/etc/xen/scripts/block-nbd 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 14 15 16 17 18 19 20 21 22 23 24 25 26 27 | #!/bin/bash
# Usage: block-nbd [bind server ctl_port |unbind node]
#
# The node argument to unbind is the name of the device node we are to
# unbind.
#
# This assumes you're running a correctly configured server at the other end!
dir=$(dirname "$0")
. "$dir/block-common.sh"
case "$command" in
  add)
    for dev in /dev/nbd*; do
      if nbd-client $2 $3 $dev; then
        write_dev $dev
        exit 0
      fi
    done
    exit 1
    ;;
  remove)
    nbd-client -d $2
    exit 0
    ;;
esac
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