What is a DNS tunnel?
DNS tunneling is a DNS attack technique that involves encoding information from other protocols or programs in DNS queries and responses. DNS tunnels often have data payloads that can lock down the target DNS server, allowing attackers to manage applications and remote servers.
DNS tunneling often relies on the compromised system's external network connectivity - DNS tunneling requires a way into an internal DNS server that has network access. The attacker must also control servers and domains that can act as authoritative servers to execute data payload executables and server-side tunneling.
Impact of DNS Tunneling
DNS was originally created for name resolution rather than data exchange, so it is generally not considered a risk for data breaches or malicious information exchange. Most organizations focus their security efforts on network and email traffic because they view it as a regular source of attacks. Therefore, DNS is often overlooked.
DNS is a secret and deeply entrenched protocol, so cybercriminals can take advantage of the fact that many organizations don't routinely investigate DNS packets for malicious behavior.
In addition to this, tunneling application packages are now an industry and widely accessible via the Internet. An attacker does not need to be particularly sophisticated to perform a DNS tunneling attack.
Threats posed by DNS tunneling vulnerabilities include:
A DNS tunneling vulnerability could provide an attacker with an accessible back channel to exfiltrate stolen information. DNS provides a covert means of communication that bypasses firewalls.
Cybercriminals establish different types of protocols (such as HTTP or SSH) through DNS, which allows them to secretly pass stolen data or pass IP traffic.
The DNS tunnel can be used as a full controller channel for an internal host that has been exploited. This allows cybercriminals to download code into malware, covertly obtain records from an organization, or gain full remote access to servers, among other things.
DNS tunneling can also be used to bypass captive portals so they don't have to pay for Wi-Fi service.
DNS tunneling uses the DNS protocol to tunnel information and malware through a client-server model.
Typical abuse cases include:
Data Breach: Cybercriminals extract sensitive information via DNS. Considering all the extra coding and overhead, this isn't the most efficient way to obtain data from the victim's PC, but it does work.
Command and Control (C2): Cybercriminals leverage the DNS protocol to send simple commands, such as installing a remote access Trojan (RAT).
IP-over-DNS Tunneling: Some utilities may have implemented the IP stack through DNS query response conventions. These make malicious actions easier.
How DNS tunneling works
DNS tunneling utilizes the DNS protocol to transmit malware and disparate data through a client-server model. This usually involves the following steps:
1. Cybercriminals register a domain name, such as malsite.com. The domain's name servers pointed to the cybercriminal's servers where the tunneling malware was installed.
2. Cybercriminals infect computers with malware that penetrates an organization's firewall. DNS requests are always allowed in and out of the firewall, so an infected computer can send queries to the DNS resolver. The DNS resolver then sends requests for IP addresses to top-level domain servers and root domain servers.
3. The DNS resolver routes the query to the cybercriminal’s server that implements the tunneling process. In this way, a connection is established between the cybercriminal and the victim through the DNS resolver. An attacker can exploit this tunnel for malicious purposes, such as exfiltrating information. There is no direct connection between the cybercriminal and the victim, making it more difficult to trace the cybercriminal's computer.
5 techniques and tools to detect DNS tunnels
1. Anomaly detection
Anomaly detection is a powerful tool for identifying potential DNS tunnels. This method involves monitoring DNS traffic and looking for patterns or behavior that deviate from the norm. If a specific DNS request or response looks unusual, it may be a sign of DNS tunneling.
For example, if a DNS query contains an unusually large amount of data, or there is an excessive number of DNS requests from a particular source, this may indicate DNS tunneling activity. The challenge here, however, is defining what constitutes "normal" behavior. This can vary significantly depending on the nature of the network and its typical uses.
2. Payload analysis
Payload analysis involves examining the actual data transmitted in DNS queries and responses. This can be a very effective method of detecting DNS tunnels, since the data transmitted in a tunnel scenario is often very different from that of a normal DNS query or response.
However, payload analysis can be very resource intensive. It requires significant processing power and storage capacity, as well as advanced knowledge of DNS protocols and data structures. Additionally, encrypted payloads can be difficult to parse, making this method less effective in some cases.
3. Rate Limiting
Rate limiting is a technique that limits the number of DNS queries issued from a specific source within a given period of time. The idea is that by limiting the query rate, it is more difficult for an attacker to use DNS tunneling to steal data or gain unauthorized access.
While this is an effective approach in many situations, it also has its drawbacks. Aggressive rate limiting can interfere with legitimate DNS traffic and negatively impact network performance. It also doesn't completely block the DNS tunnel - it just slows it down.
4. Intrusion Detection System (IDS)
An intrusion detection system (IDS) is a type of security software that monitors network traffic for signs of malicious activity. Many IDS solutions are capable of detecting DNS tunnels by looking for patterns and behaviors that indicate such activity.
An IDS is a valuable tool for detecting DNS tunnels, but it is not a panacea. 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 an IDS is not kept up to date with the latest threat information, its effectiveness may be significantly reduced.
5.DNS monitoring tools
Finally, there are a number of specialized tools designed specifically to monitor DNS traffic and detect DNS tunnels. These tools usually combine several of the above methods into a single package to provide a more comprehensive solution for detecting DNS tunnels. These tools can be very effective, but they also require a certain level of expertise to use effectively.
Best Practices for Preventing DNS Tunneling Attacks
Preventing DNS tunneling attacks requires a multifaceted approach that combines technical and human factors. Here are some best practices that can help organizations protect their networks from such attacks.
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 large number of DNS queries, large DNS text records, or DNS requests for unknown or suspicious domains. Advanced threat detection solutions can help automate this process, providing real-time monitoring and alerts.
Implement DNS Security Extensions (DNSSEC)
DNSSEC is a set of extensions that add a layer of security to the DNS protocol. It uses digital signatures to verify the authenticity of DNS data and prevent spoofing and other DNS-based attacks. However, DNSSEC does not block DNS tunnels directly because it does not inspect the payload of DNS packets. However, it can be combined with other measures to enhance overall DNS security.
Utilize firewall rules
The firewall can be configured to block all outgoing DNS traffic except to 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 helps reduce the attack surface of DNS tunnels. This includes blocking DNS queries for non-existent domains and rate limiting DNS queries from a single source. DNS servers can be configured to deny queries for certain record types commonly used in DNS tunnels, such as TXT or NULL records.
Regular patches and updates
Keeping systems, software, and network devices updated is an important aspect of cybersecurity. Regular patching can help prevent vulnerabilities that could be exploited for DNS tunneling attacks. Security patches should be applied as soon as they are released, and updates should be tested before deployment to ensure they do not introduce new vulnerabilities.