Dec 15


My Internet Service Provider changes my externally-visible IP Address from time to time without notice. I am having issues with the ISP’s service and I decided to keep a watch on my Internet connectivity unsing external monitoring services. Since I donlt have a static IP address visible to the Internet I needed to work out a way to make sure that the monitoring configuration gets to use the current IP address of my monitoring endpoint despite the ISP changing the IP address. I use Dynamic DNS service. More on this in a subsequent article.

In this article I present a simple Unix script which keeps track of my changing external IP addresses so that I can see what they were at different points in time and when they were changed. This script attempts to resolve the domain name of the host to its I address as means of obtaining the IP address which is visible form the Internet.


I have a pfSense firewall (, which is based on the FreeBSD OS. The script being discussed runs on this firewall.

The purpose of the script is to resolve the externally-visible host name of my externally-visible host, compare it to the last IP address that was resolved and, if they are different, update the last resolved IP Address and log a change to the system log and the private log created for the purpose.

Set prerequisites (manually create myip_last.log for the first time):

mkdir -pv ~/ipaddresses
# seed for the first time
myip=$(nslookup | grep Address | tail -n 1 | cut -c10-)
echo "$(date -jR) - ${myip}" > ~/ipaddresses/myip_last.log

Create the script:

> ~/ # create empoty script file
chmod u+x ~/ # set permissions to allow script execution

cat <<-'EODECK' > ~/ # create the script via a HERE-document
myip=$(nslookup | grep Address | tail -n 1 | cut -c10-)
olip=$(cat ~/ipaddresses/myip_last.log|cut -d'-' -f2|tr -d ' ')

if [ $myip != $olip ]; then
    echo "$(date -jR) - ${myip}" > ~/ipaddresses/myip_last.log
    cat ~/ipaddresses/myip_last.log >> ~/ipaddresses/myip.log
    logger -p user.crit "IP Address change: $(cat ~/ipaddresses/myip_last.log)"


Add this script to crontab and run it every minute

crontab -eAdd the following line
*/1   *    *    *    *     /root/ 2>/root/ipaddresses/log_ip_change.err

Give it 1 minute and see what the logs say

cat ~/ipaddresses/myip_last.log
cat ~/ipaddresses/myip.log


In this article I present a script which watches and records my externally-visible IP addresses, as changed bymy ISP form time to time, by resolving my externally-visible host name to the IP address – call me curious 🙂

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