[ipv6hackers] Question on DHCPv6 address assignment

Tjeldnes, Terje Terje.Tjeldnes at get.no
Sat Feb 1 18:12:54 CET 2014

On 01.02.14 11:44, "Tore Anderson" <tore at fud.no> wrote:

>* Fernando Gont
>> 1) What's the pattern with which addresses are generated/assigned? Are
>> they sequential (fc00::1, fc00::2, etc.)?  Random? Something else?
>The DHCPv6 server in an Cisco EPC3928 (but probably also EPC3925) will
>by default hand out EUI-64 based host identifiers. So it probably looks
>at the MAC address of the incoming requests, runs the EUI-64 algorithm
>on it, and presents that back in the IA_NA offer. To thoroughly confuse
>the hosts it also advertises the prefix in RA with the A bit set.

EPC3925 (and I'm assuming EPC3928 but haven't tested myself) also provides
random IA_TA-addresses to clients that request them.

>I suggested to my ISP that they ask Cisco to change this behaviour,
>whether or not they did I don't know.

We did, but we have yet to receive a fix from Cisco. To be fair, we
consider it non-critical so we haven't nagged too much about it.

1) Cisco Network Registrar will provide randomized addresses, but
it's configurable to allow requested addresses even though it hasn't
previously been leased to the client (off by default) or to use the
interface identifier (also off by default) as well. I don't see any option
to configure sequential addresses for anything other than IA_PD (which
makes a lot more sense).

2) The addresses are stable for as long as the lease is valid (in other
words until valid-lifetime expires). If it expires, it is deleted and a
new address is generated on the next SOLICIT from the client.

Terje Tjeldnes
IP Networks
http://www.get.no <http://www.get.no/>

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