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I have just started writing socket programs. Came to know that single UDP packet has source port destination port and some MAC address representing router..etc. I wonder why anybody cannot create custom packets with a fake information in and send it over internet. I would like to know how safe are our PCs. What should be done to secure it ?
There are a couple of different aspects to the answer.
One is that the web relies on TCP, not UDP. Which means that it is connection-oriented. Your package will be rejected, unless it appears to be part of an existing connection (which means, among other things, that it has to have the right source IP and port as well. And it has to have the right sequence number to fit into the receive window). This can still be faked without too much trouble, of course. But it does require you to know a bit about the packets being sent on the original connection.
Another part is that whenever we need to be sure that the sender of a packet is who they claim to be, we use encryption. 🙂
Most packets don’t really need this. It’s not a huge deal if someone sends a request to Google which appears to come from my IP. But when making credit card transactions, it becomes a bit more important.