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I have developed a Java server which listens for UDP packets which come from mobile devices. At the moment, it works correctly if I run my Server on my windows PC. I can receive and respond to the UDP packets sent from my phone (over 3g, not wifi).

However, I am trying to run the Server on a different computer which I found lying around my house. I have basically done a clean install of Ubuntu Server edition, updated it, installed the java run time environment and configured port forwarding to allow connections on the appropriate port. I have also tried to disable the firewall.

The problem is, it doesn't appear as if the server is receiving any packets and I've exhausted my brain thinking on how to solve/debug the problem. Can anyone suggest to me what the problem might be? or more likely something I can do to help me find out more information about what is going wrong.

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I could tell you a joke about UDP, but you might not get it. –  Tom O'Connor Apr 1 '12 at 20:03
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1 Answer

up vote 3 down vote accepted

Use the force, Luke.

And by "the force", I mean tcpdump, and by "Luke", I mean James

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Could you provide a bit more information. Should I run this on my ubuntu server ? –  James Apr 1 '12 at 20:28
    
ok going to try running it on my server. I'll tell you how it goes –  James Apr 1 '12 at 20:37
    
On the server with the java application on, try tcpdump ip and udp port (insert port here) –  Tom O'Connor Apr 1 '12 at 20:40
3  
I have successfully utilized the force to subdue the powers of the dark side. AKA stupidity. I saw on tcpdump that the packets were being redirected to 198.168.1.14 instead of 198.168.1.15. Turns out I had two entries for my router forwarding the same port. I'm assuming the router was sending the packet to the wrong internal IP. This tcpdump is a wonderful tool, thank you –  James Apr 1 '12 at 20:53
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