tunnel.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'net/ssh/gateway'

gateway = Net::SSH::Gateway.new('remote_host', \
                                'remote_user', \
                                :auth_methods  => ['publickey'], \
                                :forward_agent => true)

gateway.open('remote_host', remote_port, local_port) do |local_port|
  puts local_port
  while gateway.active? do
    sleep 5
  end
end

gateway.shutdown!