[CentOS] Diagnosing IPv6 routing

Wed Apr 29 02:16:09 UTC 2020
Chris Adams <linux at cmadams.net>

Once upon a time, Kenneth Porter <shiva at sewingwitch.com> said:
> I'm using OpenWrt at home and it's working mostly fine there. Except
> with my Android phone. I'm not getting a DNS setting for V6, but I
> do have the setting in the router's config file. The Win10 clients
> work fine, though. Apparently Android has issues with DHCPv6, and
> I'm betting it's interfering with my SLAAC config.

Yeah, Android refuses to support DHCPv6, so you either have to have IPv4
DNS or SLAAC.  I have IPv4 DNS on my home network, so don't have an
issue.  I did just look, and OpenWRT is putting the DNS option for SLAAC
in the RA, so that should work too (but I think that's something
relatively recent for OpenWRT).

I didn't get that you have a static assignment (presumably a business
connection) - they may not do RAs on that (I don't at my ISP job).
Business connections (or at least, connections with static assignments)
tend to operate differently.  For that, they should have given you a
static v6 address and gateway, just like they did for v4.

So... there's one thing you could try (but probably won't work to a
regular router interface) - see if there's a MAC-derived fe80::/64
link-local address on their end.  Get the MAC of the gateway from the v4
ARP entry and expand it to a LL v6 address as fe80::xxxx:xxff:fexx:xxxx
(split the MAC, put ff:fe in the middle).  Try ping6 that address with
%em2 appended (have to append the interface when using link-local
addresses).  I doubt it'll work, since I know Juniper (which IIRC AT&T
likes) doesn't assign those (I can't remember for sure about Cisco and
don't have a handy test target).

And frankly, giving you a /56 is pretty crappy, since ARIN rules say to
give every site a /48.  I'd only do a /56 for a home connection prefix
delegation.  But, that's AT&T! :)

Chris Adams <linux at cmadams.net>