proxytype

Convert your computer (raspberry pi, etc) into a bluetooth keyboard!

proxytype is a service that reads all keystrokes on an input device and proxies it to another device through bluetooth.

Usage

This has only been tested with Linux, and depends on dbus and the bluez stack

1. Start bluetoothd without the input plugin

The input plugin listens on the same ports that proxytype listens to, hence we have to disable that by starting bluetoothd without the input plugin with

sudo /usr/lib/bluetooth/bluetoothd --noplugin input

2. Put the bluetooth adapter in listening mode

$ bluetoothctl
[bluetooth]# discoverable on

3. Compile and start proxytype

A successful run should look something like this

$ go build && sudo ./proxytype
[sudo] password for ackerleytng:
2021/10/03 21:45:36 Registered profile
2021/10/03 21:45:36 Exported object
2021/10/03 21:45:36 Bound both ports
2021/10/03 21:45:52 Accepted on port 17 from addr &{17 65 [78 166 78 110 183 96] 0 {0 0 [0 0 0 0 0 0] 0 0 [0]}}
2021/10/03 21:45:52 Accepted on port 19 from addr &{19 66 [78 166 78 110 183 96] 0 {0 0 [0 0 0 0 0 0] 0 0 [0]}}
  0: Sleep Button
  1: Lid Switch
  2: HDA Intel PCH Mic
  3: HDA Intel PCH Headphone
  4: HDA Intel PCH HDMI/DP,pcm=3
  5: HDA Intel PCH HDMI/DP,pcm=7
  6: HDA Intel PCH HDMI/DP,pcm=8
  7: HDA Intel PCH HDMI/DP,pcm=9
  8: HDA Intel PCH HDMI/DP,pcm=10
  9: CX 2.4G Receiver
 10: CX 2.4G Receiver Mouse
 11: CX 2.4G Receiver
 12: Power Button
 13: CX 2.4G Receiver Consumer Control
 14: CX 2.4G Receiver System Control
 15: AT Translated Set 2 keyboard
 16: ThinkPad Extra Buttons
 17: PC Speaker
 18: Video Bus
 19: Synaptics TM3289-021
 20: Integrated Camera: Integrated C
 21: TPPS/2 Elan TrackPoint
Pick keyboard to proxy to bluetooth:
9
2021/10/03 21:45:59 Start using CX 2.4G Receiver
2021/10/03 21:46:12 Sending [161 1 0 0 23 0 0 0 0 0]
2021/10/03 21:46:12 Sending [161 1 0 0 0 0 0 0 0 0]

After picking the input device of your choice, the input device will be interpreted as a keyboard, and all keystrokes will be sent over bluetooth to the connected bluetooth device.

I didn't build any functionality for exiting, so the proxied device can't be used to exit the program.

I usually use another keyboard (not the one being proxied) to send SIGINT to proxytype.

Overview

Here's what proxytype does

• Advertise a keyboard and how to connect to this "keyboard" over bluetooth. The properties of this "keyboard" are defined in sdp_record.xml
  • This advertisement is done by registering a profile over dbus (org.bluez.ProfileManager1.RegisterProfile)
  • The advertisement is not complete without also exporting a profile object of type org.bluez.Profile1
• Having a server listen for connections on the ports as specified in the properties in the advertisement
  • After accepting a connection, ignore all inputs from the connection
  • Send keycodes over the interrupt channel
• Reads inputs using evdev, translates those scancodes to keycodes, sends the keycodes over the interrupt channel

Acknowledgements

• Bluetooth Essentials for Programmers by Albert S. Huang and Larry Rudolph
  • This book has to be the best overview of bluetooth programming ever! This helped me understand bluez the various interactions between client and server.
• Raspberry Pi Bluetooth HID gist
  • This gist and the super helpful and friendly community around the gist
• go-hidproxy
  • This repo provided another sample, in golang
• Gonna do a shoutout for zephyr too!
  • If I were to do this again from the beginning I would try zephyr first; it seems to be a more direct approach than bluez+dbus