Re: [v6ops] out-of-focus: why DHCPv6 breaks Android computers?

Alexandre Petrescu <> Wed, 30 October 2019 16:02 UTC

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To: Jen Linkova <>
Cc: "Rajiv Asati (rajiva)" <>, Ted Lemon <>, "" <>
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From: Alexandre Petrescu <>
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Date: Wed, 30 Oct 2019 17:02:44 +0100
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Subject: Re: [v6ops] out-of-focus: why DHCPv6 breaks Android computers?
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Le 30/10/2019 à 03:56, Jen Linkova a écrit :
> On Wed, Oct 30, 2019 at 5:32 AM Alexandre Petrescu 
> <> wrote:
>> At high level, it makes sense to require to allow simultaneous DHCP
>> and SLAAC usage on a same subnet; a private email suggests the
>> same. With that, Android and Windows would live ok side-by-side on
>> same subnet.
> I'm still not sure I understand why you need DHCP for
> Windows...Unless you got that Windows in an antique shop...;) I have
> plenty of Windows machines in the network (in an IPv6-only one as
> well).

Well, my Windows is very new from Dell, not from antique.

My Windows is on DHCPv4-only at work.  At home it works ok on DHCPv6 as 
well.  At home it is a mixed IPv4-IPv6.

My Windows could stop doing DHCPv6 if I request it to, but by default it 
does DHCPv6 in addition to SLAAC.

My Windows could run only SLAAC if I asked it to.

My CPE does IPv6 and SLAAC by default, and DHCPv6 is an option off by 

So - do not worry.

>> Further, thinking about how to implement the req, one would wonder 
>> whether the prefix in PIO with A flag set in an RA with M set,
>> would be the same as the prefix used by the DHCP Server to form and
>> deliver addresses?
>> (if yes, I think that is difficult to achieve: (1) difficult to put
>> same prefix in the software implementing RA sending, and in the
>> DHCP server connfig files
> Could you elaborate on this one? How is "putting the prefix in the 
> software implementing RA sending" different from "configuring a
> prefix on a router interface"? It's like saying 'difficult to
> configure the same IPv4 or IPv6 prefix on the router interface and on
> DHCP server'

I meant to say : 'to maintain'.  If I change something in the DHCP conf 
files then I have to change the same in the RA conf files, to make sure 
the same prefix is present on both.

But, probably some software packages do that ok.

>> and (2) difficult to make sure the Server does not form an address
>> for a Client, address that a Host has already formed in the same 
>> prefix).
> That's could be done to the some degree of confidence - if the pool
> is using the first addresses in the prefix (let's say, the highest
> 50 bits if the interface ID set to 0) then it's rather unlikely that
> such interface ID would be generated by SLAAC. However that's a good
> point, thanks for another item in my collection of 'why I don't need
> DHCPv6' reasons ;))

:-) that starts to sound like being against x in the a vs b discussion.

>> (because of that reason, I think that trying to implement that 
>> requirement would lead to designating a prefix for SLAAC and
>> another prefix for DHCP; that may sound a little bit as a waste).
> I'm sure operating such a network will

Sure, however, one might need this behaviour.

I think that I might need the DHCPv6 server on the CPE.  The reason I 
might need it is because I might need a Prefix Delegated from that CPE.

Do you think that would not be a valid need?

(I guess you will invite me to check whether my CPE's DHCPv6 option 
responds to an IA_PD request, thing that I will maybe try one day).

(think that currently a /56 is assigned by ISP to my home network, and 
that last time I checked I had to manually decide on a GUI which /64 out 
of that /56 goes to which IP of which in-home router).


>>>> On Oct 29, 2019, at 7:22 AM, Ted Lemon <>
>>>> wrote:
>>>>  On Oct 29, 2019, at 6:37 AM, Alexandre Petrescu 
>>>> < 
>>>> <>> wrote:
>>>>> Well no.  After  activating DHCPv6 on CPE the CPE sent three
>>>>> RAs changing the Lifetime and flipping the M(anaged) and 
>>>>> A(utonomous) flags.
>>>>> Packet dumps available upon request.
>>>> That’s the problem.   It should turn on the managed bit but
>>>> not turn off the autonomous bit.  The two can validly be on at
>>>> the same time.
>>>> Of course, since their meanings are deprecated, it’s not too 
>>>> surprising that implementations get this wrong.
>>>> _______________________________________________ v6ops mailing
>>>> list
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