Chapter 12. Printer Operation / 12.7. Special Features in SUSE LINUX | ||||
|---|---|---|---|---|
 | 12.6. Configuration for Applications | 12.8. Troubleshooting |  | |
Chapter 12. Printer Operation / 12.7. Special Features in SUSE LINUX | ||||
|---|---|---|---|---|
| 12.6. Configuration for Applications | 12.8. Troubleshooting | | |
A number of CUPS features have been adapted for SUSE LINUX. Some of the most important changes are covered here.
There are several ways to configure CUPS as the client of a network server.
For every queue on the network server, you can configure a local queue through which to forward all jobs to the corresponding network server. Usually, this approach is not recommended, because all client machines must be reconfigured whenever the configuration of the network server changes.
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.
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 the daemon 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 and the IPP packet is received earlier, 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. To use this method, port 631/UDP must be open for incoming packets.
YaST can find CUPS servers by scanning all network hosts to see if they offer this service and by listening to IPP broadcasts. The second method is used during the system installation to find CUPS servers for the proposal. It requires that port 631/UDP be open for incoming packets.
The default setting of the firewall shown in the proposal dialog is to
reject IPP broadcasts on any interface. Accordingly, the second method for
detecting remote queues and the third method for accessing remote queues
cannot work. Therefore, the firewall configuration must be modified by
marking one of the interfaces as internal, which opens
the port by default, or by explicitly opening the port of an
external interface. For security reasons, none of the
ports is open by default. Opening a port to configure access to remote
queues using the second method can be a security risk because an attacker
could broadcast a server that might be accepted by users.
The proposed firewall configuration must be modified to enable CUPS to detect remote queues during installation and access remote servers from the local system during normal operation. 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 above, this method is not recommended.
To use the administration with the Web
front-end (CUPS) or the printer administration tool (KDE), the user
root must be set up
as CUPS administrator with the CUPS administration group
sys and a CUPS password.
Do this as root
with the following command:
lppasswd -g sys -a root
If this is not done, administration with the Web
interface or with the administration tool is not possible, because the authentication
fails if no CUPS administrator has been configured. Instead of
root, any other user can
also be
appointed as CUPS administrator (see Section 12.7.3, “Changes in the CUPS Print Service (cupsd)”).
These changes were initially applied for SUSE LINUX 9.1.
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
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.
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.
In a standard installation, cupsd is activated
automatically, enabling comfortable access to the queues of CUPS network
servers without any additional manual actions. The two first items (see
Section 12.7.3.1, “cupsd Runs as the User lp” and Section 12.7.3.2, “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.
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 determine 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.
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
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 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 (see below).
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.
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.
