Re: [Xen-devel] vif tx drops

[Brian Wolfe <brianw@xxxxxxxxxxxx>]

On Thu, 2004-10-14 at 10:29, Keir Fraser wrote:
> > So the xen side of the vif isn't see on the wire except for a: the
> > backend driver domain( usually domain-0) and that vif's front and driver
> > domain eh? (I got the front and back end locations right yes?)
> 
> Noone ever sees the backend MAC address -- it is never written into
> any packet. We only need to give the backend a MAC address because we
> hook into the Linux networking code as a normal Ethernet interface,
> and normal Ethernet interfaces need a normal MAC address. :-)
> It needs to be unique because of sanity checks in the bridge that fail
> if it sees a remote address == a local address that it knows about.
> 
> > Either way it goes, how would one go about verifying that the MAC is
> > truly unique on it's visible lan? I'd be i very interested in this as a
> > safety precaution. Right now I can't think of how to do the uniqueness
> > test... 8-P if someone can give me an idea or two I could try to code it
> > since it's something that I want.(brain fried this morning)
> 
> I'm pretty sure there's no way of soliciting a response from an
> Ethernet host without using some higher-level protocol; probably IP or
> RARP. RARP is hardly ever used, but maybe if you know the IP subnet
> you could do a broadcast ping and collect the responses and look at
> their source MAC addresses?
> 

Hmm, not a bad idea.... :) I know brctl can be used to query for a list
of MACs on it's physical lan that it has cached. Maybe if xend on
domain-0 did something similar using ARP and arp-reply packets that it
sees on the bridges to vet it's mac choices against, and issue a warning
if it sees a dupe?

>  -- Keir
<snip - backlog shortened >



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