What Is Anycast DNS? A Practical Guide

DNS is the system that translates domain names such as privateinternetaccess.com into the IP addresses that computers and web servers rely on to find and communicate with each other. The sheer volume of individual domains on the web makes this a mammoth task, so DNS providers use a range of techniques to reduce latency and increase reliability.

Anycast DNS is one of those techniques, but it’s also a common focus of cyberattacks. In this guide, we explain what Anycast DNS is, how it works, how it compares to other network addressing methods, and its benefits and risks.

What Is Anycast DNS?

Anycast is a DNS resolution method that uses multiple DNS servers with the same IP address across various locations. When you type in a website domain, your traffic gets routed to the nearest DNS server, so the query is resolved as fast as possible. DNS servers, recursive resolvers, and root DNS server operators often use it to improve performance and resilience.

This routing method was introduced in the late 1980s and formally documented in 1993 in RFC 1546¹ as “Host Anycasting Service,” as a solution to performance scaling as the Internet went global. At the time, using just one IP address for a single DNS server caused latency and bottlenecks, especially for people geographically further from the DNS infrastructure in the United States and Western Europe.

By replicating servers in different locations, the load is distributed more evenly, helping prevent capacity issues and service interruptions.

How Does Anycast DNS Work?

Even though it was introduced over three decades ago, anycast DNS is still a crucial component of the modern internet. This resilient routing model has been adapted to modern technologies, from IPv4 to IPv6 to new, sophisticated cyberthreat mitigation strategies.

Today, whenever you type a website’s name in the browser’s search bar, you’re probably using anycast DNS. Here’s how it works:

1. You send a DNS query: You write the name of the domain you want to visit, and your device requests the IP address needed to take you there. 
2. Internet routing selects one destination: Using Border Gateway Protocol (BGP), the network automatically routes your query along the best available path. If a node is unavailable, its route is withdrawn from BGP, and traffic is redirected to the next optimal node.
3. The selected node processes the request: That DNS server returns the IP address.
4. You can browse the website: Now that the IP request has been resolved, you can visit the site you wanted.

Anycast works in IPv4 and IPv6 environments in a similar way. Most DNS providers today run dual-stack anycast, meaning the same service supports both IPv4 and IPv6. 

However, IPv6 includes built-in support for anycast. For example, the first address in an IPv6 subnet is automatically reserved as the subnet-router anycast address. On the other hand, IPv4 often requires a few extra configuration steps, i.e., network operators need to “manually” assign the same IP address to multiple servers and announce it over BGP.

Anycast, Unicast, Broadcast, and Multicast

While anycast is a common and crucial traffic delivery model, several models decide how packets – which carry your DNS requests – travel across an IP network. Understanding how each one of them works gives you better insight into anycast’s role in the system.

These are the main traffic delivery models in IP networking:

• Unicast: A very straightforward one-to-one system, meaning that there’s only one source and one destination, with one specific IP address each. It’s one of the most common delivery models in everyday internet traffic.
• Broadcast: Operates on a one-to-all basis in IPv4 environments. In this model, one broadcast IP address sends data to all the devices within a local network segment (subnet), and they must process it, even if they’re not interested. 
• Multicast: In this one-to-many or many-to-many communication system, one or multiple sources also send information to multiple recipients, but only to subscribers. It’s considered more efficient than broadcast and is common in streaming and conferencing.
• Anycast: In this one-to-one-of-many model, one source sends information to one destination using one IP address, but that address exists in multiple locations. The router decides which server receives the packets, considering the most efficient path.

Method	Communication Pattern	Destination
Unicast	One-to-one	A single specific device
Broadcast	One-to-all	All devices in the subnet
Multicast	One-to-many or Many-to-many	Only devices subscribed to the group
Anycast	One-to-one-of-many	One of multiple possible servers (nearest/optimal)

Why and When to Use an Anycast Network

You use the anycast method when you need your information delivered quickly and globally through a reliable system. It was designed to improve performance and reduce latency for services across the world, and has become crucial for key components of the internet, such as public DNS resolvers.

Many businesses and organizations use anycast to deliver fast DNS resolution to users, including high-traffic websites, SaaS platforms, and content delivery and streaming services – especially when they operate on a large scale. 

It’s also common in cloud environments, APIs, and mobile apps. Some providers build their entire network architecture around this method, since anycast also offers benefits like DDoS attack mitigation and high reliability for users and providers.

Benefits and Risks of Anycast DNS

Anycast DNS offers multiple benefits, which is why it’s one of the most important traffic delivery models in our global internet infrastructure. However, no system is perfect, and anycast DNS also comes with challenges. Interestingly, a few of its advantages are also disadvantages.

Benefits of Anycast DNS

• Global presence and scalability: The number one reason this method was created and why it’s still crucial today: Anycast enables international reach and easy scalability. 
• Reduced latency and enhanced speeds: You get to connect to the nearest DNS server, which reduces latency and results in you loading the website you want to visit faster. By connecting you to the closest node, your DNS requests don’t have to travel a long distance.
• Better load distribution across multiple locations: By distributing traffic across multiple nodes, anycast DNS avoids bottlenecks and prevents overloads on a single server. 
• Improved uptime and reliability: If the nearest server is unavailable or fails, your DNS request will pass to the next one. BGP routing makes sure that there’s consistent uptime.
• DDoS attack mitigation: In DDoS attacks, malicious actors send large amounts of traffic to one server, but since several Anycast DNS servers share the same IP address, this attack spreads across multiple servers and usually doesn’t have a harmful impact. Anycast networks are more difficult to disrupt through DDoS attacks.
• A layer of protection against DNS spoofing: In DNS spoofing attacks, malicious actors redirect DNS traffic to malicious websites, and this is more difficult for them to do in an anycast network, as they can’t target a single DNS server. Also, some anycast networks include a protection protocol called Domain Name System Security Extensions (DNSSEC), which adds cryptographic signatures to verify that the data hasn’t been altered and is authentic.

Risks and Challenges of Anycast DNS

• Troubleshooting complexity: The advantage of having multiple servers with the same IP address is also an issue when it comes to troubleshooting: which server is the one failing or causing problems? Identifying the node that manages a particular DNS request can be challenging. There are specialized tools and technologies available to ease this process, but they require additional work and resources.
• IP geolocation inconsistencies: In certain situations, BGP can show unpredictable behavior and redirect you to different locations, affecting your latency. The best geographically located server is not always BGP’s first option, and this can affect your browsing speed in certain regions. Also, maintaining synchronized data across all anycast servers in different locations can be challenging and cause inconsistent responses.
• Regional bottlenecks: Sometimes BGP will redirect traffic to the same servers in a region with a large ISP, because it can overlook the load and just consider its logical routes. This can make some servers receive significantly more traffic than others and even cause bottlenecks due to uneven traffic distribution. Network managers must carefully analyze capacity and expected traffic.
• Deployment challenges and risks in certain regions: Adding and removing nodes requires a complex deployment process, including BGP configuration, testing, and traffic engineering. Also, in certain regions, such as mainland China, where all traffic must go through filters and controls, performance can be significantly affected².
• BGP hijacking and route leaks: Anycast networks have security vulnerabilities. While they can mitigate DDoS attacks, the traffic in this routing method can be intercepted by attackers. In a BGP hijacking, malicious actors trick routers by using the IP prefix of the Anycast DNS service and rerouting traffic to their malicious network.

FAQ

References:

1. RFC 1546: Host Anycasting Service
2. Why you shouldn’t use Global Anycast DNS in China – IBM.com