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Re: IPv6 duplicate address detection (DAD)



On Wed, Mar 25, 2015 at 10:57:46AM +0100, Sven Hartge wrote:
> Darac Marjal <mailinglist@darac.org.uk> wrote:
> > On Wed, Mar 25, 2015 at 01:49:31AM +0000, Michael Graham wrote:
> 
> >>    I've been trying to understand what I should do about my current IPv6
> >>    wows.
> >> 
> >>    I have an IPv6 enabled network but when on a clean boot I don't get an
> >>    IPv6 address (in Jessie BTW), I've tracked this down to this message in
> >>    dmeg:
> >> 
> >>    IPv6: wlan0: IPv6 duplicate address fe80::fef8:aeff:fe7b:115f detected!
> 
> > This is the link-local address for the device with a MAC address of
> > FC:F8:AE:xx:xx:7B:11:5F (where xx:xx is obscured by the IP address).
> 
> MAC addresses are only 6 bytes big. The FF:FE is merely inserted into the
> middle and the 1st bit of the first byte of the MAC is flipped to
> generate the host part of the IPv6 address.

Oh, yes. Whoops :)

> 
> > So, perhaps the DAD is correct and there is another device on your
> > network trying for the same address (note that the address space of
> > autoconfigured link-local addresses is smaller than that of MAC
> > addresses - due to the FF:FE in the middle - so there IS a slim chance
> > of a valid collision).
> 
> No, there is not chance of a collision. If DAD thinks there is another
> device with the same address (and hence MAC) then there _is_ another
> device with the same address or another anomaly (network loop, like you
> already wrote).
> 
> Using a sniffer to clearly see, what is going on would also be my next
> step in diagnosing this.
> 
> Grüße,
> Sven.
> -- 
> Sigmentation fault. Core dumped.
> 
> 
> -- 
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