fixed IPv4 checksum calculation

actions/tamper_action_test.go

@@ -69,7 +69,9 @@ func TestParseTamperAction(t *testing.T) {
 		},
 	}
 	for _, tt := range tests {
+		tt := tt
 		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
 			s := scanner.NewScanner(tt.rule)
 			got, err := ParseTamperAction(s)
 			if (err != nil) != tt.wantErr {
@@ -117,7 +119,7 @@ func TestTamperTCP(t *testing.T) {
 			},
 			want: []byte{
 				0x30, 0x39, 0xd4, 0x31, 0xde, 0xad, 0xbe, 0xef, 0x00, 0x00, 0x00, 0x00, 0x70, 0x02, 0x00,
-				0x00, 0x82, 0x9c, 0x00, 0x00, 0x02, 0x04, 0x20, 0x00, 0x0e, 0x05, 0xff, 0xff, 0xff, 0x00,
+				0x00, 0x82, 0x9c, 0x00, 0x00, 0x02, 0x04, 0x20, 0x00, 0x00, 0x0e, 0x05, 0xff, 0xff, 0xff,
 				0x00, 0x00, 0x00, 0x54, 0x65, 0x73, 0x74,
 			},
 		}, {
@@ -140,7 +142,9 @@ func TestTamperTCP(t *testing.T) {
 	}
 
 	for _, tt := range tests {
+		tt := tt
 		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
 			tamperTCP(tt.args.tcp, tt.args.field, tt.args.valueGen)
 
 			got := append([]byte{}, tt.args.tcp.Contents...)

common/packet.go

@@ -25,28 +25,27 @@ func UpdateTCPChecksum(tcp *layers.TCP) error {
 
 // UpdateIPv4Checksum updates the IPv4 checksum field and the raw bytes for a gopacket IPv4 layer.
 func UpdateIPv4Checksum(ip *layers.IPv4) error {
-	buf := make([]byte, ip.Length)
-	copy(buf, ip.Contents)
-	copy(buf[len(ip.Contents):], ip.Payload)
-
-	chksum := CalculateIPv4Checksum(buf)
+	chksum := CalculateIPv4Checksum(ip.Contents)
 	ip.Checksum = chksum
 	binary.BigEndian.PutUint16(ip.Contents[10:12], chksum)
 
 	return nil
 }
 
-// copied directly from gopacket/layers/ip4.go because they didn't export one. for whatever some reason..
+// copied from gopacket/layers/ip4.go because they didn't export one. for whatever some reason..
 func CalculateIPv4Checksum(bytes []byte) uint16 {
+	buf := make([]byte, len(bytes))
+	copy(buf, bytes)
+
 	// Clear checksum bytes
-	bytes[10] = 0
-	bytes[11] = 0
+	buf[10] = 0
+	buf[11] = 0
 
 	// Compute checksum
 	var csum uint32
-	for i := 0; i < len(bytes); i += 2 {
-		csum += uint32(bytes[i]) << 8
-		csum += uint32(bytes[i+1])
+	for i := 0; i < len(buf); i += 2 {
+		csum += uint32(buf[i]) << 8
+		csum += uint32(buf[i+1])
 	}
 
 	for csum > 0xFFFF {