On a local network, an attacker can craft a custom IP packet with a fake source IP address and send it to a host. I know that the attacker will not receive a response, but there are still attacks for which faking/spoofing the source IP in IP packets can be useful.
But how can this be done on the internet (faking public IP addresses)? If I want so send an IP packet with a fake public source IP address, and I'm sitting behind a router that is performing NAT, I assume the router will change the fake source IP in the IP packet to the real public IP of the router. I didn't find any options to switch off NAT for particular addresses on my router. Are there routers that have such options, or is that done somehow else? Has anyone of you done something like this before?
I know that there is a similar question/answer about what happens to a packet with faked source IP that goes through NAT, but it does not really answer my question. It explains the possible problem with NAT that I mentioned, but not how to avoid the problem.