UDP send test fails on amazon ec2 - all outgoing traffic enabled

Question

I'm running an ubuntu 14.04 instance on amazon ec2- I can't seem to send any udp packets from my instance to my local machine.

Running the followings commands: On amazon ec2 instance:

echo "test" | netcat -vu m.y.i.p 5500

Connection to m.y.i.p 5500 port [udp/*] succeeded!

On my local machine:

netcat -luv 5500

Listening on [0.0.0.0] (family 0, port 5500)

So we successfully make a connection, but I never receive the test packet on my local machine.

Is there anything else I might need to configure with my instance for this to work?

Answer 1

A UDP transmission does not have a connection (as does TCP) so the message "Connection to m.y.i.p 5500 port [udp/*] succeeded!" doesn't really tell you much about the true success of the transmission of a packet from A to B. It might have never even left the originating machine (due to some firewall rule).

In my experience most common UDP problems are firewall blocks at the incoming machine so you certainly need to check on any firewall rules that might be blocking UDP incoming on port 5500.

If that looks ok, then the easiest way to debug is to use a packet sniffer (tcpdump, wireshark or similar). First confirm that a UDP packet is leaving your source machine, then try to see it incoming on the target machine.

tcpdump host m.y.i.p