Fix TOCTOU bug

This fixes the time of check to time of use (TOCTOU) bug reported by Sajjad
Hashemian.

README.md

@@ -61,8 +61,18 @@ To install this gem onto your local machine, run `bundle exec rake install`. To
 
 Bug reports and pull requests are welcome on GitHub at https://github.com/jtdowney/private_address_check. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the [Contributor Covenant](http://contributor-covenant.org) code of conduct.
 
+## Security
+
+If you've found a security issue in `private_address_check`, please reach out to @jtdowney via email to report.
+
+### Time of check to time of use
+
+A library like `private_address_check` is going to be easily susceptible to attacks like [time of check to time of use](https://en.wikipedia.org/wiki/Time_of_check_to_time_of_use). DNS entries with a TTL of 0 can trigger this case where the initial resolution is a public address by the subsequent resolution is a private address. There are some possible defenses and workarounds:
+
+- Use the TCPSocket extension in this library which checks the address the socket uses. This is most useful if your system is built on native Ruby like Net::HTTP.
+- Use a feature like the `resolve` capability in curl and [curb](https://www.rubydoc.info/github/taf2/curb/Curl/Easy#resolve=-instance_method) to force the resolution to a pre-checked IP address.
+- Implement your own caching DNS resolver with something like dnsmasq or unbound. These tools let you set a minimum cache time that can override the TTL of 0.
 
 ## License
 
 The gem is available as open source under the terms of the [MIT License](http://opensource.org/licenses/MIT).
-

lib/private_address_check/tcpsocket_ext.rb

@@ -14,11 +14,10 @@ def only_public_connections
 TCPSocket.class_eval do
   alias initialize_without_private_address_check initialize
 
-  def initialize(remote_host, remote_port, local_host = nil, local_port = nil)
-    if Thread.current[:private_address_check] && PrivateAddressCheck.resolves_to_private_address?(remote_host)
+  def initialize(*args)
+    initialize_without_private_address_check(*args)
+    if Thread.current[:private_address_check] && PrivateAddressCheck.resolves_to_private_address?(remote_address.ip_address)
       raise PrivateAddressCheck::PrivateConnectionAttemptedError
     end
-
-    initialize_without_private_address_check(remote_host, remote_port, local_host, local_port)
   end
 end

test/private_address_check/tcpsocket_ext_test.rb

@@ -3,11 +3,15 @@
 
 class TCPSocketExtTest < Minitest::Test
   def test_private_address
+    server = TCPServer.new(63453)
+    thread = Thread.start { server.accept }
     assert_raises PrivateAddressCheck::PrivateConnectionAttemptedError do
       PrivateAddressCheck.only_public_connections do
-        TCPSocket.new("localhost", 80)
+        TCPSocket.new("localhost", 63453)
       end
     end
+  ensure
+    thread.exit if thread
   end
 
   def test_public_address