Clients on Windows

What actually runs on Windows

What runs on Windows divides into three groups, and only the first is worth much attention. Free and open source clients come first: qBittorrent, written in Qt and running on the libtorrent engine, presents the same window here as on macOS and Linux, and ships both an installer and a portable build. Deluge ships a Windows installer and keeps its design of a background daemon with a separate interface even when both halves live on one machine. Transmission is developed mainly for macOS and Linux, so whether a Windows build exists for a given release is a question to settle at the project itself.

The second group is the closed source clients, some funded by advertising in the window or by extra offers in the installer. The third is the remote controls, which are not clients: they drive a client running elsewhere, usually on a server. Windows is also the only one of the five platforms here where a client normally arrives as a file you fetched and ran yourself rather than as a package requested from a repository, and that difference produces most of what follows.

What Windows does to a transfer

Windows acts on a transfer in four places, and readers meet the first within seconds. Windows Defender Firewall raises a prompt the first time a client listens for incoming connections, offering two checkboxes, Private networks and Public networks. Answering for Private only, on a connection Windows classifies as Public, leaves the client firewalled: it still downloads, but no peer can start a connection to it, which is the state The swarm describes as half a connection. Dismissing the prompt creates a block rule, and nothing in the client will tell you so.

The second is the mark of the web. A file arriving through a browser is tagged, SmartScreen checks that tag against its reputation service, and an installer it has not seen often enough produces a warning that hides the run button behind More info. The third is power. Sleep and hibernation both stop the transfer, and Fast Startup means a shutdown is not a real shutdown, so a client set to start with Windows behaves differently after a shutdown than after a restart. The fourth is the file system. NTFS is case insensitive, forbids the characters that separate paths and a handful of others, still reserves old device names such as CON and LPT1, and applies a path length limit of 260 characters unless long paths are enabled. A torrent with a deep folder structure, or with names from a case sensitive system, hits all four.

Where a Windows build comes from

A Windows build should come from the project that writes the client, which for an open source client means the file attached to a numbered release in the project's own source repository, hosted for most of them on a code service such as GitHub. Microsoft's own channels are the other honest route: the Windows Package Manager installs from a curated manifest, and the Microsoft Store lists packaged applications. Both show a publisher, and that is the thing to read, because a listing carrying a familiar client's name is not necessarily published by that project.

Two signals are worth checking. A signed installer names its publisher in the User Account Control prompt; an unsigned one says the publisher is unknown. And projects publish checksums, so a file that does not match the published hash is not the file the project built. Nothing about Windows 11 changes this or changes the protocol: what differs between releases is the shell, the wording of the prompts and the handling of network profiles.

What goes wrong first on Windows

• The firewall prompt was answered for the wrong profile, so the client shows firewalled and connects only outward.
• SmartScreen blocks the installer, and the reader concludes the client is malicious when the file is simply new.
• A path runs past the length limit, and the client errors on the files deepest in the folder tree while the rest complete.
• Real time antivirus scanning holds files open during a recheck, so the client sits in checking far longer than the disk needs.
• The save folder sits inside a synchronizing cloud folder, and the sync client and the torrent client fight over the same partly written file.

What Windows cannot do

Windows cannot transfer while asleep or hibernated, and Fast Startup means a machine that was shut down is not running the client even though it booted quickly. It cannot make the router forward a port: allowing a program through Windows Defender Firewall opens the gate on the machine only, which is why a client can be allowed locally and still report itself as firewalled. It cannot verify that an installer bearing a client's name came from that project, since only the signature establishes that. And a Store packaged application cannot be handed arbitrary folder access the way a classically installed program can, because the packaging redirects where it writes.

What the word app changes here

The word app changes where the settings live, and little else. A classic installer writes the program into Program Files and keeps its configuration in the per user application data folder, which survives a reinstall. A portable build keeps its configuration beside the executable, so it can sit on an external disk and move between machines with its transfers intact. A packaged app from the Store keeps its data in a per package location removed when the app is, so uninstalling takes the transfer list with it.

What the packaging never changes is the transfer. Whichever form the client arrives in, it verifies the same pieces against the same hashes, reports the same states of moving, complete, waiting and errored, and answers to the same swarm, as How a transfer works sets out and Is it safe? qualifies. On a phone the word app means something narrower, since the store is the only supported channel there.