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Date:   Tue, 14 Nov 2017 10:30:33 -0800
From:   Girish Moodalbail <girish.moodalbail@...cle.com>
To:     Nicolas Dichtel <nicolas.dichtel@...nd.com>, davem@...emloft.net
Cc:     netdev@...r.kernel.org, Stefano Brivio <sbrivio@...hat.com>,
        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 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?

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.

thanks,
~Girish


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