Promiscuous Bluetooth audio sinc

A2DP agent for promiscuous/permissive audio sinc for Linux. Once installed, a Bluetooth client, such as a smart phone, should be able to discover, pair, and subsequently play audio without any manual interaction. This is perfect for those with headless media boxes wanting to expand their connective options and saves explaining things to the kids 8)

This project assumes the use of PulseAudio or Pipewire via pipewire-pulse.

This project is heavily based on the Gist and comments at https://gist.github.com/mill1000/74c7473ee3b4a5b13f6325e9994ff84c#gistcomment-4032842

Authenticating

This agent essentially allows for non-interactive pairing with some caveats.

When a pairing client requests a pairing code, simply accept the code.

When a pairing client requests a pin code set as 0000.

Installation

NOTE: This is very much a WIP, please comment on any additional steps required in your distro/version.

This documentation is based on my experience using Ubuntu 21.04.

Install required packages

Ensure that the required Bluetooth, Bluetooth audio, and Python 3 dependencies are installed.

sudo apt install pulseaudio-module-bluetooth bluez python3-dbus

Install the A2DP Bluetooth agent

A Bluetooth agent is a piece of software that handles pairing and authorization of Bluetooth devices. The following agent allows a client to automatically pair and accept A2DP connections from Bluetooth devices. All other Bluetooth services are rejected.

Copy the included file a2dp-agent to /usr/local/bin and make the file executable with

sudo cp -a a2dp-agent /usr/local/bin/
sudo chmod +x /usr/local/bin/a2dp-agent

Testing the agent

Before continuing, verify that the agent is functional. Bluetooth audio should be discoverable, pairable and recognised as an audio device.

Manually run the agent by executing

sudo /usr/local/bin/a2dp-agent

Attempt to pair and connect with using your phone or computer. You will be asked to verify a pairing key, just accept it and move on.
Check on the client which profiles are available. SBC is likely your best choice quality wise. If you do not see any services then something has gone wrong 8(
The agent should output the accepted and rejected Bluetooth UUIDs

Setting device to 'discoverable' and 'pairable'...
Agent registered
RequestConfirmation (/org/bluez/hci0/dev_04_B1_67_56_62_C3, 263837)
Auto confirming...
AuthorizeService (/org/bluez/hci0/dev_04_B1_67_56_62_C3, 0000110d-0000-1000-8000-00805f9b34fb)
Authorized A2DP Service

Play some audio on the client device, you should hear or see something happening on the server side 8)

Install the A2DP Bluetooth Agent as a service

To make the A2DP Bluetooth Agent run on boot copy the included file bt-agent-a2dp.service to /etc/systemd/system. Now run the following command to enable the A2DP Agent service. NOTE: By default the the startup service runs the a2dp-agent as the pipewire user and the bluetooth group. You can remove this to run as root, or better, fix your permissions 8) If you are on a PulseAudio system, you will need to se the User=... to pulse.

sudo cp -a bt-agent-a2dp.service /etc/systemd/system/
sudo systemctl enable bt-agent-a2dp.service
sudo systemctl start bt-agent-a2dp.service
systemctl status bt-agent-a2dp.service

The status should show green:

● bt-agent-a2dp.service - A2DP Bluetooth Agent
     Loaded: loaded (/etc/systemd/system/bt-agent-a2dp.service; enabled; vendor preset: enabled)
     Active: active (running) since Wed 2022-01-19 10:26:30 NZDT; 1s ago
   Main PID: 17527 (python3)
      Tasks: 2 (limit: 18760)
     Memory: 8.9M
        CPU: 55ms
     CGroup: /system.slice/bt-agent-a2dp.service
             └─17527 /usr/bin/python3 -u /usr/local/bin/a2dp-agent

You can follow the logs of the agent with:

journalctl -fu bt-agent-a2dp.service

Configuring the Bluetooth adapter

By default hci0 is set to discoverable and pariable. In the case where there are multiple Bluetooth adapters, one or more adapters can be set to discoverable and pairable in the configuration file /etc/default/a2dp-agent as a comma separated list of devices as:

BLUETOOTH_ADAPTER=hci1

BLUETOOTH_ADAPTER=hci0,hci1

Higher quality audio

SBC has a bad reputation compared with other codes, but it turns out that good old SBC has a dual channel mode which can be enabled in the Developer options in recent LineageOS based Android phones. See Bluetooth SBC Dual Channel HD audio mode – LineageOS – LineageOS Android Distribution for a breakdown.

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