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Sophos Firewall Troubleshooting – iPerf Speedtest

Sophos firewalls are one of the firewalls on the market that offer a lot of performance for the price. At least if you choose a firewall that is not too small, but rather one that is properly sized.

Nevertheless, it can happen that you get the speed on the firewall due to a misconfiguration, but no longer behind the firewall. In this case, it is helpful not to simply go to any speed test website, but to perform a clean test. Or you want to test the performance of a VPN connection. Here iPerf helps.

At you can download the tool. The tool is available from Windows to macOS, Linux, Android and iOS.

To measure the performance of a route, you need two things. A computer that acts as a Server and, consequently, a client that loads data from the Server.

iPerf Server

The following command starts iPerf in Serverand waits for the client to connect.

iperf3 -s

⚠️ Remember the firewall, which may be active on a Windows system, for example.

If you want to test e.g. the internet speed, you can also take one of the Public Server nearby. In my experience, the ones on the site are often offline and I use the list on the iPerf Public Server on GibHub here.

iPerf Client

On the client you can start a speed test with the following command.

iperf3 -c

More commands

Of course there are several parameters for iPerf. Here are a few often used examples.

Test UDP performance

With the parameter -u you can do the speetest from above, but with UDP.

iperf3 -c -u

Tests for a 1Gbit line

We often test a 1Gbit line to see where the limitations are here.

  • -t: How many seconds should the test be performed. (default 10 seconds)
  • -d: Execute a simultaneous bidirectional test
  • -u: Using UDP instead of TCP
  • -b: Target bandwidth 1G (default 1 Mbit/s). This setting requires UDP (-u).
  • -p: Server(default 5001)
iperf3 -c -t 30 -d -u -b 1g -p 5200