OpenDNS with DNS-O-Matic

A while ago I wrote a short article that I found a nice way to \”secure\” or at least be able to monitor my childrens\’ web behavior called OpenDNS. I soon found out that you have at least one problem and that is dynamic IP addresses which your ISP shoves to you when you link up your router. Problem is these are never the same and the DHCP lifetime is 0 seconds. So even in a small link bounce of 2 or 3 seconds you get a new IP address on your WAN side.

This renders the security features of OpenDNS (DNS Domain blocking) more or less useless since the DNS queries that are now made from one of your PC on the LAN side are now exposed to the OpenDNS with another public IP address and OpenDNS can therefore not link your profile to this address.

So lets take an example:
Your internal LAN is using and is NAT-ed on your router to the outside world. Your ISP provides you with an adress of, let say,

On the OpenDNS website you create a profile called \”My Home network\” and you link  this address to the profile. The profile also allows you to block certain websites manually or entire categories like Adult, Weapons, Gambling etc. so all in all important to keep this away from your children.
Now what happens if one of your computers does a DNS query is that OpenDNS takes the from address (ie your public IP address, link this to your profile to verify if your requested page/domain falls in one of the criteria you configured and if the action is for this site to be blocked it redirects you to a page which just shows an explanation why this site is blocked. You can customize this as well.

The problem is however that if your ISP provided address changes OpenDNS cannot link this WAN ( address to your profile anymore and will just return the IP address of that site after which your computer just connects to it and shows the page.

This so called Dynamic IP address problem is also acknowledged by OpenDNS and their recommendation is in these cases to install a little tool which on regular intervals checks if this address changes or not and if it has it updates your OpenDNS profile with the new address. \”Problem solved\” you might say. Well, not exactly. The problem is that this little tool has to be installed on a PC which either runs Windows or MaxOS. Secondly this PC has to be secured from tampering since kids become smarter as well and it gives them the option to just remove this or fumble around as they seem fit which in essence renders it useless. I also don\’t want too much of these tools installed on PC\’s since I\’m being seen as the household admin I want to do as little as possible. Admins should be lazy. Improves effectiveness 🙂 I decided not to use this agent so this has put me in some sort of catch22 situation. Again I should be lazy from an admin standpoint so I don\’t have the time nor urge to check the OpenDNS website every 10 minutes if my address has changed so I worked something out with another service from OpenDNS which is called DNS-O-Matic (DOM). This service allowed me to write a simple script which enbled me to automate the entire process.

So In my case I\’ve done the following.
I have an OpenDNS account with a network profile which blocks certain categories of websites.
Next to that I created an DOM account and linked the OpenDNS service to the DOM account. This basically means that if I update DOM with my new, ISP provided, IP address it will propagate this to my OpenDNS account. (DNS-O-Matic provides many more options to link this service to but I leave this up to you to check this out.)

Now you might say \”How does this fix things?\”. Well, the solution is easy. DOM provides a simple API which you can write a script or program against. This allows you to update DOM automatically via this API which in turn updates your OpenDNS profile with your new IP address. So the first thing you need to do is obtain your current IP address. If you query the OpenDNS servers with the destination it will always return your actual (ISP provided) IP address. (This is basically the source address on which the OpenDNS service should return the answers to).
Next thing you need to do is to verify if this address is the same as your \”old\” address and if not, update DOM with this new address.

I made a little script which I hooked up to cron so it does this for me automatically every 5 minutes.

## Script to update OpenDNS and DNS-O-Matic
## Check opendns is linked to this.
## Documentation
## This script runs in cron every 5 minutes.

## First get your public IP address
ip=$(dig @ +short)
## Get my IP I know I use to have from a hidden file
oldip=$(cat /home/erwin/.oldip)

## If needed update the IP address on the web. If not do nothing.
if [ $ip != $oldip ]


## Write the new IP address to the hidden file again.
echo $ip > /home/erwin/.oldip


That\’s it. I\’m sure this can be achieved on Windows as well with either batch files or commandlets and vb script but I just had bash at hand.

My crontab entry looks like this:

*/5 * * * * /home/erwin/Desktop/scripts/DNS-O-Matic/

And it works perfectly I must say.

Now there are two \”Gotchas\”:

  1. How do you prevent from kids just choosing another DNS service like the default ones that come with your ISP.
  2. This still requires you to have your computer online.

The answer to 1 is to create a frame redirect rule in your router firewall so that every DNS query (UDP port 53) is directed to OpenDNS. And the answer to 2 is \”You are correct :-)\”. Since I work from home my Linux box is always on. (At least during the time I\’m working and during the time my kids are allowed on the net.

Some newer generation routers have this functionality build in so its a one time setup on your router and you wouldn\’t have to worry about it anymore.

Hope this helps in one of your situations.


Leave a Comment

Scroll to Top