What is DNS Caching?

DNS caching is the process of temporarily storing the results of DNS lookups in a local memory on devices, browsers, or servers. This makes future visits to the same site load faster by avoiding repeated lookups, reducing network traffic, and improving performance. We will explain what DNS caching is, what Time to Live (TTL) is, and how you can clear your cache.

There are several types of caching in the DNS system. When a user or machine starts a DNS request, the response may be stored in the temporary memory or cache of the user’s operating system and browser. Each record has a time-to-live (TTL) value that dictates how long records can be stored in a cache before being deleted.

When a user starts a DNS query, their device will check the local cache in the operating system or browser to see if the record already exists. If it does not, it will forward the request to a recursive DNS server. The server will either resolve the request based on the information stored in its own cache, or it will forward it to other nameservers and eventually to the authoritative server.

The response from the authoritative DNS server is stored in the cache of each resolver step and forwarded back to the device. This will load the correct web page or connect to the correct device.

Storing DNS responses in the cache provides several benefits including faster resolution as future lookups for that same domain can be served directly from the cache, provide better online experiences, optimize traffic, and offer offline access as devices can still access websites and online resources when disconnected from the web.

DNS caching works in three steps:

1. First Request: When you visit a new website, your system will ask a DNS server for its IP.
2. Cache Storage: The IP is then saved locally on your device, router, or your ISP’s server for a set time through the process of TTL.
3. Subsequent Requests: Any future visits to that new website will skip the DNS lookup and use the cached IP which will make the connection faster.

What is TTL in DNS?

Time to Live (TTL) in DNS dictates how long DNS records should be stored in the cache memory before it is considered outdated and must be deleted. It is typically measured in seconds. The functions of TTL in DNS caching are as follows:

1. TTL is set by the authoritative DNS servers for each record.
2. When a query is resolved, the TTL countdown begins. When it expires. The cached record is discarded and a new query is made.

TTL in DNS caching is important because it reduces time taken for DNS lookups, ensures timely updates to DNS records, and prevents outdated data issues while maintaining speed. This can help reduce the frequency of proxy error codes from showing up. 

There are eight types of DNS caching, each with its own unique functions. They are:

1. Browser-Level: This allows the browser to quickly resolve the domain names without needing to query operating systems or external DNS servers again which will reduce latency.
2. Operating System-Level: This cache serves as the next line of the resolution if the browser cache does not have the required DNS record. It will reduce the need for external DNS lookups.
3. Router-Level: Allows the router to the server to store DNS queries locally for all the connected devices and improves the network efficiency while reducing the number of external DNS queries.
4. DNS Resolver (ISP/Third-Party DNS Server): Serves the broader user base and reduces the need to query authoritative DNS servers repeatedly while improving the resolution times for multiple users.
5. Recursive Resolver: Helps reduce load on DNS infrastructure by storing results of the queries which will be used to answer future requests from any client.
6. Authoritative DNS Server: Helps authoritative servers provide faster responses of certain types of queries, including those in more complex DNS setups.
7. Content Delivery Network: Helps CDNs resolve the domain names quickly and directs users to the nearest server with the cached content. It improves load times and reduces latency.
8. Host File: Acts as the manual override for the DNS resolution where the system will first check the hosts file before making a query.

How to Clear DNS Cache

Clearing your DNS or “flushing” it, as it is more commonly known, is the practice of cleaning the history saved up. The reason you might want to flush your DNS cache is that sometimes, the cache might become corrupted, which can lead to phishing schemes.

The cache can also be outdated or create connectivity issues. Additionally, as we mentioned above, a corrupt cache can lead to error codes popping up. It is recommended to clear your cache once every two weeks but if you use a VPN or proxy frequently, the recommendation is weekly.

macOS

1. Open your Terminal and enter sudo killall -HUP mDNSResponder
2. After that, you will be prompted to enter your password.

Keep in mind that the command you enter changes depending on your macOS. The command we provided is for version 10.11 and above.

Windows

1. Open Command Prompt by clicking Start, Windows Systems, Command Prompt.
2. Enter this command ipconfig /flushdns

Keep in mind that the command you enter changes depending on your operating system. The command we provided is for Windows 10.

Linux (Ubuntu) 

1. Click on Activities and search for Terminal.
2. Enter this command sudo systemd-resolve --flush-caches

For other distributions, the command is sudo /etc/init.d/dns-clean start

Conclusion 

DNS caching is the process of saving the results from the DNS lookup into the server. Each result is saved for a limited time until the TTL expires. Caching makes it easier and faster to access websites as well as give the ability to access websites offline.

Key Takeaways: 

• DNS caching stores information from the DNS process into servers.
• This makes it easier to access websites, optimize traffic, and provide offline capabilities.
• Clearing or flushing your DNS cache is recommended to do anywhere from once a week to once every two weeks, depending on if you use a proxy or not.
• Flushing your cache is a simple process and only takes a few seconds.
• Clearing your cache is recommended to avoid picking up malformed data and affecting your website experience.

DNS caching is nothing to fear as it makes your web browsing experience faster and more effective. However, do not forget to clear your cache occasionally, especially if you have frequent experiences with website errors.

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