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Stellaris® ARM® LM3S Microcontrollers Forum
enet_lwip Packet Sniffer
Happy new year! I'm working on the LM3S9B92 Ethernet+OTG Evaluation Ki and attempting to construct a basic packet sniffer. I've run into a design challenge that I should have foreseen but didn't. I'm modifying the included enet_lwip package. Currently, my device will connect to a network, receive packets, and then flash one of the onboard LEDs when it detects a specific packet that my project requires (an H.225 video call setup packet). My device is intended to alert when a specific video phone is being called.
My question is, as my evaluation board has one Ethernet port, and as in order to be a legitimate packet sniffer for these video calls, it is vital that the evaluation board receive exactly the same packets as the target video phone. Therefore, my initial design called for placing both the video phone, and my device, behind a HUB (which transmits data to all ports).
However, upon further consideration I feel that this is inefficient. I realized that my device doesn't really need to connect to a network at all (in terms of establishing itself as a separate host). It should be able to passively accept copies of the packets and sort through them, with no need to send or communicate on the network. I could build your basic network tap (http://www.altsec.info/passive-network-tap.html or similar), plug the video phone into the "thru" port, and my device into the monitor port. In this way, the video phone's bandwidth would be unimpeded, and my device would receive the same traffic.
So, the problem is, how do I get my device (using lwip) to passively accept and filter network traffic without getting an IP address or sending information over the network of any kind? How do I set up a connection, without all the functionality to communicate over a network? All it needs to do is receive the same packets as the video phone, and flash an LED when it detects the call-setup packet.
Sorry this is long, hope it makes sense because I'm stumped and any help would be GREATLY appreciated. Thank you so much!
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