#!/bin/bash

username=your_opendns_username
passwd=your_opendns_password
log_dir=/home/some_user_dir/.opendns

if [ ! -d $log_dir ]; then
  mkdir $log_dir
fi

if [ -e $log_dir/log ]; then
  mv $log_dir/log $log_dir/log.1
fi

date >> $log_dir/log

oldip=`/usr/bin/curl http://nst.sourceforge.net/nst/tools/ip.php -silent -i`

while [ 1 ]
do

  newip=`/usr/bin/curl http://nst.sourceforge.net/nst/tools/ip.php -silent -i`

  if [ $oldip != $newip ]
  then
    /usr/bin/curl -i -m 60 -k -u $username:$passwd 'https://updates.opendns.com/account/ddns.php?' -silent >> $log_dir/log
    echo -e "\n" >> $log_dir/log
    $oldip = $newip
  fi

  sleep 60

done