Service<\/strong>: It is the service or resource being monitored on the host such as HTTP, SMTP, POP, DNS, number of logged-in users, memory usage, etc.<\/p>\n Nagios Plugin: <\/strong>These are scripts that check the status of a host or service and verify whether they are working correctly. There are some predefined plugins that can check basic resources such as processor load, disk usage, ping rates, etc. Plugins are usually located in \/usr\/local\/nagios\/libexec<\/strong> directory.<\/p>\n NRPE (Nagios Remote Plugin Executor) add-on: <\/strong>NRPE add-on executes the plugins on the remote Linux hosts which then allows Nagios to keep an eye on local resources (disk space, memory usage, etc). NRPE add-on comprises of:<\/p>\n \u00a0You must have sudo privileges on both the servers. The details of our machines are as follows:<\/p>\n Nagios Monitoring Server:<\/strong><\/p>\n Nagios Host:<\/strong><\/p>\n On the Nagios monitoring server, follow the below steps to download and install the Nagios core.<\/p>\n First, we will need to install Prerequisites on the Nagios monitoring server. To do so, update the packages list using the below command in Terminal:<\/p>\n Then install the required packages:<\/p>\n Navigate to the \/tmp<\/strong> directory:<\/p>\n Then download the Nagios source code using the wget<\/strong> command. We will download the currently available latest version of Nagios that is 4.4.6<\/strong>. You can visit the Nagios Downloads<\/a> to look for the current stable release.<\/p>\n The downloaded file will be saved as nagios.tar.gz<\/strong> in the \/tmp<\/strong> directory.<\/p>\n Extract the downloaded archive nagios.tar.gz <\/strong>using the below command:<\/strong><\/p>\n Move into the extracted folder using the cd<\/strong> command:<\/p>\n Run the configure<\/strong> script and specifies the path to the Apache server configuration directory:<\/p>\n Once the configure script is executed, you will see the below summary at the end of the output:<\/p>\n Your configure<\/strong> script may fail in case of any pre-requisite missing. In that case, install the missing pre-requisites and again run the script.<\/p>\n Now compile Nagios using the below command:<\/p>\n Now create user and group for Nagios.<\/p>\n The above command will create a system user and group Nagios.<\/p>\n We will also need to add user \u201cwww-data<\/strong>” to the nagios<\/strong> group.<\/p>\n Then to install binaries, use the below command:<\/p>\n To install service files, use the below command:<\/p>\n To install and configures the external command file, use the below command:<\/p>\n Then to install the sample Nagios configuration, use the below command:<\/p>\n This installs the Apache configuration files, use the below command:<\/p>\n Now enable the Apache modules using the below commands:<\/p>\n Now we will create an Apache user account that can access the Nagios web interface.<\/p>\n Now set the password for the nagiosadmin<\/strong> user account. This password will be used to access the Nagios\u2019 web interface.<\/p>\n Then restart Apache for the configuration changes to take effect:<\/p>\n Now we have successfully installed the Nagios core engine.<\/p>\n For the Nagios core to work properly, you will need to install the Nagios plugin.<\/p>\n On the Nagios server, Navigate to the \/tmp<\/strong> directory:<\/p>\n Then download the Nagios plugin source code using the wget<\/strong> command. We will download the currently available latest version of the Nagios plugin that is 2.3.3.<\/strong> You can visit the Github<\/a> page to find the recent stable release.<\/p>\n The downloaded file will be saved as Nagios-plugins.tar.gz<\/strong> in the \/tmp<\/strong> directory.<\/p>\n Extract the downloaded archive Nagios-plugins.tar.gz<\/strong> using the below command:<\/strong><\/p>\n Navigate to the extracted folder using the cd<\/strong> command:<\/p>\n Run the configure<\/strong> script:<\/p>\n Now compile the Nagios plugin using the below command:<\/p>\n Now we will install check_nrpe<\/strong> plugin on our Nagios server.<\/p>\n On the Nagios server, Navigate to the \/tmp<\/strong> directory:<\/p>\n Then download the check_nrpe source code using the wget command. We will download the recent available version of check_nrpe that is 4.0.3. You can visit the Github<\/a> page to find the recent stable release.<\/p>\n The downloaded file will be saved as Nagios-nrpe.tar.gz<\/strong> in the \/tmp<\/strong> directory.<\/p>\n Extract the downloaded archive nagios.tar.gz <\/strong>using the below command:<\/strong><\/p>\n Navigate to the extracted folder using the cd<\/strong> command:<\/p>\n Run the configure<\/strong> script:<\/p>\n Now compile the check_nrpe<\/strong> using the below command:<\/p>\n Now, we will configure the Nagios main configuration file. Edi the Nagios configuration file nagios.cfg<\/strong> using the below command:<\/p>\n Now find the below line in the file and uncomment it by removing the #<\/strong> character from the beginning of the line.<\/p>\n The above line specifies the directory which contains a configuration file for all the hosts that the Nagios server will monitor. Now save and close the configuration file.<\/p>\n Now we will create the directory in the path defined above:<\/p>\n Now we will configure the contacts that should be notified in case of any event. Edit the Nagios contacts configuration using the below command:<\/p>\n In order to receive alerts from the Nagios monitoring server, you will need to configure the contacts. Open the contacts <\/strong>configuration file, and specify the email address you would like to receive the emails from Nagios:<\/p>\n In the email directive, replace nagios@localhost<\/strong> with your own email address and then save and close the file.<\/p>\n Now in order to use check_nrpe<\/strong> plugin, we will add a new command in the command <\/strong>configuration file. Edit the command configuration file commands.cfg <\/strong>using the below command:<\/p>\n Append the following lines to the end of the commands.cfg<\/strong> file and then save and close it:<\/p>\n Now start Nagios service:<\/p>\n Now access the Nagios web interface by visiting the below address in any web browser:<\/p>\n In our scenario, it would be:<\/p>\n In the Authentication Required<\/strong> dialog, type nagiosadmin<\/strong> as the username and password you have set earlier.<\/p>\n Once logged in to the web nterface, you will see the default Nagios web page. Go to the Hosts<\/strong> tab from the left sidebar to view the monitored hosts.<\/p>\n At present, you will see only one host \u201clocalhost<\/strong>” which is the Nagios server itself. In order to monitor other hosts, you will need to install the NRPE<\/strong> daemon on them.<\/p>\n Now you will have to install the Nagios plugin<\/strong> and NRPE<\/strong> on the hosts that need to be monitored on the Nagios server.<\/p>\n In the remote host (client) that needs to be monitored, follow the below steps to install Nagios plugin.<\/p>\n 1. First, update the apt:<\/p>\n 2. Then issue the below command to install the required packages:<\/p>\n 3. Now, Navigate to \/tmp<\/strong> and download the Nagios plugin.<\/p>\n 4. Extract the downloaded archive:<\/p>\n 5. Move into extracted folder and run the configure<\/strong> script:<\/p>\n 6. Compile the plugin:<\/p>\n Follow the below steps to install NRPE on the remote host.<\/p>\n 1. First, update the apt:<\/p>\n 2. Issue the below command to install the pre-requisites:<\/p>\n 3. Now Navigate to \/tmp<\/strong> and download NRPE.<\/p>\n 4. Extract the downloaded archive:<\/p>\n 5. \u00a0Move into the extracted folder and run the configure script:<\/p>\n 6. Compile the NRPE with the following commands:<\/p>\n Now if a firewall is running on your remote host, you will need to open port 5666 (used by NRPE) through it.<\/p>\n Now open the NRPE configuration file in the remote host:<\/p>\n You will need to find and modify the below entries:<\/p>\n Add client IP address:<\/p>\n Also, replace \/dev\/vda1<\/strong> with your root file system:<\/p>\n Then start the NRPE service:<\/p>\n Now check if the NRPE service is properly running:<\/p>\n The active (running) in the below output shows that the service is successfully running without any issues. You can also see the NRPE is allowing connection from 192.168.72.158<\/strong> which is the IP address of the Nagios server.<\/p>\n Now to check if the check_nrpe<\/strong> on the Nagios server<\/strong> can communicate with the NRPE on the remote host<\/strong>, issue this command on the Nagios server<\/strong>:<\/p>\n You should receive the following similar output (NRPE version number).<\/p>\n Now in the Nagios monitoring server, you will need to create separate configuration files in the \/usr\/local\/nagios\/etc\/servers<\/strong>\/ for all remote hosts that you need to monitor.<\/p>\n For our host machine, we will create a new configuration file client.cfg<\/strong>:<\/p>\n Replace client<\/strong> with the name of your host:<\/p>\n Now add host definition<\/strong> to the new configuration file replacing the value of host_name<\/strong> with the remote hostname, alias<\/strong> with a short description, and the address<\/strong> with IP address of the remote host.<\/p>\n This configuration will check if the host machine is up or down.<\/p>\n Now add service definition<\/strong> to the configuration file for monitoring the remote host. Replace the value of host_name<\/strong> with the remote hostname.<\/p>\n Note:<\/strong> These service definitions will use the commands preconfigured in the \/usr\/local\/nagios\/etc\/nrpe.cfg <\/strong>file on the remote host.<\/p>\n Add the following service block to monitor the CPU load<\/strong> on the remote host<\/p>\n }<\/p>\n Add the following service block to monitor disk usage<\/strong> on the remote host:<\/p>\n }<\/p>\n Add the following service block to monitor the number of users<\/strong> currently logged in <\/strong>on the remote host:<\/p>\n Now save and close the configuration file.<\/p>\n Restart the Nagios service to apply the configuration changes:<\/p>\n\n
\n
\n
Step 1 – Installing Nagios Core on Monitoring Server<\/h2>\n
1. Installing Prerequisites<\/h3>\n
$ sudo apt update<\/pre>\n
$ sudo apt install -y autoconf gcc libc6 make wget unzip apache2 php libapache2-mod-php7.4 libgd-dev openssl libssl-dev<\/pre>\n
2. Download the Nagios Source<\/h3>\n
$ cd \/tmp<\/pre>\n
$ wget -O nagios.tar.gz https:\/\/github.com\/NagiosEnterprises\/nagioscore\/archive\/nagios-4.4.6.tar.gz<\/a><\/pre>\n
3. Compile Source<\/h3>\n
$ tar -xzf nagios.tar.gz<\/pre>\n
$ cd nagioscore-nagios-4.4.6\/<\/pre>\n
$ .\/configure --with-httpd-conf=\/etc\/apache2\/sites-enabled<\/pre>\n
![](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-612.png\")
<\/p>\n$ make all<\/pre>\n
4. Create User and Group<\/h3>\n
$ sudo make install-groups-users <\/pre>\n
![](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-613.png\")
<\/p>\n$ sudo usermod -a -G nagios www-data<\/pre>\n
5. Installing Binaries<\/h3>\n
$ sudo make install<\/pre>\n
6. Install Service \/ Daemon<\/h3>\n
$ sudo make install-daemoninit<\/pre>\n
7. Install Command Mode<\/h3>\n
$ sudo make install-commandmode<\/pre>\n
8. Install Configuration Files<\/h3>\n
$ sudo make install-config<\/pre>\n
9. Install Apache Config Files<\/h3>\n
$ sudo make install-webconf<\/pre>\n
$ sudo a2enmod rewrite<\/pre>\n
sudo a2enmod cgi<\/pre>\n
Create Nagiosadmin User Account for the Web Interface:<\/h4>\n
$ sudo htpasswd -c \/usr\/local\/nagios\/etc\/htpasswd.users nagiosadmin<\/pre>\n
$ sudo systemctl restart apache2<\/pre>\n
Step 2 – Installing the Nagios Plugin on Monitoring Server<\/h2>\n
Download Nagios Plugin<\/h3>\n
$ cd \/tmp<\/pre>\n
$ wget -O Nagios-plugins.tar.gz https:\/\/github.com\/nagios-plugins\/nagios-plugins\/releases\/download\/release-2.3.3\/nagios-plugins-2.3.3.tar.gz<\/a><\/pre>\n
Compile<\/h3>\n
$ tar zxf nagios-plugins.tar.gz<\/pre>\n
$ cd nagios-plugins-2.3.3\/<\/pre>\n
$ sudo .\/configure<\/pre>\n
$ sudo make
$ sudo make install<\/pre>\nStep 3 – Installing the check_nrpe Plugin on Monitoring Server<\/h2>\n
Download Plugin Source<\/h3>\n
$ cd \/tmp<\/pre>\n
$ wget -O Nagios-nrpe.tar.gz https:\/\/github.com\/NagiosEnterprises\/nrpe\/releases\/download\/nrpe-4.0.3\/nrpe-4.0.3.tar.gz<\/pre>\n
Compiling the Source<\/h3>\n
$ tar -xzf Nagios-nrpe.tar.gz<\/pre>\n
$ cd nrpe-4.0.3\/<\/pre>\n
$ sudo .\/configure<\/pre>\n
$ sudo make check_nrpe
$ sudo make install-plugin<\/pre>\nStep 4 – Configuring Nagios<\/h2>\n
$ sudo nano \/usr\/local\/nagios\/etc\/nagios.cfg<\/pre>\n
#cfg_dir=\/usr\/local\/nagios\/etc\/servers<\/pre>\n
![](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-614.png\")
<\/p>\n$ sudo mkdir \/usr\/local\/nagios\/etc\/servers<\/pre>\n
Configure E-mail Contacts<\/h3>\n
$ sudo nano \/usr\/local\/nagios\/etc\/objects\/contacts.cfg<\/pre>\n
![](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-615.png\")
<\/p>\nConfigure check_nrpe<\/h3>\n
$ sudo nano \/usr\/local\/nagios\/etc\/objects\/commands.cfg <\/strong><\/pre>\n
define command{
command_name check_nrpe
command_line $USER1$\/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}<\/pre>\n![](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-616.png\")
<\/p>\n$ sudo systemctl start Nagios.service<\/pre>\n
Step 5 – Accessing the Nagios Web Interface<\/h2>\n
http:\/\/nagios-server-ip\/nagios<\/a><\/pre>\n
http:\/\/192.168.72.158\/nagios<\/pre>\n
![](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-617.png\")
<\/p>\n![](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-618.png\")
<\/p>\nStep 6 – Remote Host Setup<\/h2>\n
1. Install Nagios Plugin on Remote Host<\/h3>\n
$ sudo apt update<\/pre>\n
$ sudo apt install autoconf gcc libmcrypt-dev make libssl-dev dc build-essential gettext<\/pre>\n
$ cd \/tmp<\/pre>\n
$ wget -O Nagios-plugins.tar.gz https:\/\/github.com\/nagios-plugins\/nagios-plugins\/releases\/download\/release-2.3.3\/nagios-plugins-2.3.3.tar.gz<\/pre>\n
$ tar -zxf nagios-plugins.tar.gz<\/pre>\n
$ cd nagios-plugins-2.3.3\/\/
$ sudo .\/configure<\/pre>\n$ sudo make
$ sudo make install<\/pre>\n2. Install NRPE Daemon on Remote Host<\/h3>\n
$ sudo apt update<\/pre>\n
$ sudo apt install -y autoconf automake gcc libc6 libmcrypt-dev make libssl-dev<\/pre>\n
$ cd \/tmp
$ wget -O nrpe.tar.gz https:\/\/github.com\/NagiosEnterprises\/nrpe\/releases\/download\/nrpe-4.0.3\/nrpe-4.0.3.tar.gz<\/a><\/pre>\n$ tar xzf nrpe.tar.gz<\/pre>\n
$ sudo .\/configure --enable-command-args<\/pre>\n
$ sudo make all<\/pre>\n
$ make install-groups-users
$ sudo make install
$ sudo make install-config
$ sudo make install-init<\/pre>\n3. Configure Firewall<\/h3>\n
$ sudo ufw allow 5666\/tcp<\/pre>\n
4. Update NRPE Configuration<\/h3>\n
$ sudo nano \/usr\/local\/nagios\/etc\/nrpe.cfg<\/pre>\n
server_address=192.168.72.159
Add Nagios server IP address:
allowed_hosts=127.0.0.1,192.168.72.158<\/pre>\n![](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-619.png\")
<\/p>\ncommand[check_disk]=\/usr\/local\/nagios\/libexec\/check_disk -w 20% -c 10% -p \/dev\/sda1<\/pre>\n
![](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-620.png\")
<\/strong><\/p>\n$ sudo systemctl start nrpe.service<\/pre>\n
$ sudo systemctl status nrpe.service<\/pre>\n
![](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-621.png\")
<\/p>\n$ \/usr\/local\/nagios\/libexec\/check_nrpe -H remote_host_ip<\/pre>\n
![\"How](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-622.png\")
<\/p>\nStep 7 – Add Host Configuration to the Nagios Server<\/h3>\n
$ sudo nano \/usr\/local\/nagios\/etc\/servers\/client.cfg<\/pre>\n
define host {
use linux-server
host_name client
alias client machine
address 192.168.72.159
max_check_attempts 5
}<\/pre>\ndefine service {
use generic-service
host_name client
service_description CPU Load
check_command check_nrpe!check_load
}
c on the remote host:<\/pre>\ndefine service{
use generic-service
host_name client
service_description Total Processes
check_command check_nrpe!check_total_procs<\/pre>\ndefine service {
use generic-service\u00a0\u00a0\u00a0\u00a0\u00a0
host_name\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 client
\u00a0\u00a0\u00a0\u00a0\u00a0 service_description\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Check Disk
\u00a0\u00a0\u00a0\u00a0\u00a0 check_command\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 check_nrpe!check_disk<\/pre>\ndefine service {
use generic-service\u00a0\u00a0\u00a0\u00a0\u00a0
host_name\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 client
\u00a0\u00a0\u00a0\u00a0\u00a0 service_description\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Current Users
\u00a0\u00a0\u00a0\u00a0\u00a0 check_command\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 check_nrpe!check_users
}<\/pre>\n![\"How](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-623.png\")
<\/p>\n$ sudo systemctl restart Nagios<\/pre>\n
![\"How](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-624.png\")
<\/p>\n![\"How](\"http:\/\/linuxways.net\/wp-content\/uploads\/2020\/11\/word-image-625.png\")
<\/p>\n