I try making network performance measurement using iperf in UDP option. I perform all the operations in Ubuntu Terminal. I have two computer and I have number of interfaces on each of my computers. Interfaces other than Loopback and Local Host are created using high-gain antennas connected to computers. I create server side with the line of sudo iperf -s -u -B For client side I use the line of sudo iperf -u -B -c -b 1M -t 15 -i -f m. It seems that interfaces are connected (the message is displayed only on the client side) to each other and client side shows Internal, Transfer and Bandwidth statistics for 15 seconds (comes from -t 15). However, in the end I expect Server Report to be displayed, but it is not displayed. Instead, I have WARNING: did not receive ack of last datagram after 10 tries, which is displayed only on the client side. In Server side, even "connected" message is not displayed. In server side basically nothing changes before, during, and after the iperf test. Thus, my question is why do I get this WARNING? How can I fix this? I appreciate your help.


If you're not seeing anything at all on the server side, it's likely not receiving anything at all from the client. I noticed your client and server appear to be on different networks, so I'd first ensure there's a path between them and that they can communicate with each other (can they ping?). Another possibility is a firewall exists in between them (default iperf port is 5001), so double-check to make sure the traffic isn't being blocked.

One way to quickly check is to use nc to see if you can pass UDP datagrams back and forth. On the server side, run:
nc -l -u 5001

On the client side, run:
nc -u 5001

Then type something into the client side and see if it appears on the server side.

  • Hi, thank you very much for your response. Sorry, I should have seen this earlier. I did exactly what you have told me to do, but again nothing happens in the server side. After the commands you told me, I typed something in the client side, but server didn't receive anything. Does this mean that server and client are not connected?
    – Eric
    Feb 7 '17 at 3:57
  • Also I tried ping. I used ping -I <interfaceName for> Terminal says Destination Host Unreachable :( How can I fix this?
    – Eric
    Feb 7 '17 at 4:14

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