/etc/xen/scripts/qemu-ifup is in xen-utils-common 4.4.0-0ubuntu5.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 | #!/bin/sh
echo -c 'config qemu network with xen bridge for '
echo $*
# Initialise a dummy MAC address. We choose the numerically
# largest non-broadcast address to prevent the address getting
# stolen by an Ethernet bridge for STP purposes.
# (FE:FF:FF:FF:FF:FF)
ip link set $1 address fe:ff:ff:ff:ff:ff || true
ifconfig $1 0.0.0.0 up
brctl addif $2 $1
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