/usr/sbin/nbdrootd is in ltsp-server 5.3.7-0ubuntu2.
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 28 29 30 31 32 33 34 35 36 37 38 | #!/bin/sh
#
#  Copyright (c) 2007 Canonical LTD
#
#  Author: Oliver Grawert <ogra@canonical.com>
#
#  2008, Warren Togami <wtogami@redhat.com>
#
#  This program is free software; you can redistribute it and/or
#  modify it under the terms of the GNU General Public License as
#  published by the Free Software Foundation; either version 2 of the
#  License, or (at your option) any later version.
#
#  This program is distributed in the hope that it will be useful,
#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
#
#  You should have received a copy of the GNU General Public License
#  along with this program.  If not, you can find it on the World Wide
#  Web at http://www.gnu.org/copyleft/gpl.html, or write to the Free
#  Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
#  MA 02110-1301, USA.
#
# nbd-server wrapper that serves an ltsp squashfs image
# start the blockdevice server
# First parameter is the image file, second is the delay after which 
# the connection is killed (default to 2 hours)
if [ -n "$2" ]; then
    # Set a timeout. Note that nbd 2.9.13+ will fail if the -a option is
    # passed to it, as it was removed.
    nbd_timeout="-a $2"
fi
PATH=/usr/bin:/bin:$PATH exec nbd-server 0 $1 -r -C /dev/null $nbd_timeout > /dev/null 2>&1
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