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Date:   Tue, 14 Nov 2017 12:52:20 -0800
From:   Girish Moodalbail <girish.moodalbail@...cle.com>
To:     Stefano Brivio <sbrivio@...hat.com>
Cc:     Nicolas Dichtel <nicolas.dichtel@...nd.com>, davem@...emloft.net,
        netdev@...r.kernel.org, Matteo Croce <mcroce@...hat.com>,
        Erik Kline <ek@...gle.com>
Subject: Re: [PATCH net v2] ipv6: set all.accept_dad to 0 by default

On 11/14/17 11:10 AM, Stefano Brivio wrote:
> On Tue, 14 Nov 2017 10:30:33 -0800
> Girish Moodalbail <girish.moodalbail@...cle.com> wrote:
> 
>> On 11/14/17 5:21 AM, Nicolas Dichtel wrote:
>>> With commits 35e015e1f577 and a2d3f3e33853, the global 'accept_dad' flag
>>> is also taken into account (default value is 1). If either global or
>>> per-interface flag is non-zero, DAD will be enabled on a given interface.
>>>
>>> This is not backward compatible: before those patches, the user could
>>> disable DAD just by setting the per-interface flag to 0. Now, the
>>> user instead needs to set both flags to 0 to actually disable DAD.
>>>
>>> Restore the previous behaviour by setting the default for the global
>>> 'accept_dad' flag to 0. This way, DAD is still enabled by default,
>>> as per-interface flags are set to 1 on device creation, but setting
>>> them to 0 is enough to disable DAD on a given interface.
>>>
>>> - Before 35e015e1f57a7 and a2d3f3e33853:
>>>             global    per-interface    DAD enabled
>>> [default]   1             1              yes
>>>               X             0              no
>>>               X             1              yes
>>>
>>> - After 35e015e1f577 and a2d3f3e33853:
>>>             global    per-interface    DAD enabled
>>> [default]   1             1              yes
>>>               0             0              no
>>>               0             1              yes
>>>               1             0              yes
>>>
>>> - After this fix:
>>>             global    per-interface    DAD enabled
>>>               1             1              yes
>>>               0             0              no
>>> [default]   0             1              yes
>>>               1             0              yes
>>
>> Above table can be summarized to..
>>
>> - After this fix:
>>             global    per-interface    DAD enabled
>>               1             X              yes
>>               0             0              no
>> [default]   0             1              yes
>>
>> So, if global is set to '1', then irrespective of what the per-interface value
>> is DAD will be enabled. Is it not confusing. Shouldn't the more specific value
>> override the general value?
> 
> Might be a bit confusing, yes, but in order to implement an overriding
> mechanism you would need to implement a tristate option as Eric K.
> proposed. That is, by default you would have -1 (meaning "don't care")
> on per-interface flags, and if this value is changed then the
> per-interface value wins over the global one.
> 
> Sensible, but I think it's outside of the scope of this patch, which is
> just intended to restore a specific pre-existing userspace expectation.
> 
>> On the other hand, if the global is set to '0', then per-interface value will be
>> honored (overrides global). So, the meaning of global varies based on its value.
>> Isn't that confusing as well.
> 
> I don't find this confusing though. Setting the global flag always has
> the meaning of "force enabling DAD on all interfaces".
> 
> You would have the same problem if you chose a logical AND between
> global and per-interface flag. There, setting the global flag would mean
> "force disabling DAD on all interfaces".
> 
> So the only indisputable improvement I see here would be to implement a
> "don't care" value (either for global or for per-interface flags). But
> I'd rather agree with Nicolas that we should fix a potentially broken
> userspace assumption first.

Agree.

Thanks,
~Girish


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