[OpenWrt-Devel] [BB-rc3] Disabling DHCPv6 also disables IPv6 SLAAC

[Baptiste Jonglez]

Hi,

The default behaviour of BB is to hand out statically assigned IPv6
addresses through DHCPv6.  To disable this behaviour and only keep SLAAC
through RA, I disabled dhcpv6 in /etc/config/dhcp:

config dhcp 'lan'
       option interface 'lan'
       option start '70'
       option limit '64'
       option leasetime '1h'
       option dhcpv6 'none'
       option ra 'server'


However, that didn't work as expected on BB-rc3.  The router was then
sending empty RAs, without any prefix or route.  Here is a RA captured on
a client with radvdump:

interface eth0
{
        AdvSendAdvert on;
        # Note: {Min,Max}RtrAdvInterval cannot be obtained with radvdump
        AdvManagedFlag off;
        AdvOtherConfigFlag off;
        AdvReachableTime 0;
        AdvRetransTimer 0;
        AdvCurHopLimit 0;
        AdvDefaultLifetime 0;
        AdvHomeAgentFlag off;
        AdvDefaultPreference medium;
        AdvSourceLLAddress on;
        AdvLinkMTU 1453;

        DNSSL lan
        {
                AdvDNSSLLifetime 1200;
        }; # End of DNSSL definition

}; # End of interface definition



For the sake of comparison, here is a RA from the same router with the
default configuration, i.e. DHCPv6 enabled (the IPv6 prefix is
obfuscated on purpose):

interface eth0
{
        AdvSendAdvert on;
        # Note: {Min,Max}RtrAdvInterval cannot be obtained with radvdump
        AdvManagedFlag on;
        AdvOtherConfigFlag on;
        AdvReachableTime 0;
        AdvRetransTimer 0;
        AdvCurHopLimit 0;
        AdvDefaultLifetime 1800;
        AdvHomeAgentFlag off;
        AdvDefaultPreference medium;
        AdvSourceLLAddress on;
        AdvLinkMTU 1450;

        prefix 2001:db8:80ce:9201::/64
        {
                AdvValidLifetime 7200;
                AdvPreferredLifetime 1800;
                AdvOnLink on;
                AdvAutonomous on;
                AdvRouterAddr off;
        }; # End of prefix definition


        prefix fdd1:b9fe:7201:1::/64
        {
                AdvValidLifetime 7200;
                AdvPreferredLifetime 1800;
                AdvOnLink on;
                AdvAutonomous on;
                AdvRouterAddr off;
        }; # End of prefix definition


        route 2001:db8:80ce:9200::/56
        {
                AdvRoutePreference medium;
                AdvRouteLifetime 7200;
        }; # End of route definition


        route fdd1:b9fe:7201::/48
        {
                AdvRoutePreference medium;
                AdvRouteLifetime 7200;
        }; # End of route definition


        RDNSS 2001:db8:80ce:9201::1
        {
                AdvRDNSSLifetime 1800;
        }; # End of RDNSS definition


        DNSSL lan
        {
                AdvDNSSLLifetime 1200;
        }; # End of DNSSL definition

}; # End of interface definition



I would expect prefixes and routes to be announced in the RA, even when
DHCPv6 is disabled.  Is this the intended behaviour?

Also notice that some options are sligthly different in both cases
(different MTU, etc).  Note that the IPv6 upstream is a L2TP tunnel, whose
MTU is equal to 1453.

Thanks,
Baptiste
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