I do a lot of web development on untrusted networks (coffeeshops, the neighbors\' open wifi, DEF CON), and I get twitchy when random, assuredly buggy software (my Rails app
Use the --binding=ip parameter:
--binding=ip
rails s --binding=127.0.0.1
https://github.com/rails/rails/blob/master/railties/lib/rails/commands/server.rb