pfSense

There is already something along these lines that will auto-assign interfaces if there is a mismatch. I believe it was disabled after problems arose and the contributor didn't respond. The code should still be there.

There's a lot of potential risk in doing this, the previous discussion resulted in some things to considerably limit risk. The former way relied on checks to ensure it wasn't hosing a valid config that had just lost a NIC due to hardware problems or other changes. It would only auto-assign if:
1) The assign interfaces prompt timed out (it waited 10 seconds)
2) trigger_initial_wizard is set (indicating reset to defaults or a new install)
3) The default config.xml interface assignments were in place

This sounds like over-engineered.
What is needed is just to make sure that the interface at boot matches the one before reboot.
If this is a first boot than just auto assign on embedded.

The other problems can be solved by just checking if the interface exists before doing any operation on it.

I'd like to add a situation that I recently eperienced: One interface was gone (just a usb stick for WLAN) and network interface mismatch ran on boot.
I would have expected to be asked if I care about the missing interface and done, but instead I had to reconfigure EVERYTHING, I had to reassign LAN and WAN, though they were still there and did not change! Because I also have several VLANs on my LAN, I finally gave up and restored a backup.
To keep it short: Please, don't let the user reconfigure already configured things!

Also I would like it if pfsense would continue with the known interfaces, without guessing any interfaces and then show the errors on the webinterface and in addition on the console prompt that's shown after successfull boot. Better for embedded systems and also reduces downtime for e.g. internet access: in my case booting stopped because WLAN was gone and therefore also internet access wasn't possible from system using ethernet. so the network interface mismatch stuff made my problem worse than it was.

As a compromise, a good solution until there is maybe more time to work on this would be to offer the user the choice to continue with the found / still available interfaces without reconfiguring them. I don't know the current code, but I guess just keeping the configuration for existing interfaces should be quite easy to implement? Showing stuff on webinterface etc. could then maybe added later.

about restong the backup, that only works if that yet doesn't include the missing interface. my wlan usb stick was quite new, so that was no problem. but if my wlan stick fails in a year, I won't have a backup without usb wlan stick interface and up to date firewall rules! so then I would have to reconfigure everything manually on the console ):