Once you have done the nagios client installation and configuraiton in your system then, you may getting an error HTTP-WARNING : 403 Forbidden like that same as,
Check your nrpe configure file : check_http
HTTP WARNING: HTTP/1.1 403 Forbidden - 659 bytes in 0.032 second response time |time=0.032355s;5.000000;10.000000;0.000000 size=659B;;;0
The Nagios checking into your webpage (Root Directory), you need to add index.html or index.php file.
Check your websites by the ip address or domain name its shows 403 forbidden error.
HTTP OK: HTTP/1.1 200 OK - 394 bytes in 0.007 second response time |time=0.007302s;5.000000;10.000000;0.000000 size=394B;;;0
Do you need more information about HTTP or HTTPS monitoring