IPv6 duplicate address detection (DAD)

[Michael Graham]

Hi all,

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!

And can now get an IPv6 address on my laptop by disabling doing:

echo 0 | sudo tee /proc/sys/net/ipv6/conf/*/accept_dad

However, I don't know what to do next... this seems like a bug.  But I have no idea what package is should raise it against? Is it ifconfig, networkmanager, linux, systemd?

[Darac Marjal]

On Wed, Mar 25, 2015 at 01:49:31AM +0000, Michael Graham wrote:
> Hi all,
>
> 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).

Duplicate Address Detection works by the interface joining a multicast
address and sending a Neighbour Solicitation message (similar to ARP's
"Who has this address?" message). If the interface either gets a
Neighbour Advertisement reply, or sees a Neighbour Solicitation message
with the address it wants, then that implies that some other interface
on the network wants that address and there is a clash.

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).

Another alternative is a network issue - a routing loop or something -
causing the interface to see its own NS message. I would suggest using
wireshark or similar to listen for ICMPv6 messages and see what happens
when the interface comes up.

>
> And can now get an IPv6 address on my laptop by disabling doing:
>
> echo 0 | sudo tee /proc/sys/net/ipv6/conf/*/accept_dad

If you're happy that you're NOT actually going to get duplicate
addresses on your network, then disabling DAD might be an acceptable
option.

[Sven Hartge]

Darac Marjal <maili...@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.

> 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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[Darac Marjal]

On Wed, Mar 25, 2015 at 10:57:46AM +0100, Sven Hartge wrote:
> Darac Marjal <maili...@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 :)

[Michael Graham]

On 25 March 2015 at 05:57, Sven Hartge <sv...@svenhartge.de> wrote:
> 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.

There are no other devices on the network just me and my AP, I don't have
a loop in my network either, I don't see duplicate packets for anything else.
But I will check this when I get home just in case.

The DAD has in the past triggered issues in Debian because the ifupdown
scripts weren't for the IPv6 address to transition from tentative [1].

For now I've decided this is an issue with NetworkManager and raised an
issue [2], hopefully the devs can get to the bottom of this.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=705996
[2] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=781143

Cheers,
--
Michael Graham <oobe...@gmail.com>

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