During a boot of a xen debian template you can see the following output:

Configuring network interfaces...Starting portmap daemon....
portmap: server localhost not responding, timed out
RPC: failed to contact portmap (errno -5).

If you press CTRL-D to cancel the portmap startup the loopback interfaces does not come up and several other services will not come up. I've been searching for the error for a long time but finally I found the wrong configuration. If you take a close look at the network configuration file at /etc/network/interfaces you will see the following startup order:

auto eth0 lo

This is wrong. Change the order to:

auto lo eth0

Now the system will bring up the interfaces in the proper order.