Enable port forwarding to be directed at a specific adapter (NIC), prevented entirely making standard use of TCP/IP and ensuring that port forwarding is only attempted when the NIC is using the VirtualBox NAT.

config/default.rb

@@ -7,6 +7,7 @@ Vagrant::Config.run do |config|
   config.ssh.username = "vagrant"
   config.ssh.password = "vagrant"
   config.ssh.host = "localhost"
+  config.ssh.port = 22
   config.ssh.forwarded_port_key = "ssh"
   config.ssh.max_tries = 10
   config.ssh.timeout = 30

lib/vagrant/actions/vm/forward_ports.rb

@@ -9,7 +9,7 @@ module Vagrant
             vm.forwarded_ports.each do |fp|
               @runner.env.config.vm.forwarded_ports.each do |name, options|
                 if fp.hostport.to_s == options[:hostport].to_s
-                  raise ActionException.new(:vm_port_collision, :name => name, :hostport => fp.hostport.to_s, :guestport => options[:guestport].to_s)
+                  raise ActionException.new(:vm_port_collision, :name => name, :hostport => fp.hostport.to_s, :guestport => options[:guestport].to_s, :instance => options[:instance])
                 end
               end
             end
@@ -30,12 +30,23 @@ module Vagrant
           logger.info "Forwarding ports..."
 
           @runner.env.config.vm.forwarded_ports.each do |name, options|
-            logger.info "Forwarding \"#{name}\": #{options[:guestport]} => #{options[:hostport]}"
-            port = VirtualBox::ForwardedPort.new
-            port.name = name
-            port.hostport = options[:hostport]
-            port.guestport = options[:guestport]
-            @runner.vm.forwarded_ports << port
+            adapter = options[:instance]
+
+            # Assuming the only reason to establish port forwarding is because the VM is using Virtualbox NAT networking.
+            # Host-only or Bridged networking don't require port-forwarding and establishing forwarded ports on these
+            # attachment types has uncertain behaviour.
+            if @runner.vm.network_adapters[adapter].attachment_type == :nat
+               logger.info "Forwarding \"#{name}\": #{options[:guestport]} on Adapter\##{adapter+1} => #{options[:hostport]}"
+               port = VirtualBox::ForwardedPort.new
+               port.name = name
+               port.hostport = options[:hostport]
+               port.guestport = options[:guestport]
+               port.instance = adapter
+               @runner.vm.forwarded_ports << port
+            else
+              logger.info "VirtualBox Adapter\##{adapter+1} not configured as \"NAT\"."
+              logger.info "Skipped setting forwarding \"#{name}\": #{options[:guestport]} on Adapter\##{adapter+1} => #{options[:hostport]}"
+            end
           end
 
           @runner.vm.save

lib/vagrant/config.rb

@@ -63,6 +63,7 @@ module Vagrant
       attr_accessor :username
       attr_accessor :password
       attr_accessor :host
+      attr_accessor :port
       attr_accessor :forwarded_port_key
       attr_accessor :max_tries
       attr_accessor :timeout
@@ -101,11 +102,12 @@ module Vagrant
         @provisioner = nil
       end
 
-      def forward_port(name, guestport, hostport, protocol="TCP")
+      def forward_port(name, guestport, hostport, protocol="TCP", instance=0)
         forwarded_ports[name] = {
           :guestport  => guestport,
           :hostport   => hostport,
-          :protocol   => protocol
+          :protocol   => protocol,
+          :instance   => instance
         }
       end
 

lib/vagrant/ssh.rb

@@ -114,9 +114,19 @@ module Vagrant
 
     # Returns the port which is either given in the options hash or taken from
     # the config by finding it in the forwarded ports hash based on the
-    # `config.ssh.forwarded_port_key`
+    # `config.ssh.forwarded_port_key` or use the default port given by `config.ssh.port`
+    # when port forwarding isn't used.
     def port(opts={})
-      opts[:port] || env.config.vm.forwarded_ports[env.config.ssh.forwarded_port_key][:hostport]
+      # Check if port was specified in options hash
+      pnum = opts[:port]
+      return pnum if pnum
+
+      # Check if we have an SSH forwarded port
+      pnum = env.config.vm.forwarded_ports[env.config.ssh.forwarded_port_key]
+      return pnum[:hostport] if pnum
+
+      # Fall back to the default
+      return env.config.ssh.port
     end
   end
 end

test/vagrant/actions/vm/forward_ports_test.rb

@@ -71,12 +71,17 @@ class ForwardPortsActionTest < Test::Unit::TestCase
   context "forwarding ports" do
     should "create a port forwarding for the VM" do
       forwarded_ports = mock("forwarded_ports")
+      network_adapter = mock("network_adapter")
+      
+      @vm.expects(:network_adapters).returns([network_adapter])
+      network_adapter.expects(:attachment_type).returns(:nat)
       
       @mock_vm.env.config.vm.forwarded_ports.each do |name, opts|
         forwarded_ports.expects(:<<).with do |port|
           assert_equal name, port.name
           assert_equal opts[:hostport], port.hostport
           assert_equal opts[:guestport], port.guestport
+          assert_equal opts[:instance], port.instance
           true
         end
       end
@@ -87,6 +92,18 @@ class ForwardPortsActionTest < Test::Unit::TestCase
     end
   end
 
+  context "Not forwarding ports" do
+    should "No port forwarding for non NAT interfaces" do
+      forwarded_ports = mock("forwarded_ports")
+      network_adapter = mock("network_adapter")
+      
+      @vm.expects(:network_adapters).returns([network_adapter])
+      network_adapter.expects(:attachment_type).returns(:host_only)      
+      @vm.expects(:save).once
+      @action.forward_ports
+    end
+  end
+
   context "clearing forwarded ports" do
     should "call destroy on all forwarded ports" do
       forwarded_ports = []