Re: IPv6 default routes / NM vs. kernel autoconfig vs DHCP6

* Stuart D Gathman

> Ah yes.  When the RA router IPs are link local, you don't care about
> a prefix (link local IPs are a cool feature of IP6).  But *sometimes*,
> the router IP is global,

A route learned from an RA will *always* have a link-local next-hop.
RAs sent from other than link-local addresses are invalid. See RFC 4861
section 4.2. If this wasn't the case, the off-link prefix feature of
IPv6 simply couldn't work (as the link-local prefix is the only
guarantee that you will have a on-link route you can use to reach your
default route's next hop). You're only supposed to see global next-hops
if you're doing static config (disregarding stuff like BGP), and if
you're doing static config, you're not doing DHCPv6 - unless you're
*really* weird.

> and you have to assign a prefix to the DHCP6 IP to be able to talk to that router.

Not really, what you need to talk to a router with a global next-hop is
a on-link route. This is independent of adressing and thus independent
of DHCPv6. On-link routes are only advertised in RAs, specifically in
Prefix Information Options with the L-flag set.

> However, perhaps that is too unusual for NetworkManager to support.
> All the IP6 lans I am currently using have link local router IPs
> advertised by RA.  I think the one that had a global IP was on openwrt
> - and perhaps because I had set the prefix to /80 (to put the wireless
> clients on their own subnet).

If this indeed happened, it would be a serious bug in OpenWRT. Honestly
though, I think it's more likely that you set up a static default route
and forgot about it.

Best regards,
Tore Anderson

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