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Bundled ffmpeg — how it's built and the LGPL source offer

Ichoi transcodes by shelling out to an ffmpeg binary. It is bundled beside the Ichoi binary in release artifacts and container images, but it is never linked into Ichoi (DESIGN §5.3, §13). This keeps Ichoi's own license clean (subprocess = mere aggregation) and dodges static-musl cross-compile pain.

What we ship

An LGPL ffmpeg with only the audio encoders we need — nothing that requires --enable-gpl and nothing nonfree:

  • Encoders: native aac (the default transcode target) and libmp3lame (MP3, LGPL). We never encode to Opus (DESIGN §5.2, §5.6).
  • Decoders: the formats we read, including Opus and HE-AAC sources (played via transcode-to-AAC-LC), plus ALAC, FLAC, Vorbis, MP3, AAC, and PCM.
  • No --enable-gpl, no --enable-nonfree, no Fraunhofer FDK AAC (DESIGN §13).

We use ffmpeg's native AAC encoder, not libfdk_aac — FDK has no patent grant, isn't OSI, and is Debian non-free. AAC-LC patents are considered expired (~2017–18); the exposure is ffmpeg's, not ours.

How it's built (the recipe)

The exact, reproducible recipe is the ffmpeg stage of the repo-root Dockerfile. In short:

  1. Build libmp3lame statically from source (LGPL-2.0), linked only into the ffmpeg subprocess.
  2. Configure ffmpeg with --disable-everything + an explicit audio allowlist, --enable-static --disable-shared, --enable-libmp3lame, the native aac encoder, and no --enable-gpl / --enable-nonfree.
  3. Build-time license gate: the image assertion greps ffmpeg -buildconf and fails the build if --enable-gpl or --enable-nonfree ever appears (DESIGN §5.3: "verify at build time no encoder pulls --enable-gpl").

The result is a single static ffmpeg binary for each release architecture (amd64, arm64).

Runtime resolution

Ichoi finds ffmpeg in this order (DESIGN §5.3):

  1. Bundled next to the Ichoi binary, or the path in ICHOI_FFMPEG.
  2. System PATH.

If neither is found, transcoding is disabled and the core serves direct mode only, with a warning. Ichoi never fails to start for lack of ffmpeg.

LGPL obligation

The bundled ffmpeg and libmp3lame are LGPL. Ichoi is a separate program that invokes ffmpeg as a subprocess, so no copyleft obligation attaches to Ichoi itself (DESIGN §13). The LGPL obligation we do carry is to offer ffmpeg's corresponding source and the build recipe:

  • Build recipe: this file plus the ffmpeg stage of the Dockerfile, which pins the exact ffmpeg and LAME versions and every configure flag.
  • Source: the pinned upstream releases — https://ffmpeg.org/releases/ffmpeg-<version>.tar.xz and https://downloads.sourceforge.net/project/lame/lame/<version>/lame-<version>.tar.gz (versions are the FFMPEG_VERSION / LAME_VERSION build args in the Dockerfile).

We never inline LGPL source into Ichoi (LGPL §3 would convert it to GPLv2). ffmpeg stays a separate, subprocessed binary.