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jacktrades
Asked: 2014-10-16 12:45:01 +0800 CST

UDP Proxy with cloned packets

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Having UDP messages sent from dynamic public IP periodically to public IP X:20000.

Need a proxy that listens in IP X:20000, then forwards duplicate/clone packets to X:20001 and a different machine located in IP Y:20000. Now, when application listening on X:20001, responds to a message to X:20001, I want that message to be forwarded back to PC A, from X:20000.

How could I achieve this functionality in Linux? (trying to avoid custom script)

Trying to have one application in a headless server responding to messages, and one computer with monitor to debug messages when needed in real time.

linux

1 Answers

  1. Best Answer

    I think you could pull this off w/ iptables and the samplicator tool if you're using a new enough kernel to support the raw table.

    First, why socat won't work: Teeing a packet flow with socat is fairly easy. You'd just do this:

    socat - udp4-listen:20000,fork | tee >(socat - udp-sendto:X.X.X.X:20001) >(socat - udp-sendto:Y.Y.Y.Y:20000)

    That duplicates the traffic to X.X.X.X:20001 and Y.Y.Y.Y:20000.

    That doesn't help you, though, because the service listening on X.X.X.X:20001 is going to "see" 127.0.0.1 as the source address. That's where samplicator can help out. Quoth the samplicator Google code page:

    This simple program listens for UDP datagrams on a network port, and sends copies of these datagrams on to a set of destinations. ... Another option is that it can "spoof" the IP source address, so that the copies appear to come from the original source, rather than the relay.

    That sounds like exactly what we need re: the source address. (Having said that, I haven't actually tested this tool. The box I'm testing on doesn't have compilers installed and I'm not going to spin up something right now just for Server Fault. >smile<)

    The last thing you'd need is to take care of the traffic coming from X.X.X.X:20001, making it appear to come from X.X.X.X:20000.

    Then, to NAT the replies from X.X.X.X:20001 to "come from" X.X.X.X:20000:

    iptables -t raw -A POSTROUTING -s X.X.X.X -p udp --sport 20001 -j NOTRACK
iptables -t nat -A POSTROUTING -s X.X.X.X -p udp --sport 20001 -j SNAT --to-source :20000

    Beware: I haven't tested all of this together. I mocked it up with socat and it worked fine minus the source address "spoofing" that samplicator provides.

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