Re: TUN/TAP/UML proxy arp and IPV6 don't like one another.

From: greg
Date: Tue Nov 14 2006 - 10:30:37 EST

On Nov 8, 1:16pm, Jeff Dike wrote:
} Subject: Re: TUN/TAP/UML proxy arp and IPV6 don't like one another.

Good morning Jeff, thanks for the reply.

I've added Willy as a CC to this note to bring him in on the issue.

Willy I'm able to demonstrate a problem with ARP in the which
seems to be related to the presence of IPV6 in the kernel. Details

> On Fri, Nov 03, 2006 at 12:08:01PM -0600, greg@xxxxxxxxxxxx wrote:
> > Environment:
> > host kernel with SKAS3 patch.
> > Stock 2.6.18 UML guest kernel.
> >
> > Host kernel has IPV6 enabled but there are no IPV6 routers
> > active on the subnet. The active eth0 host interface only has
> > a link-local address assigned to it.
> >
> > The host kernel was recompiled with the only change being to drop IPV6
> > from the kernel. Identical guest OS implementation boots and properly
> > pings the same target host on the network.

> So, you're saying that IPV6, even though it is not being used, is
> somehow interfering with normal IPV4 networking?

That seems to be the case. Most specifically the willingness of the
kernel to proxy-ARP for an IPV4 address assigned to the end of a
TUN/TAP interface when IPV6 is enabled in the host kernel.

With everything else held constant removing IPV6 from the host kernel
makes the problem go away.

> > I know is a bit dated so this might be better to bounce off
> > Willy.
> >
> > Thoughts, further suggestions?

> Maybe upgrade and hope the problem goes away. It sounds like this
> should be reproducable without UML, so if you can reproduce this on
> a newer kernel with a smaller test case, that might help get it
> fixed.

I'm including Willy on the mail since if its a regression in the 2.4.x
its probably worth chasing down. 2.6 is nice but we have literally a
ton of 2.4 running for stability reasons.

Willy any thoughts on a test harness for this problem beyond UML? I
have the UML environment we use all bottled up into an RPM with a
reasonably sophisticated startup script. Invoking a UML environment
to validate the bug is basically an RPM install followed by a single
command. I can easily drop the RPM on an FTP server.

> Jeff

Thanks for the input Jeff, best wishes for a productive week.

}-- End of excerpt from Jeff Dike

As always,
Dr. G.W. Wettstein, Ph.D. Enjellic Systems Development, LLC.
4206 N. 19th Ave. Specializing in information infra-structure
Fargo, ND 58102 development.
PH: 701-281-1686
FAX: 701-281-3949 EMAIL: greg@xxxxxxxxxxxx
"Intel engineering seem to have misheard Intel marketing strategy. The
phrase was 'Divide and conquer' not 'Divide and cock up'".
-- Alan Cox
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