How to fake source ip-address of a udp-packet?

The UDP packet does not actually have the source(your) IP address. The source IP address is part of the packet it is sent in. So you would have to modify the packet it is enclosed in.So while it is non-trivial, it is possible.

The packet structure for UDP and the enclosing packets for reference.

The UDP packet does not actually have the source(your) IP address. The source IP address is part of the packet it is sent in. So you would have to modify the packet it is enclosed in.So while it is non-trivial, it is possible.

The packet structure for UDP, and the enclosing packets for reference.

– youllknow Mar 22 '10 at 15:41 That is correct. Essentially you create a fake packet containing the UDP info, and send that. May I ask why you are trying to do this?

– jschoen Mar 22 '10 at 16:14 If I'm able to set up a prototype that can perfom the task, I may write a security application as my final year project which allows multiple clients to exchange data over the internet, without someone else knowing that there is a connection between the partners. Do you have any suggestion how to implement sending of the fake packet? – youllknow Mar 22 '10 at 16:47 Honestly, I am not sure how do this, and only knew about this because I am taking a Network Security class at the moment.

Maybe make another SO question about that, and you might get some suggestions from others. – jschoen Mar 22 '10 at 17:17 Thanks a lot! Can you please explain what a SO question is?

– youllknow Mar 22 '10 at 17:24.

It can be performed using IP-Spoofing... you'll find a lot of information when you google for it! Thanks to everyone for your help!

This is IP-spoofing. Unless you ISP is a dodgy russian one, it will probably prevent you from doing that (the first router will just drop the packet because it is suspicious). If you don't want to be identified you should try to find a proxy supporting UDP... Or you can buy a botnet.

:) (if you didn't get it, that's a joke, don't do that).

You will need to have access your ISP Router in order to do that. If you send a raw UDP-Packet with all the information to the other server, the Router will encapsulate it in another Package with your real ip.

– youllknow Mar 22 '10 at 15:30 as @jschoen says, it is not trivial, and it is probably against your ISP's Terms of Service. – jpabluz Mar 22 '10 at 15:41 Do you have any idea how to send a enclosed packet (like jschoen says) using C# or C/C++? – youllknow Mar 22 '10 at 16:00 I have no idea, but would look first in documentation from a Network Driver, and see if that is possible in your ethernet card... but still I think that encapsulation takes place on the Router, which your network card connects to, so there will be trouble in trying to send a packet without your real-IP information to be sent.

– jpabluz Mar 22 '10 at 17:48 3 A router simply forwards the packets it receives. It may (depending on configuration) discard a packet having an obviously fake source, but as a rule routers do not change the source address. NAT gateways and transparent proxies would.

But only a VPN endpoint would actually encapsulate the packet further. – Ben Voigt Mar 22 '10 at 18:15.

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