CloudFlare is a Content Deliery Network and distributed DNS system; Cloud Foundry is a Platform as a Service.
Routing custom top-level domains (e.g. my-custom-domain.com) and custom subdomains (e.g. blog.my-custom-domain.com) to an app running on a Cloud Foundry instance can be achieved using CloudFlare as a DNS provider as follows:
Sign up for an account with a Cloud Foundry provider (e.g. Pivotal Web Services) and the install the Cloud Foundry CLI.
Using the CLI, login and target your org and space:
$ cf api <e.g. https://api.run.pivotal.io>
$ cf login
$ cf target -o my-org -s my-space
From the working directory of your application, push it to Cloud Foundry:
$ cd my-app-dir/
$ cf push my-app
Create the domain and map the route to your application (see Cloud Foundry documentation for additional information):
$ cf create-domain my-org my-custom-domain.com
$ cf map-route my-app my-custom-domain.com
Sign up for a CloudFlare account.
Sign in to your account, and go to the DNS page for your domain (e.g https://www.cloudflare.com/dns-settings?z=my-custom-domain.com.
Add a CNAME entry for the domain to point to the default application route provided by Cloud Foundry. For example, on Pivotal Web Services, this url would look like my-app.cfapps.io.
CloudFlare uses CNAME Flattening to be able to utilize a CNAME entry for both custom top-level domains (e.g. my-custom-domain.com) and custom subdomains (e.g. blog.my-custom-domain.com).
Once this DNS record has propagated (anywhere from a few minutes to 24 hours) you should be able to access your application at both the Cloud Foundry provided url (e.g. my-app.cfapps.io) and your custom domain my-custom-domain.com.