ndpd

#!/bin/bash

# NDPD - Neighbor Discovery Protocol Daemon
# 
# A simple Bash script wrapper around `ndisc6`` that runs as a daemon. 
# It monitors all network interfaces for newly assigned IPv6 addresses and sends an NDP (Neighbor Discovery Protocol) probe to the appropriate gateway when a new address is detected.
# Includes support for automatic interface and gateway detection.
#
# @author	Black HOST Ltd.
# @version 	0.1
# @since 	2025-07-25

# determine the path to the ndisc6 executable
NDISC6_PATH="$(command -v ndisc6 || true)"

# Ensure the script is run as root, which is required for network monitoring.
if [[ $EUID -ne 0 ]]; then
	echo "This script must be run as root" 
	exit 1
fi

# dependecy check
if [[ ! -x "$NDISC6_PATH" ]]; then
	echo "Error: ndisc6 not found or not executable at ${NDISC6_PATH}."
	echo "Please install the 'ndisc6' or 'iputils-ndisc6' package and ensure the path is correct."
	exit 1
fi

# A simple function to send log messages to the system journal.
log() {
	logger -t "NDPD" "$1"
}

# trap SIGINT and SIGTERM to cleanly exit
trap "log 'Daemon exiting'; exit 0" SIGINT SIGTERM

log "Daemon started."

# watch for IPv6 address changes on ALL interfaces.
ip -6 monitor address | while read -r -a line_parts; do

	# ignore deletion events
	[[ "${line_parts[0]}" == "Deleted" ]] && continue

	# fetch interface and the new IPv6 address
	INTERFACE="${line_parts[1]}"
	IPv6="${line_parts[3]}"

	# skip if address format is malformed
	[[ -z "$INTERFACE" || -z "$IPv6" ]] && continue

	# determine the scope
	scope=""
	for i in "${!line_parts[@]}"; do
		if [[ "${line_parts[$i]}" == "scope" && -n "${line_parts[$i+1]}" ]]; then
			scope="${line_parts[$i+1]}"
			break
		fi
	done

	# if the scope is not global, skip to the next event.
	if [[ "$scope" != "global" ]]; then
		continue
	fi

	# clean the IP address by removing the prefix length (e.g., /64).
	IP=${IPv6%%/*}
	log "Detected new global IP ${IP} on interface ${INTERFACE}."

	# determine the default IPv6 gateway for this specific interface.
	GATEWAY=$(ip -6 route show default dev "${INTERFACE}" | awk '/via/ {print $3}')

	if [[ -z "$GATEWAY" ]]; then
		log "Could not determine default gateway for interface ${INTERFACE}. Skipping probe."
		continue
	fi

	log "Found gateway ${GATEWAY} for ${INTERFACE}. Sending NDP probe..."

	# anounce the new IP on the router by send an NDP probe
	if "$NDISC6_PATH" "${GATEWAY}" "${INTERFACE}" -s "${IP}"; then
		log "Successfully sent NDP probe for ${IP} via ${GATEWAY}."
	else
		log "ERROR: Failed to send NDP probe for ${IP}."
	fi

done