OBS Studio: "Couldn't initialize muxer" on Ubuntu 24.04+ / 25.10

If OBS Studio refuses to record on a recent Ubuntu (24.04, 24.10, 25.04, 25.10) with this dialog:

An encoder error occurred while recording: Couldn't initialize muxer

…and the OBS log (~/.config/obs-studio/logs/) shows:

[ffmpeg muxer: 'simple_file_output'] os_process_pipe_write for info structure failed
[ffmpeg muxer: 'simple_file_output'] ffmpeg-mux: Couldn't initialize muxer

…this is almost certainly AppArmor blocking unprivileged user namespaces, not a codec/container/path problem.

Root cause

Ubuntu 24.04+ ships with:

kernel.apparmor_restrict_unprivileged_userns = 1

OBS spawns obs-ffmpeg-mux as a helper child process and pipes muxer configuration to it over stdin. The userns restriction breaks that IPC, so the muxer never initializes — even though the helper binary itself works fine when invoked directly.

You can confirm by temporarily disabling the restriction:

sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0

If recording starts working, this is your problem.

Permanent fix (recommended)

Re-enable the global restriction and grant OBS its own AppArmor profile that allows userns:

sudo tee /etc/apparmor.d/obs >/dev/null <<'EOF'
abi <abi/4.0>,
include <tunables/global>

profile obs /usr/bin/obs flags=(unconfined) {
  userns,
  include if exists <local/obs>
}
EOF

sudo systemctl reload apparmor
sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=1

OBS now has the exception it needs while the rest of the system stays protected. Recording survives reboot.

Notes

If your OBS binary lives somewhere other than /usr/bin/obs (e.g. Flatpak), adjust the profile path. Flatpak OBS has different sandboxing — different fix.
The flags=(unconfined) attaches a profile without confining OBS further; we just need the userns line to be allowed.
This is unrelated to the more common "MP4 + weird codec" muxer errors. If your log doesn't contain os_process_pipe_write, you have a different problem.

This gist was generated by Claude Code. Please verify any information before relying on it.

danielrosehill/obs-muxer-apparmor-fix.md

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