Based on http://serverfault.com/questions/47057/is-there-a-tool-to-log-internet-connectivity-on-a-mac

netlog.sh

#!/bin/bash
LOG=/tmp/netlog-$1
rm -f $LOG
while ( true );
do 
   ping -c1 -t 1 $1 > /dev/null
   if [ "$?" -gt "0" ]; then
       echo `date`  " I am offline" | tee $LOG
	   traceroute -a $1 | tee $LOG
   fi

 sleep 5 ;
done