How do I configure a web server for .torrent files?

You must associate files ending in .torrent with the Content-Type application/x-bittorrent.

For Apache, you should add the line:
AddType application/x-bittorrent .torrent to your httpd.conf configuration file. If you cannot modify the main httpd.conf file (such as in the case of a shared or virtual hosting scenario), you can also put the above AddType directive in a .htaccess file. This presumes that the server's administrator has enabled this ability with the AllowOverride FileInfo directive. Also note that settings in a .htaccess file only apply to the directory containing the file, so make sure this is the directory that will contain the .torrent files.

Alternatively, you can add the line:
application/x-bittorrent .torrent to your mime.types file, which is used if the TypesConfig directive is present in httpd.conf.

To add MIME types with Microsoft's IIS web server:

Right-click the Web site and choose Properties.
Select the HTTP Headers tab.
In the MIME Map box, click File Types, and then click New Type.
Enter the extension ".torrent" and the MIME type "application/x-bittorrent" in the appropriate boxes.
Select OK to close all the dialogs.

If you cannot configure the application/x-bittorrent type, you could place the .torrent files in an archive (.zip, .rar, etc.) and serve that instead. Doing so will forfeit the ability to click on a torrent link and automatically launch the BitTorrent client, however. The user must manually unpack the .torrent file and then launch it to start the transfer.