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What Is a DNS Leak? | Fortinet
What Is a DNS?
The Domain Name System (DNS) is an internet protocol that translates website addresses into code so they can be read by computers. This is crucial to helping internet users visit their favorite websites, access search engines, use social media, and watch streaming services.
The DNS translates a web address into an Internet Protocol (IP) address, which enables computers to recognize the location a user wants to access and helps devices communicate with each other. Web browsers use DNS servers to discover the IP address that users search for and enable people to get a more user-friendly experience on the internet.
Users’ DNS requests go to servers that are operated by their internet service provider (ISP), which records user data in logs. This can be a privacy concern because that data can be subpoenaed by law enforcement agencies to track user activity, or in some countries, sold to advertisers and other third parties without the user’s permission. However, users can avoid this by using a virtual private network (VPN) to keep their browsing activity encrypted, private, and secure.
What Is a DNS Leak?
A DNS leak is a security flaw that occurs when requests are sent to an ISP’s DNS servers even when a VPN is being used to protect users. A VPN is designed to encrypt a user’s internet connection, which keeps their traffic in a private tunnel that hides all of their browsing activity. That means all the user’s internet searches and website visits are hidden from everyone except for their VPN provider.
However, a DNS leak occurs when the user’s DNS requests move outside the encrypted tunnel and become visible to their ISP. As a result, all their browsing activity, including their IP address, location, and web searches, goes through the ISP in the same way it would if they were not using a VPN.
How Can a DNS Leak Happen?
There are several situations that can result in a DNS leak occurring, including:
An improperly configured VPN: A DNS leak is most likely to occur when a VPN is configured improperly and assigns a DNS server belonging to the user’s ISP. VPNs require a user to connect to their ISP before they log in to the VPN, so this is likely to occur when users regularly use multiple networks.
An ineffective VPN service: A VPN service that does not have its own DNS servers will result in DNS leaks occurring and will fail to provide effective protection from DNS leaks.
No Internet Protocol version 6 (IPv6) support: IP addresses were originally 32-bit Internet Protocol version 4 (IPv4) addresses with four sets of three digits. But 128-bit IPv6 addresses have been created to extend the pool of IP addresses and accommodate more devices. The internet is still transitioning, and some VPNs may not support IPv6, which may push a user’s DNS request outside of the encrypted tunnel.
Transparent DNS proxies: Some ISPs have started forcing customers to use their DNS servers even when they change their settings to a third-party VPN. If the ISP detects DNS setting changes, it uses a transparent proxy that forces a DNS leak by redirecting the user’s web activity to its own DNS servers.
Windows smart features: Microsoft introduced a feature known as Smart Multi-Homed Name Resolution (SMHNR) in devices using operating systems from Windows 8 onwards. The feature submits DNS requests to available servers and accepts whichever DNS server responds first. This can cause a DNS leak and leave users open to spoofing attacks.
Windows Teredo: Windows operating systems include a built-in feature called Teredo that aims to ease the transition from IPv4 to IPv6. It helps the two IP systems coexist more easily but creates a huge security issue for VPN users. That is because Teredo is also a tunneling protocol that can take precedence over a user’s encrypted VPN tunnel.
Is a DNS Leak Bad?
A DNS leak can be serious because it contravenes the reasons why a user deploys a VPN service. It can result in users’ private information, such as browsing activity, IP address, and location, unknowingly being leaked to their ISP, third-party organizations, and malicious actors monitoring network activity.
Is My DNS Leaking?
Internet users can check whether their DNS is leaking by testing their VPN connection. Many VPN suppliers and vendors provide tests that show the DNS server the user is connected to and supply additional information about their browsing session.
How Does a DNS Leak Test Work?
A DNS leak test provides users with information about their VPN connection, including the active IP address and location. This can be compared against their real IP address and location for a DNS leak check. Users can also run a DNS status check, which displays whether they are using DNS servers that belong to their ISP or their VPN. The tests also provide advice on DNS leak protection as well as how to fix an issue.
Are DNS Leak Tests Safe?
Reputable VPN providers provide DNS leak tests that are safe and secure. Users should avoid DNS leak tests from untrusted providers.
How Do I Fix a DNS Leak?
A standard DNS leak can be fixed by configuring a VPN to only connect to its own DNS servers. This will force a computer to only use the VPN’s DNS servers and not connect to the user’s ISP.
In the case of the Windows SMHNR issue, this can be more tricky because it is built into the operating system. Some VPN providers enable Windows 8 and Windows 10 VPN users to install a free plugin that resolves the issue, while other users will need to contact their VPN for support.
How Fortinet Can Help
The Fortinet FortiTester solution enables users to test for DNS leaks. It checks the latency of a user’s network connection or DNS server. FortiTester enables organizations to future-proof and secure their infrastructure by assessing the people, processes, and technologies accessing their network.
What is DNS?
The Domain Name System (DNS) is an internet protocol that translates website addresses into computer language. This helps internet users visit websites and enables devices and websites to talk to each other.
What is a DNS leak?
A DNS leak occurs when virtual private network (VPN) users’ browsing activity is exposed outside of their encrypted connection.
Is a DNS leak bad?
A DNS leak can be bad because it makes private browsing data available to internet service providers (ISPs), third-party organizations, and hackers.
Is my DNS leaking?
VPN vendors provide DNS leak tests that enable users to check the status of their connection, Internet Protocol (IP) address, and DNS server.
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can I fix a DNS leak
The solution is to ensure that once connected to the VPN,
you are using ONLY the DNS server/s provided by the VPN service.
OpenVPN v2. 3. 9+
As of OpenVPN version 2. 9 you can now prevent DNS leaks by specifying a new OpenVPN option. Simply open the
(or) file for the server that you are connecting to
and add the following on a new line. For more information see the OpenVPN manual.
block-outside-dnsIf for any reason you are unable to use the solution above continue reading.
If you are using a version of OpenVPN older than v2. 9
Please note that as this problem normally only affects windows clients, only solutions for Windows appear
3 basic steps to fix the problem;
Before connecting to the VPN, set static IP address properties if you are using DHCP
After connecting, remove DNS settings for the primary interface
After disconnecting, switch back to DHCP if neccessary or reapply original static DNS servers
Solution A – Automatic
If you are using OpenVPN on Windows XP/Vista/7 then a fully automated solution is available.
Download – (md5 checksum:
After installation, when you connect to a VPN server, a batch file will be run executing the 3 steps
Three scripts are generated for each OpenVPN configuration file;
– executed when you initiate the connection but before the
connection is established – Calls – If any active DHCP adapters exist, switch to static
– executed when the connection is established – Calls –
Clear the DNS servers for all active adapter except the TAP32 adapter
– executed after the connection is disconnected – Calls
– Reconfigure adapters back to their original configuration
Solution B – Manually clearing the DNS
The solution below does not switch the adapter to static if you are using DHCP. If you do not
switch to a static IP configuration and your computer renews its IP address whilst connected to
the VPN, the DNS settings may be overwritten. It is highly recommended to switch to a static
Open the command prompt () as an administrator.
Before connecting identify the name of the connected network interface. In the case below it is
“Local Area Connection”
netsh interface show interface
Connect to the VPN. Once connected proceed to the next step.
Flush the DNS resolver cache
Disable the DNS configuration for the Interface identified in step 1
netsh interface IPv4 set dnsserver “Local Area Connection” static 0. 0. 0 both
Test for DNS leaks.
After disconnecting, reconfigure the adapter to renew the previous DNS settings
netsh interface IPv4 set dnsserver “Local Area Connection” dhcp
Once again, flush the DNS resolver cache.
What Is a DNS Leak? How to Find & Fix DNS Leaks
DNS records tell a lot about you and your online browsing behaviors — explore what a DNS leak is and how you can detect & prevent DNS leaks
A lot of people turn to Google to answer the question, “what is DNS leak? ” And for many people around the world, a DNS leak may not seem like a big deal. (“If I’m not doing anything wrong, I’ve got nothing to hide, right? Wrong. ) But for some individuals, depending on their country of residence and its laws, a leaky DNS is a big concern.
When you type a website’s domain name in the address bar, the browser opens up that website within seconds. But there are a lot of processes that take place in the background which you are not aware of. And if any of those procedures has a security vulnerability known as a DNS leak, it means that third parties can eavesdrop on your entire browsing behavior.
In this article, we will answer the following questions:
What is a DNS and how does it work? What is a DNS leak and what causes it? How to perform a DNS leak test to tell if your DNS is leaking How do you prevent a DNS leak? What risks are associated with a leaky DNS?
What Is a DNS?
Since this article’s focus isn’t about explaining what a domain name system (DNS) is (but it’s still necessary to understand the topic), we’ll keep this brief. If you already know the basics, feel free to skip this part.
Everything that is connected to the internet (such as your computer, smartphone, and organization’s web servers) has a digital identity that’s written in either a numeric or alphanumeric format. This is known as an internet protocol (IP) address. IP addresses come in different formats — public, private, static, and dynamic — and are written in different ways based on their size.
A 32-bit IPv4 address is numeric and consists of four numeric segments that are separated by periods. So, an IPv4 address for looks like this: 64. 233. 166. 113. A 128-bit IPv6 address is alphanumeric, which means it’s much larger. This type of IP address consists of eight 16-bit hexadecimal blocks, all of which are separated by colons. An IPv6 address for looks like this: 2607:f8b0:4002:c08::8a.
So, if you want to connect to a website, you need to provide that website’s IP address to the web browser. But as you can see, that’s a lot of random numbers or letters to try to remember.
It’s inconvenient and almost impossible for the average human brain to remember IP addresses for thousands of websites. That’s where the domain name system (DNS) technology comes in handy. DNS essentially translates the website’s domain address into the IP address for you.
The domain name system (sometimes called “domain name service”) is a series of servers and computers that connect domain addresses to their corresponding IP addresses (either IPv4 or IPv6). They do this through a process known as a DNS lookup. So, instead of typing in a series of seemingly random numbers every time you want to shop on Amazon, you can instead type in “” This simple approach makes things a lot easier to remember.
How Does DNS Work?
This simplified illustration shows how the domain name system works in terms of handling DNS requests.
Basically, the domain name system serves as an intermediary between you and the website you are trying to open.
When you type a website’s name, let’s say, in your web browser, the browser needs to find that website’s corresponding IP address to open it. So, it sends requests to DNS servers to track down the IP address. This request is sent via your internet service provider (ISP).
The DNS server looks into its cache and responds to your browser with the website’s IP address from its DNS cache memory. If the DNS server can’t find it from the cache, it starts a four-step process that involves recursive resolvers, root nameservers, TLD nameservers, and authoritative nameservers. (Read more about these processes: DNS servers)
After all these processes, the DNS submits the corresponding IP address of the website you are searching for to the browser. The browser connects to the server where that IP address is (website) is hosted. Hence, the DNS works like a phonebook or a directory. You gave it the name of a website, and it searches for the website’s IP address.
What Is a DNS Leak?
This simplified illustration shows how what happens during a DNS leak.
Now that we know what a DNS is and how it works, it’s time to answer the question, “what is a DNS leak? ” In a nutshell, a DNS leak is a term that’s used to describe data exposure despite the use of a virtual private network (VPN). Basically, your DNS server requests are visible to third parties.
A VPN is a software that hides your original IP address and provides a new random IP address, which keeps changing frequently. It also creates an encrypted tunnel that securely transmits all the traffic between your browser, DNS, and the website’s server you are trying to connect! So, no one can track your online actions using your IP address.
But sometimes VPNs fail to hide your IP address and encrypt the communication. When this happens, they reveal your original IP address (provided by your ISP) to anyone who can intercept the traffic between your browser and DNS, causing the DNS leaks.
What Does a DNS Leak Mean in Terms of Privacy and Security?
So, a DNS leak means that someone can intercept the communications between your browser and DNS or steal the data from a DNS cache even though you’re using a virtual private network (VPN). This is also known as an IP leak.
But what does steal this type of data accomplish? It means that intruders can:
Trace your device’s IP address, See what types of sites you visit (based on your browser’s DNS requests), andMonitor all your online activities.
But wait, don’t your ISP’s DNS servers already keep records of your DNS requests? Yes, if you’re using your ISP’s DNS servers by default instead of the VPN’s DNS servers. That’s because browsers need the internet to communicate, and your ISP (and its DNS servers) to make that happen by default if the VPN isn’t configured to use its own servers. That means your internet provider — and anyone else who can legally (or illegally) access their servers — can monitor all your actions online by tracing your IP address from the browser-IPS DNS communications.
What Causes a DNS Leak?
There are different reasons for why you can experience a DNS leak. A few potential causes of DNS leaks include:
Your network’s DNS settings are incorrect or improperly configured. Your ISP may be using transparent DNS proxies. There are issues in your IPv4 to IPv6 transition process.
Needless to say, a DNS leak is no laughing matter. But how do you know if the DNS server you’re using is leaking?
How to Conduct a DNS Leak Test
As we mentioned before, by default, all the traffic between your browser and the DNS server is unencrypted. (This means that it’s sent in plaintext format, so anyone can read it. ) Your ISP and any hacker can easily track all your online actions when they intercept this communication.
But if you’re using a VPN and worry that you have a DNS leak, you should follow these steps.
Turn off your VPN. Open one of these websites: or Note down the resulting information the page displays. This is going to be your ISP IP address, ISP’s name, hostname, and geographical location. Now, turn on the VPN and select any other geographical location of your choice. Once again, go to the DNS leak checking website and conduct the test. This time, you should see the different IP addresses, internet provider’s names, and geographical locations. If you still see your original ISP IP address and other details, your VPN is suffering from a DNS leak.
Check out the screenshot below of my DNS leak test — the first one I took without turning on the VPN, which shows my original IP address. Then, I turned on VPN and chose Mumbai as my geographic information. As you can see, all the details changed when I conducted the DNS test. It indicates that my VPN is not suffering from a DNS leak.
If your DNS leak test shows your DNS isn’t leaking, that’s great news. However, just know that it means that you’re not experiencing a DNS leak now — but that doesn’t mean you won’t experience one in the future. So, you may want to periodically re-test to ensure your DNS doesn’t have a leak.
But what if you discover that your DNS server is experiencing a DNS leak? What can you do to stop or prevent future DNS leaks?
How to Prevent DNS Leaks
As we know, DNS traffic and records tend to be insecure and unencrypted by default. This means that if you’re using your ISP’s DNS servers, you can’t prevent your ISP from tracking your actions or selling that data to advertisers (we’ll speak more to the dangers of DNS leaks shortly) except by taking the legal route. But these are some steps you can take to prevent DNS leaks from occurring.
Let’s break down the steps for how to prevent a DNS leak.
1. Use a Robust VPN
Using a secure and reliable VPN is the best way to hide your original IP address and encrypt the tunnel between your browser and DNS servers. But, sometimes, the browsers bypass the VPN’s IP address and access your original IP address to send the DNS requests. This causes DNS leaks. Hence, use the following tips while using a VPN:
Enable DNS leak prevention feature. While purchasing a VPN, you should always check whether it has a DNS leak prevention feature. Use VPN monitoring software. VPN monitoring software keeps an eye on critical metrics to ensure the integrity of the VPN connection. They make sure that all the DNS requests pass through a VPN tunnel and can’t access the user’s original IP address.
It can immediately block the requests or alert the users if someone is trying to bypass the VPN IP address.
2. Clear DNS Caches
The DNS cache stores information of all your browsing history. If an intruder intercepts it, they can track all your online actions. So, keep flushing the DNS cache on a regular basis.
Here are a few quick steps you can take to delete your DNS cache on Windows:
Click on the Start menu, search for cmd. Open the command prompt.
Enter ipconfig/flushdns in the prompt.
3. Disable Microsoft Teredo
Microsoft Teredo is the technology that smooths the transition from IPv4 to IPv6 in Windows-based devices. Until all hosts are on IPv6, Teredo helps to give full IPv6 connectivity to the hosts that are on the IPv4 Internet.
While this is a great feature in many cases, it’s not perfect. Sometimes, it bypasses the VPN’s IP address and accesses the ISP’s IP address, causing DNS leaks. However, you can manually turn-off teredo by following these three steps:
Click on the Start menu and search for cmd.
When you see the command prompt apear, right-click on the icon and select Run as an administrator.
Type in the following command: netsh interface teredo set state disabled.
Whenever you want to resume using Teredo again, type netsh interface teredo set state type=default into the command prompt to enable it.
4. Change Your Settings to Default to Use Your VPN’s DNS Servers
If you don’t use the default IPS DNS server, your ISP won’t be able to track your actions. You can use the VPN’s DNS server instead. Or if you decide to go the public DNS server route, only choose the DNS server provider that you completely trust. Check out this resource: How to use Google’s DNS server.
5. Use Secure DNS Service
There are some solutions, like Comodo’s Secure Internet Gateway, that provide a secured DNS connection on an enterprise level. It encrypts all the DNS traffic to prevent leaks. Apart from preventing DNS leaks, this tool also provides a DNS filtering service. It monitors all the DNS requests and blocks malicious websites. You can also manually change the settings and block the non-work-related websites to improve employees’ productivity.
Why Is a DNS Leak Dangerous?
For many people, it may not seem like that big of a deal. But for people who have concerns about their privacy or live in regions of the world where certain internet-related activities are prohibited, a DNS leak can be a big deal. DNS records can be used for anything from censorship to tracking or limiting internet use or even legal punishments. So, if you’re using a VPN to try to access content that’s prohibited or banned by your government, DNS records could be used against you.
For people who don’t have those same types of concerns, there are other privacy-related issues that can stem from DNS leaks. For example, your ISP can sell your browsing data to marketers and advertisers. They closely monitor all the webpages you visit, understand your interest areas and buying behaviors, and show the advertisements accordingly to manipulate your purchase habits.
And there’s also the cybercrime angle of concern. For example, if a hacker intercepts your data from DNS leaks, they can target you via sophisticated email phishing attacks that are based on this information. And if they know you or any of your coworkers frequently visit specific sites (such as a vendor’s website), they could create watering hole phishing websites to target you all as well.
That’s a scary thought, am I right? Let’s take a few moments to understand how hackers use your browsing history for phishing attacks with a hypothetical example.
Your DNS Request Data Could Be Used as Phishing Email Fodder
Let’s say an intruder uses your leaky DNS to their advantage and intercepts your DNS request data. They notice that you visit Chase bank’s website a lot. Although they can’t see your credentials and any confidential details due to the bank’s TLS/SSL certificates, but they notice that you frequently visit Chase bank’s website and web pages relating to student loans.
Bingo! The hacker knows that you’re at least interested in finding more about loans or are maybe even considering applying for one. So, the attacker sends you a sophisticated phishing email using Chase bank’s logo and writing style that’s designed to appear to be part of the bank’s loan approval process. In the email, they ask you for your social security number, bank account number, and other confidential details!
You, having been on the site and applied for the loan, might not think twice about such a request. This is why it’s not uncommon for people to fall for such traps.
Your Data Could Be Used for Malvertisement-Based Cyber Attacks
Malvertising means malware-laden advertisements, which attackers use to distribute viruses, trojan horses, worms, etc. onto victims’ devices. Data from DNS leak can be used to target people who visit specific websites via malvertising attacks.
Let’s consider another example. Say, a hacker intercepts your DNS cache and notices that your frequently visiting websites that provide tips to accelerate mobile phone’s speed. The hacker could create an advertisement on that website to lure you in: “Is your phone getting slow? It might be infected with a virus. Scan your phone now with this free antivirus software to detect and remove the viruses! ”
If you click on that malvertisement, it could automatically download malware onto your device.
DNS Data Is a Great Social Engineering Resource for Cybercriminals
In social engineering attacks, the scam artists use social media and other tools to gather information about you to plot a cyber attack. With DNS leak, the hacker can track the pages, groups, and profiles you visited on social media and learn about your interests and the type of people you interact with. Then they use this info to:
Guess your login credentials. Make fake profiles/ groups to interact with you and persuade you to share your personal info. Send you malware-loaded files or attachments in the inbox. Send you links that take you to spammy or malicious sites.
In the same way, advertisers can also track your social media activities and show the advertisements accordingly. In short, your browsing behavior is way more valuable to advertisers and hackers than you imagine!
Final Words on DNS Leaks & How to Prevent Them
Just like any other technology, DNS has its weak points. A DNS leak makes it easy for intruders (both hackers and advertisers) to gain valuable information about you from the web pages you visit. They can know a wide variety of information, including your:
Bank, School, Workplace, Favorite ecommerce sites, Insurance company, Likes, dislikes, and areas of interest, Concerns you are facing (or may be seeking solutions for), and People you communicate with on social media.
While advertisers craft their ads to manipulate your buying behavior, hackers can use this info to execute sophisticated phishing attacks, distribute malware, and plot ransomware attacks. That’s why you should always use robust VPN software and other prevention methods to stop DNS leaks from happening in the first place.
Frequently Asked Questions about dns leaking
How do I fix a DNS leak?
3 basic steps to fix the problem;Before connecting to the VPN, set static IP address properties if you are using DHCP.After connecting, remove DNS settings for the primary interface.After disconnecting, switch back to DHCP if neccessary or reapply original static DNS servers.
What does it mean if your DNS is leaking?
What Does a DNS Leak Mean in Terms of Privacy and Security? So, a DNS leak means that someone can intercept the communications between your browser and DNS or steal the data from a DNS cache even though you’re using a virtual private network (VPN). This is also known as an IP leak.Oct 28, 2020
How do I know if my DNS is leaking?
There are easy ways to test for a leak, again using websites like Hidester DNS Leak Test, DNSLeak.com, or DNS Leak Test.com. You’ll get results that tell you the IP address and owner of the DNS server you’re using. If it’s your ISP’s server, you’ve got a DNS leak.