Installing Nagios on Ubuntu 10.10

Nagios is a powerful monitoring software package that basically consists of a web interface, a scheduling and evaluation engine, and various scripts or plug-ins. It is a widely used monitoring tool due to it's flexibility and stability. I've personally seen it being used at numerous operation centers and set it up for countless clients. Until recently I had never used Ubuntu as a nagios host but that has changed and the difference in setup was worth documenting. This

I'll be starting with a basic install of Ubuntu Server 10.10, specifically the Rackspace Cloud Server image and all commands will be run as root ( sudo su - ) to simplify things. After building it I like to make sure that I'm all patched up before starting.

apt-get update
apt-get upgrade

No Compiling?
If you have ever built a nagios system in the past you have probably had to go to nagios.org, download the source, compile it and install it. Ubuntu keeps nagios in their repository so there is no need for that.

apt-get install nagios3

This will prompt you to install a large number of packages including apache2, php, postfix, samba, and a slew of libraries all used to either present the nagios interface or support the underlying scripts. The install will prompt you to set the postfix ( mail ) settings. Internet Site and your domain name are probably what you want to use there. It will also prompt you for the nagiosadmin password, this is for the htaccess on the website. Once all of the packages are done installing you have a working nagios install. You can verify this by going to the web interface at http://your.server.ip/nagios3 use the username nagiosadmin and the password you set during the install.

Where are the files?
Traditionally nagios installs into /usr/local/nagios as a complete package. The way the ubuntu package installs these files are put into the larger os layout. Below is a rough cheat sheet as to where things ended up.

Purpose ................... Old ..................................... New
binary ..................... /usr/local/nagios/bin/nagios ... /usr/sbin/nagios3
configuration files ... /usr/local/nagios/etc .............. /etc/nagios3 & /etc/nagios-plugins/config
plugins ................... /usr/local/nagios/libexec ........ /usr/lib/nagios/plugins
website files ........... /usr/local/nagios/share ........... /usr/share/nagios3/htdocs
command file ......... /usr/local/nagios/var .............. /var/lib/nagios3

One last hurdle.
At this point you have a very basic nagios system that is functioning, almost. If you are in the web interface and try to use any of the links under Service Commands or Host Commands you will see this error:

Sorry, but Nagios is currently not checking for external commands, so your command will not be committed!

Like the error says, external commands are not enabled. To fix this edit /etc/nagios3/nagios.cfg, find the line check_external_commands=0 and change it to check_external_commands=1

If you restart nagios now and try the commands again you will get another error:

Error: Could not stat() command file '/var/lib/nagios3/rw/nagios.cmd'!
The external command file may be missing, Nagios may not be running, and/or Nagios may not be checking external commands.
An error occurred while attempting to commit your command for processing.

This error is coming up because the apache process does not have the permission to write to the file that nagios checks for the commands you are requesting. The fix for this is to allow the www-data group read and execute permission to the directory where the command file lives and then add the www-data user to the nagios group because the command file gets created with nagios:nagios as the owner:group and 660 as the permissions.

chmod g+rx /var/lib/nagios3/rw
usermod -a -G nagios www-data
/etc/init.d/apache2 restart
/etc/init.d/nagios3 restart

Now the host and service commands will work and you can start configuring the rest of your nagios setup.