# products_controller.rb
before_filter :authenticate

protected

def authenticate
  authenticate_or_request_with_http_basic do |username, password|
    case request.domain
    when "domain1.com"
      username == "foo1" && password == "bar"
    when "domain2.com"
      username == "foo2" && password == "bar"
    when "domain3.com"
      username == "foo3" && password == "bar"
  end
end