Nagios : Monitoring a remote Linux host

In this post, I will give you a step by step guide on how to monitor a remote Linux host and the various services running on the remote host.

How Nagios Works?

Following steps will take place when Nagios (installed on Nagios server) monitors a service (eg: server load) on the remote host

1) Nagios will execute check_nrpe command on nagios-server and will request it to monitor disk usage on remote host using check_load command.
2) The check_nrpe on the nagios-server will contact the NRPE daemon on remote host and will request it to execute the check_load on remote host.
3) The results of the check_load command will be returned back by NRPE daemon to the check_nrpe on nagios-server.

Nagios Server(check_nrpe)~>Remote host(NRPE deamon)~>check_load

Nagios Server(check_nrpe)<~Remote host(NRPE deamon)<~check_load

Before we begin, we need to install some packages on the remote host for Nagios to work fine.

# yum install elinks gcc make gcc-c++
# yum -y install openssl-devel
# useradd nagios
# passwd nagios
# cd /usr/src/

Then install Nagios plugin and NRPE on remote host.

Install Plug-in

# wget http://sourceforge.net/projects/nagiosplug/files/nagiosplug/1.4.15/nagios-plugins-1.4.15.tar.gz/download
# tar -zxvf nagios-plugins-1.4.15.tar.gz && cd nagios-plugins-1.4.15
# ./configure --with-nagios-user=nagios --with-nagios-group=nagios
# make
# make install
# chown nagios:nagios /usr/local/nagios
# chown -R nagios:nagios /usr/local/nagios/libexec
# cd ..

Install NRPE

#wget http://sourceforge.net/projects/nagios/files/nrpe-2.x/nrpe-2.12/nrpe-2.12.tar.gz/download
# tar -zxvf nrpe-2.12.tar.gz && cd nrpe-2.12
# ./configure
# make all
# make install-plugin
# make install-daemon
# make install-daemon-config
# make install-xinetd

Edit Xinetd NRPE entry

# vi /etc/xinetd.d/nrpe

only_from = 127.0.0.1 204.93.156.227

(nagios monitoring server ip-address )

:wq (save and exit)

Edit services file entry

# vi /etc/services

nrpe 5666/tcp # Entry for NRPE daemon

:wq (save and exit)

Restart xinetd

# service xinetd restart

Verify whether NRPE is listening

# netstat -at |grep nrpe # output -: tcp 0 0 *:nrpe *.* LISTEN

Verify to make sure the NRPE is functioning properly

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

NRPE v2.12

Configuring Nagios monitoring server to monitor the remote host

# cd /usr/src
# wget http://sourceforge.net/projects/nagios/files/nrpe-2.x/nrpe-2.12/nrpe-2.12.tar.gz/download
# tar -zxvf nrpe-2.12.tar.gz && cd nrpe-2.12
# yum -y install openssl-devel
# yum install perl
# ./configure
# make all
# make install-plugin

Create a command definition

# vi /home/nagios/public_html/etc/objects/commands.cfg

Add the following:

# NRPE CHECK COMMAND
# Command to use NRPE to check remote host systems

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

Create configuration file for remote host

# cp –prf /home/nagios/www/etc/objects/ localhost.cfg /home/nagios/www/etc/objects/remotehost.cfg
# vi /home/nagios/www/etc/objects/remotehost.cfg
Replace the values “host_name” “alias” “address” with the values that match your setup:

** The “host_name” you set for the “define_host” section must match the “host_name” in the “define_service” section **

# Define a host for the remote machine
define host{
       use      linux-server     ; Name of host template to use
                                         ; This host definition will inherit all variables that are defined
                                         ; in (or inherited by) the linux-server host template definition.
        host_name         alpha235
        alias                    alpha235
        address              62.75.215.12
        }
# SERVICE DEFINITIONS
# Define a service to "ping" the local machine

define service{
        use                              generic-service         ; Name of service template to use
        host_name                  alpha235
        service_description      PING
        check_command          check_ping!100.0,20%!500.0,60%
        }

# Define a service to check the disk space of the root partition.

define service{
        use                             generic-service         ; Name of service template to use
        host_name                 alpha235
        service_description     Root Partition
        check_command          check_nrpe!check_disk
        }

# Define a service to check the number of currently logged in users on the remotehost.

define service{
        use                             generic-service         ; Name of service template to use
        host_name                 alpha235
        service_description     Current Users
        check_command         check_nrpe!check_users
        }

# Define a service to check the number of currently running processes on the remote host.

define service{
        use                              generic-service         ; Name of service template to use
        host_name                  alpha235
        service_description     Total Processes
        check_command          check_nrpe!check_total_procs
        }
# Define a service to check the load on the remote host.

define service{
        use                             generic-service         ; Name of service template to use
        host_name                 alpha235
        service_description     Current Load
        check_command         check_nrpe!check_load
        }

# Define a service to check SSH on the remote host.
# Disable notifications for this service by default, as not all users may have SSH enabled.

define service{
        use                             generic-service         ; Name of service template to use
        host_name                 alpha235
        service_description     SSH
        check_command          check_nrpe!check_ssh
        notifications_enabled  0
        }
# Define a service to check HTTP on the remote host.
# Disable notifications for this service by default, as not all users may have HTTP enabled.
define service{
        use                              generic-service         ; Name of service template to use
        host_name                  alpha235
        service_description      HTTP
        check_command           check_nrpe!check_http
        notifications_enabled   0
        }

Activate the remotehost.cfg template

# vi /usr/local/nagios/etc/nagios.cfg
 Definitions for monitoring remote Linux machine

cfg_file=/home/nagios/www/etc/objects/remotehost.cfg

Verify Nagios Configuration Files

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

...

Things look okay - No serious problems were detected during the pre-flight check

Verify whether nagios monitoring server can talk to the remote host

# /usr/local/nagios/libexec/check_nrpe -H 62.75.215.12

NRPE v2.12

Start nagios

# /home/nagios/www/bin/nagios -d /home/nagios/www/etc/nagios.cfg

Restart nagios (if already running)

# kill -HUP <nagios_pid>

# /usr/local/nagios/libexec/check_nrpe -H 62.75.215.12 -c check_load (To check the load of the remote server)

OK - load average: 0.35, 0.30, 0.23|load1=0.350;15.000;30.000;0; load5=0.300;10.000;25.000;0; load15=0.230;5.000;20.000;0;

(Usage: check_nrpe -H <host> [-n] [-u] [-p <port>] [-t <timeout>] [-c <command>] [-a <arglist...>])

Welcomes comments and suggestions..