice properties:

  • app snaps never are provided access to the global cups socket (which leads to confusing policy in snapd where it’s unclear if the cups interface is providing access to a global socket from another snap or access to a socket that is coming from the classic host world)
  • app snaps always are able to print by talking to a version of cups that does mediation to deny admin actions, since the only way they can talk to a cupsd socket is through the content interface shared socket
  • classic apps wanting to print can continue to do so using the normal location of the cups socket
  • users who already have cupsd classically installed can continue to use it and manage queues and drivers and such, and it will appear to them that their snap apps printing things are connecting directly to the classic cupsd instance (when in fact they are being proxied via cupsd)
  • we have simpler interface policies in snapd, and less complicated auto-connect rules (we just have the global auto-connect for the cups content interface slot from the cups snap)
  • a snap that wants to print will be able to do so without the user granting any special privileges to it, on any distro with or without classic cupsd installed on it.

Note that this entirely depends on the cups snap being able to nicely forward/proxy requests from snaps wishing to print, which if that doesn’t work well or is not maintainable then causes the whole plan to fall apart…

Now all of that is basically a total left turn from the current plan, so maybe it doesn’t make sense to do this just because it is a lot of work to suddenly do this instead of the existing plan, but I think that at the very least we should consider following this sort of paradigm for future classic applications that we want to start mediating more finely by moving them to snaps, and the only requirements for that classic application/service are:

  • that it can peacefully co-exist with a classically installed version of itself, effectively becoming a firewall/proxy if the classic version exists on the system
  • that it can expose both the global resources that classic non-snap applications need to use/interact with it (in the case that the snap version of this software is the only one on the system), as well as very finely controlled resources that are entirely sharable via the content interface

I’m really curious if @jdstrand has time to think about this, as someone who initially worked on the current plan if there are things I am missing here.

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@ijohnson Brilliant plan, it is great. I like it a lot.

Some remarks:

  • The firewall cupsd (the snapped one which is running in parallel to a classic one) only can forward print requests not admin requests. This is no problem as we go the firewalled printing path only for snapped clients using the content interface as they are not plugging cups-control. Snaps plugging cups-control always communicate with the “standard” CUPS (classic if present, snapped otherwise) and manage this one.
  • If we have a classic CUPS, the user creates CUPS queues on this classic CUPS and the user only manages this classic CUPS with printer setup tools or the web interface. The snapped CUPS in firewall mode will run fully automatic without being managed by the user. It does not open port 10631 but only uses the socket in $SNAP_DATA and also has its web interface turned off. It has its cups-browsed attached to see the queues of the classic CUPS and replicate them on the snapped firewall CUPS. The classic CUPS therefore needs one little change in configuration: If it is configured to not share its print queues it now must share them but to localhost only, so that the cups-browsed of the snapped CUPS can pick them up and forward them. This would require some mechanism to change the classic CUPS’ configuration automatically when the CUPS Snap gets force-installed as firewall.
  • As the CUPS Snap already automatically switched between two modes, first beint the only stand-alone/system CUPS and second being alternative CUPS in parallel to classic CUPS (on alternative port and socket) it is easy to change the alternative mode to a firewall mode. The startup scripts for both cupsd and cups-browsed in the CUPS Snap do all required configuration changes.
  • You say: “(6) All of the above means we can actually just get rid of the cups interface as it exists …” We can simply make the interface named cups being this content interface mentioned in (1), but be careful, a print client Snap not only needs access to the cupsd socket but also to the D-Bus interfaces of CUPS, as some clients want to subscribe to notifications. The D-Bus interfaces are the same as in cups-control.

CUPS and cups-browsed should have enough configuration options to be able to auto-configure them appropriately in the start-up scripts of the CUPS Snap so that we can run a cupsd in firewall mode. If something is missing, no problem, I have the upstream maintainer power over both and so I can add the missing configuration options.

For me it looks all workable, the most difficult part is probably to automatically change the classic CUPS’ configuration so that its printers get shared localhost-only.

Till

How would a user normally make this change?

@ijohnson, to change the configuration of a classically installed cupsd one needs to change cupsd,conf. This can be done with either by a user with “lpadmin” group membership or by root. As it works as root, a program which installs a package can do that. A daemon in a Snap including its startup script also runs as root, but the confinements of the Snap usually block modifying external files or restarting an external daemon, but the CUPS Snap can access the global CUPS socket /run/cups/cups.sock and as its CUPS daemon’s startup script runs as root, it should be able to modify the classic CUPS’ configuration, with code like in cupsctl.

But I also came to another idea: As a locally running use application sees the locally available CUPS printers, lists them in its print dialog, and also prints on them, even without the local CUPS sharing its printers, I could add a mode to cups-browsed which discovers printers on a given CUPS socket (which is not the same as of the CUPS the cups-browsed is attached to) and creates queues on the Snap’s local CUPS appropriately, so that jobs to these queues get passed on to the classic CUPS. This would not need any configuration change on the classic CUPS.

Here I need to find out which is the better solution.

@ijohnson, good news, one does not need the classic CUPS to share printers. The shared bit is only about making the printers available to remote machines. On the local machine they are accessible through their usual URI ipp://localhost:631/printers/QUEUE. Without sharing they are only not DNS-SD-registered, so I need to add an option to cups-browsed to discover printers on a given socket via cupsGetDests() API.

@ijohnson, I am currently working on a cups-proxyd which mirrors the queues of the system’s cupsd to the Snap’s cupsd, so that the Snap’s cupsd has the same queues but passing the jobs unfiltered to the system’s cupsd (drivers are on the system’s cupsd).

I do not expand cups-browsed to use that, because operations here are well different, and complications as taking note of network interfaces or the implicitclass backend should be avoided. Also cups-browsed is supposed to get spun out of the CUPS Snap. cups-proxyd will then be run by the startup script for cupsd, no new “app” created in snapcraft.yaml.

There is only one problem which I discovered now with @alexmurray here, the “cups” plug for client Snaps only allows access to /run/cups/cups.sock not to the alternative socket in $SNAP_DATA, /var/snap/cups/common/run/cups.sock. This needs to be added, I am talking with @jamesh about this currently.

@ijohnson, @alexmurray, @jamesh, in case of the Snaps plugging “cups” auto-installing the CUPS Snap, we should even think about the “cups” plug only allowing access to /var/snap/cups/common/run/cups.sock and not to /run/cups/cups.sock.

Hi @till.kamppeter, from my proposal above, you don’t need to provide access to /var/snap/cups/common/run/cups.sock to client app snaps wishing to print via the cups interface. Instead in my proposal you would have definitions like this:

cups snap snapcraft.yaml (approximate)

slots:
  cups-socket:
    interface: content
    content: cups-socket
    source:
      write:
        - $SNAP_COMMON/cups.sock

apps:
  cupsd:
    slots: 
      - cups-control
  cups-print-only-test:
    environment:
      CUPS_SOCKET: /var/lib/cups/cups.sock
    plugs: {} # empty because it doesn't need any special permissions - 
              # it can access the socket hence it can print
  cups-admin-test:
    environment:
      CUPS_SOCKET: /var/lib/cups/cups.sock
    plugs:
      - cups-control # here we need to use cups-control since this test app
                     # needs to do more than just print, it needs admin things too

client snap wanting to print

plugs:
  cups-socket:
    interface: content
    content: cups-socket
    default-provider: cups # this line ensures that if the system installing this snap does not
                           # already have cups snap installed, it will get automatically installed
    target: ​/var/lib/cups/cups.sock # or wherever is convenient
environment:
  CUPS_SOCKET: /var/lib/cups/cups.sock

Under this situation as I described above, we don’t actually need to use the cups interface verbatim at all, we can resort to using only the cups-control interface and the cups-socket content interface.

Hopefully this helps.

@ijohnson, thank you very much.

You are using the CUPS socket /var/lib/cups/cups.sock for the client to access CUPS through. Is it intended that this is not the standard CUPS socket /run/cups/cups.sock? Or did you simply not remember correctly or using an old socket path?

@ijohnson, what will exactly happen if a Snap which wants to print is installed of a system without CUPS Snap and with classic CUPS installed? The classic CUPS is listening on /run/cups/cups.sock. The installation of the Snap triggers the installation of the CUPS Snap. Now the CUPS Snap sees the presence of the classic CUPS and therefore it goes into proxy/firewall mode (classic CUPS stays running without changes, Snap’s CUPS mirrror’s queues of classic CUPS and passes jobs on to there).

How does it now work that unsnapped apps printing to /run/cups/cups.sock print directly to the classic CUPS while the snapped app printing to /run/cups/cups.sock prints through the content interface connection to the snapped CUPS?

Or do you really mean /var/lib/cups/cups.sock as a different socket file compared to /run/cups/cups.sock? Why do you then use something in /var/lib/...? This would need a separate permission as it is outside the Snap. Would be $SNAP_COMMON/run/cups.sock not the better choice?

@jamesh, on Mattermost you told:

You probably can’t use the content interface directly, but having the cups and cups-control interfaces perform a bind mount is certainly possible.

How does this get implemented?

@jamesh on Mattermost:

you can’t use the content interface to bind mount to /run/cups. Also, if we put the logic into the cups interfaces, it hides the complexity from application snaps on the plug side, content the content interface can only mount to filesystem locations the snap controls like $SNAP, $SNAP_DATA, and $SNAP_COMMON.
While you can use layouts to mount from those locations to other parts of the file system, snapd doesn’t do well at always ordering the mounts (especially if it is updating an existing mount namespace)

@ijohnson, this means that you cannot use /var/lib/cups/cups.sock as target for the slot. It must be something like $SNAP_COMMON/run/cups.sock or so.

@ijohnson, @jamesh, I have now implemented the new proxy mode, as you have suggested:

It was a lot of work and took me a lot of time, as I had to create a new daemon (cups-proxyd) for auto-mirroring the queues of the system’s classic CUPS to the Snap’s CUPS, including the discoovered printers for which the classic CUPS auto-creates temporary queues.

I also had to create a new CUPS backend (proxy) to allow the mirrored queues of the snapped CUPS to pass the jobs on to their original queues on the classic CUPS without needing the classic CUPS to share the queues. So nothing on the configuration of the system’s CUPS has to be changed. The CUPS Snap in proxy mode does not do any administrative tasks on the system’s CUPS at all.

What is still missing and what I will do as the next step is to create the content interfaces.

Updated the README.md of the CUPS Snap:

This documents how the new proxy mode is working, how it is invoked, and also the stand-alone (CUPS Snap is system’s CUPS) and parallel (classic CUPS and snapped CUPS are two independent instances on one system) modes.

Some technical background info to the new proxy mode:

  • The proxy mode is invoked if there is an /etc/cups/cupsd.conf file present and readable (at least for root in the Snap), independent of whether there is actually a CUPS daemon running or not. This form of proxy mode invocation prevents race conditions on whether the CUPS Snap or the system’s CUPS starts first during boot. We cannot for example check for the presence of the cupsd executable file, as the Snap can only read /etc/cups in the system.
  • The mirroring of print queues from the system’s CUPS daemon to the Snap’s CUPS daemon is done by an auxiliary daemon named cups-proxyd. This daemon listens for appearing and disappearing of arbitrary IPP print services via DNS-SD (to get note of printers which could create temporary on-demand queues on the system’s CUPS) and for print queue addition/modification/removal and printer status change via system’s CUPS D-Bus notifications (to get note of queue modifications on the system’s CUPS even if the queues are not shared). On each event it freshly mirrors all queues from the system’s CUPS to the Snap’s CUPS and also removes disappeared queues from the Snap’s CUPS. It even mirrors temporary queues which CUPS creates on-demand for discovered IPP printers, by force-creating them on the system’s CUPS via a dummy access and then mirroring them.
  • There is no apps: entry in snapcraft.yaml for cups-proxyd. cups-proxyd is completely managed by the run-cupsd and stop-cupsd scripts. We do not run it as drop-in replacement for cups-browsed by the run-cups-browsed script as we want to spin out cups-browsed into its own Snap later.
  • cups-browsed is not run in proxy mode, the run-cups-browsed script is simply running alone, without cups-browsed, with a dummy daemon process which stop-cups-browsed can kill.
  • 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 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.
  • To allow the queues of the Snap’s CUPS pass on the jobs to the original queues on the system’s CUPS without the system’s CUPS needing to share them, a special CUPS backend named proxy got created, which gets the system’s CUPS’ socket and the queue name via the device URI and prints the same way as lp would do, passing on the options of the original job.
  • The mirrored queues on the Snap’s CUPS do not filter the jobs any further than to PDF. The remaining filtering to the printer’s native language is done by the system’s CUPS. So the user’s drivers (especially proprietary ones) are continued to be used. The options are made available in the mirrored queus simply by copying the PPD files, but the filter rules in the PPD files are changed to stop the filtering at PDF.
  • cups-proxyd is simple, it has no configuration file, it is controlled only by its command line. It has a log file, in the CUPS Snap it is /var/snap/cups/current/var/log/cups-proxyd_log.
  • The source code of cups-proxyd and proxy is maintained in the CUPS Snap project, in the cups-proxyd/ subdirectory, as these only make sense in the Snap.

@ijohnson, CUPS listening on two domain sockets is no problem at all, it always was capable of this, simply two Listen ... lines in cupsd.conf pointing to the domain sockets do the trick. So (4) in your initial presentation of the proxy idea is solved by my following commit on the CUPS Snap (no change on CUPS itself needed):

It makes simply the run-cupsd script add two Listen ... lines to cupsd.conf when it finds out that we will run in stand-alone mode:

Listen /run/cups/cups.sock
Listen /var/snap/cups/common/run/cups.sock
Port 631
[...]

Now the snapped CUPS listens on /var/snap/cups/common/run/cups.sock in all three modes (stand-alone, proxy, parallel), so that snapped applications can always print to this domain socket. In stand-alone mode unsnapped applications print also to the snapped CUPS as it is also listening on /run/cups/cups.sock then.

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@ijohnson, @jamesh, now I only need to create the content interface for the snapped clients to force-install the CUPS Snap and having access to /var/snap/cups/common/run/cups.sock for printing, but at the same time the snapped clients need to also have access to D-Bus notifications of CUPS (push notifications from CUPS for printer status changes, job progress, …, no admin tasks possible via D-Bus) (as cups-control also provides).

What I like most here would be an interface named cups containing both the content interface and the D-Bus access. Or do we have to split into cups (or cups-dbus) for the D-Bus and cups-socket for the content interface (as in this example)?

@ijohnson, note that the name of the environment variable is CUPS_SERVER and not CUPS_SOCKET here.

Sorry for taking a while to respond on this. My thoughts are still that we should keep the things as simple as possible for applications, and preferably allow communication with the system CUPS instance.

Now for some specific feedback:

I think a bind-mount like the content interface provides here is going to be part of the solution, but I’m not sure we want to make use of the content interface directly. In particular:

  1. we can’t use this to talk to the host system CUPS.
  2. any application snap will need plug definition and environment variable boilerplate to make proper use of it.
  3. we can’t use it to make the CUPS socket appear at the default /run/cups/cups.sock location, since the content interface only allows bind mount targets under $SNAP/, $SNAP_DATA/, or $SNAP_COMMON/. While layouts allow creating bind mounts outside of those directories, trying to chain the content interface and layouts together is error prone.

One other option would be to modify the existing cups and cups-control interfaces to perform the mounts themselves. That would allow us to bind mount over /run/cups from some directory managed by the slot-side snap, and skip the bind mount when connecting to an implicit system slot.

As for what directory to use as the source for the bind mount, this could either be something fixed by the interface, or specified by an interface attribute on the slot. I don’t think we want to do anything fancy like perform a bind mount over /run/cups on the slot side. While that would allow an unmodified CUPS to create its socket in the normal location, it would also make it impossible to implement the “proxy cups” model, where a snapped CUPS talks to a system CUPS.

What is the advantage of having a snap being able to talk directly to the host system’s CUPS if the proxy mode of the cups snap can provide all the necessary information whilst being mediated?

This is inconvenient, but I don’t view it as a blocker or a large enough disadvantage to stop CUPS from operating in proxy mode, if it is more complicated than a few environment variables, then I think that something like a snapcraft extension and command-chain script could alleviate this situation.

I agree using the content interface with layouts has been error prone in the past, if you think this is a barrier to using it, then we should a) work on identifying those bugs as much as possible and b) if we are already having folks redirect where to find the cups socket via an environment variable, why not just make that environment variable point to somewhere we don’t have to use layouts with, i.e. $SNAP_COMMON, then this is not an issue.

I’m not opposed to this at all, it sounds like from @till.kamppeter’s comments that there are other things that are needed in addition to accessing the socket, mainly D-Bus access in order to print. Is that correct @till.kamppeter ? This does slightly change my proposal, but still I think can be done in a very clean and always mediating way.

I would really like to avoid as much as possible having an implicit slot here since AIUI the cups snap can always do mediation for any distro, then we should have every snap always use the cups snap to the point where implicit slots are not needed.

This isn’t a problem if we require that snaps always have to talk to CUPS through the cups snap’s shared socket, the work from @till.kamppeter ensures that this socket will always expose everything one can do natively outside of the snap sandbox, so from my understanding there is no advantage to a snap being able to talk directly to the host’s CUPS (which may not do any mediation remember).

Just to take a step back, I’m not trying to make this needlessly complicated. I just know that folks have wanted an easy way to print from snaps for a long time, specifically one which doesn’t require fiddling with permissions. In this proxy mode, with a few simple modifications (env vars and the like) to client applications enables those snaps to always be able to do the simple act of printing from inside a snap without needing to connect any interface on any distro that snaps support. Whereas if we just stick to the existing state of things with the cups slot being implicit on other distros, users still need to go and figure out how to connect interfaces and deal with permissions for “something as simple as printing”. I want to make the best experience available for all users of snaps, on any distro. If it’s a bit extra work to make this work, I think it’s well worth the delay to deliver the best experience for snap users wishing to print.

I have the following remarks:

  1. For sole printing and displaying print dialogs which list the available print queues and show the user-settable options for each print queue one only needs the domain socket of CUPS and if it is not at the standard location /run/cups/cups.sock we need to set the CUPS_SERVER environment variable to the actual location (the CUPS Snap is ALWAYS listening on /var/snap/cups/common/run/cups.sock and it is ALWAYS mediating).
  2. D-Bus notifications are an optional service of CUPS. If a client subscribes, CUPS gives push notifications via D-Bus about changes on printer status, available queues, configuration changes, job status, … They are not needed for printing and for displaying standard print dialogs, not used by most client applications which print. They improve possibilities though, as with them a print dialog could update on changes in real time and it could also give the user a way to track their jobs in real time.
  3. The cups-control interface is a complete interface for CUPS clients, it contains both access to the CUPS domain socket plus access to the D-Bus notification facility (and perhaps other features, so let us say cups-control allows access to the CUPS domain socket and the “CUPS extras”).
  4. We need to force snapped applications print through the snapped CUPS as the snapped CUPS always has Snap mediation, the system’s CUPS only in rare cases (and we cannot check this from the outside). To have assure that snapped applications and unsnapped applications see the same printers if we are on a system with unsnapped CUPS, we need to force-install the CUPS Snap in proxy mode.
  5. The interface of CUPS is rather complex, so writing a new “mediation shell daemon” which looks/behaves like CUPS for snapped applications and passes on non-admin requests to the system’s CUPS is way more complex and bug-prone than simply using the actual CUPS (the one in the CUPS Snap) as this “mediation shell daemon”.
  6. @jamesh came with an approach on Mattermost to use the xdg-desktop-portal which flatpak uses. This is more or less like the “Common Print Dialog” which I tried to establish back in 2006 together with some GUI experts but did not succeed due to lack of available coding workforce. The applications do not have their own print dialog but call functions via D-Bus to pop up a print dialog and to actually print. This would probably solve all the problems of the print dialogs I am complaining about, but this is not viable here as all applications would need highly invasive modifications for snapping them, an d snapping command line apps for headless servers will get impossible.
  7. So there seems to be no way to protect an existing CUPS on an arbitrary distribution against admin requests by simple AppArmor and other Snap-typical sandboxing techniques. The proxy CUPS seems to be the easiest-to-implement way.

So our print interface needs:

  1. Printing from Snaps forced through the CUPS of the CUPS Snap so that this CUPS mediates
  2. If the main system’s CUPS is some CUPS installed with the distribution (it is usually not mediating) the CUPS of the CUPS Snap works as proxy. This proxy mode is already implemented.
  3. If there is no CUPS from the distro, the CUPS of the CUPS Snap goes into stand-alone mode to work as the main CUPS. In this case the snapped CUPS also listens on /run/cups/cups.sock so that unsnapped apps can also print (this is also already implemented in the CUPS Snap).
  4. We can simply make the print interface let the CUPS_SERVER env variable be set to /var/snap/cups/common/run/cups.sockso that the snapped app prints to the snapped CUPS and in addition not allow the snapped app access /run/cups/cups.sock to protect against apps which override the env variable and switch (or hard-code) to /run/cups/cups.sock internally.
  5. Better than (4) is that if the print interface bind mounts /var/snap/cups/common/run/cups.sock to /run/cups/cups.sock in the application Snaps instead of setting the CUPS_SERVER env variable, as them badly (or maliciously) programmed apps hard-coding to /run/cups/cups.sock can also print, but only print, not administrate as the job goes through the CUPS of the CUPS Snap.
  6. Whatever the print interface does, it has also to support all the “CUPS extras” which I mentioned in (3) in the beginning of this posting.
  7. The cups-control interface should NEVER route requests through a proxy CUPS, as administrative requests through the proxy CUPS are technically not possible. It should always directly talk to the main CUPS which actually executes the jobs. So it should not do any bind mounting and ALWAYS talk to /run/cups/cups.sock. Then in distros with their own classic CUPS it talk’s to the distro’s CUPS and on systems with the CUPS Snap as main CUPS (stand-alone mode) it talks to the snapped CUPS.
  8. Both the print interface and the cups-control interface should support the very same “CUPS extras” in addition, the ones of the current cups-control interface.

I hope such a set of interfaces could be implemented, especially with (5) instead of (4). Please tell me what I should do from my side for that. Thanks.