Using Perl Modules from CPAN with Distribution Packaged Modules

When I first started using CPAN, this was a concern of mine that I was never able to fully address: how do Perl modules from CPAN interact with modules installed via distribution packages?

It turns out that the answer is simple. First, however, let’s install a set of tools called pmtools; these tools will tell you a lot and make it simple. Using these will help you to understand how your system is set up as well. In Ubuntu, the package is called pmtools; in Red Hat, it is perl-pmtools. To install, use this command on an Ubuntu system as root:

$ sudo apt-get install pmtools

On a Red Hat Enterprise Linux system, use this command instead:

$ sudo yum install perl-pmtools

Once that is installed, use the tool pmdirs to see what directories are searched for modules in order. Here is one example from an Ubuntu 10.04 LTS server system:

$ pmdirs
/etc/perl
/usr/local/lib/perl/5.10.1
/usr/local/share/perl/5.10.1
/usr/lib/perl5
/usr/share/perl5
/usr/lib/perl/5.10
/usr/share/perl/5.10
/usr/local/lib/site_perl
.

With this, you can see that several site-specific directories in /usr/local are searched before system directories in /usr.

Same thing holds true for Red Hat; here is an example from a Red Hat Enterprise Linux 5.7 system:

$ pmdirs
/usr/lib64/perl5/site_perl/5.8.8/x86_64-linux-thread-multi
/usr/lib/perl5/site_perl/5.8.8
/usr/lib/perl5/site_perl
/usr/lib64/perl5/vendor_perl/5.8.8/x86_64-linux-thread-multi
/usr/lib/perl5/vendor_perl/5.8.8
/usr/lib/perl5/vendor_perl
/usr/lib64/perl5/5.8.8/x86_64-linux-thread-multi
/usr/lib/perl5/5.8.8
.

In Red Hat, system-installed Perl modules go into /usr/lib/perl5/vendorperl/$version/ while modules installed with CPAN go into /usr/lib/perl5/siteperl/$version/. On the other hand, in Ubuntu, CPAN modules go into /usr/local/share/perl5/$version/ while packaged modules are placed in /usr/lib/perl/$version/.

This means that modules installed with CPAN will be searched (and found) first – whether they are newer or not. Usually they will be newer, but if they are the same version you’re better off staying with the packaged version of the Perl module as it will often have bugfixes and patches not present on the original.

If you want to see where a particular module lives – use pmpath:

$ pmpath CPAN
/usr/local/share/perl/5.10.1/CPAN.pm

This shows that CPAN was installed from (or updated from) CPAN as well.

If you want to see what version is installed, use pmvers:

$ pmvers CPAN
1.9600

Note: In Ubuntu you may find that the directory /usr/local/lib/site_perl is missing and that some pmtools utilities will fail because of this. Use this command to create the directory (and stop the error messages):

# mkdir -p /usr/local/lib/site_perl

Using Nagios from an Android Phone

I got an Android phone in the last year, and started looking in earnest for a Nagios client for it. With a Nagios client, you can read what the current status is of your systems in Nagios.

There are several available; the two most often mentioned are NagRoid and NagMonDroid. However, neither one of these worked for me, and there are indeed others that were good.

All of the clients use the same basic method to get data from Nagios: scrape the data from a web page. The biggest problem comes when that web page is not available – or is incorrect. Most of these applications request a URL, but sometimes are unclear as to what URL they want exactly. Add to that the fact that Nagios changed its URL structure slightly between versions and it gets even more complicated.

To discover what was happening, I used tcpdump to watch the accesses to the web server from the Nagios clients, as well as watching the Apache logs. By doing this, I was able to discern what URLs were being loaded.

Here are some of the URL paths being looked for by the various clients:

• /cgi-bin/tac.cgi
• /cgi-bin/status.cgi
• /cgi-bin/nagios3/statuswml.cgi?style=uprobs
• /cgi-bin/nagios3/status.cgi?style=detail
• /cgi-bin/nagios3/status.cgi?&servicestatustypes=29&serviceprops=262144

Further complicating matters in my case was the fact that any unrecognized URL was massaged (via mod_rewrite) into serving the main Nagios page via SSL.

However, by using mod_rewrite it was possible to rewrite the old /cgi-bin paths to a newer /cgi-bin/nagios3 path, and things started working.

In the case of the statuswml.cgi file, Google Chrome wanted to download the resulting file instead of actually using it somehow.

The main choices for Nagios clients on Android are these:

• NagRoid
• NagMonDroid
• aNag
• jNag (and jNag ad supported)
• TiNag (and TiNag Light)

I have gone with aNag – it has a nice interface, good use of notification, and worked without trouble once the URL was fixed up. Several of the others never did work right – or they gave no indication that they were working right. In the case of jNag, it also requires a modified Nagios server and the installation of mkLivestatus. aNag was the one that was easiest to work with and get working.

aNag does use a mostly text-based format to show data, but it has the ability to manipulate services as well as one-button access to the web interface directly.