StreetPass
StreetPass is a feature that allow your 3DS to connect with other 3DS using WiFi in standby mode. It can be used to share Mii(s) on Mii Plaza for example. Games' StreetPass data is stored on SD card. Wifi infrastructure with APs are used to communicate, like WMB and multi-player.
Using Wireshark tool with a WiFi card in monitor mode allow you to see the data used to scan for other 3DS in the range. The below is a broadcast probe request from an 3DS while in standby mode, with SSID "Nintendo_3DS_continuous_scan_000". When in "active" mode, 3DS sends probe requests with arbitrary random SSID strings, like "ic[kSvm9s@*cYD>/~IEVj\(fGG;qDo8j". This frame also contains a custom Nintendo tag, it seems to contain unknown console unique data, since the contents of this tag from different 3ds captures don't match.
When in standby mode, old DS wifi is used,(this includes SpotPass and StreetPass) but in "active" mode the regular DSi wifi bus is used.
0000 00 00 1a 00 2f 48 00 00 19 7d 19 de 2a 00 00 00 ..../H...}..*... 0010 12 16 9e 09 a0 00 c9 02 00 00 40 00 00 00 ff ff ..........@..... 0020 ff ff ff ff da 6b f7 22 f3 77 ff ff ff ff ff ff .....k.".w...... 0030 40 77 00 20 4e 69 6e 74 65 6e 64 6f 5f 33 44 53 @w. Nintendo_3DS 0040 5f 63 6f 6e 74 69 6e 75 6f 75 73 5f 73 63 61 6e _continuous_scan 0050 5f 30 30 30 01 08 82 84 8b 0c 12 96 18 24 32 04 _000.........$2. 0060 30 48 60 6c dd 15 00 1f 32 01 11 05 00 02 08 00 0H`l....2....... 0070 00 f0 08 c8 34 6e 05 0f c9 c6 80 5b 6f bc 5a ....4n.....[o.Z
A streetpass "AP" was spoofed on a laptop with hostapd by setting the SSID to "Nintendo_3DS_continuous_scan_000", with the extra Nintendo tag from another 3DS' probe request. The SSID and AP can't be easily spoofed with hostapd for streetpass when 3DS is "active", for the random "ic[kSvm9s@*cYD>/~IEVj\(fGG;qDo8j" strings. The 3DS didn't seem to authenticate or associate with the "AP". Streetpass "AP" comms use WPA2 encryption. Eventually the 3DS stops communicating with the fake "AP" since the AP doesn't understand the sent data,(especially since it's encrypted) and sends a 802.11 "Action" frame, with category ID 0x7f and Nintendo's vendor ID: 00 1f 32.(However the 3DS keeps communicating with the above process repeatedly)