SpotPass
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SpotPass is a Nintendo 3DS feature that allows the 3DS to automatically download content, notifications, and software when it's in standby mode. Software downloaded with SpotPass is stored on SD card.
SpotPass Internet communications are mostly small UDP frames and HTTPS transfers, the only HTTP download is the connection test page.
With the 2.0.0-2 update, SpotPass now regularly requests content over HTTPS from nppl.c.app.nintendowifi.net and sometimes nasc.nintendowifi.net. These are probably related to the automatic system updates/SpotPass automatic software download functionality. Going by the domain names, nppl* might be for SpotPass software downloads, while nasc* might be for auto updates?(Which domain is used for what exactly isn't confirmed yet) In System Settings, you can disable SpotPass content downloading,(this is _only_ for free titles etc not auto system updates) but it states that important software will still downloaded. This is likely why SpotPass still connects to nppl* and nasc* even with that option disabled.
Automatic System Update Download
With the 2.0.0-2 update, system updates updates are automatically downloaded via SpotPass. It only downloads updates, it will not install updates without the user's permission. See this: http://www.nintendo.com/consumer/systems/3ds/en_na/menu_update.jsp
No success with capturing this auto dl so far.