Fix corrupted iso images with invalid md5 checksum using Bittorrent

We have been running the RequestCD Program for more than a year now. For this program we have been heavily downloading iso's, mostly GNU/Linux distributions. We download either through direct downloads or through bittorrent. Bittorrent downloads are inherently safer because the downloaded iso would be guaranteed error free. However http and ftp downloads results in corrupted downloads once in a while. Traditionally when you end up with a corrupted iso you had to download the iso file again. Not anymore. You can easily repair a corrupted iso using Bittorrent.

1. First you have to find a torrent for the iso and start the download of the iso using a torrent client. We use Transmission or KTorrent. Make sure that the torrent client has been configured to preallocate the files during download.
2. Now turn off the client and overwrite the file-download started by the torrent client with the corrupted iso.
3. Restart the torrent client and ask the client to recheck the file.
4. During recheck the torrent client will check through all the chunks in the iso file and identify the ones that are corrupt and download just the corrupted chunks.

For internet users who have limited bandwidth plans, bittorrent is slightly more expensive than direct downloads since you end up uploading as much as you download using bittorrent. This is probably one reason why people go for direct downloads in spite of the risk of getting corrupted iso`s. But by using the above strategy you can make the best of both options and still end up with a good iso file.