fix(dns): decode up to four answer records (#186)

* fix(dns): decode up to four answer records

A single DNS question can return multiple answer records. Use an answer
decode limit of four in Client.StartResolve and add a regression test
covering a single-question response with multiple A records.

* fix(dns): add MaxResponseAnswers to ResolveConfig

This allows callers to explicitly declare the maximum number of answer
records to retain, removing the hardcoded limit in Client.StartResolve.

xnet.StackAsync is updated to set this limit to match the length of its
lookup-result buffer. This maintains the zero-allocation design while
fixing the issue where multiple A records were ignored.

dns/client.go

@@ -24,6 +24,9 @@ type ResolveConfig struct {
 	Questions       []Question
 	Additional      []Resource
 	EnableRecursion bool
+	// MaxResponseAnswers limits how many answer records are decoded from the
+	// DNS response. If zero it defaults to the number of Questions.
+	MaxResponseAnswers uint16
 }
 
 func (sudp *Client) Protocol() uint64 { return uint64(lneto.IPProtoUDP) }
@@ -37,8 +40,12 @@ func (c *Client) StartResolve(localPort, txid uint16, cfg ResolveConfig) error {
 	if nd > math.MaxUint16 {
 		return lneto.ErrInvalidConfig
 	}
+	maxAns := cfg.MaxResponseAnswers
+	if maxAns == 0 {
+		maxAns = uint16(nd)
+	}
 	c.reset(localPort, txid, CQueryPending, cfg.EnableRecursion)
-	c.msg.LimitResourceDecoding(uint16(nd), uint16(nd), 0, 0)
+	c.msg.LimitResourceDecoding(uint16(nd), maxAns, 0, 0)
 	c.msg.AddQuestions(cfg.Questions)
 	c.msg.AddAdditionals(cfg.Additional)
 	return nil

dns/dns_test.go

@@ -243,9 +243,25 @@ func TestClient_ReceivesDNSResponse(t *testing.T) {
 	const hostname = "example.com"
 	const txid = uint16(12345)
 	const clientPort = uint16(54321)
-	wantIP := [4]byte{93, 184, 216, 34}
-
-	// Build a DNS response message.
+	const maxAnswers = 4
+	allIPs := [5][4]byte{
+		{192, 0, 2, 1},
+		{192, 0, 2, 2},
+		{192, 0, 2, 3},
+		{192, 0, 2, 4},
+		{192, 0, 2, 5},
+	}
+	tests := []struct {
+		name        string
+		responseIPs [][4]byte
+		wantAnswers int
+	}{
+		{name: "single_answer", responseIPs: allIPs[:1], wantAnswers: 1},
+		{name: "multiple_answers", responseIPs: allIPs[:4], wantAnswers: 4},
+		{name: "answer_limit", responseIPs: allIPs[:5], wantAnswers: maxAnswers},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
 			name := MustNewName(hostname)
 			responseMsg := Message{
 				Questions: []Question{{
@@ -253,66 +269,74 @@ func TestClient_ReceivesDNSResponse(t *testing.T) {
 					Type:  TypeA,
 					Class: ClassINET,
 				}},
-		Answers: []Resource{
-			NewResource(name, TypeA, ClassINET, 300, wantIP[:]),
-		},
+				Answers: make([]Resource, len(tt.responseIPs)),
+			}
+			for i := range tt.responseIPs {
+				responseMsg.Answers[i] = NewResource(name, TypeA, ClassINET, 300, tt.responseIPs[i][:])
 			}
 
 			// Response flags: QR=1 (response), RD=1, RA=1.
 			responseFlags := HeaderFlags(1<<15 | 1<<8 | 1<<7)
-
-	var buf [512]byte
-	dnsPayload, err := responseMsg.AppendTo(buf[:0], txid, responseFlags)
+			var responseBuf [512]byte
+			dnsPayload, err := responseMsg.AppendTo(responseBuf[:0], txid, responseFlags)
 			if err != nil {
 				t.Fatal("failed to build DNS response:", err)
 			}
 
-	// Set up the DNS client.
 			var client Client
-	client.StartResolve(clientPort, txid, ResolveConfig{
+			err = client.StartResolve(clientPort, txid, ResolveConfig{
 				Questions: []Question{{
-			Name:  MustNewName(hostname),
+					Name:  name,
 					Type:  TypeA,
 					Class: ClassINET,
 				}},
 				EnableRecursion:    true,
+				MaxResponseAnswers: maxAnswers,
 			})
-
-	// Simulate sending by calling Encapsulate (changes state to AwaitResponse).
-	var dummy [512]byte
-	client.Encapsulate(dummy[:], 0, 0)
-
-	// Call Demux with DNS payload.
-	err = client.Demux(dnsPayload, 0)
 			if err != nil {
-		t.Fatal("Client Demux error:", err)
+				t.Fatal("failed to start DNS resolve:", err)
 			}
 
-	// Check the client received the answer.
-	var addrs [4]netip.Addr
+			// Encapsulate the query to move the client into the outstanding state.
+			var queryBuf [512]byte
+			_, err = client.Encapsulate(queryBuf[:], 0, 0)
+			if err != nil {
+				t.Fatal("failed to encapsulate DNS query:", err)
+			}
+			if err := client.Demux(dnsPayload, 0); err != nil {
+				t.Fatal("failed to demux DNS response:", err)
+			}
+
+			var addrs [maxAnswers]netip.Addr
 			answers, err := client.ResponseAnswerLookup(addrs[:], hostname)
-	if answers != 1 {
-		t.Fatalf("expected 1 answer, got %d", answers)
+			if err != nil {
+				t.Fatal("failed to look up DNS response answers:", err)
 			}
-	addr := addrs[0]
+			if answers != uint16(tt.wantAnswers) {
+				t.Fatalf("expected %d answers, got %d", tt.wantAnswers, answers)
+			}
+			for i := 0; i < tt.wantAnswers; i++ {
+				addr := addrs[i]
 				if !addr.Is4() {
-		t.Fatalf("expected 4 bytes in answer, got %d", addr.BitLen()/8)
+					t.Errorf("answer %d: expected IPv4 address, got %v", i, addr)
+					continue
+				}
+				if addr.As4() != tt.responseIPs[i] {
+					t.Errorf("answer %d: expected IP %v, got %v", i, tt.responseIPs[i], addr)
 				}
-	if addr.As4() != wantIP {
-		t.Errorf("expected IP %v, got %v", wantIP, addr.String())
 			}
 
-	// Test MessageCopyTo as well.
 			var lookup Message
-	lookup.LimitResourceDecoding(1, 1, 0, 0)
 			done, err := client.ResponseCopyTo(&lookup)
 			if err != nil {
-		t.Fatal("MessageCopyTo error:", err)
+				t.Fatal("failed to copy DNS response:", err)
 			}
 			if !done {
 				t.Fatal("expected done=true")
 			}
-	if len(lookup.Answers) != 1 {
-		t.Fatalf("MessageCopyTo: expected 1 answer, got %d", len(lookup.Answers))
+			if len(lookup.Answers) != tt.wantAnswers {
+				t.Fatalf("expected %d copied answers, got %d", tt.wantAnswers, len(lookup.Answers))
+			}
+		})
 	}
 }

x/xnet/stack-async.go

@@ -630,6 +630,7 @@ func (s *StackAsync) StartLookupIPType(host string, qtype dns.Type) error {
 			s.ednsopt,
 		},
 		EnableRecursion:    true,
+		MaxResponseAnswers: uint16(len(s.addrbufnip)),
 	})
 	if err != nil {
 		return err