A Rails route.rb rule that will redirect any GET to another domain and maintain the path, query, and hash/fragment.

rails_route_redirect_everything_to_another_domain.rb

Rails.application.routes.draw do
  get '(*path)', to: redirect{|params, request| "#{ENV['DOMAIN_TO_REDIRECT_TO']}#{request.url.gsub(request.scheme, '').gsub('://', '').gsub(request.host + '/', '')}"}
end

You can do this with .htaccess or nginx or Apache config but a special set of circumstances forced me to use a Rails app. Could possibly be a Ruby/Sinatra script too but again, special circumstances. Happy to hear improvements to the code itself.

A nice improvement would be maintaining the original sub-domain but that means you also need a wild-card SSL certificate. Heroku, which this route.rb is hosted on for my purposes, doesn't allow wild-card custom domains over SSL.