net: allow a dns TXT record to contain more than one <character-string>

RFC 1035 3.3.14 allows a TXT record to contain one or more <character-string>s.
The current implementation returns a "no such host" error if there is more
than one <character-string> in the TXT record.

Fixes #10482

Change-Id: I0ded258005e6b7ba45f687fecd10afa2b321bb77
Reviewed-on: https://go-review.googlesource.com/8966Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/net/dnsmsg.go

@@ -306,7 +306,23 @@ func (rr *dnsRR_TXT) Header() *dnsRR_Header {
 }
 
 func (rr *dnsRR_TXT) Walk(f func(v interface{}, name, tag string) bool) bool {
-	return rr.Hdr.Walk(f) && f(&rr.Txt, "Txt", "")
+	if !rr.Hdr.Walk(f) {
+		return false
+	}
+	var n uint16 = 0
+	for n < rr.Hdr.Rdlength {
+		var txt string
+		if !f(&txt, "Txt", "") {
+			return false
+		}
+		// more bytes than rr.Hdr.Rdlength said there woudld be
+		if rr.Hdr.Rdlength-n < uint16(len(txt))+1 {
+			return false
+		}
+		n += uint16(len(txt)) + 1
+		rr.Txt += txt
+	}
+	return true
 }
 
 type dnsRR_SRV struct {

src/net/dnsmsg_test.go

@@ -96,6 +96,93 @@ func TestDNSParseCorruptSRVReply(t *testing.T) {
 	}
 }
 
+func TestDNSParseTXTReply(t *testing.T) {
+	expectedTxt1 := "v=spf1 redirect=_spf.google.com"
+	expectedTxt2 := "v=spf1 ip4:69.63.179.25 ip4:69.63.178.128/25 ip4:69.63.184.0/25 " +
+		"ip4:66.220.144.128/25 ip4:66.220.155.0/24 " +
+		"ip4:69.171.232.0/25 ip4:66.220.157.0/25 " +
+		"ip4:69.171.244.0/24 mx -all"
+
+	replies := []string{dnsTXTReply1, dnsTXTReply2}
+	expectedTxts := []string{expectedTxt1, expectedTxt2}
+
+	for i := range replies {
+		data, err := hex.DecodeString(replies[i])
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		msg := new(dnsMsg)
+		ok := msg.Unpack(data)
+		if !ok {
+			t.Errorf("test %d: unpacking packet failed", i)
+			continue
+		}
+
+		if len(msg.answer) != 1 {
+			t.Errorf("test %d: len(rr.answer) = %d; want 1", i, len(msg.answer))
+			continue
+		}
+
+		rr := msg.answer[0]
+		rrTXT, ok := rr.(*dnsRR_TXT)
+		if !ok {
+			t.Errorf("test %d: answer[0] = %T; want *dnsRR_TXT", i, rr)
+			continue
+		}
+
+		if rrTXT.Txt != expectedTxts[i] {
+			t.Errorf("test %d: Txt = %s; want %s", i, rrTXT.Txt, expectedTxts[i])
+		}
+	}
+}
+
+func TestDNSParseTXTCorruptDataLengthReply(t *testing.T) {
+	replies := []string{dnsTXTCorruptDataLengthReply1, dnsTXTCorruptDataLengthReply2}
+
+	for i := range replies {
+		data, err := hex.DecodeString(replies[i])
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		msg := new(dnsMsg)
+		ok := msg.Unpack(data)
+		if ok {
+			t.Errorf("test %d: expected to fail on unpacking corrupt packet", i)
+		}
+	}
+}
+
+func TestDNSParseTXTCorruptTXTLengthReply(t *testing.T) {
+	replies := []string{dnsTXTCorruptTXTLengthReply1, dnsTXTCorruptTXTLengthReply2}
+
+	for i := range replies {
+		data, err := hex.DecodeString(replies[i])
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		msg := new(dnsMsg)
+		ok := msg.Unpack(data)
+		// Unpacking should succeed, but we should just get the header.
+		if !ok {
+			t.Errorf("test %d: unpacking packet failed", i)
+			continue
+		}
+
+		if len(msg.answer) != 1 {
+			t.Errorf("test %d: len(rr.answer) = %d; want 1", i, len(msg.answer))
+			continue
+		}
+
+		rr := msg.answer[0]
+		if _, justHeader := rr.(*dnsRR_Header); !justHeader {
+			t.Errorf("test %d: rr = %T; expected *dnsRR_Header", i, rr)
+		}
+	}
+}
+
 // Valid DNS SRV reply
 const dnsSRVReply = "0901818000010005000000000c5f786d70702d736572766572045f74637006676f6f67" +
 	"6c6503636f6d0000210001c00c002100010000012c00210014000014950c786d70702d" +
@@ -117,3 +204,63 @@ const dnsSRVCorruptReply = "0901818000010005000000000c5f786d70702d73657276657204
 	"6503636f6d00c00c002100010000012c00200005000014950b786d70702d7365727665" +
 	"72016c06676f6f676c6503636f6d00c00c002100010000012c00FF0014000014950c78" +
 	"6d70702d73657276657231016c06676f6f676c6503636f6d00"
+
+// TXT reply with one <character-string>
+const dnsTXTReply1 = "b3458180000100010004000505676d61696c03636f6d0000100001c00c001000010000012c00" +
+	"201f763d737066312072656469726563743d5f7370662e676f6f676c652e636f6dc00" +
+	"c0002000100025d4c000d036e733406676f6f676c65c012c00c0002000100025d4c00" +
+	"06036e7331c057c00c0002000100025d4c0006036e7333c057c00c0002000100025d4" +
+	"c0006036e7332c057c06c00010001000248b50004d8ef200ac09000010001000248b5" +
+	"0004d8ef220ac07e00010001000248b50004d8ef240ac05300010001000248b50004d" +
+	"8ef260a0000291000000000000000"
+
+// TXT reply with more than one <character-string>.
+// See https://tools.ietf.org/html/rfc1035#section-3.3.14
+const dnsTXTReply2 = "a0a381800001000100020002045f7370660866616365626f6f6b03636f6d0000100001c00c0010000" +
+	"100000e1000af7f763d73706631206970343a36392e36332e3137392e3235206970343a36392e" +
+	"36332e3137382e3132382f3235206970343a36392e36332e3138342e302f3235206970343a363" +
+	"62e3232302e3134342e3132382f3235206970343a36362e3232302e3135352e302f3234206970" +
+	"343a36392e3137312e3233322e302f323520692e70343a36362e3232302e3135372e302f32352" +
+	"06970343a36392e3137312e3234342e302f3234206d78202d616c6cc0110002000100025d1500" +
+	"070161026e73c011c0110002000100025d1500040162c0ecc0ea0001000100025d15000445abe" +
+	"f0cc0fd0001000100025d15000445abff0c"
+
+// DataLength field should be sum of all TXT fields. In this case it's less.
+const dnsTXTCorruptDataLengthReply1 = "a0a381800001000100020002045f7370660866616365626f6f6b03636f6d0000100001c00c0010000" +
+	"100000e1000967f763d73706631206970343a36392e36332e3137392e3235206970343a36392e" +
+	"36332e3137382e3132382f3235206970343a36392e36332e3138342e302f3235206970343a363" +
+	"62e3232302e3134342e3132382f3235206970343a36362e3232302e3135352e302f3234206970" +
+	"343a36392e3137312e3233322e302f323520692e70343a36362e3232302e3135372e302f32352" +
+	"06970343a36392e3137312e3234342e302f3234206d78202d616c6cc0110002000100025d1500" +
+	"070161026e73c011c0110002000100025d1500040162c0ecc0ea0001000100025d15000445abe" +
+	"f0cc0fd0001000100025d15000445abff0c"
+
+// Same as above but DataLength is more than sum of TXT fields.
+const dnsTXTCorruptDataLengthReply2 = "a0a381800001000100020002045f7370660866616365626f6f6b03636f6d0000100001c00c0010000" +
+	"100000e1001227f763d73706631206970343a36392e36332e3137392e3235206970343a36392e" +
+	"36332e3137382e3132382f3235206970343a36392e36332e3138342e302f3235206970343a363" +
+	"62e3232302e3134342e3132382f3235206970343a36362e3232302e3135352e302f3234206970" +
+	"343a36392e3137312e3233322e302f323520692e70343a36362e3232302e3135372e302f32352" +
+	"06970343a36392e3137312e3234342e302f3234206d78202d616c6cc0110002000100025d1500" +
+	"070161026e73c011c0110002000100025d1500040162c0ecc0ea0001000100025d15000445abe" +
+	"f0cc0fd0001000100025d15000445abff0c"
+
+// TXT Length field is less than actual length.
+const dnsTXTCorruptTXTLengthReply1 = "a0a381800001000100020002045f7370660866616365626f6f6b03636f6d0000100001c00c0010000" +
+	"100000e1000af7f763d73706631206970343a36392e36332e3137392e3235206970343a36392e" +
+	"36332e3137382e3132382f3235206970343a36392e36332e3138342e302f3235206970343a363" +
+	"62e3232302e3134342e3132382f3235206970343a36362e3232302e3135352e302f3234206970" +
+	"343a36392e3137312e3233322e302f323520691470343a36362e3232302e3135372e302f32352" +
+	"06970343a36392e3137312e3234342e302f3234206d78202d616c6cc0110002000100025d1500" +
+	"070161026e73c011c0110002000100025d1500040162c0ecc0ea0001000100025d15000445abe" +
+	"f0cc0fd0001000100025d15000445abff0c"
+
+// TXT Length field is more than actual length.
+const dnsTXTCorruptTXTLengthReply2 = "a0a381800001000100020002045f7370660866616365626f6f6b03636f6d0000100001c00c0010000" +
+	"100000e1000af7f763d73706631206970343a36392e36332e3137392e3235206970343a36392e" +
+	"36332e3137382e3132382f3235206970343a36392e36332e3138342e302f3235206970343a363" +
+	"62e3232302e3134342e3132382f3235206970343a36362e3232302e3135352e302f3234206970" +
+	"343a36392e3137312e3233322e302f323520693370343a36362e3232302e3135372e302f32352" +
+	"06970343a36392e3137312e3234342e302f3234206d78202d616c6cc0110002000100025d1500" +
+	"070161026e73c011c0110002000100025d1500040162c0ecc0ea0001000100025d15000445abe" +
+	"f0cc0fd0001000100025d15000445abff0c"