Thanks for the tip .

Here is my situation :
- server A : provides acces to external services (Web), through a proxy
- server B : provides some internal services, like FTP, HTTP, ...
- server C : provides some internal servicies, like IMAP messagery, DNS, ...

So, what I will test is :
- if the IMAP service is running on server C, with an IMAP request
- if the DNS service is running on server C, with a QHttp request targetting an intranet web site (HTTP service provided by server B, but DNS resolved on server C)
- if the B server is running, testing an intranet HTTP request with the server IP (in order not to use the DNS service on server C)
- if the server A is running, testing an external QHTTP request

That's a good idea !
I was on my "ping" solution snce this morning, for nothing