netdev - Re: why are IPv6 addresses removed on link down

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Message-ID: <1421161230.13626.30.camel@stressinduktion.org>
Date:	Tue, 13 Jan 2015 16:00:30 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	David Ahern <dsahern@...il.com>
Cc:	YOSHIFUJI Hideaki <hideaki.yoshifuji@...aclelinux.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: why are IPv6 addresses removed on link down

On Di, 2015-01-13 at 07:53 -0700, David Ahern wrote:
> On 1/13/15 5:36 AM, Hannes Frederic Sowa wrote:
> > Hi,
> >
> > On Di, 2015-01-13 at 21:15 +0900, YOSHIFUJI Hideaki wrote:
> >> YOSHIFUJI Hideaki wrote:
> >>> Hi,
> >>>
> >>> Hannes Frederic Sowa wrote:
> >>>> On Mo, 2015-01-12 at 23:10 -0800, Stephen Hemminger wrote:
> >>>>> On Mon, 12 Jan 2015 22:06:44 -0700
> >>>>> David Ahern <dsahern@...il.com> wrote:
> >>>>>
> >>>>>> We noticed that IPv6 addresses are removed on a link down. e.g.,
> >>>>>>      ip link set dev eth1
> >>>>>>
> >>>>>>
> >>>>>> Looking at the code it appears to be this code path in addrconf.c:
> >>>>>>
> >>>>>>            case NETDEV_DOWN:
> >>>>>>            case NETDEV_UNREGISTER:
> >>>>>>                    /*
> >>>>>>                     *      Remove all addresses from this interface.
> >>>>>>                     */
> >>>>>>                    addrconf_ifdown(dev, event != NETDEV_DOWN);
> >>>>>>                    break;
> >>>>>>
> >>>>>> IPv4 addresses are NOT removed on a link down. Is there a particular
> >>>>>> reason IPv6 addresses are?
> >>>>>>
> >>>>>> Thanks,
> >>>>>> David
> >>>>>
> >>>>> See RFC's which describes how IPv6 does Duplicate Address Detection.
> >>>>> Address is not valid when link is down, since DAD is not possible.
> >>>>
> >>>> It should be no problem if the kernel would reacquire them on ifup and
> >>>> do proper DAD. We simply must not use them while the interface is dead
> >>>> (also making sure they don't get used for loopback routing).
> >>>>
> >>>> The problem the IPv6 addresses get removed is much more a historical
> >>>> artifact nowadays, I think. It is part of user space API and scripts
> >>>> deal with that already.
> >>>
> >>> We might have another "detached" state which essintially drops
> >>> outgoing packets while link is down.  Just after recovering link,
> >>> we could start receiving packet from the link and perform optimistic
> >>> DAD. And then, after it succeeds, we may start sending packets.
> >>>
> >>> Since "detached" state is like the state just before completing
> >>> Optimistic DAD, it is not so difficult to implement this extended
> >>> behavior, I guess.
> >>>
> >>
> >> Note that node is allowed to send packets to neighbours or default
> >> routers if the node knows their link-layer addresses during Optimistic
> >> DAD.
> >>
> >
> > I don't think it should be a problem from internal state handling of the
> > addresses.
> >
> > I am much more concerned with scripts expecting the addresses to be
> > flushed on interface down/up and not reacting appropriate.
> 
> The current code seems inconsistent: I can put an IPv6 address on a link 
> in the down state. On a link up the address is retained. Only on a 
> subsequent link down is it removed. If DAD or anything else is the 
> reason for the current logic then why allow an address to be assigned in 
> the down state? Similarly that it currently seems to work ok then it 
> suggests the right thing is done on a link up in which case a flush is 
> not needed.
> 
> Bottom line is there a harm in removing the flush? If there is no harm 
> will mainline kernel take a patch to do that or is your backward 
> compatibility concern enough to block it?

This was already discussed several times here, e.g. one patch I just 
found:

http://lists.openwall.net/netdev/2011/01/24/8
and
http://patchwork.ozlabs.org/patch/17558/

Albeit I hate sysctls for things like this, it might I tend to find it
acceptable because it solves a problem which happened to lots of people.
And I don't like the current behavior neither.

I think this can work, but we should follow up all the old discussions
to not introduce any kind of new undesired behavior this time.

Thanks,
Hannes


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