netdev - Re: [PATCH net-next] ip6_tunnel: ensure to always have a link local address

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Date:	Wed, 21 Aug 2013 14:11:51 +0200
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	Bjørn Mork <bjorn@...k.no>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH net-next] ip6_tunnel: ensure to always have a link local
 address

Le 21/08/2013 13:37, Bjørn Mork a écrit :
> Nicolas Dichtel <nicolas.dichtel@...nd.com> writes:
>
>> Le 21/08/2013 11:02, Bjørn Mork a écrit :
>>> Nicolas Dichtel <nicolas.dichtel@...nd.com> writes:
>>>> Le 21/08/2013 08:48, David Miller a écrit :
>>>>
>>>>> Applied, but this brings up an issue I keep noticing.
>>>>>
>>>>> We talk about eth_random_addr() and "uniqueness" together all the
>>>>> time, but the former never implies the latter.
>>>>>
>>>>> And we're going to run into situations where any conflicts generated
>>>>> by this random address generater will cause reall failures.
>>>>>
>>>>> Therefore we'll have to create a system to prevent them.  Probably
>>>>> using some simple table that keeps track of the addresses we've
>>>>> generated.
>>>>>
>>>> Ok, I will look at this.
>>>
>>> Are eth_random_addr() collisions really any different than interfaces
>>> having the same address for other reasons?
>> I would tend to say yes, it's different.
>> It's easy for an administrator to fix a configuration for a physical
>> interface, because it's statically configured and there is a limited
>> number of interfaces.
>>
>> For virtual interfaces, they can be dynamically created and destroyed
>> by daemons and we can have a lot of interfaces. Hence it could be hard
>> to fix them.
>
> If they are created by daemons then it should be up to the daemons to
> fix them.  Or?
>
>> Trying to avoid these errors at kernel level could be useful.
>
> I strongly believe in fixing configuration issues in userspace if at all
> possible.  You are setting a new policy every time you implement an
> automatic fix or workaround.  It is so much better to keep that out of
> the kernel, or the next question you will face is "How do I change this
> policy? I want the addresses to be assigned by function Y"
>
> I see no reason why the daemon creating these interfaces can't also
> fixup any collisions.  Or maybe better: If your daemon create millions
> of interfaces, and cares about unique addresses, then it should
> implement it's own address management.
Ok ok, you convince me ;-)

I will wait David feedback.

>
>> I've start to write a patch, and to test it I've just run a simple
>> test which generate 1 000 000 of random addresses. I've run it several
>> times (maybe not enough ;-)) and I never get a duplicated address...
>
> Well, there are only 2^46 combinations so you are guaranteed to hit a
> collision if you just generate 70 368 744 177 665 random addresses :-)
Yes, it was just to say that the function which generate these addresses has a 
"good" entropy ;-)
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