The Blocked Host is a new feature of Rails 6. You can add this pattern to your config/environments/development.rb to have no worries of that in case of dynamic urls
config.hosts << /[a-z0-9]+\.c9users\.io/
Also for ngrok user, just replace above c9users by ngrok
Update: ngrok is currently using - and . as subdomain in their URLs so this should be accurate config.hosts << /[a-z0-9-.]+\.ngrok\.io/
Source: https://github.com/MikeRogers0/puma-ngrok-tunnel