Installation

Windows

Copy the nex-dissector-plugin folder to %appdata%\Wireshark\plugins (you may need to create the plugins folder). Don't copy it into the plugins folder in Program Files!
Create %appdata%\Wireshark\nex-keys.txt and (optionally) add your PID/password.

macOS/Linux

Copy the nex-dissector-plugin folder to ~/.local/lib/wireshark/plugins/ (you may need to create this folder).
Create ~/.config/wireshark/nex-keys.txt and (optionally) add your PID/password.

Getting your NEX PID/password

You will need to add your NEX PID and your NEX password (for your device) in nex-keys.txt in the Wireshark folder - the dissector will automatically derive the needed key from the PID/password and write it back to the file. The format is nexpid:nexpassword.

3DS

Run this homebrew application - the source code is in the get_3ds_pid_password folder in this repository. It will create a file nex-keys.txt on the root of the SD card in the correct format.

Wii U

This assumes you have a HTTP proxy set up with your Wii U. You can find your NEX PID and password in a request to https://account.nintendo.net/v1/api/provider/nex_token/@me?game_server_id=xxx. The response looks like this (formatted for clarity):

HTTP/1.1 200 OK
Server: Nintendo 3DS (http)
X-Nintendo-Date: xxx
Content-Type: application/xml;charset=UTF-8
Content-Length: xxx
Date: xxx

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<nex_token>
    <host>IP</host>
    <nex_password>nexpassword</nex_password>
    <pid>nexpid</pid>
    <port>PORT</port>
    <token>token</token>
</nex_token>
</xml>

What works

Dissection of PRUDP v0 (3DS, WiiU friends)
Dissection of PRUDP v1 (Some 3DS games, most Wii U uses)
Automatic secure connection decryption if the connections are in the same pcap.
Automatic generation of dissector code from wiki, including structure definitions.
Data<> handling

What doesn't work

Dissection of PRUDP-Lite (Switch)
Fragment reassembly (sometimes)

Shoutouts to Kinnay's documentation at https://github.com/Kinnay/NintendoClients, it was absolutely essential to understand any of this.

Stary2001/nex-dissector