How To Test Network Speed With iPerf

You probably ended up on this page because you were searching for a way to test your network speed from one host to another. The best tool I have found for this job is iperf.

What is iPerf?

iperf is an open source network testing tool used to measure bandwidth from host to host. It is licensed under the BSD license.

It is available for most operating systems. Check the download page here to see if your OS is supported.

You can test TCP or UDP throughput using iperf.

Tests can run for a set amount of time or using a set amount of data.

iPerf uses a client/server model. The server can accept connections from multiple clients on different ports and can even run as a daemon.

I’ve used iPerf at for a couple of reasons. One reason was to verify that the connection between 2 hosts was as I expected over a VPN connection. This can be handy if you need to estimate the time it will take for a backup job to run from host to host.

My favorite use for iPerf is testing wifi. To do this I keep a hardwired host running in my homelab that runs an iPerf server in Daemon Mode. That means it is always running in the background and waiting for a client to connect to it.

If I am walking around using my laptop(or phone) wirelessly, I can generate traffic using the client and send it to the hardwired server to see what speeds I am getting.

This is helpful if you are trying to troubleshoot wifi dead zones.

Maybe you have a coworker or family member complaining that their wireless internet is too slow. You can bring your laptop to the dead zone and move your access point(s) around. Every time you move the access point, run another iPerf test with the laptop in the wifi dead zone. Continue this process until you see the connection improve.

If you are setting up a new wireless network for a client or at home, you can do the same to make sure you are happy with your coverage throughout the entire site. This can help you decide if you need to move an access point to another location or even get another access point for the site.

How to install iPerf on Mac, CentOS, and iOS

We will actually be using iperf3 for this tutorial. iPerf3 is currently the newest version of iPerf.

It’s important to note that iPerf3 is not backwards compatible with other versions of iPerf.

Mac

To install iperf3 on your Mac, you’ll want to have homebrew installed. You can do this by running the following command.

/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

Now that we’ve installed Homebrew, we can run the next command to install iperf3

brew install iperf3

CentOS

Run the following command to install iperf3 using yum.

yum install iperf3

iOS

Download iPerf 3 Wifi Speed Test from the iOS App Store. I should note that is only useful as a client. You still need to have a host on the network running as a server to send data to.

How to Use iPerf

As mentioned before, iPerf needs to run a client and a server. That means you need 2 hosts. My server is always connected to my network with a wired connection. This allows me to only worry about the connection wireless from the client. It’s no fun troubleshooting the connection on 2 hosts at once.

I’ll run through some examples below.

For all of these demos, some-host will always be running the server. Clients will be run from mac-mini and my iPhone.

iPerf TCP Client and Server

Run iperf3 -s to start the iperf server.

[tynick@some-host ~]$ iperf3 -s
-----------------------------------------------------------
Server listening on 5201
-----------------------------------------------------------

You should see something similar to the snippet above. It is telling you that the server is listening and on what port. Per iPerf documentation, the default port for iPerf 3 is 5201. iPerf 2 default port is 5001. More on changing the port later.

Now we are going to move over to one of our clients.

Run the following command.

iperf3 -c 192.168.1.210

Replace that IP address with the IP of the iPerf server.

On the left you can see the iPerf 3 client is sending data to the iPerf 3 server on the right.

This screenshot is from the client running on my iPhone connecting to the iPerf 3 server. It shows that I am currently downloading from the server at 135 Mbits/s.

You can now walk around your house or office and find all the dead Wifi spots.

iPerf UDP Client and Server

When testing with UDP, no changes are required on the server side. Continue using iperf3 -s to run the server.

On the client side we just need to add the -u flag to start sending UDP packets instead of TCP packets. See below.

iperf3 -c 192.168.1.210 -u

As you can see, UDP was faster than TCP from the same 2 hosts. This is pretty normal. I’m not a network engineer but I’ll try to explain anyway so someone on internet can tell me I am wrong.

The TCP protocol checks each time to make sure the target recieved the packets that the origin sent.

UDP on the other hand, just sends packets like a crazy person. It doesn’t care or check to see if the packets are being received. UDP just wants to send packets quickly. UDP is most commonly used for video streaming services. There is no error correction with UDP. If you lose a frame of a video during transfer, it wouldn’t make much sense to show the frame later on in the movie.

Changing iPerf Port

You may run into a situation where you want to run iPerf on a different port. This is pretty easily done by adding the -p flag. You will need to do this on both the client and the server.

Change iPerf Server Port

This will change the port that the iPerf Server is listening on.

iperf3 -s -p 5202

Change iPerf Client Port

This will change the port that the iPerf Client connects to.

iperf3 -c 192.168.1.210 -p 5202

You can see that the client is connected to an iPerf server listening on port 5202.

Running iPerf Server in Daemon Mode

Daemon mode is used when you want to keep your iPerf Server up and running without having to SSH in each time to start it up.

It will run in the background just waiting for connections from iPerf Clients.

To do this, run the server command with the -D flag from the server host.

iperf3 -s -D

You wont see anything after that. It should look something like this…

[tynick@some-host ~]$ iperf3 -s -D
[tynick@some-host ~]$

There are no changes to the client side when running in this configuration. Just connect as you normally would from the iPerf Client.

Check if iPerf is Running in Daemon Mode

At some point you will want to see if iPerf is running in the background.

The obvious solution is to use the ps command.

[tynick@some-host ~]$ ps aux | grep iperf | grep -v grep
[tynick@some-host ~]$ iperf3 -s -D
[tynick@some-host ~]$ ps aux | grep iperf | grep -v grep
tynick    5129  0.0  0.0   9612   332 ?        Ss   17:35   0:00 iperf3 -s -D
[tynick@some-host ~]$

However, iPerf 3.1 offers a new feature that creates a PID file when it starts.

[tynick@some-host ~]$ ls
[tynick@some-host ~]$ iperf3 -s -D --pidfile /home/tynick/iperf.pid
[tynick@some-host ~]$ ls
iperf.pid

We can cat that file to get the iPerf PID.

[tynick@some-host ~]$ cat iperf.pid
5141

Just to show you that it is the iPerf PID…

[tynick@some-host ~]$ ps aux | grep iperf | grep -v grep
tynick    5141  0.0  0.0   9612   332 ?        Ss   17:48   0:00 iperf3 -s -D --pidfile /home/tynick/iperf.pid
[tynick@some-host ~]$

It also removes the PID file when the process dies.

[tynick@some-host ~]$ ls -la /home/tynick/iperf.pid
-rw-------. 1 tynick tynick 5 Jul  8 18:14 /home/tynick/iperf.pid
[tynick@some-host ~]$ pkill iperf
[tynick@some-host ~]$ ls -la /home/tynick/iperf.pid
ls: cannot access /home/tynick/iperf.pid: No such file or directory
[tynick@some-host ~]$

Pretty useful feature if you are looking to automate iPerf or need a way for a monitoring check to see if iPerf is still running.

Stopping iPerf Server in Daemon Mode

Eventually you will probably want to stop the iPerf Server running in Daemon Mode. This is as simple as running pkill iperf.

[tynick@some-host ~]$ iperf3 -s -D
[tynick@some-host ~]$ ps aux | grep iperf
tynick    5056  0.0  0.0   9612   332 ?        Ss   17:10   0:00 iperf3 -s -D
tynick    5058  0.0  0.0 112708   944 pts/0    S+   17:10   0:00 grep --color=auto iperf
[tynick@some-host ~]$ pkill iperf
[tynick@some-host ~]$ ps aux | grep iperf
tynick    5061  0.0  0.0 112708   940 pts/0    S+   17:10   0:00 grep --color=auto iperf
[tynick@some-host ~]$

You could also use the PID file you just learned how to make. Here is a quick one-liner to kill iPerf if you are using a PID file.

kill $(cat /home/tynick/iperf.pid)

[tynick@some-host ~]$ iperf3 -s -D --pidfile /home/tynick/iperf.pid
[tynick@some-host ~]$ ps aux | grep iperf | grep -v grep
tynick    5253  0.0  0.0   9612   332 ?        Ss   21:20   0:00 iperf3 -s -D --pidfile /home/tynick/iperf.pid
[tynick@some-host ~]$ kill $(cat /home/tynick/iperf.pid)
[tynick@some-host ~]$ ps aux | grep iperf | grep -v grep
[tynick@some-host ~]$

As you can see, iPerf is an extremely useful networking tool.

If you’d like to do some more learning, here are the iPerf command line options in their entirety from the iPerf documentation page

iPerf General Options

iPerf Server Options

iPerf Client Options

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