ipv6, stateful config and non-default prefixlen

[Stephen Clark]

On 03/19/2011 04:34 AM, Eugene M. Zheganin wrote:
>  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.
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How does a router that is using dhclient to get delegated a address 
prefix from an upstream router obtain its default route, since dhcpv6 
can't provide a default route.
And since it is a router it can't get its default route from router 
advertisements from the upstream router?



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