netdev - Re: Bridge stays down until a port is added

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Date:	Mon, 22 Aug 2011 08:57:30 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Marc Haber <mh+netdev@...schlus.de>
Cc:	netdev@...r.kernel.org, Sven-Haegar Koch <haegar@...net.de>
Subject: Re: Bridge stays down until a port is added

On Sun, 21 Aug 2011 14:13:05 +0200
Marc Haber <mh+netdev@...schlus.de> wrote:

> On Sat, Aug 20, 2011 at 09:30:59AM -0700, Stephen Hemminger wrote:
> > The problem is that IPv6 Duplicate Address Detection needs to
> > work. This is not a simple problem.  If the bridge asserted
> > carrier with no ports then:
> > 
> > 1. IPv6 address assigned and IPv6 decides it is okay.
> > 2. Port added later
> > 3. Another system has the same address.
> > *broke*
> 
> Same situation when a system-to-system-link is added after bringing up
> an interface. I agree that the issue is not an issue if a real switch
> is being used.
> 
> > If you want to avoid DAD, then you can configure disable DAD
> > by setting /proc/sys/net/ipv6/conf/br0/accept_dad to 0
> 
> I'd like to avoid that.
> 
> 2001:db8::1
> 
> Would it acceptable (and clean!) to have:
> 
> eth0: 2001:db8:1::100/64 default gw to the internet is 2001:db8:1::1
> lo: 127.0.0.1/8, ::1/128, 2001:db8:2::100/64
> br0: 2001:db8:2::1/64 (being default gw for the VMs connected to br0,
> routing 2001:db8:2::/64 to the Internet)
> 
> Note 2001:db8:2::/64 being used both on lo and br0, with 2001:db8:2::100 meant
> to be reachable from the Internet even if no VM is already up. my
> hostname will be A-Recorded to 2001:db8:2::100 with proper reverse
> DNS. The background for doing so is that I cannot control the reverse
> DNS for the IP addresses inside 200a:db8:1::/64 in a lot of my setups
> (for example, if my IPv6 comes in via Sixxs).
>

No, DAD config is per interface and addrconf doesn't know anything
about your topology.
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