Hi Stephane,
You could configure a standard HTTP monitor to send a single HTTP request to the pool members. If the server response contains the receive string, it will be marked up in the pool.
I believe that NTLM authentication requires multiple request/response loops so I don't think you could use NTLM with a standard HTTP monitor. Here is a related post (
Click here).
Does the proxy support any other authentication methods? If so, you could configure an HTTP monitor to do this health checking. You can get example monitor send strings in the LTM config guide for your version (
Click here), in the Monitoring forum (
Click here) or in the Monitoring Codeshare (
Click here).
Aaron