Since Rails 4.2, the
rails s command no longer listens at 0.0.0.0 (means listen on all interfaces) by default, its default is now localhost instead.
For IPv4 only OS localhost is always 127.0.0.1. For IPv6 enabled OS localhost may be either 127.0.0.1 or ::1, so everything works if client, server and domain name system agree to use the same IP version.
Rails 4.2 with Puma web server on Mac OS X listens at ::1. For me, it breaks http://lvh.me (the domain name is IPv4 only), http://pow.cx (it doesn’t cover IPv6 by design), gem “em-eventsource” and even Google Chrome (try to add
::1 subdomain.localhost to /etc/hosts; although it’s not a problem for Safari, you’ll fail to open http://subdomain.localhost:3000 powered by Puma).
Your mileage may vary.