Understanding DNS Cache 

Most managed services providers (MSPs) will have heard of the Domain Name System (DNS), the distributed network of servers that acts as a directory, cataloging domain names and their corresponding Internet Protocol (IP) addresses. Relatedly, a DNS cache is local storage that contains the records of a computer’s query history, including recent website visits. 

As a whole, the DNS translates domain names, a verbal nomenclature humans can more easily understand and recall, to the numerical naming and transmission method required by computers. In turn, the operating system (OS) uses caching to store DNS resource records, which avoids redundancy when attempting to access a web page and therefore decreases DNS lookup latency. If a machine has recently visited the page it wants to access, the cache can supply the IP address of its web server, completing the website request before the lookup has to query the DNS server. 

Ultimately, the DNS enables human users to keep track of more web pages and to access them as required, and DNS caching expedites the DNS lookup process to more quickly resolve a domain name to an IP address when the OS has visited a web page before.

How does DNS caching affect the network? 

While it’s fairly straightforward to answer the question “What is a DNS cache?” the way it affects network operations is a slightly more complicated topic—and in fact, DNS caching can actually be a security concern for MSPs. 

As explained above, the DNS cache exists to streamline the DNS lookup process that resolves a domain name to an IP address—thus, it serves an invaluable acceleration purpose. But DNS caching can compromise webpage access and network security if not properly managed. For this reason, MSPs must understand how caching can put them at risk and must know how to view and clear DNS cache contents. 

First, cleaning the DNS cache regularly is important to ensure consistent access to web pages. If a web page has changed the location of its web server in the time since its IP address was cached, a web browser might return an HTML 404 error—although the site is still online, the cache is feeding the browser an inaccurate IP address. This blocks the user’s access to an active page. 

Second, a clear DNS cache protects overall network security, from wiping personal web usage data to preventing DNS poisoning. Many operating systems (like MacOS and Windows) and almost all web browsers automatically create a DNS cache. Although this function seeks to serve the user by maintaining DNS lookup efficiency, it means that, in effect, a user’s computer and web browser have a comprehensive collection of their web activity whether they realize it or not. Wiping DNS resource records prevents malicious agents from acquiring and abusing this personal information. 

DNS poisoning, or DNS spoofing, refers to the cybersecurity threat in which hackers corrupt DNS resource records. By changing the IP addresses associated with particular domain names, hackers can hijack a web session and send computers requesting a particular site to the wrong web server—a form of phishing. These alternate pages may expose users to advertisements, prompt them to install malware, or succeed at stealing private data (like Social Security numbers or financial information) if they pass as the correct website and convince users to enter sensitive data. Routinely clearing DNS caches both narrows the window of opportunity for DNS poisoning and wipes any corrupted records.