Blizzard Downloader

Blizzard Downloader (or Background Downloader) is a combination BitTorrent/HTTP client used for Blizzard Entertainment's software products. It was first implemented with the MMORPG World of Warcraft. It is used to download the full game installer package, and periodic updates for the game. It is also used to download the trailers for The Burning Crusade, StarCraft II, and Diablo III. Early versions were based on the source code of the original BitTorrent client, though it is unclear what the source model is now. When a large update is issued, Blizzard Downloader is downloaded to the WoW directory and it fetches the update file, using TCP ports 3724 and 6881. Blizzard offers an individual client for each download they offer, instead of just a torrent file. Blizzard Downloader also runs in the background under the name of Background Downloader. While the downloader is running, the program also uploads parts of files over the internet to other users who need the same data. To pause a download in progress, simply close the downloader program (do not cancel the download first) and it should resume where it left off when you start the downloader again.

New look
In patch 3.0.8, the look of the downloader were upgraded to match the launcher and made to look more specific to World of Warcraft. Previously, the window was much simpler with considerably less artistry. There were no obvious functional improvements, but the splash art could possibly change with each patch or for different product downloads (assuming it is still used for non- products).

The downloader was further changed with patch 3.2.

The broken Background Downloader
Many people have found out their Background Downloader will not execute and popup after WoW has been closed. This will prevent people from pre-downloading patches. The problem is fixed by deleting the file BackgroundDownloader.exe and running the repair program to download a new one.

Known Broken Downloaders

 * 2.2.0.1092
 * 2.2.0.1097
 * 3.3.2 to 3.3.3
 * 3.3.3 to 4.0.1.

To Fix

 * Delete the BackgroundDownloader.exe in your WoW directory. In Windows its usually "C:\Program Files\World of Warcraft".
 * Run the "Repair" program and it will download a new background downloader.
 * Either run WoW and the close the game to launch it, or manually execute it from the WoW directory.

Structure
The Blizzard Downloader bundle is actually a plain BitTorrent client that has at least one .torrent file inside. The .torrent can be opened by other BitTorrent clients giving the user greater control over how the download is handled. It is possible to extract the .torrent file from the end of the downloader executable:

Windows version

 * Search for d8:announce at the end of the downloader executable, and then save from this point to the end of the executable to a separate file. Add .torrent to the file name and open in any BitTorrent client.
 * Note that these executables may contain multiple torrent files (e.g. the Burning Crusade Downloader contains a "slim" torrent for 1.24 GB and a "full" torrent for 2.23 GB).

Earlier versions included a Macintosh-variant of the .torrent as well, but this has been removed in later versions. You may also need to remove some trailing characters at the end of the file, since they do not all belong to the .torrent; essentially, the resulting file has to be Bencode-compliant.

Alternatively, start the background downloader and check the end of Logs\BackgroundDownload.log for a message like this:


 * Creating cache file C:\Program Files\World of Warcraft\Cache\BackgroundDownload.torrent

This .torrent file can be used with any BitTorrent client. Alternate locations may include the Cache folder, depending on the WoW client's locality.

Macintosh version

 * /Applications/World of Warcraft/WoW-old-to-new-local-downloader.app/Contents/
 * MacOS/
 * Blizzard Downloader - executable file
 * Resources/
 * downloader.torrent - torrent file
 * Note: GUI & localization files not listed for readability.