qBittorrent: Open Source BitTorrent Client
qBittorrent is a BitTorrent client with a complete web interface, which makes it the usual choice for a headless server.
Why it became the default
It is genuinely open source, contains no advertising or bundled software, and its Web UI exposes essentially everything the desktop client does. The alternatives each carry a caveat: Transmission is lighter but less capable, Deluge’s plugin architecture is powerful and dated, and the historically dominant commercial clients accumulated adware.
Web UI and automation
The Web UI is what makes it work on a server: full control from a browser, plus a documented API that Sonarr, Radarr, and the rest of the *arr stack drive directly. Categories map to save paths, which is how automated media libraries stay organized.
Configuration worth getting right
Set a category-based save path layout from the start rather than dumping everything in one directory. Enable sequential download only when you need it, since it hurts swarm health. Set global and per-torrent rate limits so seeding does not saturate your uplink and make the household internet unusable, which is the most common complaint about running a seedbox at home.
If you route traffic through a VPN, bind qBittorrent to the VPN interface rather than relying on routing alone; binding means the client simply stops if the tunnel drops rather than leaking.
Mobile control
qBitController, iqBit, and nzb360 all manage it remotely.
Legal note
BitTorrent is a transfer protocol with entirely legitimate uses, including Linux distribution images, and copyright law applies to what you actually transfer.
License
qBittorrent is released under the GNU General Public License v2.0.