No persistent /dev/net/tun in xen Linux domU… Hacky workaround is adding
mkdir /dev/net && mknod /dev/net/tun c 10 200
while booting (for example in rc.local or in your init.d script which need it).
For example tun device is useful for SSH VPN. Without it, you will have errors like:
channel 0: open failed: administratively prohibited: open failed