netdev - Re: [PATCH net] ipv6: set all.accept

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Date:   Mon, 13 Nov 2017 23:52:26 +0900
From:   Maciej Żenczykowski <maze@...gle.com>
To:     Stefano Brivio <sbrivio@...hat.com>
Cc:     Erik Kline <ek@...gle.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Matteo Croce <mcroce@...hat.com>
Subject: Re: [PATCH net] ipv6: set all.accept_dad to 0 by default

>> Should we consider rolling back the patch that caused this?
>> "accept_dad = 1" is the proper IETF-expected default behaviour.
>>
>> Alternatively, if we really want to make all, default, and ifname
>> useful perhaps we need to investigate a tristate option (for currently
>> boolean values, at least).  -1 could mean no preference, for example.
>
> I haven't checked how ugly it would be, yet. But another way to restore
> the previous behaviour, while keeping the new functionality, would be
> to keep the global default as 1 and instead set the per-interface
> accept_dad default value to 0. What do you think?

The default out-of-the-box behaviour should definitely be to do DAD.

You can achieve this in 4 ways:

[A] all=1, default=1, AND --> the OLD pre-patch behaviour
[B] all=1, default=1, OR --> the NEW post-patch behaviour - problematic
[C] all=1, default=0, OR --> problematic for same reason: iface=0 is a no-op
[D] all=0, default=1, OR

Note that:
AND == (all < 1 || interface < 1)
OR == (all < 1 && interface < 1)

[C] requires one to set all but one interface (incl. default) to 1,
then set all=0,
just to disable a single interface's dad

[D] is weird, because with the default already being dad enabled, there's really
no reason to ever set all=1

Being able to disable either for all interfaces (via all=0) or for a
specific interface (via iface=0) seems
the most useful.

Setting all=1, default=0, specific_interfaces=1, AND-logic also seems useful.

Hence my vote to rollback a2d3f3e33853.