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This may have a bit to do with the internals of iperf. I set up iperf -s -u -i 2 on server A (UDP server, supposed to print stats every 2 seconds), and am sending traffic to it from server B. As a prefix, I am aware of some MTU issues on the network, but I am using iperf to diagnose them. I send traffic using iperf -c <server-address> -l 1000 -u (the size of 1000 is to avoid MTU problems, for now at least). Now, as soon as I send out the traffic, I can see from server A that a connection was received, due to a printed line. However, stats on bandwidth are not printed out every 2 seconds after.

I confirmed that traffic is actually being sent using tcpdump. I see packets arriving, destined to the local server and port 5001, which the server does by default, but server A does not print anything regarding bandwidth.

I am leaning towards some sort of local routing issue, but it is a bit odd since the connection is initially established, so I am leaning towards an MTU issue/iperf issue of some kind.

  • Random guess: make sure you don't have a host firewall (e.g., iptables) dropping the traffic. [If this turns out to be right, anyone is welcome to convert it to an answer.] – derobert Jan 19 '18 at 20:49
  • Yeah I made sure to check that before, iptables allows everything. Don't know if I made it clear, but the traffic DOES make it to the server, it is just not registered by iperf. – stoneman_41 Jan 19 '18 at 21:07
  • Is ICMP blocked by other firewalls along the way? – Rui F Ribeiro Jan 19 '18 at 21:30
  • @RuiFRibeiro Two things: 1. I am not aware of the underlying network and things it might do. The stuff I am seeing above is in fact for the reason of determining issues. 2. ICMP packets seem to go fine, as ping works (similar MTU issues pop up when I make the ping packet larger than a certain size). The main thing I am trying to figure out is why iperf -s is not recognizing the packets which did come to the server. – stoneman_41 Jan 19 '18 at 21:40
  • There are known bugs with Cisco ASAs default configuration + Linux. Try to found out more about your underlying network infra-structure. – Rui F Ribeiro Jan 19 '18 at 21:43

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