When testing a little program that I’ve made, called ptool (more info here), I have found out that the LAN interface of my ADSL router was operating in promiscuous mode. The router is a standard issue from Free.fr.
I have no reason why this would happen, especially on the LAN port. If this is common practice from Free, then I might even be forced to change ISP. I’m curious how many other providers snoop on their users’ LANs.
If you’re curious whether or not your ISP is listening to your LAN traffic, you could try downloading and running the program that I’ve written. I think it currently supports most linux distributions and might work on Solaris and Mac OS too (not sure if that’s the case for the latest build though).
Please let me know what you find. I really want to get to the bottom of this!