Why does ffmpeg need DNS resolver as its dependencies? - ffmpeg

Question

I was installing FFmpeg and libav today using brew install ffmpeg libav when I noticed unbound in the dependencies list.
It's very strange because unbound is DNS resolver and why ffmpeg would need it.
❯ brew install ffmpeg libav
==> Installing dependencies for ffmpeg: aom, frei0r, gmp, libtasn1, nettle, p11-kit, unbound, gnutls, lame, libass, libbluray, libsoxr, libvidstab, libvpx, opencore-amr, opus, libsndfile, libsamplerate, rubberband, sdl2, speex, giflib, leptonica, tesseract, theora, x264, x265 and xvid
I even checked the info list for ffmpeg but unbound was not present there
❯ brew info ffmpeg
ffmpeg: stable 4.2.1 (bottled), HEAD
Play, record, convert, and stream audio and video
https://ffmpeg.org/
/usr/local/Cellar/ffmpeg/4.2.1_2 (287 files, 56.6MB)*
- Poured from bottle on 2019-12-19 at 10:45:56
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/ffmpeg.rb
==> Dependencies
Build: nasm ✘, pkg-config ✔, texi2html ✘
Required: aom ✔, fontconfig ✔, freetype ✔, frei0r ✔, gnutls ✔, lame ✔, libass ✔, libbluray ✔, libsoxr ✔, libvidstab ✔, libvorbis ✔, libvpx ✔, opencore-amr ✔, openjpeg ✔, opus ✔, rtmpdump ✔, rubberband ✔, sdl2 ✔, snappy ✔, speex ✔, tesseract ✔, theora ✔, x264 ✔, x265 ✔, xvid ✔, xz ✔
==> Options
--HEAD
Install HEAD version
==> Analytics
install: 66,434 (30 days), 280,128 (90 days), 1,079,492 (365 days)
install-on-request: 48,053 (30 days), 203,583 (90 days), 759,254 (365 days)
build-error: 0 (30 days)
Links for reference
https://formulae.brew.sh/formula/ffmpeg
https://formulae.brew.sh/formula/unbound

Answer 1

brew deps --tree can help you figure this out. In the case of ffmpeg, unbound is pulled in by gnutls for HTTPS support.
% brew deps --tree ffmpeg
ffmpeg
├── aom
├── fontconfig
│ └── freetype
│ └── libpng
├── freetype
│ └── libpng
├── frei0r
├── gnutls
│ ├── gmp
│ ├── libidn2
│ │ ├── gettext
│ │ └── libunistring
│ ├── libtasn1
│ ├── libunistring
│ ├── nettle
│ │ └── gmp
│ ├── p11-kit
│ │ └── libffi
│ └── unbound
│ ├── libevent
│ │ └── openssl#1.1
│ └── openssl#1.1
├── lame
├── libass
│ ├── freetype
│ │ └── libpng
│ ├── fribidi
│ └── harfbuzz
│ ├── cairo
│ │ ├── fontconfig
│ │ │ └── freetype
│ │ │ └── libpng
│ │ ├── freetype
│ │ │ └── libpng
│ │ ├── glib
│ │ │ ├── gettext
│ │ │ ├── libffi
│ │ │ ├── pcre
│ │ │ └── python
│ │ │ ├── gdbm
│ │ │ ├── openssl#1.1
│ │ │ ├── readline
│ │ │ ├── sqlite
│ │ │ │ └── readline
│ │ │ └── xz
│ │ ├── libpng
│ │ ├── lzo
│ │ └── pixman
│ ├── freetype
│ │ └── libpng
│ ├── glib
│ │ ├── gettext
│ │ ├── libffi
│ │ ├── pcre
│ │ └── python
│ │ ├── gdbm
│ │ ├── openssl#1.1
│ │ ├── readline
│ │ ├── sqlite
│ │ │ └── readline
│ │ └── xz
│ ├── graphite2
│ └── icu4c
├── libbluray
│ ├── fontconfig
│ │ └── freetype
│ │ └── libpng
│ └── freetype
│ └── libpng
├── libsoxr
├── libvidstab
├── libvorbis
│ └── libogg
├── libvpx
├── opencore-amr
├── openjpeg
│ ├── libpng
│ ├── libtiff
│ │ └── jpeg
│ └── little-cms2
│ ├── jpeg
│ └── libtiff
│ └── jpeg
├── opus
├── rtmpdump
│ └── openssl#1.1
├── rubberband
│ ├── libsamplerate
│ │ └── libsndfile
│ │ ├── flac
│ │ │ └── libogg
│ │ ├── libogg
│ │ └── libvorbis
│ │ └── libogg
│ └── libsndfile
│ ├── flac
│ │ └── libogg
│ ├── libogg
│ └── libvorbis
│ └── libogg
├── sdl2
├── snappy
├── speex
│ └── libogg
├── tesseract
│ ├── leptonica
│ │ ├── giflib
│ │ ├── jpeg
│ │ ├── libpng
│ │ ├── libtiff
│ │ │ └── jpeg
│ │ ├── openjpeg
│ │ │ ├── libpng
│ │ │ ├── libtiff
│ │ │ │ └── jpeg
│ │ │ └── little-cms2
│ │ │ ├── jpeg
│ │ │ └── libtiff
│ │ │ └── jpeg
│ │ └── webp
│ │ ├── jpeg
│ │ ├── libpng
│ │ └── libtiff
│ │ └── jpeg
│ └── libtiff
│ └── jpeg
├── theora
│ ├── libogg
│ └── libvorbis
│ └── libogg
├── x264
├── x265
├── xvid
└── xz

Answer 2

On Homebrew unbound is a dependency of gnutls which is a dependency of ffmpeg when using --enable-gnutls (which is used by default in the ffmpeg Homebrew formulae).
gnutls (or openssl, mbedtls, libtls, securetransport) is used in ffmpeg for HTTPS support.