Spoofing and Impersonation
Just as it's a simple matter to create a program that can construct a steady stream of SYN packets and send them rapidly to your server, it's also easy to create network packets that have false information in other fields of the IP header. For example, you might have a firewall set up to reject packets from known sources of trouble, based on the source IP address found in the header. However, there's nothing to stop the hacker, cracker, or attacker from simply putting in another source address so that your firewall lets the packet through.
IP address spoofing is very easy to do. It's also very hard to detect. One thing a firewall can do, however, is guard against packets that contain a spoofed address, making it appear that the packet originated inside your network. Think about it. If the source address of a network packet falls within the address range of your internal network, it shouldn't be coming in through a firewall interface that's connected to the Internet. It should be the other way around! All good firewalls can be configured to drop packets that arrive from the outside world with an address that makes it look like the packet came from your network.
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