Comment 12 for bug 199533

Some debugging ideas

add the line:
set -x
to the network scripts that you are using (i.e. network-brige, vif-bridge)
This will produce output in /var/log/syslog

configure udev log DEBUG in /etc/udev.conf

keep an eye on xenstore with xenstore-ls and xenstore-read to see if the vif devices are being created.

Make sure that the vif and veth devices exists

Keep an eye on all the logs in /var/log/xen/

try to use strace on different commands, scripts, entry points to see where things hang etc.