It basically was a raspberry pi with a specific usb wifi dongle plugged into the usb port on your pi you then connected that by a lan cable to your router and it would go through the net to all the official & unoffical streetpass relays in the network harvesting the most recent streetpass data and then spoofing it to your own setup so your 3DS would just get hundreds of passes each hour. The script would cycle addresses every few mins so you could keep getting passes it was an amazing bit of kit. It was designed to autoboot the spillpass program when you powered on your pi so it could be left up & running in the background i ended up installing a screen into my pi setup using a brilliant case adafruit sells so i could make it into a dedicated relay unit. Including adding in buttons so i could shut it down properly from the front of the unit.
The project was called spillpass pi but like i have said i haven't had it connected in years so i don't know if it still works. Used to remember the animosity on here from salty bitches who would accuse you of cheating because you are not supposed to use the streetpass network like that lol. If you google spillpass pi you will find pages explaining how it was set up but i can't seem to find the creators website as his link doesn't seem to work anymore.
This was my first iteration before i made one of my pis a dedicated machine to do it.