A complete breakdown of Anycast DNS

We will show you everything you need to know about Anycast DNS. A complete breakdown of Anycast DNS that will explain to you the functionality and purpose of this service. 

​What is Anycast?

Anycast is a routing scheme that is perfect for use with DNS. You can use the same IP address (Anycast address) on multiple nodes. It is more about the configuration of routers and routing than servers. 

Each packet sent to an Anycast IP address may reach a different server in an Anycast DNS scenario. The anycast address is shared or configured in multiple different servers across the network. 

They are routed to the IP address with the best network metric. This is the nearest server in most cases, but it is not a must. The metrics could be various factors like bandwidth, cost, load, or reliability, etc. 

An important element of the Anycast network is that each server must have its Unicast IP address. 

Imagine if you try to administrate or monitor one of these servers, you can’t be sure which one you are reaching. So you target the Anycast address, and it will automatically lead you to one that the network metrics point. 

​Anycast and DNS, how do they work? 

Anycast DNS in recursive servers

The recursive DNS servers are usually Anycasted in the local network. You want your recursive server to respond to your customer and not to respond to the rest of the Internet. 

In this case, the servers are configured by the IP address of the clients. The latency here is very important. You can use the Anycast DNS to distribute the load across multiple devices. 

Anycast DNS authoritative servers

You can distribute anycast nodes geographically so that you will be able to reduce the latency massively. The point of presence (PoPs) will be a lot closer to the DNS clients, which will speed up the DNS resolution

​Advantage of Anycast DNS

If you have multiple DNS servers with the same Anycast address, the availability of the domain name will increase. If one of the nodes is down, the query will just travel a bit longer, but it will still find the same address and be resolved. The process will happen automatically. You won’t need to do anything, and at the same time, you won’t lose traffic. 

It is a very simple and yet effective way to perform load balancing.

​Disadvantages of Anycast DNS

Anycast could be a double-edged sword if you don’t have any protection and you are experiencing an attack. After the first Anycast node goes down, you will experience a cascade effect, and the attack will go from node to node until you stop it or all your nodes go down. You will a DNS monitor that can detect such problems and react accordingly. 

The routers must be well-configured so that the Anycast can function. Otherwise, you won’t be able to patch individual servers and have access to them. Depending on your software, you could put a specific label on each of the servers, and that way, you can distinguish them. This works on BIND. 

Monitoring might be harder because of the same reason – the same IP address. Again it could be resolved with an additional label or IP address for the nodes.

​Conclusion

In general, Anycast DNS is a way to keep the traffic local. You direct it to the closest instance, and by doing so, you provide a very fast and reliable service for your clients. In addition, it is easy to install, and it will improve your networking. 

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