Activating the CUPS snmp backend in Ubuntu Feisty

Till Kamppeter till.kamppeter at
Mon Feb 26 23:33:31 UTC 2007


on the IRC I talked with pitti about activating the CUPS backends which
were de-activated by Debian by moving them into
/usr/lib/cups/backend-available. The three de-activated backends are
serial, scsi and snmp.

Most important is snmp, as it makes it much easier for unexperienced
users (our main audience) to set up network printers,

The CUPS backend snmp serves only for auto-detecting network-connected
printers in the local networks. It is called whenever CUPS is asked to
detect printers (via "lpinfo -v". entering the admin page or adding a
queue with the CUPS web interface, or adding a queue with most other
printer setup tools, like gnome-cups-manager, KDE Print Manager, or
printerdrake), It returns all detected network printers with there
correct URIs (using the ipp, lpd, or socket backends for printing). So
the user has only to click the auto-detection result and gets his
network printer running.

The detection is done by an SNMP broadcast asking for the printer MIB.
This is only answered by printers and so only printers and not other
SNMP-capable devices (like routers) get listed. This method is also very
fast. The result appears in 3 or 4 seconds, even in big networks. No
ping or portscan to unknown devices is performed and also no extra
daemon is started and no ports get opened. snmp runs only the mentioned
3 or 4 seconds and it runs as the cupsys user and not as root (no SUID).
So the impact to the system and the network is low. There are also no
broadcasts into the internet with the default configuration of the snmp

Issues which users have reported with problems of correctly detecting
printers of certain brands are fixed by Mike Sweet. So the backend is
currently in a very good shape.

It is also only needed for detecting printers, printing on the detected
printers is then done with the ipp, socket, or lpd backends, so the snmp
backend only gets invoked by printer admins to add a queue and never by
normal users. So security risks are very low and "printing just works"
experience is very high with it.

Pitti is, like me, in favour of activating it, but I also post here 
because in a certain form this is adding a feature after feature freeze.
So I want to know what is the general opinion about activating the CUPS 
snmp backend (and perhaps also the serial and scsi backends)?

For testing do

sudo ln /usr/lib/cups/backend-available/snmp /usr/lib/cups/backend/
sudo ln /usr/lib/cups/backend-available/serial /usr/lib/cups/backend/
sudo ln /usr/lib/cups/backend-available/scsi /usr/lib/cups/backend/

and then run the printer setup tools which are available on your 
machine. Set up the printers which are detected now and try to print on 
them via the new print queues.

For the implementation in Feisty the cupsys package only needs to create 
the above-mentioned links. No extra space on the CD is needed. Local 
admins can de-activate these backends again by removing the links, so no 
files need to be deleted or moved.


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