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Are you talking about changing source address in IP header? Most ISPs will drop such packets: https://tools.ietf.org/html/rfc3013#section-4.3

That's what I thought - but it turns out that not all ISPs behave well.

https://defcon.org/images/defcon-22/dc-22-presentations/Grah... but the video is also very entertaining. This is applicable to the IPv4 internet but I'm sure someone will eventually come up with a way to do it for IPv6.