$ sudo apt-get update
$ sudo apt-get install apache2
$ sudo service apache2 restart

We can get it from:

$ sudo wget http://assets.nagios.com/downloads/nagiosxi/xi-latest.tar.gz

Unzip it:

$ tar xvzf xi-latest.tar.gz
$ cd nagiosxi

We'll install CentOS 6 on the hosted virtual machine (WMWare Workstation 10):

$ sudo add-apt-repository ppa:nagiosinc/ppa

We can now refresh our package database and install the agent package by running these commands:

$ sudo apt-get update
$ sudo apt-get install nagios-agent

During the installation process, we will be prompted for the IP address of our Nagios server(s), which is a security directive specifying where to accept connections from. This can be changed later using the following command:

dpkg-reconfigure nagios-agent

We need to download http://library.nagios.com/Nagios XI:

It will be installed ~/Downloads folder.

$ mv xi-2014r1.4.tar.gz /tmp
$ cd /tmp 
$ tar xvzf xi-2014r1.4.tar.gz
$ cd nagiosxi

$ ./fullinstall

$ apt-get install apache2 libapache2-mod-php5 build-essential libgd2-xpm-dev libssl-dev sendmail-bin sendmail heirloom-mailx wget apache2-utils curl daemon apt-file libnet-snmp-perl libpq5 libradius1 libsensors4 libsnmp-base libtalloc2 libtdb1 libwbclient0 samba-common samba-common-bin smbclient snmp whois mrtg libmysqlclient15-dev libcgi-pm-perl librrds-perl libgd-gd2-perl

Nagios runs as its own user and has its own groups. We need to create this user and groups:

$ sudo useradd nagios
$ sudo groupadd nagcmd
$ sudo usermod -a -G nagcmd nagios

$ sudo mkdir -p /usr/local/src/nagios4
$ cd /usr/local/src/nagios4
$ sudo wget http://prdownloads.sourceforge.net/sourceforge/nagios/nagios-4.0.8.tar.gz
$ sudo tar xf nagios-4.0.8.tar.gz
$ cd nagios-4.0.8
$ sudo mkdir -p /usr/local/nagios/share/{stylesheets,images}

configure:

$ sudo ./configure --prefix=/usr/local/nagios --with-nagios-user=nagios --with-nagios-group=nagios --with-command-user=nagios --with-command-group=nagcmd

The poutput should look like this:

...
*** Configuration summary for nagios 4.0.8 08-12-2014 ***:

 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:  ${prefix}/var/nagios.lock
   Check result directory:  ${prefix}/var/spool/checkresults
           Init directory:  /etc/init.d
  Apache conf.d directory:  /etc/httpd/conf.d
             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):  


Review the options above for accuracy.  If they look okay,
type 'make all' to compile the main program and CGIs.

Let's make it:

$ sudo make all

Install the main program, CGIs, and HTML files:

$ sudo make install

Install the init script in /etc/init.d:

$ sudo make install-init

Output:

This package will be built according to these values: 

0 -  Maintainer: [ k@k ]
1 -  Summary: [  ]
2 -  Name:    [ nagios-init ]
3 -  Version: [ 4.0.8 ]
4 -  Release: [ 1 ]
5 -  License: [ GPL ]
6 -  Group:   [ checkinstall ]
7 -  Architecture: [ amd64 ]
8 -  Source location: [ nagios-4.0.8 ]
9 -  Alternate source location: [  ]
10 - Requires: [  ]
11 - Provides: [ nagios-init ]
12 - Conflicts: [  ]
13 - Replaces: [  ]
...
 Done. The new package has been installed and saved to

 /usr/local/src/nagios4/nagios-4.0.8/nagios-init_4.0.8-1_amd64.deb

 You can remove it from your system anytime using: 

      dpkg -r nagios-init
...

Install sample config files in /usr/local/nagios/etc:

$ sudo make install-config

Output:

...
This package will be built according to these values: 

0 -  Maintainer: [ root@k ]
1 -  Summary: [ Package created with checkinstall 1.6.2 ]
2 -  Name:    [ nagios-config ]
3 -  Version: [ 4.0.8 ]
4 -  Release: [ 1 ]
5 -  License: [ GPL ]
6 -  Group:   [ checkinstall ]
7 -  Architecture: [ amd64 ]
8 -  Source location: [ nagios-4.0.8 ]
9 -  Alternate source location: [  ]
10 - Requires: [  ]
11 - Provides: [ nagios-config ]
12 - Conflicts: [  ]
13 - Replaces: [  ]
...
Done. The new package has been installed and saved to

 /usr/local/src/nagios4/nagios-4.0.8/nagios-config_4.0.8-1_amd64.deb

 You can remove it from your system anytime using: 

      dpkg -r nagios-config
...

Install and configure permissions on the directory for holding the external command file:

$ sudo make install-commandmode

Output:

...
This package will be built according to these values: 

0 -  Maintainer: [ root@k ]
1 -  Summary: [ Package created with checkinstall 1.6.2 ]
2 -  Name:    [ nagios-commandmode ]
3 -  Version: [ 4.0.8 ]
4 -  Release: [ 1 ]
5 -  License: [ GPL ]
6 -  Group:   [ checkinstall ]
7 -  Architecture: [ amd64 ]
8 -  Source location: [ nagios-4.0.8 ]
9 -  Alternate source location: [  ]
10 - Requires: [  ]
11 - Provides: [ nagios-commandmode ]
12 - Conflicts: [  ]
13 - Replaces: [  ]
...
Done. The new package has been installed and saved to

 /usr/local/src/nagios4/nagios-4.0.8/nagios-commandmode_4.0.8-1_amd64.deb

 You can remove it from your system anytime using: 

      dpkg -r nagios-commandmode

Install the Apache config file for the Nagios web interface:

$ sudo /usr/bin/install -c -m 644 sample-config/httpd.conf /etc/apache2/conf-available/nagios.conf
$ sudo ln -s /etc/apache2/conf-available/nagios.conf /etc/apache2/conf-enabled/nagios.conf

Create a plugin folder and download the plugin:

$ mkdir -p /usr/local/src/nagios-plugins
$ cd /usr/local/src/nagios-plugins
$ wget https://www.nagios-plugins.org/download/nagios-plugins-2.0.3.tar.gz
$ tar -xf nagios-plugins-2.0.3.tar.gz
$ cd nagios-plugins-2.0.3

Config and compile:

$ sudo ./configure --with-nagios-user=nagios --with-nagios-group=nagios --with-openssl=/usr/bin/openssl --enable-perl-modules --enable-libtap

Output:

...
            --with-apt-get-command: /usr/bin/apt-get
              --with-ping6-command: 
               --with-ping-command: /bin/ping -n -U -w %d -c %d %s
                       --with-ipv6: yes
                      --with-mysql: /usr/bin/mysql_config
                    --with-openssl: yes
                     --with-gnutls: no
               --enable-extra-opts: yes
                       --with-perl: /usr/bin/perl
             --enable-perl-modules: yes
                     --with-cgiurl: /nagios/cgi-bin
               --with-trusted-path: /bin:/sbin:/usr/bin:/usr/sbin
                   --enable-libtap: yes

The, make:

$ sudo make
$ sudo make install

Let's set up a user account for the new nagios user with password 'admin':

$ sudo apt-get install apache2-utils
$ sudo htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
$ sudo chown nagios:nagcmd /usr/local/nagios/etc/htpasswd.users

$ service nagios restart
$ service apache2 restart

More to come...