Tuesday, October 2, 2018

Using Iperf3 for bandwidth and througput test on Linux

At my current client we had to test the network speed between Azure and a local site.
Initially we used Rsync to copy files back and forth and although it gives an ok indication, it does not show the full line speed as Rsync encrypts data during transfer (among other things).

Iperf3 is a really easy to use and simple tool to test the bandwidth or line speed between to machines. This can be either Windows or Linux.

Below shows how to install and run Iperf3:

The test was done on RHEL 7.5 VMs:

1) Install Iperf3 on both the "client" and the "server":

# sudo yum install iperf3

2) Ensure that TCP traffic is allowed inbound on the "server":

# sudo firewall-cmd --zone=public --add-port=5201/tcp --permanent

# sudo firewall-cmd --reload

If you want to run test with UDP, then the following commands should be run:

# sudo firewall-cmd --zone=public --add-port=5201/udp --permanent

# sudo firewall-cmd --reload

3) Start Iperf3 on the "server" and put it in listen mode:

# iperf3 -s

4) Start Iperf3 on the "client" with -c and specify the IP of the server:

# iperf3 -c
(replace above IP with IP of your server)

That's it. This will run the test within around a minute and show the result, see screen dumps below.

When we ran the test, we could not max out the 1 Gbit line with TCP. So we changed to UDP and increased the packet size with the following command:

# iperf3 -c --bandwidth 10G  --length 8900 --udp -R -t 180

-c specifies to run command as client
--bandwidth emulates or assumes a 10 Gbit line (even if we just have 1 Gbit)
--length is the packet size
-R specifies to run the test in reverse. So instead of sending data, you are retrieving data. This is useful in that you can test both ways without changing the setup
-t is amount of seconds. We specified 180 seconds to let it run a bit longer.

Run Iperf3 --help for more options

Below shows a standard test between two VMs in Azure. Results are shown both on the client and on the server.

Tuesday, September 25, 2018

Fixing a corrupt /etc/sudoers file in Linux VM in Azure

I was editing the /etc/sudoers file with nano on a linux VM (RHEL 7.5) in Azure trying to remove or disable being prompted for a password every time I sudo.

I added the following to the file

root        ALL=(ALL:ALL) ALL
myadminuser     ALL=(ALL:ALL) ALL     NOPASSWD: ALL

Apparently that does not follow the correct syntax so immediately after I was not able to sudo. Below is the error meesage:

[myadminuser@MYSERVER ~]$ sudo reboot
>>> /etc/sudoers: syntax error near line 93 <<<
sudo: parse error in /etc/sudoers near line 93
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin

Since on the Azure VMs you don't have the root password, then you're stuck as the regular user do not have permissions to edit the sudoers file and you can't sudo to root.

You could mount the VM disk to another VM and then edit the file that way, but that is cumbersome.


From the Azure portal start Cloud CLI, choose Powershell

Run the following command to make /etc/sudoers editable by master

az vm run-command invoke --resource-group YOUR_RESOURCE_GROUP --name YOURVM --command-id RunShellScript --scripts "chmod 446 /etc/sudoers"

This gives the regular user permission to edit the file

with nano or VI undo the changes (i just deleted the NOPASSWD: ALL): 

nano /etc/sudoers (no sudo since you have access)

after edit, run the below command to configure default access to file.

az vm run-command invoke --resource-group YOUR_RESOURCE_GROUP --name YOURVM --command-id RunShellScript --scripts "chmod 440 /etc/sudoers"

I got the fix from the following link. Note that the syntax has changed a bit.

The useful thing about this command is that you can execute any command as root on your VMs as long as you have access to the Azure portal.

How to edit /etc/sudoers:

To ensure that you don't introduce the wrong syntax in the file, use the command to edit:


This will open the file using vi editor and if you use wrong syntax you'll get a warning/error.

See this link for a quick guide using vi editor