Upgraded Rails to 6, getting Blocked host Error

Add this line to config/environments/development.rb

config.hosts << /.*\.ngrok\.io/

Restart your rails server and it will work

If you want to disable this functionality on your development environment, you can add config.hosts.clear to config/environments/development.rb.

This article worked for me:

1. The first option is to whitelist the hostnames in config/environments/development.rb:

   Rails.application.configure do
     config.hosts << "hostname" # Whitelist one hostname
     config.hosts << /application\.local\Z/ # Whitelist a test domain
   end
   

2. The second option is to clear the entire whitelist, which lets through requests for all hostnames:

   Rails.application.configure do
     config.hosts.clear
   end
   

Credit goes to Manfred Stienstra.

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