Source and destination address selection
If you try to connect to or ping a host that has an AAAA DNS record (e.g. google.com), we will always try to use that regardless whether there is any (global-scope) IPv6 address configured. Thus on a host with only IPv4 connectivity this will fail.
There are cases where we are faced with several alternatives for both source and destination address, such as:
- IPv4 vs IPv6
- link-local vs. global-scope address (both IPv4 and IPv6)
We need to devise some strategy for proper selection of source and destination address. For inspiration how this problem can be approached see
(I don't think it's necessary to implement *exactly* this way, i.e. we don't necessarily need a configurable policy table).