Date:	Mon, 30 Mar 2015 12:44:39 -0500
From:	Dan Williams <dcbw@...hat.com>
To:	Mahesh Bandewar <maheshb@...gle.com>
Cc:	Jiri Benc <jbenc@...hat.com>, linux-netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/2] ipvlan: always allow the broadcast MAC address

On Mon, 2015-03-30 at 09:54 -0700, Mahesh Bandewar wrote:
> On Mon, Mar 30, 2015 at 7:37 AM, Dan Williams <dcbw@...hat.com> wrote:
> > On Sat, 2015-03-28 at 19:32 +0100, Jiri Benc wrote:
> >> On Fri, 27 Mar 2015 22:56:15 -0700, Mahesh Bandewar wrote:
> >> > The current logic disables broadcast by default and enables only when
> >> > an IPv4 address is added. If this is inverted and -
> >> > enables broadcast by default but disables it when only IPv6
> >> > address(es) is / are added. These links can have multiple addresses
> >> > and hence have to be careful if any one of those is IPv4 then
> >> > broadcast bit has to be set.
> >>
> >> You'd have to be careful and ignore IPv6 link local addresses.
> >> Those are added automatically whenever IPv6 is enabled and their
> >> presence does not mean the network is not IPv4 only.
> >>
> >> But I don't like such magic behavior. It would lead to DHCP sometimes
> >> working and sometimes not in mixed v4/v6 environment depending on
> >> whether DHCPv4 or SLAAC was faster.
> >>
> >> Could we perhaps add a flag when creating ipvlan interface stating
> >> whether IPv4 broadcast should be always enabled? Or, rather, the other
> >> way round - whether it should be disabled by default. Call it "nodhcp"
> >> or so.
> >>
> >> Btw, speaking about IPv6 link local addresses, these actually do not
> >> work with ipvlan correctly. I'm getting DAD failures if I have more
> >> than one ipvlan interface, which is no wonder. This means that ipvlan
> >> cannot work with IPv6 reliably by default (unless you take care of ll
> >> address assignment and ensure all ipvlan interfaces get a different
> >> one).
> >
> > ipvlan doesn't set dev_id.  Once dev_id is set the kernel's IPv6LL
> > address generation code will assign a different LL address to each
> > ipvlan interface created from the same physical interface, despite that
> > they have the same MAC address.
> >
> Yes, that was what my plan was but never got around fixing that
> 
> > But of course you'd have to be careful to assign a *different* dev_id
> > than any of that physical interface's non-ipvlan children too, and I
> > have no idea how that would work since dev_id is currently done
> > per-driver.  eg, if you have a physical interface with dev_id=1 which
> > you then create an ipvlan from, that ipvlan must not use dev_id=1 or it
> > will be assigned the same IPv6LL address as the parent.
> >
> The description is very clear for dev_id (in netdevice.h). So the idea
> of using the subsequent numbers after master's id should be possible.
> After all these logical devices are going to share the same link. Most
> physical drivers don't assign dev-id so the beginning is 0x0 (for the
> physical driver) and from 0x1 can be assigned to the logical links.
> The definition is not clear in terms of what is the beginning (0x0 or
> 0x1) but from the code that generates the IPv6LL it's common that it's
> 0x0 hence logical links on top of these links can use 0x1 onward.
> However a check to see if the master-link has dev-id and staying clear
> of that should be sufficient.

My point was that if you have a parent with a non-zero dev_id, there can
be other siblings of the parent that have a different dev_id and share
the same MAC address.  So creating an ipvlan with parent->dev_id + 1
doesn't work, because the parent may have a sibling with parent->dev_id
+ 1 and the same MAC address already.

Dan

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