How do you add a custom HTTP response header in Rails?

Use:

response.headers['HEADER NAME'] = 'HEADER VALUE'

either in a specific method or to a before_filter method of your application controller depending on whether you need this to be added in a specific or to all of your responses.

UPDATE for Rails 5 – February 24th, 2018

As noted by @BrentMatzelle in the comments, for Rails 5:

response.set_header('HEADER NAME', 'HEADER VALUE')