5.5.  Printer Access Control

In a ThinLinc cluster, all printers that any user of the cluster needs to be able to print to must be defined centrally, or the user will not be able to print from applications that run in a ThinLinc session. For large installations, this leads to a very long list of available printers.

A long list of printers leads to usability problems - having to select printer from a long list can be troublesome. Also, it opens for problems with printer jobs being printed at remote locations by mistake (or on purpose, by users finding it amusing to send "messages" to other locations).

The solution to this problem is the Printer Access Control feature of ThinLinc. By integrating with CUPS (the Common Unix Printing System), the list of printers a user is presented with and allowed to print to is limited to the printers that should be available to a specific terminal, based on information in a database of printers, terminals and locations.

Note

The Printer Access Control feature will affect all users on the ThinLinc cluster. The only user excepted from limitations of the printer list is the superuser (root) - all other users will only see and be able to use printers based on the location of their terminals, when the Printer Access Control feature is enabled.

5.5.1.  Theory of Operation

Each time a user requests a new session or reconnects to an existing session, the hardware (MAC) address of the terminal is sent along with the request from the ThinLinc client. Using the same database as the nearest printer feature used to find which printer is closest to the user, the printer access control feature calculates which printers the user is allowed to use, and then configures the access control of the printing system (CUPS).

This way, the user is presented with a list of printers that only contains the printers relevant for the location where the terminal the user is currently using is located. In a situation where a user has multiple sessions running from multiple clients, all printers associated with the different terminals will be made available.

5.5.2.  Requirements

  • CUPS v1.2 or higher.

5.5.3.  Activating the Printer Access Control Feature

First, make sure you have configured the printers in your ThinLinc cluster as documented in Section 5.2, “ Printer Configuration Overview ”. For the Printer Access Control Feature, a central CUPS daemon on the VSM Server host is required, and all agent hosts must have a correctly configured /etc/cups/client.conf.

To activate the printer access feature, create two symlinks on the host running VSM Server, as follows:

ln -s /opt/thinlinc/sbin/tl-limit-printers /opt/thinlinc/etc/sessionstartup.d
ln -s /opt/thinlinc/sbin/tl-limit-printers /opt/thinlinc/etc/sessionreconnect.d

The first symlink makes sure tl-limit-printers is run when sessions are started. The second makes sure it is run at reconnects to existing sessions. More details about the session startup can be found in Section 13.4, “ Customizing the User's Session ”.

Note

With the above configuration (symlinking tl-limit-printers into sessionstartup.d and sessionreconnect.d), the client will not get an answer back from the server until tl-limit-printers has finished its execution. This is the desired behaviour if it is strictly neccessary that printer access rights are correct when the user connects to the session. In environments where it is acceptable that the final list of printers shown to the user may not be finished when the user connects to the session, place the execution of tl-limit-printers in the background, as detailed in Section 13.4.1.1, “ Scripts run at session startup/reconnect ”, as that will decrease the time the user has to wait for the session to appear on his client.

After creating the symlinks, try connecting to your ThinLinc cluster with a ThinLinc cluster and bring up an application that lists the available printers. The list of printers should now be limited according to configuration.

Note

The printer list limitation doesn't work for applications that use the deprecated cupsGetPrinters library call. This means that older applications might show the whole list of printers. The access control are still enforced, which means that even if a disallowed printer is shown in the list of printers, users can't submit jobs to it.

Most applications in a modern Linux distribution doesn't have this problem.

5.5.4.  Configuration

Configuration of the printer access control feature is mostly a matter of using tlwebadm (see Chapter 16, Administration of ThinLinc using the Web Administration Interface for details) to add the hardware address of all terminals as well as information about where they are located and which printers are to be available for each location.

5.5.4.1.  Unknown Terminals / Terminals Without Hardware Address

When a client reports a hardware address that is not present in the database of terminals, or when no hardware address is reported, the default behaviour is to disallow access to all printers, rendering an empty printer list for the user.

There is however a way to give even unknown terminals access to one or more printers - define a special location and check the Use for unknown terminals and terminals without hardware address checkbox. Then add the printers that should be available for the unknown terminals.

One common configuration is to add such a location and then add the thinlocal printer to this location. This way, unknown terminals, for example people working from their home computers, will be able to user their locally connected printer, but no other printer will be available.