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What Is DNS Tunneling and How to Detect and Prevent Attacks

What Is DNS Tunneling and How to Detect and Prevent Attacks

Admir Dizdar

What Is DNS Tunneling?

DNS tunneling is a DNS attack technique that involves encoding the information of other protocols or programs in DNS queries and responses. DNS tunneling generally features data payloads which can latch onto a target DNS server, allowing the attacker to manage applications and the remote server. 

DNS tunneling tends to rely on the external network connectivity of the compromised system—DNS tunneling needs a way into an internal DNS server that has network access. Attackers also have to control a server and a domain that may function as an authoritative server to carry out data payload executable programs and server-side tunneling. 

In this article:

Impact of DNS Tunneling

DNS was first created for name resolution rather than for data exchange, thus it’s often not viewed as a risk for data exfiltration or malicious interchanges of information. Most organizations focus their security efforts on web and email traffic, as they see this as a regular source of attacks. As a result, DNS is often overlooked. 

DNS is a confided and entrenched protocol, so cybercriminals can take advantage of the fact that many organizations don’t often investigate DNS packets for malevolent behavior. 

Aside from this, tunneling application bundles are now an industry and are widely accessible via the internet. An attacker doesn’t need to be particularly sophisticated to carry out DNS tunneling exploits.

The threats posed by DNS tunneling exploits include:

  • DNS tunneling exploits may provide attackers with an accessible backchannel to exfiltrate stolen information. DNS provides a covert means of correspondence to bypass firewalls.
  • Cybercriminals tunnel different sorts of protocols, such as HTTP or SSH, with DNS, which allow them to covertly pass stolen data or pass IP traffic. 
  • The DNS tunnel may be used as a full controller channel for an inside host that has already been exploited. This allows cybercriminals to download code to malware, secretly take records out from the organization, or have complete distant entry to the servers, and more.
  • DNS tunnels can also be used to sidestep captive portals, so they don’t need to pay for wi-fi services.
  • DNS tunneling uses the DNS protocol to tunnel information and malware via a client-server model.

Typical abuse cases include:

  • Data exfiltration—cybercriminals extract sensitive information over DNS. This is not the most effective approach to obtaining data from a victim’s PC, given all the additional encoding and overheads, but it does work.
  • Command and control (C2)—cybercriminals utilize the DNS protocol to dispatch simple commands to, for example, install a remote access trojan (RAT).
  • IP-over-DNS tunneling—some utilities may have actualized the IP stack via the DNS inquiry reaction convention. These make malicious movements simpler.

Learn more in our detailed guide to dns amplification attack.

How DNS Tunneling Works

DNS tunneling makes use of the DNS protocol for tunneling malware and different data via a client-server model. This typically involves the following steps:

  1. The cybercriminal registers a domain, for example malsite.com. The domain’s name server directs to the cybercriminal’s server, where the tunneling malware software is installed. 
  2. The cybercriminal infects a computer with malware, which penetrates the organization’s firewall. DNS requests are always permitted to move in and out of the firewall, so the infected computer is permitted to send queries to the DNS resolver. The DNS resolver then sends requests for IP addresses to top-level and root domain servers. 
  3. The DNS resolver routes queries to the cybercriminal’s server, where the tunneling program is implemented. A connection is thus created between the cybercriminal and the victim via the DNS resolver. The attacker can use this tunnel for malicious ends, such as exfiltrating information. There is no direct connection between the cybercriminal and the victim, so it is harder to trace the cybercriminal’s computer.  

5 Techniques and Tools to Detect DNS Tunneling

1. Anomaly Detection

Anomaly detection is a powerful tool in identifying potential DNS tunneling. This method involves monitoring DNS traffic and looking for patterns or behaviors that deviate from the norm. If a particular DNS request or response seems unusual, it may be a sign of DNS tunneling.

For example, if a DNS query contains unusually large amounts of data, or if there are an excessive number of DNS requests from a particular source, this could indicate DNS tunneling activity. The challenge here, however, is defining what constitutes ‘normal’ behavior. This can vary greatly depending on the nature of the network and its typical usage.

2. Payload Analysis

Payload analysis involves examining the actual data being transmitted in DNS queries and responses. This can be a very effective method for detecting DNS tunneling, as the data being transmitted in a tunneling scenario will often be quite different from that of a normal DNS query or response.

However, payload analysis can be quite resource-intensive. It requires a significant amount of processing power and storage capacity, as well as advanced knowledge of DNS protocols and data structures. In addition, encrypted payloads can be difficult to analyze, making this method less effective in some cases.

3. Rate Limiting

Rate limiting is a technique that limits the number of DNS queries that can be made from a particular source in a given time period. The idea is that by limiting the rate of queries, it becomes harder for an attacker to use DNS tunneling to exfiltrate data or gain unauthorized access.

While this can be an effective method in many cases, it’s not without its drawbacks. Aggressive rate limiting can interfere with legitimate DNS traffic and negatively impact network performance. It also doesn’t prevent DNS tunneling entirely—it merely slows it down.

4. Intrusion Detection Systems (IDS)

Intrusion Detection Systems (IDS) are a class of security software that monitors network traffic for signs of malicious activity. Many IDS solutions are capable of detecting DNS tunneling by looking for patterns and behaviors indicative of such activity.

An IDS can be a valuable tool in detecting DNS tunneling, but it’s not a silver bullet. Like any detection method, it has its limitations and potential for false positives. Furthermore, an IDS is only as good as the rules and signatures it uses to detect threats. If the IDS isn’t kept up-to-date with the latest threat information, its effectiveness can be significantly reduced.

5. DNS Monitoring Tools

Finally, there are a number of specialized tools available that are specifically designed to monitor DNS traffic and detect DNS tunneling. These tools often combine several of the above methods into a single package, offering a more comprehensive solution for detecting DNS tunneling. These tools can be quite effective, but they also require a certain level of expertise to use effectively.

Related content: Read our guide to dns flood attack.

Best Practices to Prevent DNS Tunneling Attacks

Preventing DNS tunneling attacks requires a multi-faceted approach, combining both technical and human elements. Here are some best practices that can help organizations protect their networks from this type of attack.

Regularly Monitor DNS Traffic

Continuous monitoring of DNS traffic is the first line of defense against DNS tunneling attacks. Look for anomalies such as an unusually high volume of DNS queries, large DNS text records, or DNS requests to unknown or suspicious domains. Advanced threat detection solutions can help automate this process, providing real-time monitoring and alerting.

Implement DNS Security Extensions (DNSSEC)

DNSSEC is a suite of extensions that add a layer of security to the DNS protocol. It uses digital signatures to verify the authenticity of DNS data, preventing spoofing and other DNS-based attacks. However, DNSSEC does not directly prevent DNS tunneling as it does not inspect the payload of DNS packets. It can, however, be combined with other measures to enhance overall DNS security.

Utilize Firewall Rules

Firewalls can be configured to block outgoing DNS traffic to all but authorized DNS servers. This can help prevent DNS tunneling attacks by limiting the ability of malicious actors to communicate with their DNS servers. In addition, firewalls can be used to inspect and filter DNS traffic, blocking suspicious DNS queries or responses.

Limit or Block Unnecessary DNS Queries

Blocking or limiting unnecessary DNS queries can help reduce the attack surface for DNS tunneling. This includes blocking DNS queries to non-existent domains and limiting the rate of DNS queries from a single source. DNS servers can be configured to reject queries for certain record types commonly used in DNS tunneling, such as TXT or NULL records.

Regular Patching and Updates

Keeping systems, software, and network devices updated is a critical aspect of network security. Regular patching can help protect against vulnerabilities that could be exploited to carry out DNS tunneling attacks. Security patches should be applied as soon as they are available, and updates should be tested before deployment to ensure they do not introduce new vulnerabilities.

DNS Tunneling Protection with Bright Security

Bright has been built from the ground up with a dev first approach to test your web applications and APIs for hundreds of vulnerabilities.

Bright complements DevOps and CI/CD processes, empowering developers to detect and fix vulnerabilities on every build. It reduces the reliance on manual testing by leveraging multiple discovery methods:

  • HAR files
  • OpenAPI (Swagger) files
  • Postman Collections

Get a free Bright Security account and prevent DNS tunneling attacks on your applications.

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