Why share a set of headphones between MacOS and Fedora (or another Linux)?

Well, the answer to the 'why?' question is quite simple: because I dual boot between MacOS and Fedora on my Mac Mini. This means in order to use the same pair of headphones on MacOS and Fedora, I need them to use the same key, so the headphones think they are always connected to the same machine.

There are a couple of loose posts on the internet, especially on Ask Ubuntu about this topic already. I'm just combining those posts into a single blog for my own reference and potentially preventing people from having to dig up information from multiple places.

Background

Simply said, Bluetooth pairing works by exchanging a secret key between two devices. Your operating systems (well, Linux anyway, but I assume MacOS does the same thing) associated a key with both the Bluetooth address on the local machine as well as the Bluetooth address of the remote device. Devices do the same thing.

This means that there can only be a single key for each point to point connection in the local Bluetooth "database". And if you pair a set of headphones with MacOS and then with Linux, the key from the pairing with Linux will overwrite the key from the pairing with MacOS.

In other words, we have to make sure the pairing with both MacOS and Linux use the same key.

Process

In order to share the Bluetooth key betwen MacOS and Fedora (or another Linux distro), we follow these steps:

1. Pair on Linux
2. Reboot into MacOS
3. Pair on MacOS
4. Extract the Bluetooth key from MacOS
5. Reboot into Linux
6. Swap out the old - now obsolete - key for the one from MacOS

Steps 1, 2 and 3

I'm not going to describe these in detail, as I'm assuming you know how this works :)

Step 4: extracting the Bluetooth key from MacOS

This is the crucial bit. On MacOS, open the Keychain app, and search for "bluetooth". As you just paired the headphones, you are looking for the most recent entry called "MobileBluetooth".

Double click it. It will show you the Bluetooth address in the Account field. Make a note of both what is in the Account field and what is in the Show password field. You might have to provide your password once or twice along the way.

What is the the Show password field will look something like this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>LinkKey</key>
<string>00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00</string>
<key>LinkKeyType</key>
<string>UnAuthenticated</string>
<key>LocalAddress</key>
<string>11:11:11:11:11:11</string>
</dict>
</plist>

The bit aboce that reads 00-00-00-00-[...] is your Bluetooth key. The bit that reads 11:11:11:[...] is the Bluetooth address of your Bluetooth controller (i.e. the Bluetooth address of your computer.

Step 5 and 6: reboot into Linux and swap out the key

Ok, now switch off your headphones for now, and make your sure you keep the note somewhere you can access from Linux and reboot.

Once rebooted into Linux, as root, go into /var/lib/bluetooth/11:11:11:11:11:11. Obviously swap out the 11:11:11:[...] string for the Bluetooth address of your computer.

In that directory, you'll find another directory that is named after the Bluetooth address of your headphones. This is what was in the Account field on MacOS. I'll use 22:22:22:[...] in this example. Enter that directory. You are now in /var/lib/bluetooth/11:11:11:11:11:11/22:22:22:22:22:22. In that directory, there is a file called info.

Edit the info file. It'll look something like this:

[General]
Name=Your Awesome HEadphone
Class=0x240404
AddressType=public
SupportedTechnologies=BR/EDR;LE;
Trusted=true
Blocked=false
Services=[very long string here, shortened for readability]

[DeviceID]
Source=2
Vendor=1999
Product=3000
Version=1999

[LinkKey]
Key=this_is_the_old_key
Type=4
PINLength=0

Remove the dashes from the 00-00-00-[...] string and paste it in the Key field above, where it says this_is_the_old_key.

Save the file and run systemctl restart bluetooth. Power on your headphones. Listen to music.

Hope this works as well for you as it did for me!!