I had a similar issue including the vanishing network-rules.cfg after each restart...
(btw, for this everybody replying to a previous thread of mine insists i have something installed that removes it, which is actually not the case. i still think that its unraid itself removing the file due to reasons during startup after reading it.)
The machine has 2 NICs, the onboard (which whyever cannot be disabled in the bios) and the 10GBit Nic
Here is my network-rules.cfg:
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="e4:1d:9c:92:ce:44", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="22:48:5c:05:16:45", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth1"
It is merely to make sure the 10GBit is eth0 because Unraid has some special handling for eth0 as we all know...
As well, I made sure its there before restarting the server (user script just writes it to /boot/config each hour )
hope this helps you as well