diff --git a/config_test.go b/config_test.go
index 7a4e10172..79f16de13 100644
--- a/config_test.go
+++ b/config_test.go
@@ -12,8 +12,106 @@ import (
 	"testing"
 
 	"github.com/hashicorp/packer/packer"
+	"github.com/hashicorp/packer/packer/plugin"
 )
 
+func newConfig() config {
+	var conf config
+	conf.PluginMinPort = 10000
+	conf.PluginMaxPort = 25000
+	conf.Builders = packer.MapOfBuilder{}
+	conf.PostProcessors = packer.MapOfPostProcessor{}
+	conf.Provisioners = packer.MapOfProvisioner{}
+
+	return conf
+}
+func TestDiscoverReturnsIfMagicCookieSet(t *testing.T) {
+	config := newConfig()
+
+	os.Setenv(plugin.MagicCookieKey, plugin.MagicCookieValue)
+	defer os.Unsetenv(plugin.MagicCookieKey)
+
+	err := config.Discover()
+	if err != nil {
+		t.Fatalf("Should not have errored: %s", err)
+	}
+
+	if len(config.Builders) != 0 {
+		t.Fatalf("Should not have tried to find builders")
+	}
+}
+
+func TestEnvVarPackerPluginPath(t *testing.T) {
+	// Create a temporary directory to store plugins in
+	dir, _, cleanUpFunc, err := generateFakePlugins("custom_plugin_dir",
+		[]string{"packer-provisioner-partyparrot"})
+	if err != nil {
+		t.Fatalf("Error creating fake custom plugins: %s", err)
+	}
+
+	defer cleanUpFunc()
+
+	// Add temp dir to path.
+	os.Setenv("PACKER_PLUGIN_PATH", dir)
+	defer os.Unsetenv("PACKER_PLUGIN_PATH")
+
+	config := newConfig()
+
+	err = config.Discover()
+	if err != nil {
+		t.Fatalf("Should not have errored: %s", err)
+	}
+
+	if len(config.Provisioners) == 0 {
+		t.Fatalf("Should have found partyparrot provisioner")
+	}
+	if _, ok := config.Provisioners["partyparrot"]; !ok {
+		t.Fatalf("Should have found partyparrot provisioner.")
+	}
+}
+
+func TestEnvVarPackerPluginPath_MultiplePaths(t *testing.T) {
+	// Create a temporary directory to store plugins in
+	dir, _, cleanUpFunc, err := generateFakePlugins("custom_plugin_dir",
+		[]string{"packer-provisioner-partyparrot"})
+	if err != nil {
+		t.Fatalf("Error creating fake custom plugins: %s", err)
+	}
+
+	defer cleanUpFunc()
+
+	pathsep := ":"
+	if runtime.GOOS == "windows" {
+		pathsep = ";"
+	}
+
+	// Create a second dir to look in that will be empty
+	decoyDir, err := ioutil.TempDir("", "decoy")
+	if err != nil {
+		t.Fatalf("Failed to create a temporary test dir.")
+	}
+
+	pluginPath := dir + pathsep + decoyDir
+
+	// Add temp dir to path.
+	os.Setenv("PACKER_PLUGIN_PATH", pluginPath)
+	defer os.Unsetenv("PACKER_PLUGIN_PATH")
+
+	config := newConfig()
+
+	err = config.Discover()
+	if err != nil {
+		t.Fatalf("Should not have errored: %s", err)
+	}
+
+	if len(config.Provisioners) == 0 {
+		t.Fatalf("Should have found partyparrot provisioner")
+	}
+	if _, ok := config.Provisioners["partyparrot"]; !ok {
+		t.Fatalf("Should have found partyparrot provisioner.")
+	}
+}
+
 func TestDecodeConfig(t *testing.T) {
 
 	packerConfig := `
@@ -107,18 +205,13 @@ func TestLoadExternalComponentsFromConfig_onlyProvisioner(t *testing.T) {
 	}
 }
 
-/* generateFakePackerConfigData creates a collection of mock plugins along with a basic packerconfig.
-The return packerConfigData is a valid packerconfig file that can be used for configuring external plugins, cleanUpFunc is a function that should be called for cleaning up any generated mock data.
-This function will only clean up if there is an error, on successful runs the caller
-is responsible for cleaning up the data via cleanUpFunc().
-*/
-func generateFakePackerConfigData() (packerConfigData string, cleanUpFunc func(), err error) {
-	dir, err := ioutil.TempDir("", "random-testdata")
+func generateFakePlugins(dirname string, pluginNames []string) (string, []string, func(), error) {
+	dir, err := ioutil.TempDir("", dirname)
 	if err != nil {
-		return "", nil, fmt.Errorf("failed to create temporary test directory: %v", err)
+		return "", nil, nil, fmt.Errorf("failed to create temporary test directory: %v", err)
 	}
 
-	cleanUpFunc = func() {
+	cleanUpFunc := func() {
 		os.RemoveAll(dir)
 	}
 
@@ -127,19 +220,36 @@ func generateFakePackerConfigData() (packerConfigData string, cleanUpFunc func()
 		suffix = ".exe"
 	}
 
-	plugins := [...]string{
-		filepath.Join(dir, "packer-builder-cloud-xyz"+suffix),
-		filepath.Join(dir, "packer-provisioner-super-shell"+suffix),
-		filepath.Join(dir, "packer-post-processor-noop"+suffix),
-	}
-	for _, plugin := range plugins {
-		_, err := os.Create(plugin)
+	plugins := make([]string, len(pluginNames))
+	for i, plugin := range pluginNames {
+		plug := filepath.Join(dir, plugin+suffix)
+		plugins[i] = plug
+		_, err := os.Create(plug)
 		if err != nil {
 			cleanUpFunc()
-			return "", nil, fmt.Errorf("failed to create temporary plugin file (%s): %v", plugin, err)
+			return "", nil, nil, fmt.Errorf("failed to create temporary plugin file (%s): %v", plug, err)
 		}
 	}
 
+	return dir, plugins, cleanUpFunc, nil
+}
+
+/* generateFakePackerConfigData creates a collection of mock plugins along with a basic packerconfig.
+The return packerConfigData is a valid packerconfig file that can be used for configuring external plugins, cleanUpFunc is a function that should be called for cleaning up any generated mock data.
+This function will only clean up if there is an error, on successful runs the caller
+is responsible for cleaning up the data via cleanUpFunc().
+*/
+func generateFakePackerConfigData() (packerConfigData string, cleanUpFunc func(), err error) {
+	_, plugins, cleanUpFunc, err := generateFakePlugins("random-testdata",
+		[]string{"packer-builder-cloud-xyz",
+			"packer-provisioner-super-shell",
+			"packer-post-processor-noop"})
+
+	if err != nil {
+		cleanUpFunc()
+		return "", nil, err
+	}
+
 	packerConfigData = fmt.Sprintf(`
 	{
 		"PluginMinPort": 10,