Nagios allows monitoring servers and detecting any unexpected behavior (considering a probe was setup to detect it). We will monitor here disk space, services, and ib network.

Add nagios user and nagcmd, a user used to launch nagios commands.

groupadd nagios
useradd -m -g nagios nagios
passwd nagios
groupadd nagcmd
usermod -g nagcmd nagios
usermod -g nagcmd apache

Install needed packages (built previously, see preparing install).

yum install nagios nagios-contrib nagios-debuginfo nagios-devel nagios-plugins nagios-plugins-debuginfo

Edit /usr/local/nagios/etc/objects/contacts.cfg and add here your email address to let nagios know where to send alerts, and also set admin user as nagiosadmin.

define contact{
       contact_name  nagiosadmin	; Short name of user
       use             generic-contact	; Inherit default values from generic-contact template (defined above)
       alias         Administrateur Nagios	; Full name of user
       email         root@localhost	; Adresse Email pour les notifications
       }
define contactgroup{
       contactgroup_name       admins
       alias                   Administrateurs Nagios
       members                 nagiosadmin
       }

Then generate a password for the nagiosadmin user, which will be used in the web interface, and restart httpd:

htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
service httpd restart

Now, check the configuration, it should be OK:

/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
[…]
Total Warnings: 0
Total Errors:   0

And start nagios:

/etc/init.d/nagios start

It is now possible to login into nagios interface at http://localhost/nagios

Time now to configure nagios to monitor our servers.

mkdir /usr/local/nagios/etc/linux_servers/

Edit /usr/local/nagios/etc/objects/commands.cfg and add check_nrpe command:

###############################################################################
# COMMANDS.CFG - SAMPLE COMMAND DEFINITIONS FOR NAGIOS 4.1.1
#
#
# NOTES: This config file provides you with some example command definitions
#        that you can reference in host, service, and contact definitions.
#
#        You don't need to keep commands in a separate file from your other
#        object definitions.  This has been done just to make things easier to
#        understand.
#
###############################################################################

# 'check_nrep' command definition
define command{
       command_name    check_nrpe
       command_line    $USER1$/check_nrpe -H $HOSTADDRESS$ -u -c $ARG1$ -a $ARG2$ $ARG3$ $ARG4$ $ARG5$ $ARG6$
       }

Then, in /usr/local/nagios/etc/nagios.cfg, add our linux_serverss directory. This directory is where we will store our configuration files of servers monitored.

# You can also tell Nagios to process all config files (with a .cfg
# extension) in a particular directory by using the cfg_dir
# directive as shown below:

#cfg_dir=/usr/local/nagios/etc/servers
#cfg_dir=/usr/local/nagios/etc/printers
#cfg_dir=/usr/local/nagios/etc/switches
#cfg_dir=/usr/local/nagios/etc/routers

cfg_dir=/usr/local/nagios/etc/linux_servers

Then create a group for Linux servers in /usr/local/nagios/etc/linux_servers/groupe_linux_servers.cfg:

# Define a hostgroup for Linux machines
# All hosts that use the linux-server template will automatically be a member of this group

define hostgroup{
        hostgroup_name  linux-servers   ; The name of the hostgroup
        alias           Serveurs Linux  ; Long name of the group
        members         compute1,compute2        ; separes par des virgules
        }

And add servers and services in /usr/local/nagios/etc/linux_servers/servprod.cfg:

# Host definition

define host{
    use        linux-server ;
    host_name    compute1         ;
    alias        Serveur compute1 ;
    address    compute1               ;
    }

define host{
    use        linux-server ;
    host_name    compute2         ;
    alias        Serveur compute2 ;
    address    compute2               ;
    }

# Check disc space

define service{
    use            generic-service
    hostgroup_name linux-servers
    service_description    Espace disque /
    check_command        check_nrpe!check_disk!80%!90%!/
    }

# Check cpu load

define service{
   use                     generic-service
    hostgroup_name linux-servers
   service_description     Charge CPU
   check_command           check_nrpe!check_load!80!90
   }

# Check number of users logged in

define service{
   use                     generic-service
    hostgroup_name linux-servers
   service_description     Nombre utilisateurs
   check_command           check_nrpe!check_users!2!10
   }

Then, comment the following lines in /usr/local/nagios/etc/objects/localhost.cfg:

# Define an optional hostgroup for Linux machines

#define hostgroup{
#        hostgroup_name  linux-servers ; The name of the hostgroup
#        alias           Linux Servers ; Long name of the group
#        members         localhost     ; Comma separated list of hosts that belong to this group
#        }

Set rights, test configuration, and restart nagios:

chown -R nagios.nagios /usr/local/nagios/etc/serveurs_*
/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
/etc/init.d/nagios start

We will add specific probes in nagios later. For now, nagios allow you to know which hosts are up, and their basic information.

Add nagios group and user:

groupadd nagios && useradd nagios -g nagios -p nagios

Then install nagios-plugin and nrpe:

yum install nagios-plugins nagios-plugins-debuginfo nrpe nrpe-debuginfo nrpe-plugin

Edit /etc/xinetd.d/nrpe and add the server ip, to allow it to query information.

# default: on
# description: NRPE (Nagios Remote Plugin Executor)
service nrpe
{
        flags           = REUSE
        socket_type     = stream
        port            = 5666
        wait            = no
        user            = nagios
        group           = nagios
        server          = /usr/local/nagios/bin/nrpe
        server_args     = -c /usr/local/nagios/etc/nrpe.cfg --inetd
        log_on_failure  += USERID
        disable         = no
        only_from       = 127.0.0.1 10.1.0.1
}

Then edit /etc/services and add (or uncomment) the following line at the right place:

nrpe 5666/tcp # NRPE

Then restart xinetd, and check it is listening using netstat:

systemctl restart xinetd
yum install net-tools

 [root@compute1 ~]#  netstat -at | grep nrpe
tcp6       0      0 [::]:nrpe               [::]:*                  LISTEN
[root@compute1 ~]#

Edit /usr/local/nagios/etc/nrpe.cfg, and uncomment the following lines:

command[check_users]=/usr/local/nagios/libexec/check_users -w $ARG1$ -c $ARG2$
command[check_load]=/usr/local/nagios/libexec/check_load -w $ARG1$ -c $ARG2$
command[check_disk]=/usr/local/nagios/libexec/check_disk -w $ARG1$ -c $ARG2$ -p $ARG3$
command[check_procs]=/usr/local/nagios/libexec/check_procs -w $ARG1$ -c $ARG2$ -s $ARG3$

Now test locally:

[root@compute1 ~]#  /usr/local/nagios/libexec/check_nrpe -H 127.0.0.1
NRPE v2.15
[root@compute1 ~]#  /usr/local/nagios/libexec/check_disk -w 10% -c 5% -u GB
DISK OK - free space: / 2 GB (70% inode=85%); /dev 0 GB (100% inode=99%); /dev/shm 0 GB (100% inode=99%); /run 0 GB (98% inode=99%); /sys/fs/cgroup 0 GB (100% inode=99%); /boot 3 GB (97% inode=99%); /run/user/0 0 GB (100% inode=99%);| /=1GB;2;2;0;3 /dev=0GB;0;0;0;0 /dev/shm=0GB;0;0;0;0 /run=0GB;0;0;0;0 /sys/fs/cgroup=0GB;0;0;0;0 /boot=0GB;2;2;0;3 /run/user/0=0GB;0;0;0;0
[root@compute1 ~]#

Nagios server should now be able to reach the client.