MAC-Address of eth0 changes on every boot.

I can confirm that the DT overlay method mentioned above also works for an Odroid M1 (Rockchip rk3568), with slight modifications:

• the ethernet0 alias is already created in the 'parent' dtb, no need to repeat that here
• the target is called gmac0 instead of emac

Create /tmp/rk3568-eth0-stable-mac.dts:

/dts-v1/;
/plugin/;

/ {
    compatible = "rockchip,rk3568";

    fragment@0 {
        target = <&gmac0>;
        __overlay__ {
            local-mac-address = [00 00 00 00 00 00];
        };
    };
};

Execute:

armbian-add-overlay /tmp/rk3568-eth0-stable.mac.dts

And reboot.

I tested on two identical Odroid M1 devices, they each got a different MAC address, which stayed stable over reboots.

I'm not sure about the T6. It has an rk3588, so at least the same SoC family. But its on-chip Gigabit network interface is not wired on the T6. Instead, two external 2.5Gbps Ethernet chips are added, each of which are connected via a PCIE lane. As such, it could maybe require a different solution.