ipv6, stateful config and non-default prefixlen

[sthaug at nethelp.no]

> I'm trying to get a working freebsd workstation with an ipv6 network 
> where addresses are received from DHCP.

Are you using IA_PD or IA_NA on your DHCPv6 server?

> ATM my IPv6 setup copies the IPv4 layout with vlans and /24 masks, so 
> I'm using /120 prefixes.
> Is that even possible ?
> As the Handbook lacks any information about such setup, I decided to ask 
> for help here.
> 
> Currently I'm having problems with prefixes.
> I'm using the net/isc-dhcp41-client port (as the dhclient from the base 
> system is unable to work with DHCPv6).
> I have a working rtadvd on a router, and a working DHCPv6 setup.
> But my client FreeBSD still gets  64 prefix length, with or without 
> running rtsold.

rtadvd can give you the default router.

DHCPv6 IA_NA gives you a single /128 address and no netmask.

DHCPv6 IA_PD gives you a prefix (with the netmask of your choice), but
I don't know whether FreeBSD can (easily) use this.

> Since DHCPv6 doesn't advertise any routing information at all (including 
> the prefix information),

As mentioned, DHCPv6 IA_PD gives you a prefix. And ISC dhclient can ask
for it, see the the -P option.

> and its only function is to provide the client 
> with an address, I assume that the prefix information has to be received 
> from the router advertisements.

A router advertisement (RA) gives you a default router, and 0 or more
prefixes - except that rtadvd seems unable to send RAs with 0 prefixes.

> I am currently looking at the wireshark 
> dump, and it appears that my workstation is receiving such 
> advertisements. However it doesn't affect the 64 prefix displayed in 
> ifconfig, and all operation involving the nodes outside the actual link 
> are simply not working.

If you use DHCPv6 IA_NA, you receive a single /128 address (it is /128
by definition, the DHCP answer doesn't include a netmask). You should
*not* expect to reach other computers on the link through such a /128
address.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no