[arch-projects] [netctl][patch] Remove DAD detection

Sébastien Luttringer seblu at seblu.net
Thu Feb 7 19:03:44 EST 2013

On Thu, Feb 7, 2013 at 12:36 PM, Jouke Witteveen <j.witteveen at gmail.com> wrote:
> On Wed, Feb 6, 2013 at 11:48 PM, Sébastien Luttringer <seblu at seblu.net> wrote:
>> DAD detection is detected via the presence of tentative flag in ip command.
>> This flag can only be setted if interface is up (obviously).
>> So, setting an IPv6 address will fail when you:
>> - the cable is not plugged
>> - any interface of a bridge is not set
>> The code was (before rewriting of netctl) reference this bug report:
>> https://bugs.archlinux.org/task/28887
> This bug will reopen after your proposed changes.
I don't think so. The bug was about DAD no waiting enough time. This
patch drop the DAD check logic by userland (same behaviour than ipv4).

> If the cable is not plugged there is no point in trying to connect anyway
Connect, but configure? Setting a IP address to an interface doesn't
mean trying to connect to anything or having an internet connection.
It just mean setting an ip.
Port can be up or down, or change of states every minutes. We don't
care. netctl should not fail to setup a static ip on iface when no
link is detected at boot.
Sometimes, setting an IP on iface can be used to allow kernel to
accept packet to this destination without really own it (lvs by

> so it would be nice if you describe in some more detail what problem your changes solve.
Imagine a router on archlinux, with a bridge of n ports. People can
plug (or not) their computers when they wants. Mostly, when you boot
your router, noboby is plugged.
When the router starts, it defines IPv4 and IPv6 on the bridge (where
all ethernet ports are added).
With the current netcfg/netctl, it's impossible because when nobody is
plugged, DAD cannot do is job of checking ip address, because all the
underlying interface can't send icmpv6 ND messages.

I've tested to disable dad (sysctl accept_dad)[1] but interface stay
in tentative mode and netctl timeout and rollback configuration.[2]

Here I don't see why we *care* about DAD when setting ipv6 in _static_
mode. This is really usefull (and required) in stateless mode and was
defined in ND RFC[3] for stateless autoconfig RFC[4].

If you really want check DAD, which is a completly correct behaviour
in stateless, we maybe use dadfailed[5] instead of tentative[6] in our


[1] Not really surprising as parameter is about accept and not using
DAD crafted ND.
[2] There is currently no check in the netcfg code about activating or
not the sysctl accept_dad. Should we add it as we do with accept_ra?
[3] http://tools.ietf.org/html/rfc4861 look at §7
[4] http://tools.ietf.org/html/rfc4862
[5] http://git.kernel.org/?p=linux/kernel/git/shemminger/iproute2.git;a=commit;h=f4af851bac12b0a72515912d6e1458d41aa2e009
[6] http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=blob;f=net/ipv6/addrconf.c;h=420e563263840442a150c13098148921559c3f2d;hb=HEAD#l1297

Sébastien "Seblu" Luttringer
GPG: 0x2072D77A

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