From ALB to Caddy - Our Wandering Path to Supporting Thousands of Domain Names

FusionAuth has recently introduced support for unlimited custom domains in its Cloud managed hosting service. This feature allows customers to configure one or more custom domain names, enabling them to maintain branding consistency across authentication flows. The introduction of this feature was made possible by the development of a new reverse proxy architecture using AWS's Elastic Container Service (ECS) and Caddy, which supports dynamic TLS certificate provisioning without limitations on the number of hostnames. This new design includes features such as multi-AZ ECS clusters, EFS volumes for storing certificates, Network Load Balancers, Global Accelerators, and internal services for authorizing certificate management. While some additional components were required to implement a Web Application Firewall (WAF) with this architecture, the final result enables FusionAuth Cloud customers to handle any use case, including those with thousands of custom domains.