I have a "black-box device" (a credit card reader, to which some klutz lost the admin unlock card...) running an unknown, but primitive, operative system. I need to find it's MAC address.
At my disposal, I have the device itself (but no access to its admin interface), a computer running Ubuntu 12.04 (where I have full sudo rights), and a regular ethernet cable. If necessary, I can probably shake up an ethernet switch, too.
How do I go about to find the MAC address of the card reader?