Flush DNS | What it is and how it's done

Operating systems such as Windows au­to­mat­i­cal­ly generate temporary entries of visited websites in a so-called DNS cache. The in­for­ma­tion contained in the cache is valid for a defined period of time. A DNS flush, i.e. emptying of the cache, removes the data from the system before the time limit expires.

DNS flush is the process of manually deleting the temporary entries of a DNS cache. Without a direct in­ter­ven­tion in the cache, the entries exist until their defined life span (“time to live”, TTL) has expired.

Generally, the deletion process is completed using system-specific flush DNS command line tools. On Mac, for example, you use Terminal and the ap­pro­pri­ate command for the par­tic­u­lar version of the Apple system. In Windows, you flush the DNS via CMD, i.e. via the command prompt, and using the command “ipconfig /flushdns”.

DNS servers are used to convert domain names such as www.example.com into numerical addresses. These “name servers” are contacted by default via the browser each time the cor­re­spond­ing projects are accessed, which slightly increases their loading time and can also lead to a DNS server overload in the case of a high number of visitors. Error messages like “DNS Server Not Re­spond­ing” are a common and despised con­se­quence.

For this reason, operating systems like Windows and macOS attach their own cache for accessed and resolved addresses – the DNS caches. These save all in­for­ma­tion that’s relevant for name res­o­lu­tion such as an IP address, host names and protocol versions. Each entry remains valid for a given time. Within this time, cor­re­spond­ing queries are answered directly from the cache without requiring a detour to the DNS server.

There are three reasons for regularly setting the DNS register to zero with a DNS flush, re­gard­less of the actual validity period of the in­di­vid­ual records:

1. Hide search behavior: Recorded addresses including ad­di­tion­al in­for­ma­tion such as validity period provide an ap­prox­i­mate overview of your page history. The more extensive the col­lec­tion of cached addresses, the more you reveal about yourself.
2. Security against ma­nip­u­la­tion: If cy­ber­crim­i­nals gain access to the DNS cache, they could ma­nip­u­late entries and redirect you to fake websites. So-called DNS spoofin (or DNS cache poisoning) is intended to tap sensitive login data, for example, user details for online banking.
3. Solve technical problems: A DNS flush can resolve technical problems when accessing web ap­pli­ca­tions, e.g., if an incorrect version of the called website is displayed due to outdated entries. Following a flush, the request is answered again by the re­spon­si­ble DNS server, and the con­nec­tion setup to the web project works as planned again.

Step 2: DNS flush with “ipconfig /flushdns”

To flush the DNS via CMD, use the command ipconfig. To do this, enter the following command and confirm it with enter: