DNS Tunnelling

A recent project at Nexor required us to look at the challenges of providing access to the DNS from a secure environment. It reminded me of the issues related to DNS tunnelling.

Heatpipe tunnel copenhagen 2009

DNS tunnelling enables a user to run a full TCP/IP stack over the top of the DNS protocol. This is especially useful when a firewall is blocking outbound ports.

DNS tunnelling is not new, and surprisingly simple to do. For example, the following DNS query:


Will pass the message ‘secret_data_sent_via_dns’ to the server infoleak.nexor.com (data leaving network). The server might respond:

Response.infoleak.nexor.com. 0 IN TXT
Message received – thanks

Thus two way data communication has occurred. Once you have two way communication, you can run any communication protocol of your choice, including TCP/IP.

Sadly DNS tunnelling means a firewall is now pretty much useless as a tool to control the network traffic that leaves a business. As an attacker, once you have managed to get software to execute on the inside of a firewall, you can use the DNS to set up a communication tunnel to get data out (or more malware in).

Is this a problem that concerns your business? If so, contact Nexor to find out how we can help solve the problem.