Use IPv4 default route for deciding IPv4 connectivity

[oskar456]

It is much easier from the operational POV to just observe IPv4 default route than to look out for particular IPv4 address type.

Think of for instance configuring IPv4 address to reach some IPv4-only devices on link (like offline file sharing between two mobile devices). Such devices might provide an IPv4 address via DHCP but without default route. In that case, CLAT should not turn off just because there is a non-link-local address configured. After all it is the default route, not the address, which is providing the connectivity.

[mstojens]

This is complicated. Sure the default is what matters usually but now we put the CLAT implementation in the position of needing to know when to ignore some IPv4 traffic, no? I think it's more complicated to conditionally translate some IPv4 traffic which is a new concept this effectively adds without saying so explicitly. If this is desirable, then we would need to say something like "enabling CLAT only applies to the default route" to account for the mentioned use case for enabling CLAT even though there's a non-link-local address plumbed.