Install Nagios (network monitoring system) in Ubuntu 16.04

Nagios is basically a free and open source monitoring and alerting system.  It has many built-in tools to monitor various network services, such as HTTP, ICMP, SNMP. SMTP, POP3, FTP, SSH, etc etc.  Even when a tool is not available in the community, you can easily create a script for Nagios to use for monitoring with your favorite language, such as php, python, perl etc.

Nagios is powerful, but it's free version lacks an easy and intuitive UI to help you configure the system.    Although it does have quite a learning curve, you will still be able to get a hold of it if you learn it step by step patiently.

Installation
# install prerequisites packages
sudo -i
sudo apt-get update
sudo apt-get install -y autoconf gcc libc6 make wget unzip apache2 php libapache2-mod-php7.0 libgd2-xpm-dev


# create nagios user and group
useradd nagios
groupadd nagcmd
usermod -a -G nagios,nagcmd www-data


# download nagios source (you may check which is the latest release here: (https://www.nagios.org/downloads/nagios-core/)

wget https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.3.4.tar.gz
# extract the source, e.g. tar xzf nagios-4.3.4.tar.gz
# cd to source dir and compile
./configure --with-nagios-group=nagios --with-command-group=nagcmd --with-httpd-conf=/etc/apache2/sites-enabled

*** Configuration summary for nagios 4.3.4 2017-08-24 ***:

 General Options:
 -------------------------
        Nagios executable:  nagios
        Nagios user/group:  nagios,nagios
       Command user/group:  nagios,nagcmd
             Event Broker:  yes
        Install ${prefix}:  /usr/local/nagios
    Install ${includedir}:  /usr/local/nagios/include/nagios
                Lock file:  /run/nagios.lock
   Check result directory:  ${prefix}/var/spool/checkresults
           Init directory:  /etc/init.d
  Apache conf.d directory:  /etc/apache2/sites-enabled
             Mail program:  /bin/mail
                  Host OS:  linux-gnu
          IOBroker Method:  epoll

 Web Interface Options:
 ------------------------
                 HTML URL:  http://localhost/nagios/
                  CGI URL:  http://localhost/nagios/cgi-bin/
 Traceroute (used by WAP):

make all
make install
make install-init
make install-commandmode
make install-config
make install-webconf



# enable apache2 rewrite and cgi module 
a2enmod rewrite
a2enmod cgi


# create password to restrict access to nagios web
htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
systemctl restart apache2.service

# copy event handler scripts to nagios dir (this is not a necessary step)
cp -R contrib/eventhandlers/ /usr/local/nagios/libexec/
chown -R nagios:nagios /usr/local/nagios/libexec/eventhandlers

# in case you are curious, what is event handler?  It is basically a script that are run when host or service change states.  For example, when a http service is detected DOWN, you may want to call a script automatically to restart service, or to reboot a machine.  Or just to create a trouble ticket in helpdesk system to notify front-line colleagues.


# install Nagios plugins
wget https://nagios-plugins.org/download/nagios-plugins-2.2.1.tar.gz
 ./configure --with-nagios-user=nagios --with-nagios-group=nagios --with-openssl
make
make install

# when something you want to monitor cannot be done by the standard plugins, search the community first, which may save you a lot of time. (https://exchange.nagios.org/directory/Plugins/)

# Now, let's start Apache and Nagios
systemctl start apache2
systemctl enable nagios
systemctl start nagios

# Open a browser and point to http://your_ip/nagios, that's it!

You may see now in "Hosts" and "Services", there is only a localhost and a few local services.  Yes, it's monitoring the Ubuntu host itself...  Next, let's start monitoring something more interesting...