What is DNS leakage? How to prevent and fix it?

Every time you browse the internet, your actions go through domain name resolution. Regardless of whether the user is using a proxy or other intermediary methods, the domain query will expose the target being accessed.

Many people are not very familiar with DNS leak and do not know how to protect themselves on a daily basis, which over time can lead to significant security risks in their network.

Next, let’s take a look at what DNS leak is and how to prevent and fix it.

1. DNS and Its Operation方式

DNS (Domain Name System) is mainly responsible for converting domain names into IP addresses. Each time you open a webpage, the browser first sends a query request to the DNS server. After the DNS server provides the IP address of the target site, the browser can establish a connection with the target site.

Requests for DNS resolution sent by the browser typically go directly to the DNS server configured in the local network, such as your internet service provider, router, or public DNS service. Since this step occurs before the content loading process, DNS data can allow others to infer which sites the user has visited, even if the web content itself is transmitted in an encrypted manner.

2. The Meaning of DNS Leak

When users utilize virtual network services, dedicated proxy lines, or other encrypted tunnels, the expected scenario is that all traffic enters a closed channel, which is uniformly handled by the proxy end. However, some systems or browsers may continue to use the default DNS instead of transmitting it along with the encrypted tunnel, leading to DNS leakage.

In certain contexts, leaks may also expose the user's system language, regional differences, or network provider, leading to an inconsistent access environment. For example, the IP address may appear as a proxy node, but the DNS records come from the local city. The website may thus determine that the access environment is abnormal.

Three,Common causes of DNS leakage

1. The system has not fully taken over the DNS.

   The operating system may still retain the default DNS after the encrypted tunnel is established, and data may still return locally along the system's default path.

2. The browser independently uses DoH (DNS over HTTPS)

   Some browsers support DoH and send DNS resolution requests directly to specified providers, such as Cloudflare or Google.

3. Virtual network services or proxies lack DNS hijacking capabilities.

   Some services do not independently handle DNS. They only encrypt the data channel while the resolution is still done by the system, resulting in a lack of overall environment protection.

4. Mandatory Redirection of Public Wi-Fi

   Some public networks enforce DNS request hijacking. Even when using encrypted tunnels, the router may still modify or redirect the resolution path.

5. Device or router configuration has been tampered with.

   Malware, misconfigurations, or accidental changes can alter DNS settings, causing resolution data to be sent to unintended servers.

四、How serious is the impact of DNS leakage?

1. Access target visible

   DNS requests contain explicit domain names. Monitors can directly see the types and frequencies of websites users are visiting.

2. Behavior pattern inference is easier.

   By analyzing the records, external entities can infer access habits, active time periods, and regional differences.

3. Trigger access restriction

   When the DNS and IP come from different regions, some sites may regard this as proxy behavior, thereby blocking access or requiring additional verification.

4. There is a risk of being hijacked or polluted.

   If DNS requests are intercepted or redirected by a third party, it may return a forged address, leading to access to incorrect sites or malicious resources.

5. How to confirm whether a DNS leak has occurred

Whether DNS is leaking needs to be determined through detection tools. The detection method is usually very straightforward; you just need to visit the corresponding site to see the current DNS source.

Recommended Query Tool: ToDetect Browser Fingerprint Detection Tool

By opening ToDetect's DNS leak detection page, you can see your local/DNS server IP, network operator, country, and region information. If the DNS shown in the results does not match the proxy or encrypted tunnel being used, it indicates that the resolution request did not enter the expected channel.

As shown in the picture, a user not in the United States uses ToDetect for DNS leak detection. Although his true IP is not exposed, the inconsistency between his IP and DNS information reveals that he may be using a virtual private network or proxy.

Six.Reduce DNS leakageSeveral ways

Preventing DNS leaks requires a multi-faceted approach involving the system, browser, and network services.

1. Use an encrypted tunnel with DNS takeover capability.

   Some virtual network services or proxies automatically encapsulate DNS requests within a tunnel. Choosing such services can keep the resolution and traffic on the same path.

2. Adjust the DoH settings of the browser as needed.

   If the browser uses DoH independently, it may bypass the tunnel. Depending on the usage scenario, DoH can be turned off or adjusted to ensure DNS always follows the intended channel.

3. Configure stable public DNS.

   Users can manually set the parsing servers in the system, such as 1.1.1.1, 8.8.8.8, or 9.9.9.9. If the encrypted tunnel can correctly take over the system DNS, these addresses will be transmitted along with the tunnel.

4. Regularly check the router and system configuration.

   Ensure that DNS settings have not been maliciously modified and avoid unnecessary plugins or extensions that interfere with the resolution behavior.

Summary

DNS leakage refers to the situation where domain name resolution requests are not transmitted through an encrypted tunnel, returning to the local network environment, thus allowing observers to see the target being accessed along with the user's environment. 

To avoid leakage, it is necessary to confirm whether the resolution enters the intended channel and repeatedly test the environment using DNS leak detection features from tools like ToDetect. By integrating system configuration, browser adjustments, and stable tunnel services, DNS leakage can be controlled.