gethostbyname "localhost" is not a reliable solution for localhost addresses

[andrew-aladev]

Steps to reproduce

Related jruby issue.

class Server < Sinatra::Base
  configure do
    set :server, :puma
    set :bind, "localhost"
  end
end

1. rvm use jruby-9.1.6.0

2. Try to start sinatra app with jruby.

Expected behavior

Server will be accessible on both 127.0.0.1 and ::1.

Actual behavior

Server is accessible only on 127.0.0.1

System configuration

Ruby version: jruby 9.1.6.0 (2.3.1) 2016-11-09 0150a76 OpenJDK 64-Bit Server VM 25.101-b13 on 1.8.0_101-b13 +jit [linux-x86_64]

Fix

You are using TCPSocket.gethostbyname "localhost" here to get the list of localhost addresses.

In MRI ruby we have a comment:

 * Note that it is not guaranteed to be able to convert to IP address using gethostbyname, getaddrinfo, etc.
 * If you need local IP address, use Socket.ip_address_list.

We can use something like:

Socket.ip_address_list.select do |address|
  if address.ipv4?
    not address.ipv4_multicast? and not address.ipv4_private?
  else
    not address.ipv6_linklocal?
  end
end
.map do |address|
  address.ip_address
end

Thank you.

[nateberkopec]

If you could provide some feedback on #1318 please