Say you have access to multiple links to the Internet, such as several wireless networks in range. Wouldn't it be nice to combine all that bandwidth into one big fat pipe for your downloading pleasure?
Unfortunately it's not so easy. You can't just trunk them together because they each have a different public IP address, gateway, etc.
What you can do however, thanks to some nifty Linux NetFilter extensions, is assign outgoing connections to different interfaces. This will allow protocols such as BitTorrent to utilize bandwidth from each of the links.
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