One cool ability of Nagios is that it allows you to group machines into services. Say, for instance, you have a number of machines that serve as Web servers or Samba servers. Instead of having to scroll around to find them, you can group those machines together, by service, to make for much easier monitoring. In this tutorial I am going to show you how to take advantage of this nifty feature.

Assumptions

I am going to assume that you already have Nagios installed and working. I will also assume you know which machines are on your network and what services they are running. For the sake of this tutorial, we will piece together all machines that fall under the categories: ssh-servers and debian-servers.

Machine-specific .cfg files

The first thing to do is to create a specific .cfg file for each machine you need to add.  This subject was already addressed in the “Easily extend Nagios’ functionality” article. Without these .cfg files, Nagios will not be able to see the machines – this is critical for being able to group machines by service.

Setting up the groups

If you look in /etc/nagios3/conf.d you will find a file called hostgroups_nagios2.cfg (NOTE: The “2″ is not a typo. For some reason the configuration files have not been renamed to reflect the migration from 2 to 3.) This is the file in charge of grouping machines together by service. A typical entry will look like:

# A list of your Debian GNU/Linux servers
define hostgroup {
hostgroup_name  debian-servers
alias           Debian GNU/Linux Servers
members         localhost, Elive, courtney-desktop
}

Figure 1

As you can see I already have three machines grouped together. You can have as many as you like. Now, when you take a look at the Nagios’ Hostgroup Overview, you will see the machines you have defined listed (see Figure 1).

Notice the Host names listed. These names are taken from the configuration you see above. You can’t, however, just edit the names to make them whatever you want. Those names have to reflect the host_name directive in the particular hosts .cfg file. So for courtney-desktop above you might find a courtney_desktop.cfg file containing the line host_name courtney-desktop.

Now, do the same for all of your SSH servers, by adding in any hosts that serve up ssh to clients. It’s done the same way. In the /etc/nagios3/hostgroups_nagios2.cfg file you will all all hosts to the ssh-servers section.

Once you have added all your hosts to your groups, you have to make sure you restart Nagios in order for the changes to take effect. Do this with the command /etc/init.d/nagios3 restart. You can now view your groups in your browser.

Final thoughts

We keep chugging along with Nagios, making it ever more useful to the already-too-busy administrator. In upcoming articles we will tackle other Nagios-specific tasks.

But for those who only need to monitor a few devices, configuring Nagios to do so is actually quite simple. In this article I am going to show you a few configurations to handle and then how to add devices and checks for your installation.

Post-install configurations and plugins

Before you get busy setting up devices for Nagios to monitor, you should first install some extra plugins for the system. Open up Synaptic, search for “nagios” (no quotes), mark nagios-plugins-extra for installation, and click Apply to install. Once that is done you will already have more functionality with Nagios.

Now let’s take care of a couple of configurations. The first is in the /etc/nagios3/conf.d directory. The file is called contacts_nagios2.cfg. In this file you will configure who is to receive the email alerts. The line to set this configuration is:

email    root@localhost

You will want to change that email address. That is it for this configuration file. Now in order for Nagios to be able to send out alerts via email, the machine housing Nagios has to be able to send those alerts out. You can check that by using the mail command like so:

mail email@address -s TEST

Where email@address is an actual address you can use for testing.

Once you enter that line, hit Enter and then type some random text. When you’re done typing hit <Ctrl>d and then Enter. The mail should send away. If not you will have to troubleshoot your mail server (beyond the scope of this tutorial).

Add a device

Figure 1

Now for the good stuff. We are going to add a device to Nagios. The device we will add is a Windows XP desktop to monitor. Within the /etc/nagios3/conf.d directory you will see a number of .cfg files. These are the files Nagios uses to dictate what is see and how it acts upon what it see. For every device you want to use, I find it best to add a .cfg file. So let’s create the file /etc/nagios3/conf.d/windows_xp.cfg. The contents of this fill will look like what you see in Figure 1.

As you can see there are a few directives here. The directives are as follows:

• check_command – The command used to check the status of the device.
• max_check_attempts – How many times will Nagios retry the status check.
• check_period – How often are the checks made.
• process_perf_data – Enable the processing of performance data.
• retain_nonstatus_information – Enable the retention of non-status information across reboots.
• notification_interval – How often are notifications sent to inform the administrator this host is not okay.
• notification_period – How often are notifications sent out.
• notification_options – The options shown are w – send notifications on “warning” state, u – send notifications on “unknown” state, and “r” – send notifications when a service stops flapping (when a service changes frequently).

NOTE: Depending upon the version of Nagios you have installed the notification_options will be different. Some instances will not accept the “w” option. If you get an error, exchange “w” with “d” (for down).

As you can see there are configuration options that will be specific to your devices and networking topology (host_name and address are examples). Once you have created this file save it and restart Nagios with the command sudo /etc/init.d/nagios3 restart command. Now take a look at your Nagios page and you will see the new device you just added.

Final thoughts

Someday someone will come up with an automated way to detect devices in Nagios. Until then we are relegated to adding them using this means. Does that mean you should shy away from Nagios? Not at all. Nagios is a powerful tool ready to help network administrators keep tabs on their network devices.

In this article I will show you how to quickly get Nagios up and running. In no time you will have Nagios up and running and monitoring your network.

Features of Nagios

Before we get to the installation, let’s take a look at the features of Nagios. Nagios can:

• Monitor network services (SMTP, POP3, HTTP, NNTP, PING, etc.)
• Monitor host resources (processor load, disk usage, etc.)
• Allow for simple plugin design
• Do parallelized service checks
• Define a network host hierarchy using
• Contact administrators when service or host problems occur
• Define event handlers to be run during service or host events for proactive problem resolution
• Automatd log file rotation
• Support the implementation of redundant monitoring hosts
• Offer an optional web interface for viewing current network status, notification and problem history, log file, etc

Sold yet? You should be, especially considering this tool is free and open source! Now, let’s get on with the installation.

Installation

Follow these simple steps to installing Nagios:

1. Fire up Synaptic.
2. Search for “nagios” (no quotes)
3. Mark nagios3 for installation (this will also pick up all of the dependencies)
4. Click Apply to install

During the installation you will be asked for an administrator password. This will be the password for the nagiosadmin user that will have access to the web interface. Once the installation is complete, you need to run a check to make sure everything is okay. The command for this check is:

sudo nagios3 -v /etc/nagios3/nagios.cfg

When this command runs you will see something like:

Nagios 3.0.2
Copyright (c) 1999-2008 Ethan Galstad (http://www.nagios.org)
Last Modified: 05-19-2008
License: GPL

Reading configuration data…

Running pre-flight check on configuration data…

Checking services…
Checked 7 services.
Checking hosts…
Checked 2 hosts.
Checking host groups…
Checked 5 host groups.
Checking service groups…
Checked 0 service groups.
Checking contacts…
Checked 1 contacts.
Checking contact groups…
Checked 1 contact groups.
Checking service escalations…
Checked 0 service escalations.
Checking service dependencies…
Checked 0 service dependencies.
Checking host escalations…
Checked 0 host escalations.
Checking host dependencies…
Checked 0 host dependencies.
Checking commands…
Checked 142 commands.
Checking time periods…
Checked 4 time periods.
Checking for circular paths between hosts…
Checking for circular host and service dependencies…
Checking global event handlers…
Checking obsessive compulsive processor commands…
Checking misc settings…

Total Warnings: 0
Total Errors:   0

With the results giving 0 errors and 0 warnings, you are ready to log in.

Logging in

Figure 1

Fire up your web browser and point it to http://IP_TO_SERVER/nagios3 at which point you will be asked for login credentials. Remember that password you gave during installation? That is the password you will use to login (with the user nagiosadmin).

When you log in you will be greeted with the Nagios welcome screen (see Figure 1), at which point you are ready to start monitoring.

Final thoughts

Wasn’t so bad was it? And you though Nagios was impossible to install. Fortunately it’s not. And even more fortunately is how powerful Nagios is. You will be blown away at how useful this tool is.

In this article you will see how to install Nagios on a working Ubuntu Server installation. The release I used was 9.04, but you should be able to use 8.04 just as easily.

Features

You want features? Nagios is full of them. Nagios can monitor your entire network, you can handle problem remediation, you can plan network downtime, you can watch hosts, systems, services, applications..there’s very little you can’t do with Nagios.

But let’s think about this installation. The best (and really only good) way to install Nagios is from source. This means you are going to have to do some compliation. Don’t worry, it’s not hard…when you have a step-by-step guide that is.

Before you start

If you are using, as I did, a Ubuntu server installation, you will need to install a few tools first. So from the command line (on your Ubuntu server) issue the following commands:

sudo apt-get install php5-gd

sudo apt-get install gcc

sudo apt-get install make

The above commands will install the tools you need in order to get Nagios installed. If you don’t install the above, you won’t be able to complete the installation.

Installing Nagios – preflight

Before you actually run the installation you will need to take care of some user/group accounting first.

Create the user nagios:

sudo useradd -m nagios

Give the nagios user a password:

sudo passwd nagios

NOTE: You will have to enter the new password twice for the above command.

Create the group nagios:
sudo groupadd nagios

Add the user nagios to the group nagios:

sudo usermod -G nagios nagios

Create the group nagcmd:

sudo groupadd nagcmd

Add the user nagios to the group nagcmd:

sudo usermod -a -G nagcmd nagios

Add the Apache user to the group nagcmd:

sudo usermod -a -G nagcmd www-data

Installing Nagios and Nagios Plugins

The first thing to do is to download the files you need. Issue the commands:

wget http://osdn.dl.sourceforge.net/sourceforge/nagios/nagios-3.0.6.tar.gz

and

wget http://osdn.dl.sourceforge.net/sourceforge/nagiosplug/nagios-
plugins-1.4.11.tar.gz

to download the files you need.

Untar the Nagios file with:

tar xvzf nagios-3.0.6.tar.gz

Change into the newly created Nagios directory with the command:

cd nagios-3.0.6

Now run the configure script using the nagcmd defined as the command group with the command:

sudo ./configure –with-command-group=nagcmd

Time to compile:

sudo make all

Time to install everything:

make install

make install-init

make install-config

make install-commandmode

Almost ready to finalize the installation of Nagios (before moving on to installing the plugins.) There is one simple configuration to take care of (for the basic installation). Open up the /usr/local/nagios/etc/objects/contacts.cfg file and add your email address as the contact email address. That’s the only configuration to change for now.

Configuring for the Web interface

There are just a few commands to run to configure the Web interface:

make install-webconf

htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin

/etc/init.d/apache2 reload

The final command above simply restarts Apache.

Install the plugins

Before you log into your Nagios installation you have to install your plugins first. Change to the directory you downloaded the Nagios files to and untar the plugins file:

tar xvzf nagios-plugins-1.4.11.tar.gz

Now change into the newly created directory:

cd nagios-plugins-1.4.11

Compile the plugins with the command:

sudo ./configure –with-nagios-user=nagios –with-nagios-group=nagios

Now install the plugins with the following commands:

make

make install

Fire it up!

Of course you want to make sure Nagios starts any time the system is restarted. Do this with the command:

sudo ln -s /etc/init.d/nagios /etc/rcS.d/S99nagios

And finally, before you start up Nagios, make sure there are no errors with the command:

sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg

If you see no errors reported, it’s time to start it up. Issue the command:

sudo /etc/init.d/nagios start

Nagios is ready to go. Point your browser to:

http://ADDRESS_OF_NAGIOS_SERVER/nagios

You should be ready to enjoy all that is Nagios. Congratulations.