xen networking and tap0

[brook%biology.nmsu.edu@localhost (Brook Milligan)]

I am trying to set up a xen machine in which dom0 uses the tap0
interface for the internal bridged network.  My thinking is that this
isolates the domUs completely.  So the setup is the following:

fxp0 ---- dom0 ---- tap0 ---- bridge0 ---- xvifN.M ---- xennet0 --- domU

If the domU xvifN.M network interface is configured manually it can
ping the dom0.  However, dom0 will not respond to dhcp requests coming
in via tap0, even though tcpdump detects them.  Note that if I use a
real network interface, e.g., fxp1, instead of tap0 in the above setup
then dhcp requests are responded to just fine.

I have never tried to use the tap0 device before, so perhaps I am
misunderstanding something.

By the way, this is with netbsd/i386 from a fairly recent checkout of
the nebsd-5 branch.  Xen is from the xen3 packages in pkgsrc, also
recently checked out and built locally.

Any help is greatly appreciated.  Thanks alot.

Cheers,
Brook