Passive DNS is a means to store Domain Name System (DNS) data. It was built specifically to help security analysts and researchers use previous details from DNS records to uncover events and incidents related to their investigations in hopes of mapping out malicious infrastructures.

You can think of passive DNS simply as a repository of all the domains the Internet Protocol (IP) addresses you’re looking into resolved to in the past and vice versa. In effect, it’s like a journal where you find details on the events that happened in its owner’s life.

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Passive DNS, also commonly abbreviated as “pDNS,” was primarily designed to help with cybersecurity. Before 2005 when it was created, all of the DNS records that tell what domains point to specific IP addresses and the other way around were lost forever after a certain amount of time. Florian Weimer invented passive DNS to work around that. You’re probably wondering now how it works. Read on to find out.

How Does Passive DNS Work?

Each time you type a domain name into your browser’s input field to access a website, your request gets sent to a DNS server. You probably know that computers can only understand numerical data, which doesn’t include a domain name like google[.]com. Therefore, the DNS server translates google[.]com or any other domain name into a corresponding IP address to take you to the site. In Google’s case, that could be 8[.]8[.]8[.]8 or any of its other IP addresses.

You need to know, too, that several domains can point to a single IP address. They are said to share a host. The IP address 8[.]8[.]8[.]8, for instance, also resolves to the domains 0–9[.]ru, 1-189tais[.]com, and 2[.]t2t[.]top, apart from google[.]com according to the results of a passive DNS query made on this web service. That right there is an illustration of how passive DNS works.

What is Passive DNS

Without it, you wouldn’t know that 0–9[.]ru first pointed users to 8[.]8[.]8[.]8 on 4 January 2019 and last resolved to it on 14 May 2021. Suppose you’re a cybersecurity analyst or researcher investigating the IP address for signs of malicious activity. In that case, you can check if any of the domains connected to it is malicious (if it appears on a popular blocklisting site or malicious Uniform Resource Locator [URL] database).

How Can Passive DNS Aid in Cybersecurity?

There are tons of ways to use passive DNS data to uncover cyberthreats or get to the bottom of a cyber attack. Here are a few of them:

Create an Attack Profile

Malicious campaigns typically employ tons of domains and IP addresses to get as much bang for their buck, so to speak. The more malicious web properties they can plant malware into and then lure potential victims to, the bigger the profit or the more massive the amount of confidential data they can get.

With passive DNS data’s help, a single IP address connected to the attack can help you uncover at least one domain that you or your company’s users should avoid accessing. The same is true given a single domain. You can find all the IP addresses it resolved to over time with the help of a passive DNS database or tool. The connections you find make up your attack profile.

Uncover Other Malicious Web Properties

While several news articles and cybersecurity blogs can provide a wealth of information on ongoing cyber attacks, they may not give the complete list. If you want utmost protection against threats, you may need to do deeper dives on your own.

Passive DNS data can definitely help with that as you can expand published lists of indicators of compromise (IoCs) or web properties you need to avoid so you won’t become the next victim with it.

Now that you know the answer to “What is passive DNS?” you can get one step closer to a more threat-free network, providing users safer browsing experiences.