The problem is that the server response is OK when de IIS site is running but also when not; because an error 500 page is displayed "correctly" I suppose! So no Alert are sent!
This is the command I use:
/usr/lib/nagios/plugins/check_http -H 'mysite.moss.com' -I '172.16.0.7' -w '10' -c '60' -u '/default.aspx'
Any ideas ?