IPv6 router advertisement

Mihai Moldovan ionic at ionic.de
Tue May 5 17:53:25 UTC 2009

Sorry for not responding so long, I've been putting v6 aside for a while
because of compilation and configuration problems and thought it might
be cool to get it finally working, until...

* On 20.04.2009 19:37, David W. Hankins wrote:
> On Sun, Apr 19, 2009 at 09:38:54AM +0200, Mihai Moldovan wrote:
>> radvd is due to the small subnet size not working of course, so not
>> employable.
> I thought radvd can announce the prefix with the SLAAC bit cleared?
> The currently (deployed) IPv6 model is that you MUST have an RA that
> delivers the default router and locally attached prefix information.
> So working out how to get radvd to emit what you want is the key.

Currently, my radvd is set up this way: (information acquired by
radvdump on one machine which is of course not the server/router itself)

# radvd configuration generated by radvdump 1.1
# based on Router Advertisement from fe80::21b:21ff:fe2a:c4fe
# received by interface eth0

interface eth0
    AdvSendAdvert on;
    # Note: {Min,Max}RtrAdvInterval cannot be obtained with radvdump
    AdvManagedFlag off;
    AdvOtherConfigFlag off;
    AdvReachableTime 0;
    AdvRetransTimer 0;
    AdvCurHopLimit 64;
    AdvDefaultLifetime 30;
    AdvHomeAgentFlag off;
    AdvDefaultPreference low;
    AdvSourceLLAddress on;

    prefix 2001:41d0:1:678e:abcd::/96
        AdvValidLifetime 2592000;
        AdvPreferredLifetime 604800;
        AdvOnLink on;
        AdvAutonomous on;
        AdvRouterAddr off;
    }; # End of prefix definition

}; # End of interface definition

Also, the DHCP server is set up to give out one fixed address out of
this very /96 pool. (ISC DHCPd of course! :))

subnet6 2001:41D0:1:678E:ABCD::/96 {
  default-lease-time        3600;
  max-lease-time        21600;

  host sui-wlan {
    host-identifier option dhcp6.client-id
00:01:00:01:4a:00:49:71:01:72:e6:b7:6c:21; # < seems like
https://fedorahosted.org/dhcpv6/ is generating some other DUID than ISC
dhclient-4?! > 00:01:00:01:11:92:FB:91:00:90:F5:48:96:D0; # default MAC
> 00:90:F5:48:96:D0;
    fixed-address6 2001:41D0:1:678E:ABCD::2;

Now to come to some terms...

> Note however that at least dhclient, and one or two others, DHCPv6
> software will presume a /64 prefix for any offered addresses, in order
> to work around a bug-in-the-protocol where an RA failure leads to the
> addressed clients being unable to speak to each other.
Exactly this is what is happening. I've tried all to me known DHCPv6
clients, that is ISC dhclient and dhcpv6 (see URL above) - dhcpcd yet
has no v6 support, though it is to be included soon according the
project owner.

This behavior is of course causing a lot of trouble around - although
the address is given out correctly, other hosts in the same /64 subnet
cannot be accessed, but you already now this. :)

Another problem is that RA totally refused to work. Although I can see
advertisements via radvdump, nothing happens automagically. I don't know
how to track this, I even have got absolutely no evidence of ra in my
kernel log ring buffer output (Linux for instance), that is, in

I have even tried removing all IPv6 addresses and routes and adding the
IP manually via ip -6 addr a 2001... dev eth0 and waiting some minutes
to see if ra is working then, but nothing happened. The only way seems
to be adding the route by hand which... sucks.

> There's currently a default gateway and prefix option draft before the
> DHC WG, which will help us resolve the above bug-in-the-protocol and
> eliminate the strict requirement for RA
I've stumpled upon an RFC implementing the routers option in DHCPv6, but
can't remember the URL anymore - is this the thing you refer to? :)

It would be perfectly cool to have a working setup which is independant
from autoconfig - either my current one is wrong (both dhcp and ra wise)
or I'm doing some things completely wrong.

Best regards,

Mihai Moldovan

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