What Is DNS Cache Poisoning And How Does It Work?

What Is DNS Cache Poisoning And How Does It Work?

What Is DNS Cache Poisoning And How Does It Work?

 Unaware of the many dangers when we go online, malware authors have stepped up their efforts to trick people into giving up their data. Domain Name System (DNS) Cache Poisoning, also known as DNS Spoofing which is one of the sneakiest ways to hijack users' browsing experience to send them to malicious sites. Then what is DNS Cache Poisoning? How does it work? And how to avoid it?

What is DNS Cache?

How URLs And IP Addresses Work

To get started, let's look at the DNS cache itself. When you want to visit a website, you usually enter its URL. If you want to check your bank account online, you have to type the URL www.bankname.com in your browser.

The problem is, your computer doesn't actually "speak" in the URL. It knows about IP addresses, these are the strings of numbers that act as the "home address" of devices on the internet. We know that using URLs is much easier to remember a site's name than its IP address.

How DNS Servers Work

In order to know where you want to go, your computer has to translate your URL into a usable IP address. To do this, it forwards your URL to the DNS server. The DNS server acts like a giant phone book for websites. When your computer sends a DNS server URL, the server will look it up in its database and find a suitable IP address. That then means your computer now knows what IP address is associated with www.namabank.com and can visit that website.

How the DNS Cache Works

Because website IP addresses rarely change, but if they do, your computer decides to save this previous ip address for later. It records the IP address for the URL www.bankname.com in the DNS cache. Now, when you go to access your bank someday, your computer won't need to use a DNS server. Because your computer will look through the cache and find the last received IP address. In a way, the DNS cache acts as a mini phone book for all the sites you've visited.

How They Do DNS Cache Poisoning

How Hackers Use It

When the computer uses the DNS cache, it doesn't see if the IP address has changed since it was last used. In a way, the DNS cache is the computer's memory, if a value in the cache is tweaked, the computer will act as if it has always been that way. Let's say a malicious agent decides to attack www.namabank.com users. To do this, they create fake websites that look identical to the real ones. They create fake login screens to harvest details of people using these fake websites.

How DNS Cache Poisoning Works

With the site online, they then attack the user's DNS cache. They can do this through malware, or by gaining access to someone's PC. Whatever their goal is to access the DNS cache and find where www.namabank.com is stored. Once logged in, they then swapped the real IP address for the bank with a fake website address they created.

Let's say your cache is compromised, and the IP address to your bank's address has been swapped. Now, when you enter your bank's URL, your computer looks for it in the cache. It finds malicious IP addresses planted by hackers, and redirects your browser to fake websites. If done smoothly enough, you won't even notice that you have arrived at a bogus website. You then enter your account login details into the fake website.

Are DNS Servers Also Vulnerable?

Since the computer talks to the DNS server to get the ip address, is it possible for a hacker to poison the server? Unfortunately, the answer is yes, and the results can be disastrous! The DNS server operates similarly to your computer. If it gets a query for an IP address, and doesn't know where to direct the user, it will ask another DNS server for the answer. These servers use their own cache to store information.

If a hacker manages to gain access to the DNS server, they can modify the database to direct users wherever they want. Now, any computer that accesses the DNS server for an IP address will get poisoned results.

Even worse, a server that doesn't have an IP address for a particular website will ask the poisoned server for the answer. They then received a poisoned answer as a result! This leads to a nasty chain of infection around the DNS servers when they pass this bogus information.

How to Avoid DNS Cache Poisoning

1. Keep Antivirus Active and Updated

A good antivirus should thwart DNS cache poisoning attempts. The internet is always full of risks, so it's important to have something to protect yourself! Download and install a recognized antivirus to keep your computer safe.

2. Don't Download Suspicious Files

To protect your own DNS cache, stay safe while browsing the internet. Do not click on suspicious files, links or banner ads. This may be an attack vector for malware that will change your DNS cache.

3. Use a Trusted ISP/DNS Server

Protecting yourself is a good move, but what about an infected DNS server? A good DNS server will never trust the first thing it receives from another server. It would treat every piece of information with suspicion and would not accept it unless it knew it was not poisoned. By using this server, you can be sure that the results your computer gets will always be legitimate.

Usually, your computer uses the DNS server provided by your ISP. Therefore, it is best to use an ISP that has a good reputation and maintains good security practices. If you want, you can use a different DNS server than the one your ISP gives you. This allows you to choose a reputable service in the knowledge that your connection is safe from poisoning.

4. Clear DNS Cache

If you suspect your DNS cache is poisoned, clean the palette of broken entries and start over. Make sure you use a trusted DNS server when refilling the cache, or you may end up poisoning again! To clean it, you can use an application like CCleaner for Windows or BleachBit for Linux.

5. Check All Websites You Visit

When you arrive at a website, you can double check to make sure the website is not fake. Unfortunately, website URLs can still display what you entered, because your computer believes this is the real IP address of the website you want to access. If you don't see any HTTPS encryption, or if something seems suspicious, there's a good chance you're on the wrong site! Do not enter any login details, log out of the website, and immediately perform a virus scan and DNS cache cleanup.

6. Restart Your Router to Clear DNS Cache

Routers may also carry their own DNS cache. It is just as vulnerable to DNS poisoning as a PC or DNS server. To make sure you're safe, give your router a solid power cycle. This should clear its DNS cache and fix the problem.


DNS servers are useful tools for speeding up your browsing experience, but they can also do serious damage if they are compromised. Luckily, there's a lot you can do to make sure you're never the victim of a DNS Cache Poisoning attack.

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Resa Risyan

Just an ordinary person who wants to share a little knowledge, hopefully the knowledge I provide can be useful for all of us. Keep in mind! Useful knowledge is an investment in the afterlife.

Also, read the article about What Is a DNS Server And How Does It Work?. And see you in another article. Bye
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