[External] Re: IPv6 uptake

Mon Feb 19 16:22:10 UTC 2024

On Mon, Feb 19, 2024 at 8:08 AM Hunter Fuller <hf0002+nanog at uah.edu> wrote:
> On Mon, Feb 19, 2024 at 9:17 AM William Herrin <bill at herrin.us> wrote:
> > There's also the double-ISP loss scenario that causes Joe to lose all
> > global-scope IP addresses. He can overcome that by deploying ULA
> > addresses (a third set of IPv6 addresses) on the internal hosts, but
> > convincing the internal network protocols to stay on the ULA addresses
> > is wonky too.
>
> In the real world today, most applications seem to use mDNS and
> link-local addresses to keep this connectivity working. (I am guessing
> Joe's Taco Shop uses something like Square, and just needs his
> register to talk to his printer. This already works with mDNS and
> link-locals today.)

Hi Hunter,

Yes and no. The client application has to be programmed to understand
link-local addresses or it can't use them at all. You can't just say
"connect to fe80::1." Even if there's an fe80::1 on your network, it
doesn't work. The client app has to also carry the interface identity
into the stack (e.g. fe80::1%eth0) in order to use it.

IPv6 link local addresses can't be expressed as a regular DNS target
the way ULA and RFC1918 addresses can. No way to add that "%eth0" to
the AAAA record. They only work with multicast DNS because the
matching interface is known based on which interface was used to send
the multicast query.

And of course link local is -strictly- link local. If you want one
subnet to communicate with another, you have to do it with global
scope or ULA addresses.

Regards,
Bill Herrin

-- 
William Herrin
bill at herrin.us
https://bill.herrin.us/