Whenever we newly setup the nagios server we mostly get this error “Nagios HTTP WARNING: HTTP/1.1 403 Forbidden”.
In dashboard it shows alert on localhost i.e our own newly set up nagios server.

To solve this alert, create a index.html file inside /var/www/html in server

touch /var/www/html/index.html

Restart the httpd and nagios service. Even without doing this it will clear the alert after a few minutes.

/etc/init.d/nagios restart
/etc/init.d/httpd restart

Wait for a few minutes the alert will be gone. Yes we are done here.

OK do you want to know some more,let me explain some more part of troubleshooting steps.

You can use check_http plugin here with command line to check do the problem exist or not.
Have a look in below given blocks, how troubleshooting is done. I hope these steps are enough to explain.

[root@localhost html]# whoami
root
[root@localhost html]# /usr/lib/nagios/plugins/check_http -I 127.0.0.1
HTTP WARNING: HTTP/1.1 403 Forbidden - 5237 bytes in 0.001 second response time |time=0.000719s;;;0.000000 size=5237B;;;0
[root@localhost html]# 
[root@localhost html]# pwd
/var/www/html
[root@localhost html]#  touch index.html
[root@localhost html]# 
[root@localhost html]# /usr/lib/nagios/plugins/check_http -I 127.0.0.1
HTTP OK: HTTP/1.1 200 OK - 265 bytes in 0.001 second response time |time=0.000574s;;;0.000000 size=265B;;;0
[root@localhost html]# 
[root@localhost html]#

Note: check your check_http plugin where it is located in server. In my case it was in /usr/lib/nagios/plugins/