#!/bin/sh
# check which IP is reachable since the notebook can be
# connected via either Ethernet or WiFi, or both;
# adjust portforwarding (which must otherwise be
# already set up correctly -- like not forgetting
# -A INPUT -d $IP -p tcp -m tcp --dport $DPT -j ACCEPT
#
# try to ping an already existing portforward target;
# if unreachable or none, try to ping/add another one
#
# (c) 2009 Michael Shigorin <mike altlinux org>
# use, modify, distribute, cronjob this freely


PATH=/sbin:$PATH

IP=EX.TER.NAL.IP	# 123.45.67.8
IP_E=NOTE.BOOK.ETH.IP	# 192.168.1.2
IP_W=NOTE.BOOK.WIFI.IP	# 192.168.1.3
PORT=EXT_PORT		# 12345
DPT=DEST_PORT		# 22

COUNT=0
IFS=":"

ipt()
{
	iptables "$1" PREROUTING -t nat \
		-d "$IP" -p tcp -m tcp \
		--dport "$PORT" \
		-j DNAT --to-destination "$2:$DPT"
}

remove() { ipt -D "$1"; }
add() { ipt -A "$1"; }

try() { ping -w1 "$1" >&/dev/null; }

# is current one reachable?
COUNT="`iptables -L PREROUTING -t nat -n \
| fgrep "dpt:$port" \
| (
    while read header to dest rest; do
	try "$dest" && let COUNT=$COUNT+1 || remove "$dest"
    done; echo $COUNT
  )`"

# any job left?
[ "$COUNT" = 0 ] || exit

for ip in "$IP_W" "$IP_E"; do
	try "$ip" && let COUNT=$COUNT+1 && add "$ip" && break
done

# any job still left?
[ "$COUNT" = 0 ] || exit

# safety net: add via wifi if nothing else currently possible
add "$IP_W"