AWS Guide: Resolving Duplicate CNAME Issues in CloudFront Distributions

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If you’re using Amazon CloudFront with multiple distributions and encounter issues with domain routing, here’s a simple way to diagnose and fix the problem.

Suppose you have two Application Load Balancers (ALBs)—let’s call them A ALB and B ALB—and two CloudFront distributions: A CloudFront and B CloudFront. Typically, you want some.test.domain to go to A CloudFront, which should then route requests to A ALB. Meanwhile, B CloudFront handles its own domain with B ALB as the origin.

Now, imagine you accidentally add some.test.domain as an Alternate Domain Name (CNAME) to both CloudFront distributions. In your case, you set *.test.domain as a CNAME for A CloudFront, and by mistake, you also added some.test.domain to B CloudFront.

Even though Route53 was configured so that some.test.domain pointed to A CloudFront, you noticed requests were being sent to B ALB instead. Upon investigating, you found that having some.test.domain listed as an Alternate Domain Name in both distributions caused this mix-up. When you removed some.test.domain from B CloudFront’s Alternate Domain Names, the requests correctly routed to A ALB again.

So, what’s happening here? In CloudFront, when two distributions have overlapping Alternate Domain Names, it can cause conflicts in how requests are resolved. CloudFront might send the requests to the distribution with the matching domain, but this behavior can sometimes be inconsistent or lead to unexpected routing, especially if the same domain is listed in multiple distributions.

The key takeaway is: Always ensure that each domain or subdomain is associated with only one CloudFront distribution. Overlapping CNAMEs can create confusing routing issues and might sometimes seem like a bug. Correcting the configuration by removing duplicate domain names from other distributions typically resolves the problem.

If you face similar issues, verify your Alternate Domain Name setups and make sure there’s no overlap across distributions. This simple step can save you a lot of headaches and ensure that your domain requests are routed exactly where you want them.

Let me know if you need further help or clarification!