query-ndp-for-ipv6-neighbor.sh
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query-ndp-for-ipv6-neighbor.sh
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	#!/bin/sh
	# shellcheck disable=SC2086

	# -------------------------------------------------------------
	# rOPS — determine IPv6 router with neighbor discovery protocol
	# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
	# Project: Nasqueron
	# Description: Read the output of `ndp -an` to find the first
	# IP address not matching one of the server MAC.
	# That should be a de facto gateway.
	# -------------------------------------------------------------

	set -e

	# -------------------------------------------------------------
	# Parse arguments
	# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

	if [ -z "$1" ]; then
	echo "Usage: $0 <interface>"
	echo "Example: $0 igb0"
	exit 1
	fi

	IFACE="$1"

	# -------------------------------------------------------------
	# Collect local MAC addresses
	# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

	LOCAL_MACS=$(ifconfig "$IFACE" \| while read -r line; do
	case "$line" in
	ether) set -- $line; echo "$2";;
	esac
	done)

	LOCAL_MACS_STR=""
	for m in $LOCAL_MACS; do
	LOCAL_MACS_STR="$LOCAL_MACS_STR $m"
	done

	# -------------------------------------------------------------
	# Parse ndp -an output
	# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

	skip=1
	ndp -an \| while read -r ipv mac rest; do
	# Skip header or incomplete entrie
	if [ $skip -eq 1 ]; then skip=0; continue; fi
	[ "$mac" = "(incomplete)" ] && continue

	# Skip local MACs
	found=0
	for m in $LOCAL_MACS_STR; do
	[ "$mac" = "$m" ] && found=1 && break
	done
	[ $found -eq 1 ] && continue

	# We've got a winner!
	case "$ipv" in
	%) ipv=${ipv%%%*} ;;
	esac

	echo "$ipv"
	break
	done

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