[ipxe-devel] Fallback to IPv4 or disable IPv6

Andreas Fink afink at list.fink.org
Mon Nov 19 12:22:41 UTC 2018

> The DNS servers at $employer, at home and elsewhere have both IPv4 and IPv6 addresses and, as I recall, you'd have to be pure IPv6 to get an IPv6 DNS server.  Most machines are both IPv4 and IPv6 capable and tend to get IPv4 DNS servers and not IPv6 name servers.   With a scheme that uses the DNS server to decide whether or not to use IPv6 that would preclude using IPv6 completely.  On the other hand, it seems to make more sense to use the address families offered by DHCP/RA -- if you're given an IPv6 address, make AAAA queries etc.  And unless you're going to adopt Happy Eyes, then preferring IPv6 over IPv4 is better.

It's more than that. It's not only use IPv6 -OR- IPv4.  It's IPv6 -AND- IPv4.  It should attempt IPv6 first and if it fails try the IPv4 because the IPv6 networks of the target and the source might be interconnected with a IPv4 only backbone in which case a IPv4 end to end connection works but a IPv6 end to end doesn't (assuming both sides are dual-stack connected and the DNS has AAAA and A records).
At least that was the transition idea when IPv6 was designed because it was assumed the international backbones wont all have IPv6 from day one (and this is still the case). And thats exactly the reason why DNS servers usually stay on IPv4 addresses because they always can be reached from IPv4 hosts and dual-stack hosts.

Also there might be multiple hosts listed in the DNS, so the logic should try all AAAA records and if they have all failed try, all A records.

I'm just reading int the code on how its done currently to see how this can be improved.

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