Is there any option in linux to change the NIC duplex settings from half to full?

I am using redhat linux 9.

If you have a switch it should be a simple matter.

Something like mii-tool 100baseTX-FD eth0 should do it.

How to make it permanent every time my system restarts?

Depending on your switch-if you have it set to 100, don't set the switch end and let them autonegotiate.

You will get better performance. otherwise, place it in your rc.local file is always simplest.

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