i assume that this is due to a redirect:
→ curl -I http://download.virtualbox.org/virtualbox/4.2.18/VirtualBox-4.2.18-88780-OSX.dmg
HTTP/1.1 302 Moved Temporarily
Location: http://dlc.sun.com.edgesuite.net/virtualbox/4.2.18/VirtualBox-4.2.18-88780-OSX.dmg
→ curl -I http://dlc.sun.com.edgesuite.net/virtualbox/4.2.18/VirtualBox-4.2.18-88780-OSX.dmg
HTTP/1.1 200 OK
Content-Length: 110667450
downloading the real url works like this:
require 'net/http'
url = "http://dlc.sun.com.edgesuite.net/virtualbox/4.2.18/VirtualBox-4.2.18-88780-OSX.dmg"
File.open('tmp.dmg', 'w') { |f| f.write Net::HTTP.get_response(URI.parse(url)).body }