config.ru (as plain Rack)

use Rack::ContentType, "text/plain"
use Rack::ContentLength

run lambda {|env| [200, {}, [env['REMOTE_ADDR'].split(",").first]]} 

config.ru (as Sinatra)

require 'sinatra/base'

run Sinatra.new {
  get('/') do
    content_type "text/plain" # you can also use Rack::ContentType
                              # with Sinatra except here we want
                              # more control since Sinatra gives it
                              # to us.  Rack::ContentType sets the
                              # Content-Type on every route/url for
                              # the current mapping.

    request.env['REMOTE_ADDR'].split(",").first
  end
}