Good practices for sharing files between virtual machines (Xen)

You should setup two bridges on dom0. You can use the standard entries in /etc/network/interfaces for this. Let's assume your real card is eth0 (and there's a DHCP server behind it) and you have bridge-utils installed. The file could look like this:

auto br0
iface br0 inet dhcp
    bridge_ports eth0
    bridge_maxwait 0

auto br1
iface br1 inet static
    bridge_ports none
    bridge_maxwait 0
    address 192.168.0.1
    netmask 255.255.255.0

You configure /etc/xen/xend-config.sxp with network-brigde and vif-bridge. In each domU config file, you select whether you want it to have direct external access (via br0) or if it should get only access via br1. For this you can use vif lines like those:

vif = ['bridge=br0']
vif = ['bridge=br1']

Of course, you still have to setup the NAT/masquerading over br1 and the network configuration of the domU should match (i.e. those on br0 should used DHCP and those on br1 should have a static IP in my example above).

"The Xen Networking wiki and the Debian Xen wiki both point me toward using Xen's bridging scripts"

The problem with wikis is that any idiot can edit them -- and they usually do. The Debian Xen wiki page is wrong, so very, very wrong; I don't even have the heart to look at the Xen networking wiki. There used to be useful networky docs in the README.Debian for the xen-utils package, but they seem to have gone walkabout at some stage.

My answer in Bringing up network interface without IP configured in Debian, for XEN dom0 probably won't work for you because it's solving a very different problem. You should probably re-read the question to discover the differences.

That mailing list thread is full of gibberish (and Nabble's usual brain-meltingly stupid formatting) so I'm not going to try and work out exactly where you're seeing a need to put an IP address on both the interface and the bridge, but I can assure you that you don't need to do that -- just an IP address on the bridge will be perfectly sufficient.

The simple way to fix your network config is probably to:

1. Set (network-script network-dummy) and then chattr +i /etc/xen/xend-config.sxp (to discourage you from trying to fiddle with it any more)
2. Delete the br-* stanzas you have currently
3. Delete the gateway line from the eth0:1 stanza
4. /allow_hotplug/d
5. s/eth0/br0/; s/eth1/br1/
6. Add bridge_ports eth0 to your now-br0 config
7. Add bridge_ports eth1 to your now-br1 config
8. /etc/init.d/networking restart