ipv6, stateful config and non-default prefixlen

[Eugene M. Zheganin]

  Hi.

On 18.03.2011 23:56, sthaug at nethelp.no wrote:
> Are you using IA_PD or IA_NA on your DHCPv6 server?
Since I didn't configure anything on a DHCPv6 server about PD, I assume 
I'm using NA.

> 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.
I don't see a relation between these 2 things. Prefix delegation is used 
to assign prefixes to client _routers_, without knowing about the 
topology. I'm configuring a _workstation_. I don't need a prefix to 
assign addresses to other computers, I don't have a network behind this 
workstation, I need to know about my prefix, and I have that information 
in ndp cache, although it is somehow useless.

> As mentioned, DHCPv6 IA_PD gives you a prefix. And ISC dhclient can ask
> for it, see the the -P option.
"The Prefix Delegation options provide a mechanism for 
automateddelegation of IPv6 prefixes using the Dynamic Host 
Configuration Protocol (DHCP). This mechanism is intended for delegating 
a long-lived prefix from a delegating router to a requesting router, 
acrossan administrative boundary, where the delegating router does not 
require knowledge about the topology of the links in the network to 
which the prefixes will be assigned." And that's from RFC.

> 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).
That seems to be a mistake. Look what explanation I found in the ietf 
maillist:

"Interface addresses are completely SEPARATE from routing 
information.Please do NOT confuse the two. This has been a source of 
confusion formany IPv6 implementors who know IPv4.The configuration of 
addresses for an interface MUST NOT be tied to the configuration of 
prefix information for routing. Just because a prefix is on a link, does 
not mean the interface necessarily has an address for that prefix (it 
may have none, 1, or many). Just because an interface has an address, 
does not mean that the system has any prefix information for a prefix 
that "contains" that address. Prefix information and addresses assigned 
to interfaces are completely separate."

So it's just an address. Not a /128, just an address.

> You should  *not*  expect to reach other computers on the link through such a /128
> address
So, in other words, DHCPv6 is useless. No, I don't think so. I have a 
bunch of windows on the same link, working with the same DHCPv6 server, 
and doing just fine. And that's sad, because I used to think that 
FreeBSD is always a queen of the network, far ahead of the 
non-truly-networked OS bunch. I'm still hoping that this /64 prefix 
issue is related to my low knowledge.

P.S. And I know that autoconfiguration won't work on a link with /120. 
And of course, THAT is the reason why I'm using the DHCPv6.

Eugene.