Connection status checker with iptables-conditions for dynamic routing, load balancing and failover

_up.sh

#!/bin/sh
#
# cron ex: @reboot /usr/bin/nice -n 19 /usr/bin/ionice -c2 -n7 /root/scripts/100_up.sh >> /root/scripts/cron.log
while true ; do
 # do ping with interface
 if /bin/ping -qc 3 8.8.8.8 -I eth0.100 > /dev/null; then
  if /bin/grep -q 0 /proc/net/nf_condition/100_up; then /bin/echo 1 > /proc/net/nf_condition/100_up; fi
 else
  # do ping with fwmark, and to another target
  if /bin/ping -qc 3 8.8.4.4 -m 100 > /dev/null; then
    if /bin/grep -q 0 /proc/net/nf_condition/100_up; then /bin/echo 1 > /proc/net/nf_condition/100_up; fi
   else
    if /bin/grep -q 1 /proc/net/nf_condition/100_up; then /bin/echo 0 > /proc/net/nf_condition/100_up; fi
   fi
 fi
  sleep 3
done

This example assumes you have a eth0.100 interface, which has reachable valid default route and reachable number 100 fwmark.