plugin/rewrite: prevent illegal names (#1972)

Log and returns an error when the name rewrite creates a name that is
illegal. Add test in name_test.go to see if an error is returned.

Possible followup could be the only check this if a name-rewrite is
done.

Fixes: #1638

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/rewrite/name_test.go

@@ -0,0 +1,33 @@
+package rewrite
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+func TestRewriteIllegalName(t *testing.T) {
+	r, _ := newNameRule("stop", "example.org.", "example..org.")
+
+	rw := Rewrite{
+		Next:     plugin.HandlerFunc(msgPrinter),
+		Rules:    []Rule{r},
+		noRevert: true,
+	}
+
+	ctx := context.TODO()
+	m := new(dns.Msg)
+	m.SetQuestion("example.org.", dns.TypeA)
+
+	rec := dnstest.NewRecorder(&test.ResponseWriter{})
+	_, err := rw.ServeDNS(ctx, rec, m)
+	if !strings.Contains(err.Error(), "invalid name") {
+		t.Errorf("Expected invalid name, got %s", err.Error())
+	}
+}