KVM Networking Hell

Soren Hansen soren at ubuntu.com
Thu Jun 10 08:16:01 UTC 2010

On Wed, Jun 09, 2010 at 11:09:48PM +0100, Jamie McDonald wrote:
>> I'm not sure if this output got linebroken somewhere. Can you perhaps
>> make sure the terminal you're using is large enough to hold the
>> output and put it on a pastebin so we can be sure noone's e-mail
>> application is messing with the formatting?
> I have pasted a new copy here: http://pastebin.org/322148

I still think it looks kind of weird. I was expecting vnet0 to be
directly underneath eth0, but meh. It's probably fine.

>> Ok. And you haven't used Eucalyptus? It's the only thing I know of
>> that might fiddle with brtables behind the scenes.
> No I have not used Eucalyptus - this is a standard 9.10 build of
> Ubuntu server from Fasthosts.

Oh. So this is running in a hosted environment?

> During my experiments this afternoon I have actually become more
> confused.  I have removed all firewall rules from the host in order to
> test as suggested by Alex (thankyou for your input kind sir). IP
> Forwarding is enabled (even though it should make no difference) and
> the following rules were added (although again I really don't think I
> should need them).
> /sbin/iptables -A FORWARD -d -j ACCEPT
> /sbin/iptables -A FORWARD -s -j ACCEPT

Right, your host does /not/ act as a router or gateway of any kind for
the guest, so iptables and routing and whatnot does not factor into it
at all.

> Any other suggestions I could try? Is there anything which Fasthosts
> could have in place which could inhibit a bridged network from
> operating correctly?

Certainly. I hadn't considered that this might be running in a hosted
environment.  Perhaps the port you're connected to only allows one
specific MAC (your host's, of course) in an effort to prevent MAC
spoofing on the network.  This is a problem because your VM has its own
MAC address which is seen on the network (since the host does not act as
a router for it).

I'm assuming you got assigned multiple IP's. Are they expecting you to
route those IP's or something?

Soren Hansen
Ubuntu Developer
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