Xen users - Aug 2009 - xen network interfaces: How do I retrieve from an executable running within domain0 the MAC of an frontend-(domainU)-network-interface ?

Hello,

I use xen as provided by openSUSE 11.1 . What I want to do is this: Operate 
several domainU. All of them have a xennet network interface, whose 
respective backend-interfaces at the domain0 are added to a boilerplate linux 
bridge. No physical interfaces are added to the bridge (in other words: a 
virtual ethernet). Now each of those backend-interfaces gets assigned the 
infamous FE:FF:FF:FF:FF:FF MAC. That turns out to be a problem if I try to 
send packets from domain0 through the bridge-interface to any of the domainU 
instances: the log tells me something along "ethernet source and target are
equal, can''t send packets to myself". However, I had some success
when I
manually assigned (by calling at domain0, for instance, "ip link set
vif13.1
address 00:16:3E:7D:D0:98") the MAC assigned to the 
frontend-(domainU)-interface to the backend-(domain0)-interface as well. I 
obtained the MAC manually, by looking into the configuration files 
in /etc/xen/vm/ and manually identifying associated 
backend-(domain0)-interfaces.
Now my question is: Supposed an executable is running within domain0, and this 
executable has been handed the name of a Xen backend interface, is there a 
way to obtain the MAC of the associated frontend-(domainU)-interface ?

Thank you very much,
Kay

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On Sun, Aug 23, 2009 at 6:22 PM, Kay Schubert<kayegypt@web.de>
wrote:
> Now each of those backend-interfaces gets assigned the
> infamous FE:FF:FF:FF:FF:FF MAC. That turns out to be a problem if I try to
> send packets from domain0 through the bridge-interface to any of the
domainU
> instances: the log tells me something along "ethernet source and
target are
> equal, can''t send packets to myself".
I had that problem on some dom0s as well :)
Interestingly it doesn''t occur when I split dom0 and domUs traffic on
separate physical interfaces. For example :
- eth0 for dom0''s IP address
- eth1 setup as trunk, created vlans and bridge on top of vlans, no IP
address on those bridges, with domU using the bridges.
> However, I had some success when I
> manually assigned (by calling at domain0, for instance, "ip link set
vif13.1
> address 00:16:3E:7D:D0:98") the MAC assigned to the
> frontend-(domainU)-interface to the backend-(domain0)-interface as well.
Actually, it doesn''t matter what MAC you assign to the vif, as long as
it''s unique. At least that was the case on my setup. So I simply
modify /etc/xen/scripts/vif-bridge, and generate a random MAC for
every vif. It''s not the same as domU''s MAC.
> Now my question is: Supposed an executable is running within domain0, and
this
> executable has been handed the name of a Xen backend interface, is there a
> way to obtain the MAC of the associated frontend-(domainU)-interface ?
If you still need domU''s mac (for any reason), you could try this
- xm network-list 13
- xenstore-ls /local/domain/13/device/vif

where "13" in that example is domU''s ID. You could usually
find out
domU id from vif name (in your case vif13.1). I haven''t tried running
it from vif-bridge script though.

-- 
Fajar

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Am Sunday 23 August 2009 16:37:25 schrieb Fajar A.
Nugraha:
> On Sun, Aug 23, 2009 at 6:22 PM, Kay Schubert<kayegypt@web.de> wrote:
> > Now each of those backend-interfaces gets assigned the
> > infamous FE:FF:FF:FF:FF:FF MAC. That turns out to be a problem if I
try
> > to send packets from domain0 through the bridge-interface to any of
the
> > domainU instances: the log tells me something along "ethernet
source and
> > target are equal, can''t send packets to myself".
>
> I had that problem on some dom0s as well :)
> Interestingly it doesn''t occur when I split dom0 and domUs traffic
on
> separate physical interfaces. For example :
> - eth0 for dom0''s IP address
> - eth1 setup as trunk, created vlans and bridge on top of vlans, no IP
> address on those bridges, with domU using the bridges.
>
> > However, I had some success when I
> > manually assigned (by calling at domain0, for instance, "ip link
set
> > vif13.1 address 00:16:3E:7D:D0:98") the MAC assigned to the
> > frontend-(domainU)-interface to the backend-(domain0)-interface as
well.
>
> Actually, it doesn''t matter what MAC you assign to the vif, as
long as
> it''s unique. At least that was the case on my setup. So I simply
> modify /etc/xen/scripts/vif-bridge, and generate a random MAC for
> every vif. It''s not the same as domU''s MAC.
Ah, you are conforming a suspicion I had ...
>
> > Now my question is: Supposed an executable is running within domain0,
and
> > this executable has been handed the name of a Xen backend interface,
is
> > there a way to obtain the MAC of the associated
> > frontend-(domainU)-interface ?
>
> If you still need domU''s mac (for any reason), you could try this
> - xm network-list 13
> - xenstore-ls /local/domain/13/device/vif
Yes, five minutes after I had sent my question I looked up "xenstore"
at
Google, and came to http://wiki.xensource.com/xenwiki/XenStore, where a 
little script resides, that dumps the xenstore''s content. And there it
was.
Its always the same: When you finally resolve to ask someone else, and think 
about how to shape your question, suddenly the answer is lurking somewhere 
near ...
>
> where "13" in that example is domU''s ID. You could
usually find out
> domU id from vif name (in your case vif13.1). I haven''t tried
running
> it from vif-bridge script though.
openSUSE doesn''t use the scripts provided by Xen. They process xen
network
interfaces through scripts in /etc/sysconfig/network/scripts (as they do for 
all network interfaces, whatever kind) where one can add custom scripts. I 
will try my luck there.

Thank you very much!

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