DNS-based multi-CDN steering is less about picking the "best" CDN and more about not picking the worst one for a given user.
Let’s say you’ve got two CDNs, Akamai and Cloudflare, and you want to send your users to whichever one is faster for them. You can’t just tell your DNS server "send them to Akamai if they’re in Chicago, Cloudflare if they’re in New York." That’s too simplistic. The reality is far more granular. A user in Chicago might get better performance from Cloudflare on a given day due to network congestion or specific routing paths.
Here’s how it actually plays out. Imagine a user in Denver typing example.com into their browser.
- User’s Resolver: Their local DNS resolver (e.g., their ISP’s DNS server, Google DNS at
8.8.8.8, or Cloudflare DNS at1.1.1.1) receives the request forexample.com. - Resolver Queries Your DNS: The resolver then queries your authoritative DNS server for
example.com. This is where the magic happens. - Your DNS is Smart: Your authoritative DNS server isn’t just a static record. It’s configured with a "traffic manager" or "steering policy" that uses real-time data. This data comes from:
- Probe Data: Services like RIPE Atlas, ThousandEyes, or even your CDN providers themselves constantly measure latency and packet loss to various Points of Presence (PoPs) across the globe.
- Real User Monitoring (RUM): Data from actual user browsers (if you’re collecting it) showing how long it took them to load your site from different CDNs.
- CDN Performance APIs: Some CDNs offer APIs that report their current performance metrics.
- Geo-Proximity and Performance: Your DNS server looks at the source IP address of the query it received from the user’s resolver. It then correlates this IP address with the real-time performance data and geographic location.
- The Decision: Based on this, your DNS server determines which CDN PoP is currently offering the best performance for that specific geographic region or IP subnet. It might be Akamai’s Denver PoP, Cloudflare’s Denver PoP, or even a Fastly PoP in Denver if you’re using three.
- The Response: Your DNS server returns a CNAME record, pointing
example.comto a hostname managed by the chosen CDN. For example, it might returnexample.com CNAME cdn-akamai-denver.example.com. - CDN Resolution: The user’s resolver then queries
cdn-akamai-denver.example.com. This hostname points to the actual IP address of Akamai’s Denver PoP. - Content Delivery: The user’s browser connects to that Akamai IP address and fetches the content.
The key here is that the decision is dynamic. If Akamai’s Denver PoP suddenly becomes overloaded, or there’s a routing issue between Denver and their origin, your DNS steering can automatically switch traffic to Cloudflare’s Denver PoP (or another available one) for subsequent requests from that user’s resolver.
Example Configuration Snippet (Conceptual - actual implementation varies by DNS provider):
Let’s say you’re using a DNS provider that supports weighted or latency-based routing with external health checks or performance data feeds. You’d configure something like this:
{
"domain": "example.com",
"records": [
{
"name": "www",
"type": "CNAME",
"steering_policy": {
"type": "PERFORMANCE",
"fallback_to_latency": true,
"regions": {
"us-west": {
"primary": "akamai-us-west.example.net",
"secondary": "cloudflare-us-west.example.net"
},
"us-east": {
"primary": "cloudflare-us-east.example.net",
"secondary": "akamai-us-east.example.net"
}
// ... more regions
},
"performance_data_source": "https://api.example-steering-service.com/performance-metrics"
}
}
]
}
In this conceptual example, akamai-us-west.example.net and cloudflare-us-west.example.net are hostnames that resolve to the IP addresses of your respective CDN’s PoPs in the Western US. The performance_data_source would be a service you run (or subscribe to) that aggregates metrics from your CDNs and monitoring tools. When a DNS query comes from a resolver in the "us-west" region, your DNS server consults this data to pick the "primary" or "secondary" hostname.
The most surprising thing is how often the "best" CDN for a given user isn’t the one with the most PoPs near them, but rather the one with the most efficient peering or least congested backbone at that specific moment in time. Your DNS steering system is constantly trying to find the path of least resistance, and that path can change minute by minute. It’s a continuous optimization problem where the "optimal" solution is a moving target.
The next hurdle is understanding how to ingest and process the diverse performance data streams to make truly informed steering decisions.