Reliably detecting the absence of IPv6

[Jonas Lochmann]

Am Wed, Dec 10, 2025 at 08:44:00PM -0700, schrieb Philip Prindeville via openwrt-devel:
> We had an issue where residential customers on US ISP’s with IPv4 (even if it’s NAT’d) would see a lot of logging from named about not being able to reach IPv6 name servers to resolve AAA records.

Are you talking about A or AAAA records?

> The workaround is to run bind with ‘-4’ to eliminate this.

You can query for IPv6 addresses using IPv4 DNS requests and the other
way around.

> I previously tested with -z “$(ip -6 -o route show default)” but as someone pointed out, if at the time that Bind starts up, someone later brings up an IPv6 default route (say via DHCP or a tunnel), then this caused a denial of service.
> 
> I don’t imagine this is the only case of having to know if IPv6 is deliberately absent on a given router or not.
> 
> What is the best canonical way to solve this?

Happy Eyeballs - just try it and see if it works. It would expect bind
to do exactly that.

You can ask the routing table or ask netifd. Both are observable (change
notifications are available) so that you could adjust your configuration
depending on the current status. But the existence of an IPv6 default
route does not imply that it works. mwan uses configurable connectivity
checks to solve that issue - in most setups, this are pings to some
predefined servers.