syslog-probe

#!/bin/sh

IPV4=192.0.2.1
IPV6=2001:db8:ffff:a000:fdc0::3b5

add () {
	[ "$IPV4" ] && [ -z "$(ip addr show dev lo scope global | grep " inet ${IPV4}/32 ")" ] \
			&& ip addr add ${IPV4}/32 dev lo
	[ "$IPV6" ] && [ -z "$(ip addr show dev lo scope global | grep " inet6 ${IPV6}/128 ")" ] \
			&& ip addr add ${IPV6}/128 dev lo

	return 0
}

del () {
	[ "$IPV4" ] && [ -n "$(ip addr show dev lo scope global | grep " inet ${IPV4}/32 ")" ] \
			&& ip addr del ${IPV4}/32 dev lo
	[ "$IPV6" ] && [ -n "$(ip addr show dev lo scope global | grep " inet6 ${IPV6}/128 ")" ] \
			&& ip addr del ${IPV6}/128 dev lo

	return 0
}

stop () {
	del

	kill -STOP $(pgrep -f $SELF)
}

quit () {
	del

	exit 0
}

check () {
	# complexity to try to avoid race conditions, amendments welcomed (I prefer not to use TCP)
	MSG="<135>$(date +"%b %_d %R:%S") 127.0.0.1 $SELF: test"

	# we only test the IPv4 side (local0|debug = 16*8 + 7)
	(nc -u --recv-only -t 1 -w 2 -l -p 5515 -s 127.0.0.1 127.0.0.1 | grep -q "$MSG") &
	JOB=$!

	LOOP=3
	until grep -q ': 0100007F:158B 00000000:0000 ' /proc/net/udp; do
		[ $LOOP -eq 0 ] && { kill -KILL $JOB; del; return; }

		LOOP=$((LOOP-1))
		sleep 1
	done

	echo -n "$MSG" | nc -u --send-only -s 127.0.0.1 127.0.0.1 5514

	wait $JOB

	[ $? -eq 0 ] && add || del
}

go () {
	trap quit TERM
	trap stop TSTP

	while true; do
		check

		sleep 1
	done
}

SELF=$(basename "$0")
PROCS=$(pgrep -f "^/bin/sh $0\$")
RC=$?
if [ $RC -gt 0 ]; then
	echo "unexpected pgrep results (ret=$RC)" > /dev/stderr
	exit 70
fi
if [ "$PROCS" != "$$" ]; then
	echo "'$SELF' is already running" > /dev/stderr
	exit 69
fi

0<&- 1<&- 2<&- go &

exit 0