net: fix packDomainName encoding of root and invalid names

Fixes #14372

Change-Id: I40d594582639e87ef2574d37ac868e37ffaa17dc
Reviewed-on: https://go-review.googlesource.com/19623Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/net/dnsmsg.go

@@ -406,6 +406,13 @@ func packDomainName(s string, msg []byte, off int) (off1 int, ok bool) {
 		s += "."
 	}
 
+	// Allow root domain.
+	if s == "." {
+		msg[off] = 0
+		off++
+		return off, true
+	}
+
 	// Each dot ends a segment of the name.
 	// We trade each dot byte for a length byte.
 	// There is also a trailing zero.
@@ -422,8 +429,13 @@ func packDomainName(s string, msg []byte, off int) (off1 int, ok bool) {
 			if i-begin >= 1<<6 { // top two bits of length must be clear
 				return len(msg), false
 			}
+			if i-begin == 0 {
+				return len(msg), false
+			}
+
 			msg[off] = byte(i - begin)
 			off++
+
 			for j := begin; j < i; j++ {
 				msg[off] = s[j]
 				off++
@@ -494,6 +506,9 @@ Loop:
 			return "", len(msg), false
 		}
 	}
+	if len(s) == 0 {
+		s = "."
+	}
 	if ptr == 0 {
 		off1 = off
 	}
@@ -803,20 +818,32 @@ func (dns *dnsMsg) Pack() (msg []byte, ok bool) {
 	// Pack it in: header and then the pieces.
 	off := 0
 	off, ok = packStruct(&dh, msg, off)
+	if !ok {
+		return nil, false
+	}
 	for i := 0; i < len(question); i++ {
 		off, ok = packStruct(&question[i], msg, off)
+		if !ok {
+			return nil, false
+		}
 	}
 	for i := 0; i < len(answer); i++ {
 		off, ok = packRR(answer[i], msg, off)
+		if !ok {
+			return nil, false
+		}
 	}
 	for i := 0; i < len(ns); i++ {
 		off, ok = packRR(ns[i], msg, off)
+		if !ok {
+			return nil, false
+		}
 	}
 	for i := 0; i < len(extra); i++ {
 		off, ok = packRR(extra[i], msg, off)
-	}
-	if !ok {
-		return nil, false
+		if !ok {
+			return nil, false
+		}
 	}
 	return msg[0:off], true
 }
@@ -848,6 +875,9 @@ func (dns *dnsMsg) Unpack(msg []byte) bool {
 
 	for i := 0; i < len(dns.question); i++ {
 		off, ok = unpackStruct(&dns.question[i], msg, off)
+		if !ok {
+			return false
+		}
 	}
 	for i := 0; i < int(dh.Ancount); i++ {
 		rec, off, ok = unpackRR(msg, off)

src/net/dnsmsg_test.go

@@ -10,6 +10,103 @@ import (
 	"testing"
 )
 
+func TestStructPackUnpack(t *testing.T) {
+	want := dnsQuestion{
+		Name:   ".",
+		Qtype:  dnsTypeA,
+		Qclass: dnsClassINET,
+	}
+	buf := make([]byte, 50)
+	n, ok := packStruct(&want, buf, 0)
+	if !ok {
+		t.Fatal("packing failed")
+	}
+	buf = buf[:n]
+	got := dnsQuestion{}
+	n, ok = unpackStruct(&got, buf, 0)
+	if !ok {
+		t.Fatal("unpacking failed")
+	}
+	if n != len(buf) {
+		t.Error("unpacked different amount than packed: got n = %d, want = %d", n, len(buf))
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got = %+v, want = %+v", got, want)
+	}
+}
+
+func TestDomainNamePackUnpack(t *testing.T) {
+	tests := []struct {
+		in   string
+		want string
+		ok   bool
+	}{
+		{"", ".", true},
+		{".", ".", true},
+		{"google..com", "", false},
+		{"google.com", "google.com.", true},
+		{"google..com.", "", false},
+		{"google.com.", "google.com.", true},
+		{".google.com.", "", false},
+		{"www..google.com.", "", false},
+		{"www.google.com.", "www.google.com.", true},
+	}
+
+	for _, test := range tests {
+		buf := make([]byte, 30)
+		n, ok := packDomainName(test.in, buf, 0)
+		if ok != test.ok {
+			t.Errorf("packing of %s: got ok = %t, want = %t", test.in, ok, test.ok)
+			continue
+		}
+		if !test.ok {
+			continue
+		}
+		buf = buf[:n]
+		got, n, ok := unpackDomainName(buf, 0)
+		if !ok {
+			t.Errorf("unpacking for %s failed", test.in)
+			continue
+		}
+		if n != len(buf) {
+			t.Error(
+				"unpacked different amount than packed for %s: got n = %d, want = %d",
+				test.in,
+				n,
+				len(buf),
+			)
+		}
+		if got != test.want {
+			t.Errorf("unpacking packing of %s: got = %s, want = %s", test.in, got, test.want)
+		}
+	}
+}
+
+func TestDNSPackUnpack(t *testing.T) {
+	want := dnsMsg{
+		question: []dnsQuestion{{
+			Name:   ".",
+			Qtype:  dnsTypeAAAA,
+			Qclass: dnsClassINET,
+		}},
+		answer: []dnsRR{},
+		ns:     []dnsRR{},
+		extra:  []dnsRR{},
+	}
+	b, ok := want.Pack()
+	if !ok {
+		t.Fatal("packing failed")
+	}
+	var got dnsMsg
+	ok = got.Unpack(b)
+	if !ok {
+		t.Fatal("unpacking failed")
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got = %+v, want = %+v", got, want)
+	}
+}
+
 func TestDNSParseSRVReply(t *testing.T) {
 	data, err := hex.DecodeString(dnsSRVReply)
 	if err != nil {