/usr/lib/grub/i386-efi/grub-mknetdir is in grub-efi-ia32-bin 1.99-21ubuntu3.19.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
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# Install GRUB on your drive.
# Copyright (C) 1999,2000,2001,2002,2003,2004,2005,2006,2007,2008,2009,2010  Free Software Foundation, Inc.
#
# GRUB 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 3 of the License, or
# (at your option) any later version.
#
# GRUB 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 GRUB.  If not, see <http://www.gnu.org/licenses/>.
# Initialize some variables.
transform="s&^&&"
prefix="/usr"
exec_prefix="${prefix}"
datarootdir="${prefix}/share"
bindir="${exec_prefix}/bin"
libdir="${exec_prefix}/lib"
PACKAGE_NAME=GRUB
PACKAGE_TARNAME=grub
PACKAGE_VERSION=1.99-21ubuntu3.19
target_cpu=i386
platform=efi
host_os=linux-gnu
native_platform=efi
pkglib_DATA="moddep.lst command.lst fs.lst partmap.lst parttool.lst handler.lst video.lst crypto.lst terminal.lst"
self=`basename $0`
grub_mkimage="${bindir}/`echo grub-mkimage | sed ${transform}`"
rootdir=/srv/tftp
grub_prefix=`echo /boot/grub | sed ${transform}`
modules=
install_device=
no_floppy=
recheck=no
debug=no
debug_image=
subdir=`echo /boot/grub | sed ${transform}`
pc_dir="${libdir}/$(echo ${PACKAGE_TARNAME} | sed ${transform})/i386-pc"
# Usage: usage
# Print the usage.
usage () {
    cat <<EOF
Usage: $self [OPTION] install_device
Install GRUB on your drive.
  -h, --help              print this message and exit
  -v, --version           print the version information and exit
  --modules=MODULES       pre-load specified modules MODULES
  --net-directory=DIR     root directory of TFTP server
  --subdir=DIR            relative subdirectory on network server
  --grub-mkimage=FILE     use FILE as grub-mkimage
$self copies GRUB images into net_directory/subdir/${target_cpu}-${platform}
Report bugs to <bug-grub@gnu.org>.
EOF
}
argument () {
  opt=$1
  shift
  if test $# -eq 0; then
      echo "$0: option requires an argument -- '$opt'" 1>&2
      exit 1
  fi
  echo $1
}
# Check the arguments.
while test $# -gt 0
do
    option=$1
    shift
    case "$option" in
    -h | --help)
	usage
	exit 0 ;;
    -v | --version)
	echo "$self (${PACKAGE_NAME}) ${PACKAGE_VERSION}"
	exit 0 ;;
    --modules)
	modules=`argument $option "$@"`; shift;;
    --modules=*)
	modules=`echo "$option" | sed 's/--modules=//'` ;;
    --net-directory)
	rootdir=`argument $option "$@"`; shift;;
    --net-directory=*)
	rootdir=`echo "$option" | sed 's/--net-directory=//'` ;;
    --subdir)
	subdir=`argument $option "$@"`; shift;;
    --subdir=*)
	subdir=`echo "$option" | sed 's/--subdir=//'` ;;
    --grub-mkimage)
	grub_mkimage=`argument $option "$@"`; shift;;
    --grub-mkimage=*)
	grub_mkimage=`echo "$option" | sed 's/--grub-mkimage=//'` ;;
    # This is an undocumented feature...
    --debug)
	debug=yes ;;
    --debug-image)
	debug_image=`argument $option "$@"`; shift;;
    --debug-image=*)
	debug_image=`echo "$option" | sed 's/--debug-image=//'` ;;
    # Intentionally undocumented
    --override-directory)
        override_dir=`argument $option "$@"`
	shift
	PATH=${override_dir}:$PATH
	export PATH
	;;
    --override-directory=*)
	override_dir=`echo "${option}/" | sed 's/--override-directory=//'`
	PATH=${override_dir}:$PATH
	export PATH
	;;
    -*)
	echo "Unrecognized option \`$option'" 1>&2
	usage
	exit 1
	;;
    *)
	if test "x$install_device" != x; then
	    echo "More than one install_devices?" 1>&2
	    usage
	    exit 1
	fi
	install_device="${option}" ;;
    esac
done
set $grub_mkimage dummy
if test -f "$1"; then
    :
else
    echo "$1: Not found." 1>&2
    exit 1
fi
# Create the GRUB directory if it is not present.
mkdir -p "${rootdir}/${subdir}" || exit 1
process_input_dir ()
{ 
    input_dir="$1"
    platform="$2"
    grubdir="${rootdir}/${subdir}/${platform}"
    config_opt=
    mkdir -p "$grubdir" || exit 1
    for file in "${grubdir}"/*.mod "${grubdir}"/*.lst "${grubdir}"/*.img "${grubdir}"/efiemu??.o; do
	if test -f "$file" && [ "`basename $file`" != menu.lst ]; then
	    rm -f "$file" || exit 1
	fi
    done
    for file in "${input_dir}"/*.mod; do
        if test -f "$file"; then
            cp -f "$file" "$grubdir/"
        fi
    done
    for file in ${pkglib_DATA}; do
	if test -f "${input_dir}/${file}"; then
            cp -f "${input_dir}/${file}" "$grubdir/"
	fi
    done
    mkdir -p "$grubdir/locale"
    for file in ${input_dir}/po/*.mo; do
        if test -f "$file"; then
	    cp -f "$file" "$grubdir/locale/"
	fi
    done
    rm -f ${grubdir}/load.cfg
    if [ "x${debug_image}" != x ]; then
       echo "set debug='${debug_image}'" >> ${grubdir}/load.cfg
       config_opt="-c ${grubdir}/load.cfg "
    fi
    case "${platform}" in
    	 i386-pc) mkimage_target=i386-pc-pxe;
	 	  netmodules="pxe";
		  prefix="(pxe)/${subdir}/${platform}";
		  ext=0 ;;
    	 *) echo Unsupported platform ${platform};
    	    exit 1;;
    esac
    cat << EOF > ${grubdir}/grub.cfg
source ${subdir}/grub.cfg
EOF
    "$grub_mkimage" ${config_opt} -d "${input_dir}" -O ${mkimage_target} "--output=${grubdir}/core.$ext" "--prefix=$prefix" $modules $netmodules || exit 1
    echo "Netboot directory for ${platform} created. Configure your DHCP server to point to ${subdir}/${platform}/core.$ext"
}
if [ "${override_dir}" = "" ] ; then
    if test -e "${pc_dir}" ; then
        process_input_dir ${pc_dir} i386-pc
    fi
else
    process_input_dir ${override_dir} ${target_cpu}-${native_platform}
fi
# Bye.
exit 0
 |