Nagios Client

Add Linux host to your Nagios Server

First of all you need to install and conigure Nagios server. If not please install a server. You can follow this post to know how to install and configure a Nagios server.

http://linux-library.blogspot.in/2014/05/nagios-server.html

Once you’ve installed, you can proceed further to install NRPE agent on your Remote Linux host. Before heading further, let us give you a short description about NRPE.

What is NRPE?

The NRPE (Nagios Remote Plugin Executor) plugin allows you to monitor any remote Linux/Unix services or network devices. This NRPE add-on allows Nagios to monitor any local resources like CPU load, Swap, Memory usage, Online users, etc. on remote Linux machines. After all, these local resources are not mostly exposed to external machines, an NRPE agent must be installed and configured on the remote machines.

Note: The NRPE addon requires that Nagios Plugins must be installed on the remote Linux machine. Without these, the NRPE daemon will not work and will not monitor anything.

Installation of NRPE Plugin

To use the NRPE, you will need to do some additional tasks on both the Nagios Monitoring Host and Remote Linux Host that the NRPE installed on. We will be covering both the installation parts separately.

We assume that you are installing the NRPE on a host that supports TCP wrappers and Xinted daemon installed on it. Today, most of the modern Linux distributions have these two installed by default. If not, we will install it later during the installation when required.

On Remote Linux Host

Please use the below instructions to install Nagios Plugins and NRPE daemon on the Remote Linux Host.

Step 1: Install Required Dependencies

We need to install required libraries like gcc, glibc, glibc-common and GD and its development libraries before installing.

# yum install -y gcc glibc glibc-common gd gd-devel make net-snmp openssl-devel

Step 2: Create Nagios User

Create a new nagios user account and set a password.

# useradd nagios
# passwd nagios

Step 3: Install the Nagios Plugins

Create a directory for installation and all its future downloads.

# cd /root/nagios

Now download latest Nagios Plugins 1.5 package here.

https://www.nagios-plugins.org/download/nagios-plugins-1.5.tar.gz

Step 4: Extract Nagios Plugins

Run the following tar command to extract the source code tarball.

# tar –xvf nagios-plugins-1.5.tar.gz

Step 5: Compile and Install Nagios Plugins

Next, compile and install using following commands

# cd nagios-plugins-1.5
# ./configure 
# make
# make install

Set the permissions on the plugin directory.

# chown nagios.nagios /usr/local/nagios
# chown -R nagios.nagios /usr/local/nagios/libexec

Step 6: Install Xinetd

Most of the systems, its by default installed. If not, install xinetd package using following yum command.

# yum install xinetd

Step 7: Install NRPE Plugin

Download latest NRPE Plugin 2.15 packages here.
http://sourceforge.net/projects/nagios/files/nrpe-2.x/nrpe-2.15/nrpe-2.15.tar.gz/download
Copy the downloaded NRPE plugins to the below directory.

# cd /root/nagios

Unpack the NRPE source code tarball.

# tar xzf nrpe-2.15.tar.gz
# cd nrpe-2.15

Compile and install the NRPE addon.

# ./configure
# make all

Next, install the NRPE plugin daemon, and sample daemon config file.

# make install-plugin
# make install-daemon
# make install-daemon-config

Install the NRPE daemon under xinetd as a service.

# make install-xinetd

Now open /etc/xinetd.d/nrpe file and add the localhost and IP address of the Nagios Monitoring Server.

only_from = 127.0.0.1 localhost <nagios_ip_address>

Next, open /etc/services file add the following entry for the NRPE daemon at the bottom of the file.

nrpe            5666/tcp                 NRPE

Restart the xinetd service.

# service xinetd restart

Step 8: Verify NRPE Daemon Locally

Run the following command to verify the NRPE daemon working correctly under xinetd.

# netstat -at | grep nrpe

tcp        0      0 *:nrpe                      *:*                      LISTEN

If you get output similar to above, means it working correctly. If not, make sure to check the following things.

1. Check you’ve added nrpe entry correctly in /etc/services file
2. The only_from contains an entry for “nagios_ip_address” in the /etc/xinetd.d/nrpe file.
3. The xinetd is installed and started.
4. Check for the errors in the system log files for about xinetd or nrpe and fix those problems.

Next, verify the NRPE daemon is functioning properly. Run the “check_nrpe” command that was installed earlier for testing purposes.

# /usr/local/nagios/libexec/check_nrpe -H localhost

You will get a following string on the screen, it shows you what version of NRPE is installed:

NRPE v2.15

Step 9: Configure Firewall Rules

Make sure that the Firewall on the local machine will allow the NRPE daemon to be accessed from remote servers. To do this, run the following iptables command.

# iptables -A INPUT -p tcp -m tcp --dport 5666 -j ACCEPT

Run the following command to Save the new iptables rule so it will survive at system reboots.

# service iptables save

Step 10: Customize NRPE commands

The default NRPE configuration file that got installed has several command definitions that will be used to monitor this machine. The sample configuration file located at.

# vi /usr/local/nagios/etc/nrpe.cfg

The following are the default command definitions that are located at the bottom of the configuration file. For the time being, we assume you are using these commands. You can check them by using the following commands.

# /usr/local/nagios/libexec/check_nrpe -H localhost -c check_users

USERS OK - 1 users currently logged in |users=1;5;10;0
# /usr/local/nagios/libexec/check_nrpe -H localhost -c check_load
# /usr/local/nagios/libexec/check_nrpe -H localhost -c check_hda1
# /usr/local/nagios/libexec/check_nrpe -H localhost -c check_total_procs
# /usr/local/nagios/libexec/check_nrpe -H localhost -c check_zombie_procs

You can edit and add new command definitions by editing the NRPE config file. Finally, you’ve successfully installed and configured NRPE agent on the Remote Linux Host. Now it’s time to install a NRPE component and add some services on your Nagios Monitoring Server…

On Nagios Monitoring Server

Now login into your Nagios Monitoring Server.

Step 1: Install NRPE Plugin

Go to the nagios download directory and download latest NRPE Plugin

# cd /root/nagios

Unpack the NRPE source code tarball.

# tar xzf nrpe-2.15.tar.gz
# cd nrpe-2.15

Compile and install the NRPE addon.

# ./configure
# make all
# make install-daemon

Step 2: Verify NRPE Daemon Remotely

Make sure that the check_nrpe plugin can communicate with the NRPE daemon on the remote Linux host. Add the IP address in the command below with the IP address of your Remote Linux host.

# /usr/local/nagios/libexec/check_nrpe -H <remote_linux_ip_address>

You will get a string back that shows you what version of NRPE is installed on the remote host, like this:

NRPE v2.15

If your receive a plugin time-out error, then check the following things.

1. Make sure your firewall isn’t blocking the communication between the remote host and the monitoring host.
   
2. Make sure that the NRPE daemon is installed correctly under xinetd.
   
3. Make sure that the remote Linux host firewall rules blocking the monitoring server from communicating to the NRPE daemon.
   

Adding Remote Linux Host to Nagios Monitoring Server

To add a remote host you need to create a two new files “hosts.cfg” and “services.cfg” under “/usr/local/nagios/etc/” location.

# cd /usr/local/nagios/etc/
# touch hosts.cfg
# touch services.cfg

Now add these two files to main Nagios configuration file. Open nagios.cfg file with any editor.

# vi /usr/local/nagios/etc/nagios.cfg

Now add the two newly created files as shown below.

# You can specify individual object config files as shown below:
cfg_file=/usr/local/nagios/etc/hosts.cfg
cfg_file=/usr/local/nagios/etc/services.cfg

Now open hosts.cfg file and add the default host template name and define remote hosts as shown below. Make sure to replace host_name, alias and address with your remote host server details.

# vi /usr/local/nagios/etc/hosts.cfg
## Default Linux Host Template ##
define host{
name                            rhel-machine             ; Name of this template
use                               generic-host              ; Inherit default values
check_period                 24x7        
check_interval               5       
retry_interval                1       
max_check_attempts     10      
check_command            check-host-alive
notification_period         24x7    
notification_interval       30      
notification_options       d,r     
contact_groups             admins  
register                        0                       ; DONT REGISTER THIS - ITS A TEMPLATE
}

## Default
define host{
use                             rhel-machine              ; Inherit default values from a template
host_name                  tiltec                   ; The name we're giving to this server
alias                           RHEL 6                 ; A longer name for the server
address                         192.168.0.120            ; IP address of Remote Linux host
}

Next open services.cfg file add the following services to be monitored.

# vi /usr/local/nagios/etc/services.cfg
define service{
        use                       generic-service
        host_name                tiltec
        service_description      CPU Load
        check_command            check_nrpe!check_load
        }

define service{
        use                       generic-service
        host_name                tiltec
        service_description      Total Processes
        check_command            check_nrpe!check_total_procs
        }

define service{
        use                       generic-service
        host_name                tiltec
        service_description      Current Users
        check_command            check_nrpe!check_users
        }

define service{
        use                       generic-service
        host_name                tiltec
        service_description      SSH Monitoring
        check_command            check_nrpe!check_ssh
        }

define service{
        use                       generic-service
        host_name                tiltec
        service_description      FTP Monitoring         check_command            check_nrpe!check_ftp        }

Now NRPE command definition needs to be created in commands.cfg file.

# vi /usr/local/nagios/etc/objects/commands.cfg

Add the following NRPE command definition at the bottom of the file.

define command{
        command_name check_nrpe
        command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
        }

Finally, verify Nagios Configuration files for any errors.

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

Total Warnings: 0
Total Errors:   0

Restart Nagios:

# service nagios restart

That’s it. Now go to Nagios Monitoring Web interface at “http://Your-server-IP-address/nagios” or “http://FQDN/nagios” and Provide the username “nagiosadmin” and password. Check that the Remote Linux Host was added and is being monitored.