Continuing a conversation from here as an RFC for implementation (per @richb-hanover-priv in this comment).
Abstract:
Method of automatic LAN network reassignment upon conflict with WAN
Purpose:
Prevent a conflict where the LAN and WAN occupy the same or overlapping subnets.
This issue will cause routing to fail. Most often impacts novice/new OpenWrt users when they connect an OpenWrt device to an existing network using the common 192.168.1.0/24 subnet.
Can also impact road-warrior type users with travel routers since the upstream network may be unknown and may change frequently based on the location.
Assumptions:
- Network configuration exists with one LAN and one WAN.
- WAN uses DHCP client or PPPoE for address assignment.
- LAN uses static IP.
- Upstream network will be a properly configured RFC1918 IP such that there will always be some RFC1918 address that is available.
- If upstream network is publicly routable or CG-NAT, this issue will not present.
Basic Logical Flow:
- On boot, do not bring up LAN interface until WAN is up with an address or an n-second timeout has occurred without a WAN address assignment.
- If WAN is not up, continue with ifup for LAN.
- The following occurs when WAN comes up (maybe as a hotplug event):
-- Evaluate the WAN's assigned IP address AND subnet
---- If upstream is publicly routable or CG-NAT, do nothing.
---- If WAN has RFC1918 address, calculate the upstream network range.
---- Compare the LAN (address + subnet) with that of the WAN network. If no collision/overlap occurs, do nothing.
---- If there is a conflict, assign an alternate IP/subnet to the LAN.
------- Bounce LAN related interfaces (ethernet and/or wifi, if enabled) to cause new DHCP client requests.
Basic Functional Configuration Elements:
- LAN subnet reassignment would only be triggered upon a conflict with the WAN.
- Preferred LAN address/subnet (default value 192.168.1.1/24)
- Alternate LAN address/subnet (default value: ideally any /24 from RFC1918 10.0.0.0/8 or 172.16.0.0/12 ranges)
- Enabled on the default installation of OpenWrt (some future major release version).
- Option to cover additional WAN networks (to allow multi-wan and/or VPN upstream coverage).
- Option to disable this feature.
- Possible feature: automatic update of the preferred LAN address if the user changes the LAN in /etc/config/network)
- User configurable alternate LAN addresses, list of any number of elements allowed.
- Option to enable random address assignment in the event that the above list does not prevent conflict.
Not Covered:
- VLANs/multiple LANs would not be covered.
-- It is likely a reasonable assumption that this is not common for a road-warrior travel router config, and most likely users implementing multiple networks would know to avoid conflicts. - No guarantees about forcing DHCP renewals after LAN address change.
-- Bouncing the LAN interfaces should work, but will not help for devices that are not directly connected (i.e. via a downstream switch or AP). - Cannot force reassignments of static IP based hosts (obviously).
- May cause issues with static DHCP host assignments.
Other:
- All configuration would be in a new config file (i.e. /etc/config/auto_lan_address or similar)
- Update documentation to indicate that https://openwrt.lan is the easiest way to access router
- Update documentation to indicate the default address for the alternate LAN address.
- Update documentation to describe options, parameters, operating principle of this feature.
Comments welcome, of course (that's why it is an RFC 
). But since this is the continuation of a previous topic (linked above), please see that thread for the historical discussion. My hope is that this specific thread will focus more on arriving at a go/no-go decision, technical implementation details and documentation. Obviously there is a political element here as far as changing a core OpenWrt behavior, so that is also fair game. But hopefully we won't end up with a duplicate set of comments here compared to the existing thread.