Collision check is now done in `prepare` of forward port action.

lib/vagrant/actions/vm/forward_ports.rb

@@ -2,6 +2,31 @@ module Vagrant
   module Actions
     module VM
       class ForwardPorts < Base
+        def prepare
+          ActiveList.vms.each do |vagrant_vm|
+            vm = vagrant_vm.vm
+            next unless vm.running?
+
+            vm.forwarded_ports.each do |fp|
+              Vagrant.config.vm.forwarded_ports.each do |name, options|
+                if fp.hostport.to_s == options[:hostport].to_s
+                  raise ActionException.new(<<-msg)
+Vagrant cannot forward the specified ports on this VM, since they
+would collide with another Vagrant-managed virtual machine's forwarded
+ports! The "#{name}" forwarded port (#{fp.hostport}) is already in use on the host
+machine.
+
+To fix this, modify your current projects Vagrantfile to use another
+port. Example, where '1234' would be replaced by a unique host port:
+
+config.vm.forward_port("#{name}", #{options[:guestport]}, 1234)
+msg
+                end
+              end
+            end
+          end
+        end
+
         def execute!
           clear
           forward_ports

test/vagrant/actions/vm/forward_ports_test.rb

@@ -6,6 +6,52 @@ class ForwardPortsActionTest < Test::Unit::TestCase
     mock_config
   end
 
+  context "checking for colliding ports" do
+    setup do
+      @forwarded_port = mock("forwarded_port")
+      @forwarded_port.stubs(:hostport)
+      @forwarded_ports = [@forwarded_port]
+
+      @vm = mock("vm")
+      @vm.stubs(:forwarded_ports).returns(@forwarded_ports)
+      @vm.stubs(:running?).returns(true)
+      vagrant_vm = mock("vagrant_vm")
+      vagrant_vm.stubs(:vm).returns(@vm)
+      vms = [vagrant_vm]
+      Vagrant::ActiveList.stubs(:vms).returns(vms)
+
+      mock_config do |config|
+        config.vm.forwarded_ports.clear
+        config.vm.forward_port("ssh", 22, 2222)
+      end
+    end
+
+    should "ignore vms which aren't running" do
+      @vm.expects(:running?).returns(false)
+      @vm.expects(:forwarded_ports).never
+      @action.prepare
+    end
+
+    should "not raise any errors if no forwarded ports collide" do
+      @forwarded_port.expects(:hostport).returns(80)
+      assert_nothing_raised { @action.prepare }
+    end
+
+    should "raise an ActionException if a port collides" do
+      @forwarded_port.expects(:hostport).returns(2222)
+      assert_raises(Vagrant::Actions::ActionException) {
+        @action.prepare
+      }
+    end
+
+    should "convert ports to strings prior to checking" do
+      @forwarded_port.expects(:hostport).returns("2222")
+      assert_raises(Vagrant::Actions::ActionException) {
+        @action.prepare
+      }
+    end
+  end
+
   context "execution" do
     should "clear all previous ports and forward new ports" do
       exec_seq = sequence("exec_seq")