Fix NSID not being set on cached responses (#3822)

* Add NSID to the cached responses
Signed-off-by: Erfan Besharat <erbesharat@gmail.com>

* Add test for NSID being set on cached responses
Signed-off-by: Erfan Besharat <erbesharat@gmail.com>

plugin/nsid/nsid.go

@@ -20,6 +20,7 @@ type Nsid struct {
 type ResponseWriter struct {
 	dns.ResponseWriter
 	Data    string
+	request *dns.Msg
 }
 
 // ServeDNS implements the plugin.Handler interface.
@@ -27,7 +28,7 @@ func (n Nsid) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (i
 	if option := r.IsEdns0(); option != nil {
 		for _, o := range option.Option {
 			if _, ok := o.(*dns.EDNS0_NSID); ok {
-				nw := &ResponseWriter{ResponseWriter: w, Data: n.Data}
+				nw := &ResponseWriter{ResponseWriter: w, Data: n.Data, request: r}
 				return plugin.NextOrFailure(n.Name(), n.Next, ctx, nw, r)
 			}
 		}
@@ -37,16 +38,31 @@ func (n Nsid) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (i
 
 // WriteMsg implements the dns.ResponseWriter interface.
 func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
+	if w.request.IsEdns0() != nil && res.IsEdns0() == nil {
+		res.SetEdns0(w.request.IsEdns0().UDPSize(), true)
+	}
+
 	if option := res.IsEdns0(); option != nil {
+		var exists bool
+
 		for _, o := range option.Option {
 			if e, ok := o.(*dns.EDNS0_NSID); ok {
 				e.Code = dns.EDNS0NSID
 				e.Nsid = hex.EncodeToString([]byte(w.Data))
+				exists = true
 			}
 		}
+
+		// Append the NSID if it doesn't exist in EDNS0 options
+		if !exists {
+			option.Option = append(option.Option, &dns.EDNS0_NSID{
+				Code: dns.EDNS0NSID,
+				Nsid: hex.EncodeToString([]byte(w.Data)),
+			})
+		}
 	}
-	returned := w.ResponseWriter.WriteMsg(res)
-	return returned
+
+	return w.ResponseWriter.WriteMsg(res)
 }
 
 // Name implements the Handler interface.

plugin/nsid/nsid_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/cache"
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	"github.com/coredns/coredns/plugin/test"
 	"github.com/coredns/coredns/plugin/whoami"
@@ -71,3 +72,65 @@ func TestNsid(t *testing.T) {
 		}
 	}
 }
+
+func TestNsidCache(t *testing.T) {
+	em := Nsid{
+		Data: "NSID",
+	}
+	c := cache.New()
+
+	tests := []struct {
+		next          plugin.Handler
+		qname         string
+		qtype         uint16
+		expectedCode  int
+		expectedReply string
+		expectedErr   error
+	}{
+		{
+			next:          whoami.Whoami{},
+			qname:         ".",
+			expectedCode:  dns.RcodeSuccess,
+			expectedReply: hex.EncodeToString([]byte("NSID")),
+			expectedErr:   nil,
+		},
+	}
+
+	ctx := context.TODO()
+
+	for i, tc := range tests {
+		req := new(dns.Msg)
+		if tc.qtype == 0 {
+			tc.qtype = dns.TypeA
+		}
+		req.SetQuestion(dns.Fqdn(tc.qname), tc.qtype)
+		req.Question[0].Qclass = dns.ClassINET
+
+		req.SetEdns0(4096, false)
+		option := req.Extra[0].(*dns.OPT)
+		option.Option = append(option.Option, &dns.EDNS0_NSID{Code: dns.EDNS0NSID, Nsid: ""})
+		em.Next = tc.next
+
+		rec := dnstest.NewRecorder(&test.ResponseWriter{})
+
+		c.Next = em
+		code, err := c.ServeDNS(ctx, rec, req)
+
+		if err != tc.expectedErr {
+			t.Errorf("Test %d: Expected error %v, but got %v", i, tc.expectedErr, err)
+		}
+		if code != int(tc.expectedCode) {
+			t.Errorf("Test %d: Expected status code %d, but got %d", i, tc.expectedCode, code)
+		}
+		if tc.expectedReply != "" {
+			for _, extra := range rec.Msg.Extra {
+				if option, ok := extra.(*dns.OPT); ok {
+					e := option.Option[0].(*dns.EDNS0_NSID)
+					if e.Nsid != tc.expectedReply {
+						t.Errorf("Test %d: Expected answer %s, but got %s", i, tc.expectedReply, e.Nsid)
+					}
+				}
+			}
+		}
+	}
+}