Blizzard Downloader Blizzard Downloader ( or Blizzard Background Downloader , Background Downloader , BackgroundDownloader . exe , and others named " something " - BKGND - 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 Cataclysm look . Sometime during the Cataclysm expansion , the downloader ' s look was updated to match the overall look of the expansion . Because the downloader is rarely seen standalone , most players won ' t notice it ' s new look . The downloader is now usually only called from the launcher , if set to run while the main game is running . New Wrath of the Lich King look . In patch 3 . 0 . 8 , the look of the downloader was 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 . On Windows PCs circa the Wrath of the Lich King expansion , many people had found out their Background Downloader would not execute and popup after WoW had been closed . This would prevent people from pre - downloading patches . The problem was fixed by deleting the file BackgroundDownloader . exe and running the repair program to download a new one . 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 . 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 : This . torrent file can be used with any BitTorrent client . Alternate locations may include the Cache folder , depending on the WoW client ' s locality . on Wikipedia .