Re: [PATCH net] ipv6: set all.accept_dad to 0 by default

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 13 Nov 2017 23:52:26 +0900
Maciej Żenczykowski <maze@google.com> wrote:

> >> 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

Old pre-patch behaviour simply ignored the 'all' value though.

> [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

But this way you could still globally disable DAD, starting from
default values, by simply setting 'all' to zero, which is what Nicolas
wanted.

> [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.

We're mostly talking about 35e015e1f577 here.

-- 
Stefano