Re: Understanding IPv6-PD over PPPoE

Steve Hill <steve opendium com> writes:
On 15/06/2021 18:19, Bjørn Mork wrote:

Yes, that is buggy.  I wonder... I did hit a similar issue many many
years ago while testing IPv6 over PPPoE (which we didn't end up doing in
the end).

I'm not entirely sure how the default route is assigned - on ethernet
it would be assigned on receipt of the router advertisement, but is
that still the case with PPP, or is the default route negotiated by

The only defined options are Interface-Identifier and
IPv6-Compression-Protocol, ref

The interface-id is required to form a valid link-local address since
there is no EUI-48 or EUI-64

Address and routing is otherwise identical to any other Ipv6 interface.
I.e. SLAAC, DHCPv6, and/or static.

Maybe you are hitting something similar with your ISP?  The typical
symptom would be that DHCPv6 worked once again after the lease expires.
If this is the issue, then you can probably assign a static interface-id
on your end to work around it.

I've got 2 internet connections on the same ISP.

One of them was set up years ago and the ISP was trialling IPv6 at the
time, so I requested an IPv6 prefix.  This one is working ok (the
DHCPv6 advertise contains my prefix).  I have no idea whether the ISP
ever rolled out IPv6 to everyone by default, but this works for me
since I enrolled on the trial years ago.

The other connection is new.  The ISP sends router advertisements and
we get a default route, but the DHCPv6 advertise doesn't contain a
prefix or IP.  I think it's a reasonable assumption that the ISP never
enabled IPv6 for everyone by default and the result is that when you
send a DHCPv6 solicit you don't get a prefix on this connection.

Yes, that sounds likely. They shouldn't be sending RAs either then. But
clients blindly trying IPv6 over PPP is probably not a common problem.
I believe most clients disable IPV6CP by default. 

I think it would be too risky to try to be smart here.
How can the system know whthere there will be an address available
later?  Should it cache all the routing?  What if the system has an
address which isn't routable on the interface with the default route?
Installing the routes you receive is the only sane thing to do,

But it results in a configuration that causes real-world brokenness
(long timeouts waiting for connections that can never happen).

Whether you want to trust an RA or not is a local policy
decision. Forcing a new default policy is sure to break existing

If the default route from a specific router or interface doesn't work
for you, then ignore it.

The correct thing would probably be for the kernel to treat all global
addresses as unroutable if the machine only has link-local addresses, 
irrespective of the existence of a default route.  Is there a reason
why you would ever want to generate traffic with a link-local source
address and a global destination address?

You can reach any directly connected system by their global address
using a link-local source.  There is no way to know that a global
address is more than one hop away without trying.


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