IPv6 duplicate address detection

Bob Van Zant bob at veznat.com
Thu May 7 00:18:02 UTC 2009

I never actually tested the NIC pairing scenario and was only testing the
naive "add a new IP, fire off an unsolicited NA for that IP."

I did just test the NIC pairing and you are right, if the interface is down
when it is configured we don't run DAD. Which does seem weird but that would
be a different thread.

I guess that changes my question quite a bit. If you randomly fire off an
unsolicited NA right after configuring an interface should that cause a DAD


On 5/6/09 4:55 PM, "JINMEI Tatuya / 神明達哉" <Jinmei_Tatuya at isc.org> wrote:

> At Wed, 06 May 2009 15:49:45 -0700,
> Bob Van Zant <bob at veznat.com> wrote:
>>> I'm afraid we need clarification first...what do you mean by
>>> "reconfigure an interface with an IPv6 address"?  Do you mean adding a
>>> new IPv6 address to an interface?  If so, I'm not sure why you
>>> referred to the following part of RFC2461 (btw the RFC was updated by
>>> RFC4861):
>> We have a crude form of NIC pairing in our software. We allow someone to
>> logically pair two interfaces together. This is implemented by `ifconfig
>> down` both interfaces, configure them both the same, then `ifconfig up` the
>> primary interface. We then monitor the link state of the primary interface.
>> If the state goes to down, we `ifconfig down` the primary NIC and then
>> `ifconfig up` the secondary NIC. This has the effect of changing the link
>> layer address associated with a given IPv6 address. After we do this we send
>> out the unsolicited NA to update whatever switch we're plugged into.
> Okay, thanks for the explanation.  But I still don't understand one
> thing: why is DAD triggered for the address on the secondary NIC?
> Unless someone has changed the code recently, the FreeBSD
> (KAME-derived) IPv6 stack shouldn't trigger DAD for an existing
> address simply because the interface becomes 'up' (this behavior may
> be debatable per se, but that's a different question).  Did you
> perhaps make the address "tentative" by hand after configuring the
> address?
> ---
> JINMEI, Tatuya
> Internet Systems Consortium, Inc.

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