Dns.join (#944)

* Add dnsutil.Join

* Create dnsutil.Join

Create Join helper function and move bits in the code over.

middleware/pkg/dnsutil/join.go

@@ -0,0 +1,19 @@
+package dnsutil
+
+import (
+	"strings"
+
+	"github.com/miekg/dns"
+)
+
+// Join joins labels to form a fully qualified domain name. If the last label is
+// the root label it is ignored. Not other syntax checks are performed.
+func Join(labels []string) string {
+	ll := len(labels)
+	if labels[ll-1] == "." {
+		s := strings.Join(labels[:ll-1], ".")
+		return dns.Fqdn(s)
+	}
+	s := strings.Join(labels, ".")
+	return dns.Fqdn(s)
+}

middleware/pkg/dnsutil/join_test.go

@@ -0,0 +1,20 @@
+package dnsutil
+
+import "testing"
+
+func TestJoin(t *testing.T) {
+	tests := []struct {
+		in  []string
+		out string
+	}{
+		{[]string{"bla", "bliep", "example", "org"}, "bla.bliep.example.org."},
+		{[]string{"example", "."}, "example."},
+		{[]string{"."}, "."},
+	}
+
+	for i, tc := range tests {
+		if x := Join(tc.in); x != tc.out {
+			t.Errorf("Test %d, expected %s, got %s", i, tc.out, x)
+		}
+	}
+}