Reliably detecting the absence of IPv6

[Philip Prindeville]

Hi,

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.

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

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?

Put something into ‘config system’ into /etc/config/system?  Like ‘disable_ipv6 boolean`?  Or… what might be better?

Thanks,

-Philip