CDN and GeoDNS Explained

A Content Delivery Network (CDN) is a network of servers distributed around the world. Instead of serving every visitor from a single server, a CDN directs users to the nearest or best-performing server.

This improves:

• Website loading speed
• Availability
• Reliability
• Protection against DDoS attacks

Popular CDN providers include Cloudflare, Akamai, Fastly, Amazon CloudFront, and Google Cloud CDN.

Suppose a website uses a CDN.

www.example.com
        │
        ▼
   CDN Network
        │
 ┌──────┼────────┐
 ▼      ▼        ▼
USA   Germany   Japan

When users visit the website, they are automatically directed to the nearest CDN server.

GeoDNS (Geographic DNS) is a DNS technique that returns different IP addresses depending on the geographic location of the DNS resolver or the user.

Example:

Although the domain name is identical, different users receive different IP addresses.

GeoDNS is often used by CDNs, but they are not the same.

If a domain uses GeoDNS or a CDN, different DNS servers may receive different answers.

Example:

This behavior is expected and does not indicate a DNS configuration problem.

Many modern CDNs use Anycast networking.

With Anycast:

• Multiple servers advertise the same IP address.
• Internet routing automatically directs users to the nearest server.
• DNS always returns the same IP.

Example:

www.example.com → 104.16.25.10

USA      Germany      Japan
  │          │          │
  └──────────┼──────────┘
             │
        Same Anycast IP

In contrast, GeoDNS returns different IP addresses depending on location.

A domain is likely using a CDN if:

• Different DNS resolvers return different IP addresses.
• Multiple geographically distributed probes receive different responses.
• The IP belongs to a known CDN provider.
• The response contains CNAME records pointing to a CDN domain.

Example:

www.example.com
        │
        ▼
example.cdn.cloudflare.net
        │
        ▼
104.16.25.10

Many websites use a CNAME record to point to a CDN.

Example:

www.example.com
        CNAME
        ▼
example.cdnprovider.net
        │
        ▼
203.0.113.25

The CDN's authoritative DNS servers then decide which IP address should be returned for each user.

Some providers also support root domains (`example.com`) using technologies such as ALIAS, ANAME, or CNAME Flattening.

Telenegar DNS Check can help identify CDN behavior by:

• Querying multiple DNS providers.
• Comparing answers from different geographic locations.
• Detecting CNAME chains.
• Identifying GeoDNS behavior.
• Showing whether DNS responses differ between locations.

This makes it easier to distinguish between:

• DNS propagation
• CDN routing
• GeoDNS
• DNS misconfiguration

• Different IP addresses always indicate a DNS problem.
  • Not true. CDNs intentionally return different IP addresses.

• Every CDN returns different IP addresses.
  • Not always. Many CDNs use Anycast and return the same IP globally.

• The DNS server itself chooses the nearest CDN server.
  • Usually, the CDN's authoritative DNS server makes that decision based on the resolver's or user's location.

• DNS Explained
• CDN and GeoDNS Explained

dns cdn geodns anycast cname authoritative