Nagios - HTTP WARNING: HTTP/1.1 403 Forbidden

I solved it by executing

$ touch /var/www/html/index.html
$ chmod 755 /var/www/html/index.html

on the remote server.

You can put a username/password and tell nagios what to expect via -e. It is not a must to authorize in any case and it is also not critical to have a WARNING.

 /usr/local/nagios/libexec/check_http -H 128.19.5.131 -e 'HTTP' --authorization=username:password

Update:

The reason why you get forbidden is because you don't have any index file and directory listing is disabled. You can create a file nagios.txt (without putting anything in) and let nagios fetch that file instead of a directory which it is not allowed to view.

I created a file with name index.html in the /var/www/html directory of the remote Linux machine (128.19.5.131). Restarted the HTTP service with service httpd restart and then executed the same command, i.e. :

nagiossrv root [libexec] >/usr/local/nagios/libexec/check_http -H 128.19.5.131

I got following output:

HTTP OK HTTP/1.1 200 OK - 250 bytes in 0.001 seconds |time=0.000977s;;;0.000000 size=250B;;;0

I do not know whether it is proper to add index.html file in the /var/www/html directory of each host to be monitored or not, but it is showing desired results in the Nagios XI console.