plugin/kubernetes: Validate transfers are allowed (#2292)

* check allowed transfers

* add tests for parsing, and comment about refactor

plugin/kubernetes/setup_transfer_test.go

@@ -0,0 +1,47 @@
+package kubernetes
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestKubernetesParseTransfer(t *testing.T) {
+	tests := []struct {
+		input     string // Corefile data as string
+		expected  string
+		shouldErr bool
+	}{
+		{`kubernetes cluster.local {
+			transfer to 1.2.3.4
+		}`, "1.2.3.4:53", false},
+		{`kubernetes cluster.local {
+			transfer to 1.2.3.4:53
+		}`, "1.2.3.4:53", false},
+		{`kubernetes cluster.local {
+			transfer to *
+		}`, "*", false},
+		{`kubernetes cluster.local {
+			transfer
+		}`, "", true},
+	}
+
+	for i, tc := range tests {
+		c := caddy.NewTestController("dns", tc.input)
+		k, err := kubernetesParse(c)
+		if err != nil && !tc.shouldErr {
+			t.Fatalf("Test %d: Expected no error, got %q", i, err)
+		}
+		if err == nil && tc.shouldErr {
+			t.Fatalf("Test %d: Expected error, got none", i)
+		}
+		if err != nil && tc.shouldErr {
+			// input should error
+			continue
+		}
+
+		if k.TransferTo[0] != tc.expected {
+			t.Errorf("Test %d: Expected Transfer To to be %s, got %s", i, tc.expected, k.TransferTo[0])
+		}
+	}
+}