How to spoof an IP Address using a socket?

I'm working on a website testing framework, and I'd like to be able to spoof the user ip. I've read, that software like Loadrunner can do this, and I'd like to be able to do this as well.

A tutorial on how to use a socket normally can be found here, but I if I want to spoof the ip, I'll have to somehow edit ip header. Is this done on socket level, or is there an other, deeper level in .Net, which would let me do this?

Solution

You can't anymore, as access to raw sockets has been restricted in the desktop versions of Windows:

On Windows 7, Windows Vista, Windows XP with Service Pack 2 (SP2), and Windows XP with Service Pack 3 (SP3), the ability to send traffic over raw sockets has been restricted in several ways:

TCP data cannot be sent over raw sockets.

UDP datagrams with an invalid source address cannot be sent over raw sockets. The IP source address for any outgoing UDP datagram must exist on a network interface or the datagram is dropped. This change was made to limit the ability of malicious code to create distributed denial-of-service attacks and limits the ability to send spoofed packets (TCP/IP packets with a forged source IP address).

A call to the bind function with a raw socket for the IPPROTO_TCP protocol is not allowed.
Note The bind function with a raw socket is allowed for other protocols (IPPROTO_IP, IPPROTO_UDP, or IPPROTO_SCTP, for example).

(From Dev Center - Desktop - TCP/IP Raw Sockets)

You can create a raw socket fairly easily:

Socket s = new Socket(ip.AddressFamily, SocketType.Raw, ProtocolType.Ip);

At which point its on you to construct the appropriate datagram (which would be a separate question if you don't have the documentation on how to do this to hand)