DNS traffic is often able to pass through security controls with minimal inspection. After all, DNS is essential for internet communications, and inspecting every DNS query would be burdensome for most organizations. Unfortunately, this implicit trust creates opportunities for attackers.
Malicious actors can exploit the DNS protocol, embedding malicious data into DNS queries and responses to stealthily exfiltrate sensitive information or inject harmful instructions into targeted systems. And because DNS traffic is one of the most commonly allowlisted protocols, tunneling attacks can persist undetected for weeks at a time. Understanding the warning signs of a DNS tunneling attack is essential for organizations aiming to safeguard their digital assets.
What Exactly is DNS Tunneling?
Whether you prefer to think of DNS as the internet’s phone book, GPS, or plumbing, the basic gist remains the same: DNS delivers users to their destination. With DNS tunneling attacks, malicious actors hide data inside DNS requests, and since DNS traffic is rarely inspected closely, hidden data travels alongside harmless traffic, going unnoticed.
The end goal is simple: move data or malware from malicious domains and DNS servers into an organization’s network. The commoditization of cybercrime has made it easy for malicious actors to deploy toolkits that establish covert channels and send data through encoded queries to attacker-controlled nameservers. Detecting tunneling attacks requires organizations to scrutinize DNS traffic for unusual patterns, such as abnormal query volume, odd payload sizes, suspicious domains, or unexpected DNS record types.
How DNS Tunneling Threatens Networks
Once malicious actors establish a DNS tunneling channel, they can leverage it for various malicious purposes. What makes DNS tunneling especially dangerous is its stealth: because the malicious traffic blends with normal DNS activity, it can easily fly under the radar. Without proper monitoring and controls in place, organizations may not realize their network has been compromised until after significant damage has been done.
Data Exfiltration and Command Control (C2)
Stealing data and maintaining long-term access are key motivations for many cybercrime activities. In C2 operations, malware embedded in compromised systems leverages DNS tunneling to maintain communication with external servers. Rather than establishing direct outbound connections (which are more likely to be blocked or monitored), the malware uses DNS queries to send updates and receive instructions disguised as ordinary DNS traffic. This allows malicious actors to maintain remote control over infected devices, all while avoiding detection.
What Makes DNS Tunneling So Stealthy
One of the biggest challenges in detecting DNS tunneling is that this attack essentially hides in plain sight. Because it embeds malicious codes, it becomes difficult to differentiate between legitimate and harmful DNS activities. Additionally, threat actors may employ advanced eviction techniques such as randomizing DNS requests and using encrypted channels to further obfuscate their presence.
Breaking Down DNS Tunneling Techniques
To fully understand how DNS tunneling enables covert data transfers, it’s important to look at how attackers encapsulate data within DNS queries and the techniques they use to build and maintain these hidden channels.
Encapsulation of Data in DNS Queries
DNS tunneling hides malicious data within normal DNS traffic. This process, known as encapsulation, allows malicious actors to exfiltrate sensitive information or maintain covert communication channels without raising alarms. By embedding data inside DNS queries and responses, attackers exploit the protocol’s typically under-scanned nature. Though this method requires additional encoding and overhead, it is highly effective at bypassing security controls that treat DNS traffic as benign.
Types of DNS Tunneling
All DNS tunneling attacks work toward creating covert channels for data exfiltration or C2. Malicious actors embed data payloads inside DNS queries and responses, allowing compromised systems to communicate with attacker-controlled servers. For DNS tunneling to be effective, a compromised device typically needs access to an internal DNS resolver that can forward queries to the internet, creating a path for malicious network traffic to leave the organization.
Detecting these attacks can be tricky because they utilize encryption and encapsulation to disguise malicious traffic and blend in with normal DNS flows. Without proper monitoring, this suspicious activity can go unnoticed for extended periods. Because DNS tunneling can originate from devices within internal networks, gaining visibility into outbound DNS network traffic is critical to identifying and stopping these covert channels.
Common types of DNS tunneling include:
- Subdomain-based tunneling: Data is encoded into long, unique subdomains and sent via DNS queries to an attacker-controlled domain.
- TXT record tunneling: DNS TXT records are used to transmit data payloads in DNS responses, which are often less scrutinized than A or AAAA records.
- NULL or uncommon record types: Certain tunneling tools use less common record types (like NULL or CNAME) to hide malicious traffic and evade basic detection.
- Fast flux tunneling: Malicious actors frequently rotate the IP addresses or domains involved in the tunneling to avoid pattern-based detection.
How to Spot DNS Tunneling on Your Network
Detecting DNS tunneling starts with knowing what normal DNS traffic looks like and watching for anomalies. After all, you can’t spot anomalous behavior if you don’t know what normal traffic looks like on your network. As with any cyberattack (even stealthy ones), there are warning signs, albeit subtle ones, such as unusually large DNS query or response sizes, spikes in DNS query volume, or odd patterns, such as frequent requests to rarely used domains.
By monitoring for high volumes of queries, unusually large query sizes, or uncommon query types, security teams can gain deeper insights into normal network behavior and spot anomalies that may indicate DNS tunneling. As a best practice, teams should also analyze historical DNS logs for persistent or repetitive queries to unusual domains.
For example, it’s normal to see steady DNS traffic to business-critical domains (such as www.company.com or mail.company.com). However, persistent traffic or spikes in requests for strange or randomized subdomains may point to malicious activity. Combining DNS monitoring with other security controls helps organizations detect and disrupt covert channels, minimizing the risk of disruptions to the broader network.
Tools for DNS Tunneling Detection
Anomaly detection is crucial for detecting and preventing DNS tunneling attacks. Some of the same tools that malicious actors leverage to perpetuate DNS tunneling attacks can also aid security teams in defending their networks.
Open-source tools like DNScat2, Iodine, and Heyoka, which malicious actors commonly use to test tunneling methods, can also be leveraged by security teams to test their detection capabilities. They help internal teams spot encoded DNS names that don’t follow typical dictionary patterns, randomized subdomains, or queries that exceed normal size limits. Recognizing these signals can help identify potential tunneling activities, as detecting malicious DNS traffic requires close examination for suspicious patterns.
Teams may also elect to leverage external DNS monitoring services, and DNS analytics tools, such as Catchpoint, can help identify suspicious DNS query behavior at the authoritative level. DNS providers can also offer DNS traffic analytics and historical query logs at the authoritative level, giving organizations greater visibility into unusual query patterns. When combined with external monitoring services and internal detection tools, this insight helps security teams identify and respond to potential tunneling activity.
Preventing DNS Tunneling Attacks
As with most prevention techniques, the best way to safeguard your organization against DNS tunneling attacks begins with understanding how these attacks work and implementing a layered defense strategy to safeguard against potential threats. Together, these practices form a frontline defense against DNS-based attacks.
Implementing DNS Security Extensions (DNSSEC)
DNSSEC is used to add cryptographic authentication to verify the authenticity and integrity of DNS data. DNSSEC helps defend against common DNS attacks such as cache poisoning, man-in-the-middle (MitM), and domain hijacking by ensuring DNS responses are authenticated and haven’t been tampered with.
While DNSSEC alone does not stop tunneling attacks, it contributes to overall DNS integrity, making other attacks more difficult for malicious actors to execute. Combining DNSSEC with traffic analysis and anomaly detection helps organizations improve their security posture and resilience against DNS-based threats.
Configuring Firewall Rules
Configuring firewall rules is an essential tactic for reducing DNS tunneling risk. Firewalls can be used to block unauthorized DNS traffic by forcing all DNS traffic through designated resolvers or by blocking direct DNS queries to unapproved servers on the internet. While firewall rules alone won’t detect tunneling in DNS traffic, properly configured firewalls can inspect and filter DNS queries, allowing only legitimate traffic. This restricts an organization’s available attack surface and aids internal teams in identifying suspicious activities that could indicate tunneling. Effective firewall rules ensure that only trusted DNS requests are permitted, fortifying your network against potential threats.
Implement a Regular Patch Cadence
Keeping systems patched is a core component of any good security strategy and helps improve an organization’s overall security posture. Many malware strains that initiate DNS tunneling exploit unpatched vulnerabilities. Applying security patches across operating systems, DNS servers, and endpoints helps reduce the risk of compromise and the likelihood of malware establishing covert DNS channels.
Integrating Detection and Prevention Measures
Regular threat hunting, supported by DNS analysis and threat intelligence, should also be part of a proactive security strategy. Analyzing DNS logs and up-to-date threat intelligence helps uncover and mitigate potential issues before they escalate. When combined with automated response systems, these measures allow organizations to quickly detect and contain tunneling attempts, reducing the risk of data exfiltration or C2 activity.
Ready to Strengthen Your DNS Defenses?
Proactive DNS monitoring is key to detecting covert tunneling activity and protecting your network. With UltraDNS, you can enjoy the visibility, control, and security necessary to detect and disrupt tunneling attacks.
With built-in capabilities to detect anomalous DNS traffic and support for private DNS data lakes, UltraDNS enables faster detection of tunneling, C2 activity, and infrastructure abuse, all while improving DNS availability for your users.
Ready to strengthen your defenses against DNS-based threats? Contact us today to learn more and schedule a demo.
