Thanks for your reply.
I have already the ethernet shield (actually I have arduino ethernet) with the chip Wiznet W5100. I'm assuming that there is no problem in using something like the promiscuos mode (since it isn't forbidden explicitly). My question is if it exists some particular software library to sniff the fields of the packets (both in UDP and TCP), or if I have to use the standard ethernet library. In case of that do you have some reference where I can found some tutorial in using such library?
Thanks again,
Gianluca