Network Troubleshooting

Last Updated October 2nd, 2026 for Rocky Linux 9.

Note: One of the steps requires administrative access.

General First Step

1. Install iperf3. This will be your network testing tool.

sudo dnf install -y iperf3

You can check if it's properly installed by running iperf3 and seeing any output. If it says 'command not found', then it didn't install or doesn't exist.

Server Side (where to test the connection from)

2. After installing iperf3, run the following command to start your origin server for testing link speeds. This will need an open port on that machine.

iperf3 -p {your_port_of_choice} -s

Client Side (where to test the connection with)

2. Run the following command to find the network you want to test.

ifconfig

Local networks general start with `10.`, for example, `10.70.***.**`. Pick the relevant one to test and copy it's network device name (such as eno2).

3. Run ethtool with the name of the network device as an argument.

ethtool {network device name}

example: `ethtool eno2`

Check the 'speed' variable. This is how fast that network device is made to be. NOTE: This is in gigaBITS, not gigaBYTES. One gigabyte is eight gigabits, so keep this in mind!

4. Run ethtool with the '-i' flag to get the lspci slot identifier.

ethtool -i {network device name}

example: `ethtool -i eno2`

Here, you want to copy the last six numbers of 'bus info'. For example, if your bus info is 0000:65:00.1, you just copy '65:00.1'.

5. Get the LnkSta and LnkCap values from lspci.

sudo lspci -vv -s {bus_info_value} | grep “Lnk”

The LnkSta value (in GT/s) shows you how fast the PCI port is, and LnkCap shows you how fast it can go. When things are perfect, these two values should be the same.

6. Run iperf3 to get active network speed data.

iperf3 -c {hostname/local ip of server machine} -P 4 -p {port of machine} -t 30

All the data we've gathered earlier is data in theory. With this, we can actively test the actual network speed between our systems.

The speed can be a little less than what's shown on ethtool, that's normal. For example, a network device may display having speeds of 10Gbi/s but in actuality it's closer to 9.40Gbi/s