Act based on subdomain

I am new to Cloudflare workers :slight_smile:
I want to create a worker that will check the client TLS version only on a specific subdomain, e.g. before handing the request over to a proxied server that resides elsewhere.
What is the better option in my case, routes or custom domains?

According to this document the answer is routes, but aren’t they used for routing based on paths within a domain?

Custom Domains are meant for when the Worker is your origin. You should use routes if you’re going to pass the request through.