Based on the Wikipedia description of Anycast, it includes both the distribution of a domain-name-to-many-IP-mapping across many DNS servers as well as replying to clients with the most geographically close (or fastest) server.
In the context of a globally distributed, highly available site like google.com (or any CDN service with many global edge locations) this sounds like the two key features one would need.
DNS services like Amazon's Route53, EasyDNS and DNSMadeEasy all advertise themselves as Anycast-enabled networks.
Therefore my assumption is that each of these DNS services transparently offer me those two killer features: multi-IP-to-domain mapping AND routing clients to the closest node.
However, each of these services seem to separate out these two functionalities, referring to the 2nd one (routing clients to closest node) as "GeoDNS", "GeoIP" or "Global Traffic Director" and charge extra for the service.
If a core tenant of an Anycast-capable system is to already do this, why is this functionality being earmarked as this extra feature? What is this "GeoDNS" feature doing that a standard Anycast DNS service won't do (according to the definition of Anycast from Wikipedia -- I understand what is being advertised, just not why it isn't implied already).
I get extra-confused when a DNS service like Route53 that doesn't support this nebulous "GeoDNS" feature lists functionality like:
Fast – Using a global anycast network of DNS servers around the world, Route 53 is designed to automatically route your users to the optimal location depending on network conditions. As a result, the service offers low query latency for your end users, as well as low update latency for your DNS record management needs.
... which sounds exactly like what GeoDNS is intended to do, but geographically directing clients is something they explicitly don't support it yet.
Ultimately I am looking for the two following features from a DNS provider:
- Map multiple IP addresses to a single domain name (like google.com, amazon.com, etc. does)
- Utilize a DNS service that will respond to client requests for that domain with the IP address of the nearest server to the requestee.
As mentioned, it seems like this is all part of an "Anycast" DNS service (all of which these services are), but the features and marketing I see from them suggest otherwise, making me think I need to learn a bit more about how DNS works before making a deployment choice.
Thanks in advance for any clarifications.
Answer
The key benefit of GeoDNS is that it resolves the same name to different IP addresses based on the requester's IP address.
An Anycasted DNS server will return the same IP address regardless of who is doing the asking.
The two work at different network layers. Anycast is below GeoDNS since it has no sense of the requester IP address's location, just its place in the network diagram. What may be close to the DNS server may not be close to the resource being requested, and the Anycasted DNS server has no way to tell this.
GeoDNS uses a map of resource locations and performs logic to determine which of those resources is closest to a given requesting IP address, and then returns that IP address.
Unless you're colocating your DNS servers with your network resources, what's close to the DNS server won't always be close to the resource, so using Anycast as your data-locality method is less effective than straight up GeoDNS.
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