What is a DNS leak?

Business users should take steps to secure their web connection and choose VPN services that avoid DNS leaks. This helps guard privacy and secure critical data.

How does a DNS leak happen?

DNS leaks occur due to a range of causes, including incorrect network configurations, VPN malfunctions, and problems with IPv6 compatibility.

When you visit a website, your device sends a DNS query to a DNS server to resolve the website's domain name into an IP address, and then uses that IP address to establish a connection to the website.

During this process, DNS queries pass through an ISP server. As a consequence, ISPs can easily monitor the sites you visit. VPNs provide a solution to this privacy issue.

Users route their traffic via encrypted tunnels to VPN servers. The VPN server assigns a new IP address to your device. The device then sends DNS queries through the VPN's DNS servers. ISP servers only see the VPN IP address, and the user's IP address remains private.

However, several factors can interfere with the way VPNs route DNS requests. Let's look at common causes in more detail to understand why DNS leaks occur.

VPN configuration issues

Most DNS leaks stem from VPN configurations. VPN clients must ensure that all web traffic, including DNS requests, from the user device passes through the encrypted VPN tunnel rather than the ISP's network.

Misconfigured clients continue to route traffic through the ISP's default DNS servers. They may also lack a kill switch to block DNS leaks, or provide poor protocol coverage (for example, securing IPv4 traffic but not IPv6).

Changes to network settings

Local network settings can also lead to DNS leaks. For example, networks may default to unprotected browsing activity when VPN coverage drops unexpectedly.

Leaks can also occur when devices move between networks, requiring a new IP address and DNS server information. VPNs may temporarily fail to reroute DNS requests through the encrypted tunnel, resulting in DNS leaks.

IPv6 compatibility

IPv6 is a newer IP address standard than IPv4. However, some outdated VPNs do not handle IPv6 traffic effectively during DNS resolution.

Outdated VPNs may not support IPv6 traffic properly, causing DNS queries over IPv6 to bypass the VPN tunnel unencrypted. Domain names are delivered in plain text format that is easily readable by outsiders.

How to test for a DNS leak

DNS leaks expose your web browsing activity, rendering VPN coverage ineffective. This is a significant problem if you use encryption to safeguard data transfers or ensure confidential communication. Fortunately, DNS leak testing identifies leaks, informing you when you need to implement prevention measures.

Note: The DNS leak protection measures discussed below only work if your VPN is activated. Ensure you log into a regularly used VPN server before checking your DNS leak status.

The easiest way to test your DNS leak protection is by using online DNS leak test websites. Popular sites like DNSLeakTest provide a one-click testing service via easy-to-use web interfaces. Test results display the servers you are currently connected to, and their geographical locations. If you see any anomalies (for instance, servers located in other countries or the name of your ISP), you may have a DNS leak.

DNS testing services also provide extended leak testing. These tests use a longer time period and probe VPN coverage in greater detail. They aim to detect the intermittent DNS leaks characteristic of poorly functioning VPN services.

Web users can also use online tools to check IPv6 connectivity. These services determine whether your VPN can safely access IPv6 websites, enabling you to change your VPN settings or select a different provider.

How to prevent a DNS leak

Checking for DNS leaks is a good start. However, what can you do if the DNS leak test delivers evidence of online privacy issues? In that case, DNS leak prevention is essential.

The best practices below provide a blueprint to prevent DNS leaks and safeguard your browsing activity:

Choose a reliable VPN provider

This first step is fundamental. Most DNS leaks occur due to failings on the VPN side. Providers may fail to update their clients or patch their software for IPv6 compatibility. Even worse, low-quality VPNs collect log information about the websites that clients visit. They store browsing data for marketing purposes, making it available to third parties.

Avoid free VPNs and research the market before choosing a business provider. Security partners need a robust privacy record and cutting-edge encryption features. Look for VPNs with private servers and built-in DNS leak protection.

Enable your VPN's kill switch

Kill switches are a vital insurance policy to prevent DNS leaks. These features shut down internet connectivity if VPN coverage drops. Users cannot browse the web or make DNS queries until VPN encryption is restored. Not all VPNs offer a kill switch, and most require users to activate the feature. Ensure you do so, especially when browsing the web for business purposes.

Check IPv6 coverage

As the internet transitions from IPv4 to IPv6, some VPNs are being left behind. VPNs that do not support the IPv6 IP address format will suffer increasing DNS leaks, compromising their ability to protect online privacy.

Verify that your VPN provider supports IPv6. If not, blocking all IPv6 traffic is a secure alternative (although this may lead to availability issues over time).

Check your VPN's DNS servers

VPNs should route web traffic through their own private DNS servers, not ISP servers or custom servers maintained by third parties. Ensure VPNs automatically force DNS queries through their own servers where possible.

Use router-level VPN protection

Consider configuring VPN coverage on your router to ensure consistent DNS leak protection. Router installation automatically extends VPN coverage to all devices on the network. This limits the risk of security gaps caused by human error.

Regularly update your security tools

Apply VPN patches as they become available. Outdated VPN clients are far more likely to experience compatibility issues with network settings. Patches should fix tunneling issues before they compromise user privacy. Updating your operating system and network tools is also critical. Compatibility issues work both ways, and your VPN may not be the root cause.

Disable untrusted network connections

Network devices may support untrusted connections via wi-fi or cellular infrastructure alongside standard VPN tunneling. This situation can cause DNS leaks if routers give untrusted connections a higher priority.

To avoid this outcome, disable unused network adapters and simplify the network environment as much as possible.

Routinely carry out DNS leak tests

Testing for DNS leaks is a critical aspect of secure web browsing. It makes sense to schedule regular testing, even if you are confident in your VPN's reliability and technical quality. Integrate leak tests into security audits and record outcomes for different workstations and locations. Performance can fluctuate, and you need to identify privacy issues before data breaches occur.

Prevent DNS leaks to ensure online privacy

DNS leaks occur when VPN encryption fails, sending unencrypted DNS queries to ISP servers. When this happens, the ISP can log your browsing activity, while unencrypted web traffic is also exposed to malicious outsiders. This raises the risk of data theft and other forms of cyberattack.

VPN users should proactively detect DNS leaks via dependable tools. Security teams should also prevent DNS leaks where possible. Prevention tools include using reliable VPNs, kill switches, patch management, and monitoring DNS servers. Active prevention minimizes leak risks, allowing VPN encryption to work as intended.