Record local audio in a docker container

Question

How can I record the audio of an application like Firefox inside a docker container with ffmpeg? I've found examples how to forward pulseaudio to the host - netflix, skype.

When I'm trying to use pactl:

pactl list sources

Or

docker exec -it <container-id> bash
apt-get install pulseaudio
pactl load-module module-native-protocol-unix auth-anonymous=1 socket=/tmp/.pulse-socket

I'm getting an error:

Connection failure: Connection refused
pa_context_connect() failed: Connection refused

This also fail

ffmpeg -f pulse -i default /tmp/pulse.wav

Answer 1

By default you don't have access to the sound card inside the container. If you pass the --privileged flag to docker run, it should work. Note that this will give the container full privileges on the host, you might want to restrict this to a set of capabilities.

For more information: https://docs.docker.com/reference/run/#runtime-privilege-linux-capabilities-and-lxc-configuration

Answer 2

You don't need a sound card in the docker container - pulse audio will create a null sink/source for you, and you can add a loopback device if you like as well.

However! I have gotten connection errors if I'm using a pulse audio library that needs X. I used Xvfb. And did something like this:

Xvfb :0 -screen 0 1600x1200x24+32 -ac -fbdir /shared_volume&
pulseaudio --start --disallow-exit -vvv --log-target=newfile:"$LOG_DIR/pulseaudio.log" --daemonize&

I just needed to detect when things were playing sound, but this guide seemed a nice detailed coverage of how to use loopback devices to record stuff.