Is it possible to build sniffer to listen other site responses?

Question

There is a site that filter products by attributes. I want to log the responses that return to this site (url, ip and sessionid) by all the users.

Is there a way to build a sniffer/crawler that can do it?

And just to make sure - is it legal?

EDIT:

i fix my wish a little. i dont want to see the response that i return to the site. i want to see all the responses from all the world that return to this specific site..

Answer 1

If I understand you correctly, you want to capture the traffic to a 3rd party site you have no control of. For example the site of a competitor.

Short answer: No. It's not possible to build a sniffer locally, that captures the requests to a 3rd party site located somewhere else.

Long answer: To sniff all the packets going to a server/site, you have to be on the route, that all these packets use. If you are not on the route, you simply can't see them.
So the only option would be to gain control of some network infrastructure like switches or router, that are very close to the target server/cluster/site (so all or at least many of the packets use this route) or gain control of the target server(s). Both is obviously illegal.