Generate IPv6 Unique Local Address prefix (https://tools.ietf.org/html/rfc4193#section-3.2.2)

gen-ula-prefix.sh

#!/bin/sh

# Copyright (c) 2020 ryot4
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.

set -e

interface=
mac_addr=
ntp_server=localhost
timestamp=
quiet=0
while getopts hi:m:s:t:q opt; do
    case "${opt}" in
    h)
        echo "$0 [-q] [-i interface] [-m mac_addr] [-s ntp_server] [-t timestamp]"
        exit
        ;;
    i)
        interface="${OPTARG}"
        ;;
    m)
        mac_addr="${OPTARG}"
        ;;
    s)
        ntp_server="${OPTARG}"
        ;;
    t)
        timestamp="${OPTARG}"
        ;;
    q)
        quiet=1
        ;;
    esac
done

for cmd in bc sha1sum; do
    if ! which "${cmd}" > /dev/null 2>&1; then
        echo "${cmd} command is not found" 1>&2
        exit 1
    fi
done

if [ -z "${timestamp}" ]; then
    if ! which ntpq > /dev/null 2>&1; then
        echo 'timestamp not specified and ntpq command is not found' 1>&2
        exit 1
    fi
    timestamp=$(ntpq -c 'rv 0 clock' "${ntp_server}" | sed -e 's/^clock=\([0-9a-f]\+\)\.\([0-9a-f]\+\).*$/\1\2/')
fi

if [ -z "${mac_addr}" ]; then
    if ! which ip > /dev/null 2>&1; then
        echo 'mac_addr not specified and ip command is not found' 1>&2
        exit 1
    fi
    if [ -n "${interface}" ]; then
        mac_addr=$(ip -br link show "${interface}" | awk '{ print $3 }')
    else
        mac_addr=$(ip -br link show | grep -v LOOPBACK | grep UP | head -1 | awk '{ print $3 }')
    fi
fi

# See RFC4193 Section 3.2.2 for the algorithm.

# Invert the seventh bit from the left
# 0 (0000) <-> 2 (0010)
# 1 (0001) <-> 3 (0011)
# 4 (0100) <-> 6 (0110)
# 5 (0101) <-> 7 (0111)
# 8 (1000) <-> a (1010)
# 9 (1001) <-> b (1011)
# c (1100) <-> e (1110)
# d (1101) <-> f (1111)
first_octet=$(echo ${mac_addr} | cut -d: -f1 | tr '[:lower:]' '[:upper:]')
case "${first_octet}" in
*[014589CD])
    first_octet_inverted=$(echo "obase=16; ibase=16; ${first_octet}+2" | bc -q | tr '[:upper:]' '[:lower:]')
    ;;
*[2367ABEF])
    first_octet_inverted=$(echo "obase=16; ibase=16; ${first_octet}-2" | bc -q | tr '[:upper:]' '[:lower:]')
    ;;
esac
second_to_third_octet=$(echo ${mac_addr} | cut -d: -f2-3)
fourth_to_sixth_octet=$(echo ${mac_addr} | cut -d: -f4-6)

eui64=$(echo "${first_octet_inverted}:${second_to_third_octet}:ff:fe:${fourth_to_sixth_octet}" | tr -d :)
global_id=$(printf '%s%s' "${timestamp}" "${eui64}" | sha1sum | cut -c31-40)
ula_prefix=$(echo "fd${global_id}" | sed 's,\(....\)\(....\)\(....\),\1:\2:\3::\/48,')

if [ ${quiet} -eq 0 ]; then
    echo "NTP timestamp:     ${timestamp}"
    echo "MAC address:       ${mac_addr}"
    echo "EUI-64 identifier: ${eui64}"
    echo "ULA Prefix:        ${ula_prefix}"
else
    echo "${ula_prefix}"
fi