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iperf -c -i 2
connect failed: Connection refused

3.2 is another machine. I think the port might be blocked.

How can I make this work?

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This may be a stupid question, but: are you running "iperf -s" on – Handyman5 Jun 16 '11 at 8:52
Another reason to get this error is running the IPerf server in UDP mode and client in TCP mode and vice versa. – user1407196 Jan 24 '13 at 14:46
Make sure Server & Client versions are the same... I had the same problem! – user301514 Jul 26 '15 at 21:28
Make sure you are using same version of iperf – Ishan Apr 9 at 18:12

I doubt that this is a firewall issue. Usually, firewalls employ the DROP rule for packets that are not allowed. A "connection refused" means that there is no application listening on the port it tries to connect to. The destination host indicates this with a corresponding ICMP message to the source.

In order to run iperf you must have a server running on the machine which you iperf to. To do this run iperf -s on the server. After that you can then run iperf -c <server IP or hostname> -i 2

After running this you will get an ouput that looks something like this:

Client connecting to <server IP>, TCP port 5000
TCP window size:   256 KByte (default)
[  3] local port 4000 connected with port 5000
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  1.1 GBytes  1.09 Gbits/sec
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Try turning off any firewall on the machines. On many linux distros, you can do

service firewall off

to disable the firewall.

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You also get a "connection refused" error when you use an iperf v3 client to send to an iperf v2 server.

The message is slightly different though:

iperf3: error - unable to connect to server: Connection refused

As the iperf3 doc states, they are not compatible (at least up to version 3.1.2). You can check the version on both server and client with

iperf -v
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