11.6. Special Features in SUSE Linux

A number of CUPS features have been adapted for SUSE Linux. Some of the most important changes are covered here.

11.6.1. CUPS Server and Firewall

There are several ways to configure CUPS as the client of a network server.

  1. For every queue on the network server, you can configure a local queue through which to forward all jobs to the corresponding network server (forwarding queue). Usually, this approach is not recommended, because all client machines must be reconfigured whenever the configuration of the network server changes.

  2. Print jobs can also be forwarded directly to one network server. For this type of configuration, do not run a local CUPS daemon. lp or corresponding library calls of other programs can send jobs directly to the network server. However, this configuration does not work if you also want to print on a local printer.

  3. The CUPS daemon can listen to IPP broadcast packets that other network servers send to announce available queues.

    This is the best CUPS configuration for printing over remote CUPS servers. However, there is a risk that an attacker sends IPP broadcasts with queues and the local daemon accesses a counterfeit queue. If it then displays the queue with the same name as another queue on the local server, the owner of the job may believe the job is sent to a local server, while in reality it is sent to the attacker's server.

YaST can find CUPS servers by either scanning local network hosts to see if they offer the IPP service or by listening to IPP broadcasts. This requires the firewall to let incoming packets on port 631/UDP (service IPP client) pass through. This is automatically enabled when you have configured your machine to be in the internal firewall zone. Opening a port to configure access to remote queues in the external zone can be a security risk because an attacker could broadcast a server that might be accepted by users. By default IPP broadcasts are rejected in the external zone. See Section 4.1.4.1, “Configuring with YaST” for details on firewall configuration.

Alternatively, the user can detect CUPS servers by actively scanning the local network hosts or configure all queues manually. However, because of the reasons mentioned in the beginning of this section, this method is not recommended.

11.6.2. Changes in the CUPS Print Service

These changes were initially applied for SUSE Linux 9.1.

11.6.2.1. cupsd Runs as the User lp

On start-up, cupsd changes from the user root to the user lp. This provides a much higher level of security, because the CUPS print service does not run with unrestricted permissions, only with the permissions needed for the print service.

However, the authentication (the password check) cannot be performed via /etc/shadow, because lp has no access to /etc/shadow. Instead, the CUPS-specific authentication via /etc/cups/passwd.md5 must be used. For this purpose, a CUPS administrator with the CUPS administration group sys and a CUPS password must be entered in /etc/cups/passwd.md5. To do this, enter the following as root:

lppasswd -g sys -a CUPS-admin-name

This setting is also essential if you want to use the CUPS administration Web front-end or the KDE printer administration tool.

When cupsd runs as lp, /etc/printcap cannot be generated, because lp is not permitted to create files in /etc/. Therefore, cupsd generates /etc/cups/printcap. To ensure that applications that can only read queue names from /etc/printcap continue to work properly, /etc/printcap is a symbolic link pointing to /etc/cups/printcap.

When cupsd runs as lp, port 631 cannot be opened. Therefore, cupsd cannot be reloaded with rccups reload. Use rccups restart instead.

11.6.2.2. Generalized Functionality for BrowseAllow and BrowseDeny

The access permissions set for BrowseAllow and BrowseDeny apply to all kinds of packages sent to cupsd. The default settings in /etc/cups/cupsd.conf are as follows:

BrowseAllow @LOCAL
BrowseDeny All

and

<Location />
  Order Deny,Allow
  Deny From All
  Allow From 127.0.0.1
  Allow From 127.0.0.2
  Allow From @LOCAL
</Location>

In this way, only LOCAL hosts can access cupsd on a CUPS server. LOCAL hosts are hosts whose IP addresses belong to a non-PPP interface (interfaces whose IFF_POINTOPOINT flags are not set) and whose IP addresses belong to the same network as the CUPS server. Packets from all other hosts are rejected immediately.

11.6.2.3. cupsd Activated by Default

In a standard installation, cupsd is activated automatically, enabling comfortable access to the queues of CUPS network servers without any additional manual actions. The items in Section 11.6.2.1, “cupsd Runs as the User lp” and Section 11.6.2.2, “Generalized Functionality for BrowseAllow and BrowseDeny are vital preconditions for this feature, because otherwise the security would not be sufficient for an automatic activation of cupsd.

11.6.3. PPD Files in Various Packages

The YaST printer configuration sets up the queues for CUPS using only the PPD files installed in /usr/share/cups/model/ on the system. To find the suitable PPD files for the printer model, YaST compares the vendor and model determined during hardware detection with the vendors and models in all PPD files available in /usr/share/cups/model/ on the system. For this purpose, the YaST printer configuration generates a database from the vendor and model information extracted from the PPD files. When you select a printer from the list of vendors and models, receive the PPD files matching the vendor and model.

The configuration using only PPD files and no other information sources has the advantage that the PPD files in /usr/share/cups/model/ can be modified freely. The YaST printer configuration recognizes changes and regenerates the vendor and model database. For example, if you only have PostScript printers, normally you do not need the Foomatic PPD files in the cups-drivers package or the Gimp-Print PPD files in the cups-drivers-stp package. Instead, the PPD files for your PostScript printers can be copied directly to /usr/share/cups/model/ (if they do not already exist in the manufacturer-PPDs package) to achieve an optimum configuration for your printers.

11.6.3.1. CUPS PPD Files in the cups Package

The generic PPD files in the cups package have been complemented with adapted Foomatic PPD files for PostScript level 1 and level 2 printers:

  • /usr/share/cups/model/Postscript-level1.ppd.gz

  • /usr/share/cups/model/Postscript-level2.ppd.gz

11.6.3.2. PPD Files in the cups-drivers Package

Normally, the Foomatic printer filter foomatic-rip is used together with Ghostscript for non-PostScript printers. Suitable Foomatic PPD files have the entries *NickName: ... Foomatic/Ghostscript driver and *cupsFilter: ... foomatic-rip. These PPD files are located in the cups-drivers package.

YaST prefers a Foomatic PPD file if a Foomatic PPD file with the entry *NickName: ... Foomatic ... (recommended) matches the printer model and the manufacturer-PPDs package does not contain a more suitable PPD file.

11.6.3.3. Gimp-Print PPD Files in the cups-drivers-stp Package

Instead of foomatic-rip, the CUPS filter rastertoprinter from Gimp-Print can be used for many non-PostScript printers. This filter and suitable Gimp-Print PPD files are available in the cups-drivers-stp package. The Gimp-Print PPD files are located in /usr/share/cups/model/stp/ and have the entries *NickName: ... CUPS+Gimp-Print and *cupsFilter: ... rastertoprinter.

11.6.3.4. PPD Files from Printer Manufacturers in the manufacturer-PPDs Package

The manufacturer-PPDs package contains PPD files from printer manufacturers that are released under a sufficiently liberal license. PostScript printers should be configured with the suitable PPD file of the printer manufacturer, because this file enables the use of all functions of the PostScript printer. YaST prefers a PPD file from the manufacturer-PPDs package if the following conditions are met:

  • The vendor and model determined during the hardware detection match the vendor and model in a PPD file from the manufacturer-PPDs package.

  • The PPD file from the manufacturer-PPDs package is the only suitable PPD file for the printer model or a there is a Foomatic PPD file with a *NickName: ... Foomatic/Postscript (recommended) entry that also matches the printer model.

Accordingly, YaST does not use any PPD file from the manufacturer-PPDs package in the following cases:

  • The PPD file from the the manufacturer-PPDs package does not match the vendor and model. This may happen if the manufacturer-PPDs package contains only one PPD file for similar models, for example, if there is no separate PPD file for the individual models of a model series, but the model name is specified in a form like Funprinter 1000 series in the PPD file.

  • The Foomatic PostScript PPD file is not recommended. This may be because the printer model does not operate efficiently enough in PostScript mode, for example, the printer may be unreliable in this mode because it has too little memory or the printer is too slow because its processor is too weak. Furthermore, the printer may not support PostScript by default, for example, because PostScript support is only available as an optional module.

If a PPD file from the manufacturer-PPDs package is suitable for a PostScript printer, but YaST cannot configure it for these reasons, select the respective printer model manually in YaST.