#!/bin/sh
# man 8 ip-neighbour:
# STATE := { permanent | noarp | stale | reachable | none | incomplete | delay | probe | failed }
ARPSTATES='(PERMANENT|NOARP|STALE|REACHABLE|NONE|INCOMPLETE|DELAY|PROBE|FAILED)'
[ "$(printf printf)" = printf ] || {
	echo "This script requires a POSIX compliant shell (lacking 'printf')!" >&2
	exit 1
}
die() {
    C=$1 ; shift 
    P="$1"; shift
    [ "$C" -eq 1 ] || [ $V ] && printf "\n$P\n" "$@" >&2
    exit $C
}
for x in 'getent head grep ping ip sort uniq'; do
	which $x>/dev/null || die 1 'This scripts depends on %s.\nExecutable "%s" is not found in $PATH ... aborting!\n' "$DEPS" $x 1>&2
done
V='y' D=''
[ "$1" = "-d" ] && { D=y ; shift ;}
[ "$1" = "--lladdr" ] && { V='' ; shift ;}
[ "$1" = "-d" ] && { D=y ; shift ;}
debug() {
    [ "$D" ] || return
    P="$1"; shift
    printf "$P\n" "$@" >&2
}
A="$1"
[ $# -gt 0 ] && shift
[ "$1" ] && die 1 'USAGE: %s [-d] [--lladdr] HOSTNAME | IPv4ADDRESS' "${0##*/}"
[ "$A" ] || die 1 'USAGE: %s [-d] [--lladdr] HOSTNAME | IPv4ADDRESS' "${0##*/}"
printf %s "$A" | grep -q '^-' && die 1 'USAGE: %s [-d] [--lladdr] HOSTNAME | IPv4ADDRESS' "${0##*/}"

R=$(getent ahostsv4 "$A" | head -1 | grep -oE '^[0-9.]+')
[ "$R" ] || die 1 'ERROR: Unable to resolve "%s"!' "$A"
A=$R

if [ $D ]; then 
    ping -w1 -c1 "$A" >&2
else
    ping -w1 -c1 "$A" >/dev/null 2>&1
fi

IFC=$(ip route get $A | grep -v via | grep -oE 'dev [^ ]+' | cut -d' ' -f2)
[ "$IFC" ] || die 2 '"%s" not in local scope!' "$A"

STATE=$(ip neigh get dev $IFC $A | grep -oE "$ARPSTATES")
[ "$STATE" ] || die 3 'No arp entry for "%s"!' "$A"
debug 'STATE=%s' $STATE
printf '%s' "$STATE" | grep -qE 'STALE|NONE|INCOMPLETE|DELAY|PROBE' && {
    ping -q -w1 -c1 "$A" >/dev/null
    sleep 1
    STATE=$(ip neigh get dev $IFC $A | grep -oE "$ARPSTATES")
    debug 'STATE=%s' $STATE
}
while printf %s "$STATE" | grep -qE 'STALE|NONE|INCOMPLETE|DELAY|PROBE'; do
    sleep 0.5
    STATE=$(ip neigh get dev $IFC $A | grep -oE "$ARPSTATES")
    debug 'STATE=%s' $STATE
done

R=$(ip neigh show to "$A")
printf %s "$R" | grep -q ' lladdr ' || die 4 '%s\n\nNo link local address for "%s"!' "$R" "$A"

if [ "$V" ]; then printf '%s\n' "$R"
else printf %s "$R" | grep -oE 'lladdr\s+([0-9a-f:]+)' | grep -oE '([0-9a-f:]+)$' | sort | uniq; fi
exit 0