📝 add documentation on udev

docs/udev.md

@@ -0,0 +1,41 @@
+# How do udev rules work and why are they needed
+
+In Linux, `udev` (part of `systemd`, read `man 7 udev`) handles "hot-pluggable" devices, of which Solo and U2F Zero are examples. In particular, it creates nodes in the `/dev` filesystem (in Linux, everything is a file), which allow accessing the device.
+
+By default, for security reasons often only the `root` user can access these nodes, unless they are whitelisted using a so-called "udev rule". So depending on your system setup, such a udev rule may be necessary to allow non-root users access to the device, for instance yourself when using a browser to perform two-factor authentication.
+
+## What does a udev rule do?
+It matches events it receives (typically, comparing with the `==` operator), and performs actions (typically, setting attributes of the node with the `=` or `+=` operators).
+
+## What is `hidraw`?
+HID are human-interface devices (keyboards, mice, Solo keys), attached via USB. The `hidraw` system gives software direct ("raw") access to the device.
+
+## Which node is my Solo or U2F Zero security key?
+You can either compare `ls /dev` before and after inserting, or use the `udevadm` tool, e.g., by running
+```
+udevadm monitor --environment --udev | grep DEVNAME
+```
+Typically, you will detect `/dev/hidraw0`.
+
+## How do you know if your system is configured correctly?
+Try reading and writing to the device node you identified in the previous step. Assuming the node is called `/dev/hidraw0`:
+
+* read: try `cat /dev/hidraw0`, if you don't get "permission denied", you can access.
+* write: try `echo "hello, Solo" > /dev/hidraw0`. Again, if you don't get denied permission, you're OK.
+
+## Which rule should I use, and how do I do it?
+Simplest is probably to copy [Yubico's rule file](https://github.com/Yubico/libu2f-host/blob/master/70-u2f.rules) to `/etc/udev/rules.d/fido.rules` on your system. This contains rules for Yubico's keys, the U2F Zero, and many others. The relevant line for U2F Zero is:
+```
+KERNEL=="hidraw*", SUBSYSTEM=="hidraw", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="8acf", TAG+="uaccess"
+```
+It matches on the correct vendor/product IDs of 10c4/8acf, and adds the TAG `uaccess`. Older versions of udev use rules such as 
+```
+KERNEL=="hidraw*", SUBSYSTEM=="hidraw", ATTRS{idVendor}=="10c4", MODE="0644", GROUP="plugdev"
+```
+which sets MODE of the device node to readable by anyone.
+
+## What about vendor and product ID for Solo?
+Current prototypes reuse the IDs of the U2F Zero (10c4/8acf). The final Solo will probably be assigned new IDs; read about it here first :)
+
+## You got this all wrong, I can't believe it!
+Are you suffering from [us being wrong](https://xkcd.com/386/)? Please, send us a [pull request](https://github.com/SoloKeysSec/solo/pulls) and prove us wrong :D

mkdocs.yml

@@ -8,8 +8,9 @@ copyright: 'Copyright © 2018 SoloKeys'
 nav:
     - Home: index.md
     - README.md: repo-readme.md
-    - Contributing: contributing.md
-    - Hosting: hosting.md
+    - Contributing Code: contributing.md
+    - Contributing Docs: documenting.md
+    - What the udev?!: udev.md
 
 theme:
   name: material