How to setup url or website monitoring in nagios server

First of all create a configuration directory for writing the rules. You can also create the rules in localhost.cfg but I recommend  to create a separate directory and create the files in it.

#mkdir /etc/nagios/monitor_websites
and cd to this directory

And create file host.cfg in this directory for setting the urls.
#vi host.cfg

Suppose I want to monitor three sites
www.abc.com, www.xyz.com, www.pqr.com

Configure host.cfg as below.
#vi host.cfg

define host{
host_name  abc.com
alias         abc
address    www.abc.com
use        generic-host
}

define host{
host_name  xyz.com
alias      xyz
address    www.xyz.com
use        generic-host
}

define host{
host_name  pqr.com
alias           pqr
address    www.pqr.com
use        generic-host
}

#Defining group of urls  - you should add this if you want to set up an HTTP check service.
define hostgroup {
hostgroup_name    monitor_websites
alias           monitor_urls
members         www.abc.com, www.xyz.com, www.pqr.com
}
:wq #save it

And now create the file services.cfg for setting the service ( http_check )
#vi services.cfg
## Hostgroups services ##
define service {
hostgroup_name                 monitor_websites
service_description             HTTP
check_command                 check_http
use                             generic-service
notification_interval           0
}

Now give the permissions for directory and configuration files.
#chown  -R nagios:nagios monitor_websites

List and check.
[root@mail nagios]#  ll monitor_websites
total 16
-rw-r--r-- 1 nagios nagios 669 Apr 25 23:13 host.cfg
-rw-r--r-- 1 nagios nagios 253 Apr 25 23:15 services.cfg
[root@mail nagios]#

Now give the configuration directory path in main nagios configuration file.
#vi /etc/nagios/nagios.cfg
cfg_dir=/etc/nagios/monitor_websites
:wq

Now restart the nagios service.
#service nagios restart

Thats it. Check the nagios site. You are done. You rocks.