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Secondary DNS Explained

  • By Gcore
  • July 10, 2023
  • 8 min read
Secondary DNS Explained

Your domains are the entry point to your online services, so their reliability and performance are vital for success. With secondary DNS, you can add redundancy to your name servers while minimizing domain resolution latency. Whether you’re a product manager wanting to refine the first stages of a sales funnel or an engineer needing to achieve a service level objective, secondary DNS can help you to achieve your business goals. This article will explain what secondary DNS is, how it fits into the DNS design, and what benefits you get from adding it to your architecture.

What Is Secondary DNS?

A secondary DNS server is a type of DNS server that automatically stores copies of all the DNS records from a primary DNS server. If your primary server cannot be reached or is busy, the secondary DNS server steps in to handle requests. This adds redundancy and ensures the continuous availability of your DNS services, safeguarding against potential disruptions in network services.

How Does DNS Work?

To better understand secondary DNS, let’s make a quick excursion to the basics of DNS. This part is necessary to understand where secondary name servers fit into the overall picture.

DNS is short for domain name service. DNS is a distributed system that resolves domain names into IP addresses, other domain names, or arbitrary text. It adds a layer of indirection between the actual addresses of each server on the Internet and the clients that want to access them. It’s an essential part of the internet. Without it, you would have to remember long numbers like 192.168.43.10 or 2001:db8::ff00:42:8329 and update them everywhere when they change.

How Does Name Resolution Work?

The name resolution happens via simple lookup tables called zone files filled with RRs. Each of these RRs contains the following fields:

  • The name field contains a fully qualified domain name, which forms the key of the zone files.
  • The type field contains the record type. Important types are:
    • SOA for administrative data, e.g., the zone file version
    • A and AAAA for IP addresses
    • CNAME for aliasing other domain names
    • MX for mail server addresses or domain names
    • TXT for arbitrary text
    • NS for addresses or domain names of authoritative name servers; this is also the record you can check for secondary name servers
  • The data field contains data like an IP address, another domain, or text. It forms the value of the zone files.
  • The time to live (TTL) contains the time a client can cache the resolved data locally.
  • The class field contains a protocol class. On the internet, its value is always IN.

Look at the following zone file for a fictional example.com zone with multiple NS records. Explanations are on the left, and the records are on the right and below.

Figure 1: Example zone file with DNS records
$TTL 86400example.com. IN SOA ns1.example.com. hostmaster.example.com. (   2023061901 ; serial number YYYYMMDDnn   3600       ; refresh every 1 hours   1800       ; retry every 30 minutes   604800     ; expire after 1 week   86400 )    ; minimum TTL of 1 dayexample.com. IN NS ns1.example.com. ; could be a primary serverexample.com. IN NS ns2.example.com. ; could be a secondary serverexample.com. IN MX 10 mail.example.com.example.com.      IN A 192.0.2.1ns1.example.com.  IN A 192.0.2.2ns2.example.com.  IN A 192.0.2.3mail.example.com. IN A 192.0.2.4hello.example.com. IN TXT "Hello, world!"

The first line defines the default TTL, which a name server automatically applies to each RR record that doesn’t have its own TTL defined.

The first RR is the SOA record. It’s mandatory and includes administrative information like a name server, the email address (written with a dot instead of an @ symbol) of the responsible domain admin, a version number of the zone file, and timings for caching.

The two NS records define the authoritative name servers for this zone. Here, we have two that also use domains from within the zone. ns1.example.com is the primary name server, ns2.example.com the secondary. This is where you would add your secondary name servers so clients can find them.

The MX record defines the email server for this zone.

The A records then map the domains of the name and email servers to IP addresses, which clients can use to connect to servers. It also includes an apex domain, which maps the bare example.com domain to an IP address.

The final TXT record resolves to the string Hello, world! when queried.

How Do Secondary Servers Relate to Primary (Authoritative) Servers?

Adding the address of a secondary name server to an NS record turns it into an authoritative name server for a zone; it becomes part of the global DNS hierarchy. An authoritative name server is any server with its address mentioned in an NS record for a zone and usually holds all the DNS records for its designated zone. Both primary and secondary servers can be authoritative for a zone.

Adding NS records for secondary servers is crucial because clients only know authoritative servers, they normally don’t know about the concept of primary and secondary name servers. If you add a secondary server to a primary one, but the secondary server’s address isn’t added as an NS record, the clients can’t find it.

Recursive Name Servers

The counterpart of an authoritative name server is a recursive name server, which isn’t responsible for a zone. Recursive servers relay queries to other name servers for resolution and may cache the results for performance reasons. Since they don’t have zone files that need synchronization, they can’t be secondary name servers.

The DNS Resolution Process

DNS is a distributed system, meaning that no single server is responsible for all domains. Instead, the domain space consists of multiple zones that form a tree structure. Each zone contains one or more domain names and one or more name servers responsible for it. If it isn’t responsible for a (sub)domain, it will contain a RR in its zone file that indicates another server is responsible for that domain. The name servers responsible for a zone are called the zone’s authoritative name servers. We’ve written an article that explains DNS zones in more detail; check it out if you want to learn more about zones.

The process of adding a new RR to the zone file and resolving it for a client is illustrated in Figure 2.

Figure 2: Adding and resolving a resource record
  1. A domain admin adds a new RR to the primary (authoritative) name server, for example, an A-record for the domain example.com.
  2. The secondary name servers either poll the primary servers for updates or are notified and download the updates via AXFR or IXFR.
  3. Secondary servers that can’t reach the primary server can receive updates from other secondary servers that are authoritative for their zone.
  4. An application—such as a browser—sends a query for the resolution of the example.com domain to the local resolver.
  5. The local resolver relays the query to a recursive name server (RNS) that relays them to authoritative servers which hold the zone files with the RRs.
  6. The RNS queries a root server, which only holds NS records for top-level domain (TLD) name servers. It returns the NS records for name servers authoritative for the com domain.
  7. The RNS queries the TLD server, which only holds NS records for the domains under the com TLD. It returns the NS records for the authoritative name servers of the example.com domain.
  8. The RNS queries one of the name servers responsible for example.com, in this case, a secondary name server. The server is chosen by round-robin. This server returns the data of the A or AAAA record for example.com. The RNS returns the data to the resolver, and the resolver returns it to the application.

How Do Secondary Name Servers Synchronize with a Primary Name Server?

The mechanism to keep name servers in sync is called a zone transfer. The secondary servers either poll other servers in their zone for updates or get notified by their primary server. Both polling and notification rely on the version number of a zone file.

Suppose the secondary name server sees the version has changed. In such a case, it will initialize a zone transfer with either the DNS zone transfer protocol (AXFR) or the incremental zone transfer protocol (IXFR) to fetch the latest RRs from the primary server or other secondary servers that are more up to date.

It is also possible to have multiple primary name servers, where each of their zone files are synchronized manually. In the context of secondary DNS, “manually” means the servers don’t synchronize via DNS-specific mechanisms. It’s possible to synchronize them by other automatic means, like Terraform scripts. To do so, a domain admin would update the zone file definition in the script, and Terraform would apply it to multiple primary name servers on a redeploy.

What Are the Benefits of Secondary DNS?

Now that we understand the zone transfer mechanism, let’s look at the benefits of secondary DNS beyond just automated synchronization.

Improved DNS Redundancy and Resiliency

Adding secondary name servers located in different data centers improves DNS redundancy. If one name server crashes or isn’t available for other reasons, clients can still use the remaining servers to resolve domain names.

Secondary name servers also improve DNS resiliency because they don’t just synchronize with the primary server but also with the other secondary servers in their zone. This way, updated RRs can still propagate to a secondary server that can’t connect to the primary server.

If a primary server is authoritative, it has to resolve queries and send updates to the secondary servers. Resolving client queries usually has the highest priority, so if the primary server load is too high, a secondary might get a notification but can initialize a zone transfer. If the zone transfer times out, the secondary server can ask other secondaries for the update and lower the load on the primary.

Low DNS Latency

If you branch your operations out to another continent, you can improve latency by deploying a secondary name server in that new location. If users are geographically dispersed, you can spread your secondary name servers all over the globe so that each user can use one close to them. With only a primary server, you must decide on one location that might deliver low latency resolution to only some of your users, and fail to provide low latency for others.

Clients select name servers via NS records and use round-robin to choose a new server for every subsequent query. This mechanism doesn’t help a client to find the server with the lowest latency, but you can add multiple servers to one NS record by giving them an Anycast IP address. Anycast routes a request to all servers that share an IP address but only returns the response that came back first to the client. This way, the clients always get the server with the lowest latency.

While Anycast isn’t directly related to DNS, it works hand in hand with the zone transfer mechanism. The secondary name servers keep each other synchronized with the primary server, and the Anycast protocol assigns each client the fastest server.

DNS Load Balancing

A zone file can have NS records for multiple name servers. The domain resolution process chooses one of these servers using a round-robin algorithm. In this algorithm, the client remembers which name server it already used and selects another one for the next query. This way, each subsequent query hits another server that can handle it while the previous one is still busy.

If your user base grows, your infrastructure must also support the additional load. The mantra of cloud is horizontal scaling, meaning that if the load rises, new servers must be added. So secondary DNS, which spreads the load over multiple name servers, works in the spirit of this mantra.

Improved Security for Primary Servers

In a typical DNS setup, you would add an NS record for your primary name server so that clients can query the primary name server directly. Clients use NS records to find name servers, but primary and secondary servers use other means of communication, so it’s possible to synchronize without any of them being present in an NS record. This means if you only add NS records for secondary servers, clients don’t know about your primary server. You can use the primary server to update your RRs and synchronize them with the secondary servers, while the secondary servers are responsible for resolving client queries.

With this technique, your primary server is hidden from the public, can focus on zone transfers, and is protected from potential attackers.

Leveraging Cloud DNS with On-Premises DNS Servers

Many organizations—especially bigger, more mature ones—already run their own name servers on-premises. Often, these are tightly coupled to the infrastructure with custom scripts and processes that create, update, and remove RRs.

Secondary name servers allow these organizations to keep their existing on-premises server as primary and add secondary name servers that run in the cloud. This way, they can benefit from the cloud and secondary DNS benefits with minimal changes to their on-premises infrastructure.

Conclusion

Your domains are the entry point to your websites and applications, so it’s crucial to ensure that users can always resolve them promptly. Secondary DNS helps you achieve this goal by letting you add extra name servers to your setup that synchronize with each other automatically. Each secondary name server acts like an authoritative name server and can resolve domain names just like your primary server, but without administrative overhead. When deployed smartly around the globe in tandem with technologies like Anycast, secondary DNS even boosts performance by lowering latency and can help to protect your primary servers from attacks.

Gcore’s DNS hosting allows you to set up zone transfers for secondary DNS with the open-source tool OctoDNS, so you get all the mentioned benefits without thinking about global deployments. Check out our docs to learn how to get started!

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This helps prevent crashes and delivers smooth performance for all users.Common Use Cases for CDNsCDNs are vital across a range of industries, providing measurable improvements in content delivery and user experience. E-commerce websites use CDNs to guarantee quick page loading and frictionless shopping experiences, even during periods of high traffic. Speed is crucial for online businesses. A study found that the average cost of downtime for e-commerce websites is around $500,000 per hour. This includes lost sales, operational costs, and long-term damage to brand reputation Streaming services rely on CDNs to deliver high-quality video content while minimizing buffering. Netflix states that its CDN contributes to the daily delivery of over 125 million hours of streaming content, guaranteeing a seamless experience for customers worldwide. Gaming companies use CDNs to lower latency and provide a consistent real-time user experience, especially during live multiplayer matches, where it is essential to preserve an engaging and fair gameplay experience. News outlets and blogs benefit from CDNs by ensuring their content loads quickly for readers around the world, during large-scale traffic surges, especially during major events like elections or breaking news.  The Benefits of a CDNFaster Website PerformanceEvery second counts when delivering content online. Slow websites frustrate users and harm your business. CDNs speed up content delivery by caching data closer to users, reducing page and file load times. Whether you’re delivering static content (such as CSS, HTML or JPG files) or dynamic content (like data generated by user interactions or API calls), a CDN ensures optimal performance regardless of user location. While factors like DNS settings, server configurations, and code optimization all play a role, the physical distance between your origin server and your users is a factor that only a CDN can solve. Increased Availability and ReliabilityDowntime can seriously affect online businesses. Hardware failures, traffic surges, and cyberattacks can reduce your website’s availability, harming your customers’ experience and causing financial or reputational damage. In fact, around 98% of organizations report that just one hour of downtime costs over $100,000. A CDN ensures that your website remains available, fast, and reliable by leveraging essential features such as: Load balancing: This process dynamically distributes traffic across multiple servers to optimize performance and prevent overload.Intelligent failover: Automatically redirects traffic if a server goes offline, ensuring continuity with minimal disruption.Anycast routing: Directs users to the closest or most efficient server, further reducing latency and enhancing response times.Security FeaturesAs cyber threats continue to grow in sophistication and frequency, securing your website or application is more critical than ever. According to recent statistics from Cobalt’s 2024 Cybersecurity Report, weekly attacks worldwide increased by 8% in 2023, while attackers used more sophisticated strategies to exploit vulnerabilities. Strong security measures that not only safeguard your website but also guarantee optimal performance are necessary in light of these evolving threats. CDN security features not only improve website performance but also defend against a wide range of attacks by distributing traffic across multiple servers, which mitigates DDoS attacks and filters out malicious traffic before it reaches your website. These features, from DDoS protection to safeguarding APIs, help maintain uptime, protect sensitive data, and guarantee a seamless user experience. Most modern solutions like Gcore CDN integrate robust security measures into content delivery, such as:SSL/TLS encryption facilitates secure data transmission by encrypting traffic, protecting sensitive information from being intercepted.L3/L4 DDoS protection blocks large-scale cyberattacks designed to flood your network and disrupt services.L7 DDoS protection guards your website from more complex attacks targeting how the website functions, helping it continue to operate smoothly.Web application firewall (WAF) acts as a shield, blocking harmful traffic such as hacking attempts or malicious scripts before they can affect your site.API security protects the communication between your application and other software, preventing unauthorized access or data theft.Bot protection identifies harmful automated traffic (bots), preventing activities like data scraping or login attempts with stolen credentials while allowing useful bots (like search engine crawlers) to function normally. Elevate Your Online Experience With a CDNA CDN is no longer a luxury—it’s a necessity for businesses that want to deliver fast, reliable, and secure online experiences. Whether your goal is to optimize performance, manage high traffic, or protect your site from attacks, a well-configured CDN makes all the difference.Ready to enhance your website’s performance? Our futureproof CDN runs on a global network of over 180 points of presence, so your customers get outstanding performance no matter where in the world they’re located. Get in touch with our team today to learn how our CDN can benefit your business.Discover Gcore CDN

How to Migrate Your Video Files to Gcore Video Streaming

Migrating large volumes of video files from different platforms can be daunting and time-consuming, often discouraging companies from moving to a superior provider. But it doesn’t have to be this way. We’ve created this three-step guide to help you efficiently migrate your video files to Gcore from other popular streaming platforms.Step 1: Get Links to Your VideosFirst, obtain links to your videos and download them. Look for your provider in the list below, or refer to the general SFTP/S3 storage section if applicable. After completing the steps for your provider, go straight to step 2.Google DriveShare the file: Open Google Drive and locate the MP4 file you want to download. Right-click on the file and select “Share.”Get the shareable link: In the sharing settings, click “Get link.” Ensure the link-sharing option is turned on.Set sharing permissions: Adjust the sharing permissions so “Anyone with the link” can view or download the file. Copy the generated link.Amazon S3Edit S3 block public access settings: Go to the S3 management console, select the bucket containing your MP4 file, and edit the Block Public Access settings if necessary.Add a bucket policy: Implement a bucket policy that grants public read access to your files.Get the list of objects: Navigate to the Objects tab, find your MP4 file, and click on the file to obtain the Object URL, which will be your download link.VimeoAccess the video: Log in to your Vimeo account and go to the video you wish to download.Select options: Click on the “Settings” button (gear icon) below the video player.Get video file link: In the settings menu, go to the “Video File” tab, where you can find the download link for your MP4 file.MUXEnable master access: Log in to your MUX account, navigate to the video asset, and enable master access if it’s not already enabled.Retrieve URL to master: Once master access is enabled, the URL to the master file will be available in the video asset details. Copy this URL for downloading the file.DropboxCreate a shareable link: Log in to your Dropbox account and locate the MP4 file you want to share. Click on the “Share” button next to the file.Set access permissions: In the sharing settings, create a link and set the permissions to “Anyone with the link.” Copy the generated link to download the file.General SFTP or S3 StorageAccess storage: Log in to your SFTP or S3 storage service control panel.Manage buckets/directories: Navigate to the appropriate bucket or directory containing your MP4 files.Retrieve download links: Generate HTTP/S links for the files you want to download. You can then use these links to download the files directly.Step 2: Check Availability to DownloadEnsure that your video files are available and ready for download, preventing any interruptions or issues during the migration process.Open HTTP/S link in a browser: Copy the HTTP/S link for the MP4 file and paste it into your browser’s address bar. Press Enter to navigate to the link.Check the video plays correctly in the browser: Verify that the video starts playing once the link is opened. This step ensures that the file is accessible and the link is functioning properly.Right-click to download: While the video is playing, right-click on the video player. Select “Save video as…” from the context menu. Choose a destination on your local disk to save the MP4 file.Step 3: Upload to Gcore Video StreamingNo matter which provider you’re migrating from, you need to upload your videos to Gcore Video Streaming storage. There are three primary methods to upload videos to Gcore storage:Copy from external storage: If your videos are available via public HTTPS URLs, you can directly copy the video files from external storage to Gcore. This method efficiently transfers files without downloading them to your local device first.Upload from a local device: Videos can be uploaded from your local host, backend, browser, or mobile app using the TUS resumable upload protocol. This method is resilient to interruptions, ensuring a smooth upload process by resuming from the point of failure.Batch upload: This method will soon be available to migrate extensive collections of videos, allowing you to transfer vast numbers of video files efficiently.The simplest migration option is to obtain video URLs and copy them to Gcore Video Hosting, eliminating the need to download and reupload videos.Example API Request to Copy Video from External StorageTo copy a video from another server, specify the origin_url attribute in the POST API request. The original video will be downloaded for video hosting on our server. Here is an example of the API request to set a task for copying a video from external storage:curl -L 'https://api.gcore.com/streaming/videos/' \-H 'Content-Type: application/json' \-H 'Authorization: APIKey 1234$0d16599c' \-d '{ "video": { "name": "Gcore Demo", "description": "Video copied from an external S3 Storage", "origin_url": "https://s-ed1.cloud.gcore.lu/demo-video/gcore.mp4" } }Refer to the complete documentation for detailed steps and examples of API requests. The original file must be in MP4 format or one of the following formats: 3g2, 3gp, asf, avi, dif, dv, flv, f4v, m4v, mov, mp4, mpeg, mpg, mts, m2t, m2ts, qt, wmv, vob, mkv, ogv, webm, vob, ogg, mxf, quicktime, x-ms-wmv, mpeg-tts, vnd.dlna.mpeg-tts. Streaming formats like HLS (.m3u8/.ts) and DASH (.mpd/.m4v) are intended for final video distribution and cannot be used as original file formats. Here are examples of good and bad links:Good link: https://demo-files.gvideo.io/gcore.mp4Bad link (chunked HLS format): https://demo-files.gvideo.io/hls/master.m3u8Note: Currently, only one video can be uploaded per request, so transferring your library in batches will require automation.Migrate to Gcore Video Streaming TodayGcore Video Streaming makes video migration easy with support for multiple sources and automatic transcoding. Whether you’re moving files from cloud storage, hosting platforms, or API-based services, Gcore streamlines video administration. Store, process, and distribute videos in various formats, complete with features like subtitles and timeline previews.With seamless migration and automatic transcoding, Gcore ensures your videos are optimized and ready for distribution, saving you time and effort. Simplify your video management and ensure your content is always accessible and in the best format for your audience with Gcore’s robust video streaming solutions.

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