Apache 2.x and earlier can’t distribute files larger than 2GB. This means DVD images won’t work. (lighttpd doesn’t have this limitation). Also, some people find FTP to be more efficient than HTTP for really large files like ISO images.
RewriteCond %{REQUEST_METHOD} GET
RewriteRule ^(.*\.iso)$ ftp://myserver/$1 [L,R=301]
These Rewrite lines will redirect all HTTP GET requests for *.iso files to a different FTP daemon.