Impact of Changing IP Address of Outgoing IP Packets From My Network

Question

If I modify the source ip address of all outgoing ip packets from my network to an ip address belonging to someone else (while ensuring that the checksum is correct) then what will happen.

Assume that I have a public IP address connected by a point-to-point link to an ISP. Will the ISP check that the IP address in my IP packets is correct or will it just forward the packets. I believe that ISP should just forward the packets.

what mechanisms are present in the Internet that prevent this from happening?

Answer 1

why should your ISP just ignore your attempts to spoof packages with false information? Were I that ISP I'd not just block the packages but instantly kick you off my network as you're either compromised by some trojan or other malware or are deliberately doing something bad. I'd do that as a public service to the world at large (trying to block computer criminals) as well as to prevent possible legal steps against me (knowingly helping in criminal activity is still a crime in most places and rightly so).

Answer 2

If you do this, you will probably be unable to establish a TCP connection -- the replies will go to the spoofed IP address, preventing the three-way handshake from succeeding.

Your ISP may, or may not, employ egress filtering at their border routers and drop the spoofed packets.