Eucalyptus version: 1.6.1 from
deb http://www.eucalyptussoftware.com/downloads/repo/eucalyptus/1.6.1/ubuntu jaunty universe
Ubuntu jaunty with debian kernel 2.6.26-2-xen-amd64
Topology: 2 hosts on the same private network with DHCP assigned addresses
Networking mode = SYSTEM

On both machines, after I installed Xen, I disabled Xen's network-bridge script per http://wiki.libvirt.org/page/Networking and change my /etc/network/interfaces to:

auto br0
iface br0 inet dhcp
bridge_ports eth0

Xen is set for bridging:

root@squiggy:~# grep vif-script /etc/xen/xend-config.sxp
(vif-script vif-bridge)

I started an instance. xm list:

root@squiggy:~# xm list
Name ID Mem VCPUs State Time(s)
Domain-0 0 3445 2 r----- 588.8
i-47250846 3 512 2 -b---- 4.8

root@squiggy:~# brctl show
bridge name bridge id STP enabled interfaces
br0 8000.00e0813283b2 no eth0
vif3.0
virbr0 8000.000000000000 yes

root@squiggy:~# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT udp -- anywhere anywhere udp dpt:domain
ACCEPT tcp -- anywhere anywhere tcp dpt:domain
ACCEPT udp -- anywhere anywhere udp dpt:bootps
ACCEPT tcp -- anywhere anywhere tcp dpt:bootps

Chain FORWARD (policy ACCEPT)
target prot opt source destination
ACCEPT all -- anywhere anywhere PHYSDEV match --physdev-in vif3.0

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

In the jaunty domU image, I added "post-up ping 10.1.1.96" in /etc/network/interfaces to notify the cloud controller of the instance's IP address. This worked.

I can ssh into the instance from the host it is running on. E.g.,
root@squiggy:~# ssh -i /home/joshua/id_euca_joshua root@10.1.1.206

IPv4 forwarding is enabled:

root@squiggy:~# cat /proc/sys/net/ipv4/ip_forward
1

I can ping dom0 from the domU:

root@ubuntu:~# ping 10.1.1.97
PING 10.1.1.97 (10.1.1.97) 56(84) bytes of data.
64 bytes from 10.1.1.97: icmp_seq=1 ttl=64 time=0.101 ms

I can ping the domU from the dom0:

root@squiggy:~# ping 10.1.1.206
PING 10.1.1.206 (10.1.1.206) 56(84) bytes of data.
64 bytes from 10.1.1.206: icmp_seq=1 ttl=64 time=0.136 ms

However, I can't ping the instance from the cloud controller, and I can't ping the cloud controller from the instance. I don't understand how the cloud controller got the instance's IP address. It looks as if IP forwarding is disabled, but it's not.

Any ideas? Was it a mistake to disable Xen's network-bridge script?