Reserialize (#8)

* Reserialize the packet's raw bytes to pick up any layer changes

---------

Co-authored-by: Seth Wright <seth@getlantern.org>

actions/tamper_action.go

@@ -360,6 +360,13 @@ func (a *TCPTamperAction) Apply(packet gopacket.Packet) ([]gopacket.Packet, erro
 		}
 	}
 
+	sb := gopacket.NewSerializeBuffer()
+	err := gopacket.SerializePacket(sb, gopacket.SerializeOptions{}, packet)
+	if err != nil {
+		panic(err)
+	}
+	packet = gopacket.NewPacket(sb.Bytes(), layers.LayerTypeEthernet, gopacket.Default)
+
 	return a.Action.Apply(packet)
 }
 
@@ -414,14 +421,6 @@ func tamperTCP(tcp *layers.TCP, field TCPField, valueGen tamperValueGen) {
 			opt.OptionLength = uint8(tcpOptionLengths[field]) + 2
 		}
 	}
-
-	// let gopacket handle converting the modified TCP headers into []byte for us since we changed the struct fields
-	// instead of the underlying []byte directly. SerializeTo doesn't write the changes to the raw packet
-	// so we have to copy the formatted bytes back into the packet header.
-	sb := gopacket.NewSerializeBuffer()
-	tcp.SerializeTo(sb, gopacket.SerializeOptions{})
-	tcp.Contents = make([]byte, len(sb.Bytes()))
-	copy(tcp.Contents, sb.Bytes())
 }
 
 // tcpFlagsToUint32 converts a string of TCP flags to a uint32 bitmap.