In my country Albania, this is legal. We need this address to filter out a unique responder, because with the ip address this is not possible. In Albania dynamic ip is widely used.

Well, let me know when you found a solution. From what I have seen this is only possible when the person uses IE/Windows and to be honest, I think this is good.

Besides being legal in your country, depending on what you are exactly trying to do it might go against the ESOMAR guide of conduct for market research, just to have said it.

Maybe we find a solution that works differently. Why is it so important to find out the unique responder? To avoid a double participation?

If you are convinced that it is possible for all browser / OS combinations you might want to ask in a Javascript-Forum. I think they might be the ones to ask.

If you can get the MAC-address via Javascript for all OS/Browser combinations, including this information in the Limesurvey results is the smallest problem.

I need this address to avoid double participation.
I also found it in java, but i dunno where to put the applet,
i mean in which file?

Problem with Java is that the probability it might be blocked is higher than for Javascript. I have no idea about how java applets work, so I can't be much of a help here.

For the double participation: I guess tokens are not a solution for you in this case then?

How can tokens replace the double participation?

Well, you send out a unique link to every participant. Once the participant has finished the survey with that link, it can't be used anymore.

The concept is this:
People will enter in the site, not that we will send it to others

OK, this is why I asked what you are trying to do.

In this case, the only chance are cookies (with all the flaws) or getting hold of the MAC address.

However, if the mac address solution would be reliable, I am quite sure the IT guys from my last company would have used that already. But they didn't.

But as I said, best thing is probably to find someone who can definitely tell you, whether this is possible with Javascript (and reliable) or not.

I found a site using java:
developerarticles.com/how-to-get-mack-address-using-java/
I can use it as an applet, but the problem here is where do i need to put it
to make it work? Also, i want to modify the source of limesurvey so i can choose
to use the mac address finder or not, just like the ip.
Can you help me here?

Sorry, I am not a developer.

Maybe someone else skip in and help out.

However, when you make changes to the core of Limesurvey, you will not be able to update Limesurvey in the future without loosing those changes you have made.

I would embed the JavaApplet on the first page of your questionnaire and write the result of the applet into a hidden question. But I can't give any detailed tips, because I am not really a programmer.