In this tutorial, we will set up a nagios server and a remote NRPE host.
Lets install the prerequisites first
$sudo yum install -y gcc glibc glibc-common gd gd-devel make net-snmp openssl-devel xinetd unzip mailx httpd php5 php5-gd postfix
We must create a user and group that will run the Nagios process. Create a "nagios" user and "nagcmd" group, then add the user to the group with these commands
$sudo useradd nagios
$sudo groupadd nagcmd
$sudo usermod -a -G nagcmd nagios
Install Nagios Core
$curl -L -O https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.1.1.tar.gz
$tar xvf nagios-*.tar.gz
$cd nagios-*
$./configure --with-command-group=nagcmd
$make all
$sudo make install
$sudo make install-commandmode
$sudo make install-init
$sudo make install-config
$sudo make install-webconf
$sudo usermod -G nagcmd apache
Now lets install the nagios plugins
$cd ..
$curl -L -O http://nagios-plugins.org/download/nagios-plugins-2.1.1.tar.gz
$tar xvf nagios-plugins-*.tar.gz
$cd nagios-plugins-*
$./configure --with-nagios-user=nagios --with-nagios-group=nagios --with-openssl
$make
$sudo make install
Now lets install NRPE
$cd ..
$curl -L -O http://downloads.sourceforge.net/project/nagios/nrpe-2.x/nrpe-2.15/nrpe-2.15.tar.gz
$tar xvf nrpe-*.tar.gz
$cd nrpe-*
$./configure --enable-command-args --with-nagios-user=nagios --with-nagios-group=nagios --with-ssl=/usr/bin/openssl --with-ssl-lib=/usr/lib/x86_64-linux-gnu
$make all
$sudo make install
$sudo make install-xinetd
$sudo make install-daemon-config
Now lets edit the start-up script
$sudo vi /etc/xinetd.d/nrpe
Modify the only_from line by adding the private IP address of the your Nagios server to the end (substitute in the actual IP address of your server):
only_from = 127.0.0.1 x.x.x.x
Restart the xinetd service to start NRPE:
$sudo service xinetd restart
Now let's perform the initial Nagios configuration. You only need to perform this section once, on your Nagios server.
$sudo vi /usr/local/nagios/etc/nagios.cfg
Now find an uncomment this line
#cfg_dir=/usr/local/nagios/etc/servers
Now create the directory that will store the configuration file for each server that you will monitor:
$sudo mkdir /usr/local/nagios/etc/servers
Open the Nagios contacts configuration in your favorite text editor. We'll use vi to edit the file:
$sudo vi /usr/local/nagios/etc/objects/contacts.cfg
Find the email directive, and replace its value (the highlighted part) with your own email address:
email nagios@localhost ; <<***** CHANGE THIS TO YOUR EMAIL ADDRES
Let's add a new command to our Nagios configuration:
$sudo vi /usr/local/nagios/etc/objects/commands.cfg
Add the following to the end of the file:
define command{
command_name check_nrpe
command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}
For mail Notification
We have to change the command "notify-host-by-email" and "notify-service-by-email" as below inorder for the mail notification to work
$sudo vim /usr/local/nagios/etc/objects/commands.cfg
define command{
command_name notify-host-by-email
command_line /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\nHost: $HOSTNAME$\nState: $HOSTSTATE$\nAddress: $HOSTADDRESS$\nInfo: $HOSTOUTPUT$\n\nDate/Time: $LONGDATETIME$\n" | /usr/bin/mail -s "** $NOTIFICATIONTYPE$ Host Alert: $HOSTNAME$ is $HOSTSTATE$ **" $CONTACTEMAIL$
}
# 'notify-service-by-email' command definition
define command{
command_name notify-service-by-email
command_line /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\n\nService: $SERVICEDESC$\nHost: $HOSTALIAS$\nAddress: $HOSTADDRESS$\nState: $SERVICESTATE$\n\nDate/Time: $LONGDATETIME$\n\nAdditional Info:\n\n$SERVICEOUTPUT$\n" | /usr/bin/mail -s "** $NOTIFICATIONTYPE$ Service Alert: $HOSTALIAS$/$SERVICEDESC$ is $SERVICESTATE$ **" $CONTACTEMAIL$
}
Configure Apache
Use htpasswd to create an admin user, called "nagiosadmin", that can access the Nagios web interface:
$sudo htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
Enter a password at the prompt. Remember this login, as you will need it to access the Nagios web interface.
Nagios is ready to be started. Let's do that, and restart Apache:
$sudo systemctl start nagios.service
$sudo systemctl restart httpd.service
To enable Nagios to start on server boot, run this command:
$sudo systemctl enable nagios.service
Open your favorite web browser, and go to your Nagios server
http://nagios_server_ip/nagios
#################################################################################
On a server that you want to monitor, install the EPEL repository:
$sudo yum install epel-release
Now install Nagios Plugins and NRPE:
$sudo yum install nrpe nagios-plugins-all
Now, let's update the NRPE configuration file. Open it in your favorite editor (we're using vi):
$sudo vi /etc/nagios/nrpe.cfg
Find the allowed_hosts directive, and add the private IP address of your Nagios server to the comma-delimited list (substitute it in place of the highlighted example):
allowed_hosts=127.0.0.1,x.x.x.x
Restart NRPE to put the change into effect:
$sudo systemctl start nrpe.service
$sudo systemctl enable nrpe.service
Add Host to Nagios Configuration
On your Nagios server, create a new configuration file for each of the remote hosts that you want to monitor in /usr/local/nagios/etc/servers/. Replace the highlighted word, "yourhost", with the name of your host:
$sudo vi /usr/local/nagios/etc/servers/yourhost.cfg
Add in the following host definition, replacing the host_name value with your remote hostname ("web-1" in the example), the alias value with a description of the host, and the address value with the private IP address of the remote host:
define host {
use linux-server
host_name client1.example.com
alias Monitor 1
address x.x.x.x
max_check_attempts 5
check_period 24x7
notification_interval 30
notification_period 24x7
contacts nagiosadmin
}
With the configuration file above, Nagios will only monitor if the host is up or down. If this is sufficient for you, save and exit then restart Nagios. If you want to monitor particular services, read on.
Add any of these service blocks for services you want to monitor. Note that the value of check_command determines what will be monitored, including status threshold values. Here are some examples that you can add to your host's configuration file:
Ping:
define service {
use generic-service
host_name yourhost
service_description PING
check_command check_ping!100.0,20%!500.0,60%
}
SSH
define service {
use generic-service
host_name yourhost
service_description SSH
check_command check_ssh
notifications_enabled 0
}
$sudo systemctl reload nagios.service
HTTP
define service {
use generic-service
host_name client1.example.com
service_description HTTP
check_command check_http
notifications_enabled 1
}
Port Check: We are going to check the availablility of port 80 with a check interval of 10 seconds
define service {
use generic-service
host_name client1.example.com
service_description HTTP_PORT_CHECK
check_command check_tcp!80
check_interval 0.2
notifications_enabled 1
}
Note:
check_interval 1 = 60 seconds
check_interval .5 = 30 seconds
To check memory utilization on the remote host
Download the Memory check Plugin
$wget https://raw.githubusercontent.com/justintime/nagios-plugins/master/check_mem/check_mem.pl
$sudo mv check_mem.pl /usr/lib64/nagios/plugins/check_mem
chmod +x /usr/lib64/nagios/plugins/check_mem
You can check whether the script generates output properly by manually running the following command on localhost. When used with NRPE, this command is supposed to check free memory, warn when free memory is less than 20%, and generate critical alarm when free memory is less than 10%.
$/usr/lib64/nagios/plugins/check_mem -f -w 20 -c 10
OK - 34.0% (2735744 kB) free.|TOTAL=8035340KB;;;; USED=5299596KB;6428272;7231806;; FREE=2735744KB;;;; CACHES=2703504KB;;;;
If you see something like the above as an output, that means the command is working okay.
Now that the script is ready, we define the command to check RAM usage for NRPE. As mentioned before, the command will check free memory, warn when free memory is less than 20%, and generate critical alarm when free memory is less than 10%.
$sudo vim /etc/nagios/nrpe.cfg
command[check_mem]=/usr/lib64/nagios/plugins/check_mem -f -w 20 -c 10
Now in the Nagios server add the following section to the /usr/local/nagios/etc/servers/yourhost.cfg
define service{
use generic-service
host_name client1.example.com
service_description Check RAM
check_command check_nrpe!check_mem
}
Restart the nrpe service in the remote host
Restart the nagios service in the Nagios server
Nagios should start checking RAM usage of a remote-server using NRPE. If you are having any problem, you could check the following.
Make sure that NRPE port is allowed all the way to the remote host. Default NRPE port is TCP 5666.
You could try manually checking NRPE operation by executing the check_nrpe command: /usr/local/nagios/libexec/check_nrpe -H remote-server
You could also try to run the check_mem command manually: /usr/local/nagios/libexec/check_nrpe -H remote-server –c check_mem
Lets install the prerequisites first
$sudo yum install -y gcc glibc glibc-common gd gd-devel make net-snmp openssl-devel xinetd unzip mailx httpd php5 php5-gd postfix
We must create a user and group that will run the Nagios process. Create a "nagios" user and "nagcmd" group, then add the user to the group with these commands
$sudo useradd nagios
$sudo groupadd nagcmd
$sudo usermod -a -G nagcmd nagios
Install Nagios Core
$curl -L -O https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.1.1.tar.gz
$tar xvf nagios-*.tar.gz
$cd nagios-*
$./configure --with-command-group=nagcmd
$make all
$sudo make install
$sudo make install-commandmode
$sudo make install-init
$sudo make install-config
$sudo make install-webconf
$sudo usermod -G nagcmd apache
Now lets install the nagios plugins
$cd ..
$curl -L -O http://nagios-plugins.org/download/nagios-plugins-2.1.1.tar.gz
$tar xvf nagios-plugins-*.tar.gz
$cd nagios-plugins-*
$./configure --with-nagios-user=nagios --with-nagios-group=nagios --with-openssl
$make
$sudo make install
Now lets install NRPE
$cd ..
$curl -L -O http://downloads.sourceforge.net/project/nagios/nrpe-2.x/nrpe-2.15/nrpe-2.15.tar.gz
$tar xvf nrpe-*.tar.gz
$cd nrpe-*
$./configure --enable-command-args --with-nagios-user=nagios --with-nagios-group=nagios --with-ssl=/usr/bin/openssl --with-ssl-lib=/usr/lib/x86_64-linux-gnu
$make all
$sudo make install
$sudo make install-xinetd
$sudo make install-daemon-config
Now lets edit the start-up script
$sudo vi /etc/xinetd.d/nrpe
Modify the only_from line by adding the private IP address of the your Nagios server to the end (substitute in the actual IP address of your server):
only_from = 127.0.0.1 x.x.x.x
Restart the xinetd service to start NRPE:
$sudo service xinetd restart
Now let's perform the initial Nagios configuration. You only need to perform this section once, on your Nagios server.
$sudo vi /usr/local/nagios/etc/nagios.cfg
Now find an uncomment this line
#cfg_dir=/usr/local/nagios/etc/servers
Now create the directory that will store the configuration file for each server that you will monitor:
$sudo mkdir /usr/local/nagios/etc/servers
Open the Nagios contacts configuration in your favorite text editor. We'll use vi to edit the file:
$sudo vi /usr/local/nagios/etc/objects/contacts.cfg
Find the email directive, and replace its value (the highlighted part) with your own email address:
email nagios@localhost ; <<***** CHANGE THIS TO YOUR EMAIL ADDRES
Let's add a new command to our Nagios configuration:
$sudo vi /usr/local/nagios/etc/objects/commands.cfg
Add the following to the end of the file:
define command{
command_name check_nrpe
command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}
For mail Notification
We have to change the command "notify-host-by-email" and "notify-service-by-email" as below inorder for the mail notification to work
$sudo vim /usr/local/nagios/etc/objects/commands.cfg
define command{
command_name notify-host-by-email
command_line /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\nHost: $HOSTNAME$\nState: $HOSTSTATE$\nAddress: $HOSTADDRESS$\nInfo: $HOSTOUTPUT$\n\nDate/Time: $LONGDATETIME$\n" | /usr/bin/mail -s "** $NOTIFICATIONTYPE$ Host Alert: $HOSTNAME$ is $HOSTSTATE$ **" $CONTACTEMAIL$
}
# 'notify-service-by-email' command definition
define command{
command_name notify-service-by-email
command_line /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\n\nService: $SERVICEDESC$\nHost: $HOSTALIAS$\nAddress: $HOSTADDRESS$\nState: $SERVICESTATE$\n\nDate/Time: $LONGDATETIME$\n\nAdditional Info:\n\n$SERVICEOUTPUT$\n" | /usr/bin/mail -s "** $NOTIFICATIONTYPE$ Service Alert: $HOSTALIAS$/$SERVICEDESC$ is $SERVICESTATE$ **" $CONTACTEMAIL$
}
Configure Apache
Use htpasswd to create an admin user, called "nagiosadmin", that can access the Nagios web interface:
$sudo htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
Enter a password at the prompt. Remember this login, as you will need it to access the Nagios web interface.
Nagios is ready to be started. Let's do that, and restart Apache:
$sudo systemctl start nagios.service
$sudo systemctl restart httpd.service
To enable Nagios to start on server boot, run this command:
$sudo systemctl enable nagios.service
Open your favorite web browser, and go to your Nagios server
http://nagios_server_ip/nagios
#################################################################################
Monitor a CentOS 7 Host with NRPE
On a server that you want to monitor, install the EPEL repository:
$sudo yum install epel-release
Now install Nagios Plugins and NRPE:
$sudo yum install nrpe nagios-plugins-all
Now, let's update the NRPE configuration file. Open it in your favorite editor (we're using vi):
$sudo vi /etc/nagios/nrpe.cfg
Find the allowed_hosts directive, and add the private IP address of your Nagios server to the comma-delimited list (substitute it in place of the highlighted example):
allowed_hosts=127.0.0.1,x.x.x.x
Restart NRPE to put the change into effect:
$sudo systemctl start nrpe.service
$sudo systemctl enable nrpe.service
Add Host to Nagios Configuration
On your Nagios server, create a new configuration file for each of the remote hosts that you want to monitor in /usr/local/nagios/etc/servers/. Replace the highlighted word, "yourhost", with the name of your host:
$sudo vi /usr/local/nagios/etc/servers/yourhost.cfg
Add in the following host definition, replacing the host_name value with your remote hostname ("web-1" in the example), the alias value with a description of the host, and the address value with the private IP address of the remote host:
define host {
use linux-server
host_name client1.example.com
alias Monitor 1
address x.x.x.x
max_check_attempts 5
check_period 24x7
notification_interval 30
notification_period 24x7
contacts nagiosadmin
}
With the configuration file above, Nagios will only monitor if the host is up or down. If this is sufficient for you, save and exit then restart Nagios. If you want to monitor particular services, read on.
Add any of these service blocks for services you want to monitor. Note that the value of check_command determines what will be monitored, including status threshold values. Here are some examples that you can add to your host's configuration file:
Ping:
define service {
use generic-service
host_name yourhost
service_description PING
check_command check_ping!100.0,20%!500.0,60%
}
SSH
define service {
use generic-service
host_name yourhost
service_description SSH
check_command check_ssh
notifications_enabled 0
}
$sudo systemctl reload nagios.service
HTTP
define service {
use generic-service
host_name client1.example.com
service_description HTTP
check_command check_http
notifications_enabled 1
}
Port Check: We are going to check the availablility of port 80 with a check interval of 10 seconds
define service {
use generic-service
host_name client1.example.com
service_description HTTP_PORT_CHECK
check_command check_tcp!80
check_interval 0.2
notifications_enabled 1
}
Note:
check_interval 1 = 60 seconds
check_interval .5 = 30 seconds
To check memory utilization on the remote host
Download the Memory check Plugin
$wget https://raw.githubusercontent.com/justintime/nagios-plugins/master/check_mem/check_mem.pl
$sudo mv check_mem.pl /usr/lib64/nagios/plugins/check_mem
chmod +x /usr/lib64/nagios/plugins/check_mem
You can check whether the script generates output properly by manually running the following command on localhost. When used with NRPE, this command is supposed to check free memory, warn when free memory is less than 20%, and generate critical alarm when free memory is less than 10%.
$/usr/lib64/nagios/plugins/check_mem -f -w 20 -c 10
OK - 34.0% (2735744 kB) free.|TOTAL=8035340KB;;;; USED=5299596KB;6428272;7231806;; FREE=2735744KB;;;; CACHES=2703504KB;;;;
If you see something like the above as an output, that means the command is working okay.
Now that the script is ready, we define the command to check RAM usage for NRPE. As mentioned before, the command will check free memory, warn when free memory is less than 20%, and generate critical alarm when free memory is less than 10%.
$sudo vim /etc/nagios/nrpe.cfg
command[check_mem]=/usr/lib64/nagios/plugins/check_mem -f -w 20 -c 10
Now in the Nagios server add the following section to the /usr/local/nagios/etc/servers/yourhost.cfg
define service{
use generic-service
host_name client1.example.com
service_description Check RAM
check_command check_nrpe!check_mem
}
Restart the nrpe service in the remote host
Restart the nagios service in the Nagios server
Nagios should start checking RAM usage of a remote-server using NRPE. If you are having any problem, you could check the following.
Make sure that NRPE port is allowed all the way to the remote host. Default NRPE port is TCP 5666.
You could try manually checking NRPE operation by executing the check_nrpe command: /usr/local/nagios/libexec/check_nrpe -H remote-server
You could also try to run the check_mem command manually: /usr/local/nagios/libexec/check_nrpe -H remote-server –c check_mem
