fsnmp 1.0.0
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fsnmp 1.0.0

 

FAQ

General FAQ

What is fsnmp?

The fsnmp program is a very simple print filter for the LPRng print system. It performs the following processing:

  • Save standard input (the print job data) to a temporary file.
  • Retrieve pagecounter value from the printer using SNMP, report the pagecounter value to accounting mechanism configured for LPRng.
  • Transfer the temporary file contents to the printer over a network socket.
  • Retrieve pagecounter value from the printer using SNMP, report the pagecounter value to the accounting mechanism configured for LPRng.
  • Remove the temporary file.

Which license conditions are applied to the package?

  • fsnmp
    Copyright (C) 2008 Dipl.-Ing. Dirk Krause
  • Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
    • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
    • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
    • Neither the name of the Dirk Krause nor the names of contributors may be used to endorse or promote products derived from this software without specific prior written permission.
  • This software is provided by the copyright holders and contributors ``as is'' and any express or implied warranties, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose are disclaimed.
    In no event shall the copyright owner or contributors be liable for any direct, indirect, incidental, special, exemplary, or consequential damages (including, but not limited to, procurement of substitute goods or services; loss of use, data or profits; or business interruption) however caused and on any theory of liability, whether in contract, strict liability, or tort (including negligence or otherwise) arising in any way out of the use of this software, even if advised of the possibility of such damage.

Why is there a need for another print filter?

Our faculty used LPRng for a long time, in the PC lab we had HP printers. The ifhp filter available from the LPRng web site worked well with these printers, the pagecounter value was retrieved via PCL commands.
After changing to Xerox phaser printers we needed to retrieve the pagecounter values using SNMP. I attempted multiple configuration settings for ifhp, this took me some days. Unfortunately I did not succeed to configure ifhp to work with my printers. In this time I read the manuals for LPRng and ifhp multiple times and learned a lot about LPRng's print filtering.
Although I knew which tasks the filter must perform I did not find out how to configure ifhp to do the tasks. So I decided to write a dedicated print filter.

Which filtering is performed by fsnmp?

The name "filter" is a little bit misleading. The fsnmp program is more an output driver than a filter, only minimal filtering is done:

  • If you configured the option "ctrl-d at start" fsnmp checks whether or not the first byte in the print job is a Control-D. If it isn't, fsnmp sends an additional Control-D to the printer before starting the print job data.
  • If you configured the option "ctrl-d at end" fsnmp checks whether or not the last byte in the print job is a Control-D. If it isn't, fsnmp sends an additional Control-D to the printer after sending the print job data.

Is it still usefull to develop for LPRng?

I know there was no new version for over a year and no mailing list activity for a long time.
Many Linux distribution use CUPS, i.e. on Fedora (my preferred Linux distribution) there are no longer LPRng packages available. Although CUPS is possibly a good choice for print clients (PPD files can be read, users can use all features of a printer if the PPD file is present) I prefer to use LPRng on my print server over CUPS and other print systems for several reasons:

  • LPRng seems to be smaller.
    There's only the lpd daemon handling LPD protocol requests instead of multiple daemons for multiple protocols.
    Normally I also would expect smaller and simpler software to have less security problems than large and over-featured software.
    The reference manual can be read and understood within some hours.
  • Installation and configuration is easy and well-documented.
    For instance it is relatively easy to turn off automatic filtering and conversions (Why? See below.).
  • LPRng is flexible. It is sometimes referred to as the "swiss army knife of printing".
    It is well-documented how to write filters (like fsnmp), specifications how to write accounting software (either as a pipe command or a network server) is available too. Accounting software can also be used to limit printer usage. Limitation can be done "just-in-time" rather than periodically build deny- and allow-files.
  • LPRng compiles without problems on all my systems.
    Attempts to compile CUPS on a Solaris system using the workshop compilers failed.
    C++ != C++.
    If I have a native C/C++ compiler for my system I want to use this compiler, I don't want to spend several MBs and days just to install and learn about another compiler producing binaries showing less performance.

The ifhp filter recognizes file types and invokes conversion programs? Why doesn't fsnmp do so?

From a security-related point of view it is not a good idea to run conversion programs as user "lp" (or "daemon" or in worst case "root") and feed the conversion programs with input provided by users.
It is better to leave it up to the users to do conversions if necessary; users should only send "ready-to-print" jobs to the print system.

I know the security problems but I want automatic conversion. How do I do that?

You can create bounce queues, see the LPRng reference manual.
You need one print queue using fsnmp as filter to send the print job data to the printer.
Another queue can be configured to use different filters for different input classes or a filter recognizing file types and invoking conversion programs. This queue sends it's output to the first queue.
Are you really sure you need automatic filtering? See next question, please!

Which security problems are there in automatic filtering?

Here are some reasons why not to use automatic filtering:

  • Security problems
    Each of the conversion programs may have security problems. Some of the programs used for conversions even allow to execute shell commands.
    If the programs are run by normal users, each user can only destroy his own files...
    If the programs are run by "lp", "daemon" or "root" (you really should not do that!) anyone can submit a destructive "print job" which can at least disturb the printing system.
  • Secure mode restricts functionality
    Some of the conversion programs can be turned into secure mode, denying dangerous operations. Typically this restricts functionality.
    Leaving all conversions up to the users, the user can decide whether or not to trust the input file and find a compromise between security and functionally. (Example: If I wrote a LaTeX source file I know I can trust this file and I do not need secure mode for latex and dvips. If I receive LaTeX sources from other people I prefer to turn secure mode on or to inspect the source.)
  • Secure mode is not always the default setting
    For some programs the secure mode is the default setting, other programs need special options to enter secure mode.
    Example: For automatic conversion of *.dvi files in the print system, are you sure dvips is run using the "-R" option?
  • Changed environment
    Some of the conversion programs use environment variables or personal settings from setup files. The settings for the print system's user account will differ from individual user settings, this might result in "resource not found" problems or other unexpected output.

Installation FAQ

Where can I download fsnmp?

The sources are available at SourceForge, see http://fsnmp.sourceforge.net.

Which software packages are required to install fsnmp?

How do I build and install fsnmp?

After unpacking the sources change into the fsnmp directory and run

./configure
make
make install

Some libraries are specified multiple times. How do I fix the linker problems?

Manually edit the Makefile and make sure each library is used only once.
The configure script uses the net-snmp-config script to find the libraries needed by Net-SNMP. So may be some libraries are found here and found by other configure script sections.

How do I handle compiler/linker errors about wrong ELF classes?

The configure script uses the net-snmp-config script to find CFLAGS (C compiler options) for programs linked against the Net-SNMP libraries. At least on one system I found the net-snmp-config script producing very strange options like

-R../.../sfw -xarch=...

If the Makefile contains CFLAGS and/or LDFLAGS looking strange and differing from your normal CFLAGS/LDFLAGS settings, do the following:

  • Remove compiler-specific, platform-specific options, i.e. the "-xarch=...".
  • In -R... and -L... replace relative paths by absolute paths or remove the option.

After changing the Makefile run

make clean
make
make install

or

rm -fr fsnmp *.o
make
make install

Usage FAQ

Which configuration files are used by fsnmp?

There are two configuration files:

  • printcap
    This file is typically located in /etc or /etc/lprng.
  • fsnmp.conf
    This file is typically located in ${prefix}/etc/fsnmp.

How do I configure the printcap file entry?

In the printcap file we must make sure that LPRng does not open a network connection to the printer. So we use the /dev/null device instead.
We configure fsnmp as filter for all input types.

lp
	:lp=/dev/null
	:filter=/usr/local/libexec/filters/fsnmp

How do I configure the fsnmp.conf file entry?

In fsnmp.conf you must at least configure the host name (or IP address) and the port number of the printer, the SNMP version and the SNMP community.

[lp]
lp = 192.168.1.99%9100
snmp community = public
snmp version = 1

Other settings are optional, it is a good idea to start from the following settings:

[lp]
lp = 192.168.1.99%9100
snmp community = public
snmp version = 1
ctrld-d at end = yes
shutdown data socket = no
minimum pagecount time = 30
shutdown accounting socket = yes

The purpose of the settings is explained on the usage page.

Do I need to restart LPRng after applying configuration changes?

You need to restart LPRng only if you applied changes to the printcap file.
Changes in the fsnmp.conf file do not require to restart LPRng.

Developer FAQ

Should I change the *.c or the *.ctr files?

Please make changes only in the *.ctr files, these are the original source files.
The tracecc program is used to generate *.c files from *.ctr files.

Dirk Krause
Schmalkalden (Germany), 2008-11-01
http://fsnmp.sourceforge.net