Rails 4 - on the server, how do I list all the cookies and their values that have been sent from the client?

In your controller try the following:

cookies.each do |cookie|
  puts cookie
end

cookie[0] will be the name and cookie[1] will be the value.

For a nicer output:

Rails.logger.debug(cookies.map { |cookie| cookie.join('=') }.join("\n"))

And watch your Rails log.