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+// Copyright 2012 Google, Inc. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style license
+// that can be found in the LICENSE file in the root of the source
+// tree.
+
+/*
+Package gopacket provides packet decoding for the Go language.
+
+gopacket contains many sub-packages with additional functionality you may find
+useful, including:
+
+ * layers: You'll probably use this every time.  This contains of the logic
+     built into gopacket for decoding packet protocols.  Note that all example
+     code below assumes that you have imported both gopacket and
+     gopacket/layers.
+ * pcap: C bindings to use libpcap to read packets off the wire.
+ * pfring: C bindings to use PF_RING to read packets off the wire.
+ * afpacket: C bindings for Linux's AF_PACKET to read packets off the wire.
+ * tcpassembly: TCP stream reassembly
+
+Also, if you're looking to dive right into code, see the examples subdirectory
+for numerous simple binaries built using gopacket libraries.
+
+Minimum go version required is 1.5 except for pcapgo/EthernetHandle, afpacket,
+and bsdbpf which need at least 1.7 due to x/sys/unix dependencies.
+
+Basic Usage
+
+gopacket takes in packet data as a []byte and decodes it into a packet with
+a non-zero number of "layers".  Each layer corresponds to a protocol
+within the bytes.  Once a packet has been decoded, the layers of the packet
+can be requested from the packet.
+
+ // Decode a packet
+ packet := gopacket.NewPacket(myPacketData, layers.LayerTypeEthernet, gopacket.Default)
+ // Get the TCP layer from this packet
+ if tcpLayer := packet.Layer(layers.LayerTypeTCP); tcpLayer != nil {
+   fmt.Println("This is a TCP packet!")
+   // Get actual TCP data from this layer
+   tcp, _ := tcpLayer.(*layers.TCP)
+   fmt.Printf("From src port %d to dst port %d\n", tcp.SrcPort, tcp.DstPort)
+ }
+ // Iterate over all layers, printing out each layer type
+ for _, layer := range packet.Layers() {
+   fmt.Println("PACKET LAYER:", layer.LayerType())
+ }
+
+Packets can be decoded from a number of starting points.  Many of our base
+types implement Decoder, which allow us to decode packets for which
+we don't have full data.
+
+ // Decode an ethernet packet
+ ethP := gopacket.NewPacket(p1, layers.LayerTypeEthernet, gopacket.Default)
+ // Decode an IPv6 header and everything it contains
+ ipP := gopacket.NewPacket(p2, layers.LayerTypeIPv6, gopacket.Default)
+ // Decode a TCP header and its payload
+ tcpP := gopacket.NewPacket(p3, layers.LayerTypeTCP, gopacket.Default)
+
+
+Reading Packets From A Source
+
+Most of the time, you won't just have a []byte of packet data lying around.
+Instead, you'll want to read packets in from somewhere (file, interface, etc)
+and process them.  To do that, you'll want to build a PacketSource.
+
+First, you'll need to construct an object that implements the PacketDataSource
+interface.  There are implementations of this interface bundled with gopacket
+in the gopacket/pcap and gopacket/pfring subpackages... see their documentation
+for more information on their usage.  Once you have a PacketDataSource, you can
+pass it into NewPacketSource, along with a Decoder of your choice, to create
+a PacketSource.
+
+Once you have a PacketSource, you can read packets from it in multiple ways.
+See the docs for PacketSource for more details.  The easiest method is the
+Packets function, which returns a channel, then asynchronously writes new
+packets into that channel, closing the channel if the packetSource hits an
+end-of-file.
+
+  packetSource := ...  // construct using pcap or pfring
+  for packet := range packetSource.Packets() {
+    handlePacket(packet)  // do something with each packet
+  }
+
+You can change the decoding options of the packetSource by setting fields in
+packetSource.DecodeOptions... see the following sections for more details.
+
+
+Lazy Decoding
+
+gopacket optionally decodes packet data lazily, meaning it
+only decodes a packet layer when it needs to handle a function call.
+
+ // Create a packet, but don't actually decode anything yet
+ packet := gopacket.NewPacket(myPacketData, layers.LayerTypeEthernet, gopacket.Lazy)
+ // Now, decode the packet up to the first IPv4 layer found but no further.
+ // If no IPv4 layer was found, the whole packet will be decoded looking for
+ // it.
+ ip4 := packet.Layer(layers.LayerTypeIPv4)
+ // Decode all layers and return them.  The layers up to the first IPv4 layer
+ // are already decoded, and will not require decoding a second time.
+ layers := packet.Layers()
+
+Lazily-decoded packets are not concurrency-safe.  Since layers have not all been
+decoded, each call to Layer() or Layers() has the potential to mutate the packet
+in order to decode the next layer.  If a packet is used
+in multiple goroutines concurrently, don't use gopacket.Lazy.  Then gopacket
+will decode the packet fully, and all future function calls won't mutate the
+object.
+
+
+NoCopy Decoding
+
+By default, gopacket will copy the slice passed to NewPacket and store the
+copy within the packet, so future mutations to the bytes underlying the slice
+don't affect the packet and its layers.  If you can guarantee that the
+underlying slice bytes won't be changed, you can use NoCopy to tell
+gopacket.NewPacket, and it'll use the passed-in slice itself.
+
+ // This channel returns new byte slices, each of which points to a new
+ // memory location that's guaranteed immutable for the duration of the
+ // packet.
+ for data := range myByteSliceChannel {
+   p := gopacket.NewPacket(data, layers.LayerTypeEthernet, gopacket.NoCopy)
+   doSomethingWithPacket(p)
+ }
+
+The fastest method of decoding is to use both Lazy and NoCopy, but note from
+the many caveats above that for some implementations either or both may be
+dangerous.
+
+
+Pointers To Known Layers
+
+During decoding, certain layers are stored in the packet as well-known
+layer types.  For example, IPv4 and IPv6 are both considered NetworkLayer
+layers, while TCP and UDP are both TransportLayer layers.  We support 4
+layers, corresponding to the 4 layers of the TCP/IP layering scheme (roughly
+anagalous to layers 2, 3, 4, and 7 of the OSI model).  To access these,
+you can use the packet.LinkLayer, packet.NetworkLayer,
+packet.TransportLayer, and packet.ApplicationLayer functions.  Each of
+these functions returns a corresponding interface
+(gopacket.{Link,Network,Transport,Application}Layer).  The first three
+provide methods for getting src/dst addresses for that particular layer,
+while the final layer provides a Payload function to get payload data.
+This is helpful, for example, to get payloads for all packets regardless
+of their underlying data type:
+
+ // Get packets from some source
+ for packet := range someSource {
+   if app := packet.ApplicationLayer(); app != nil {
+     if strings.Contains(string(app.Payload()), "magic string") {
+       fmt.Println("Found magic string in a packet!")
+     }
+   }
+ }
+
+A particularly useful layer is ErrorLayer, which is set whenever there's
+an error parsing part of the packet.
+
+ packet := gopacket.NewPacket(myPacketData, layers.LayerTypeEthernet, gopacket.Default)
+ if err := packet.ErrorLayer(); err != nil {
+   fmt.Println("Error decoding some part of the packet:", err)
+ }
+
+Note that we don't return an error from NewPacket because we may have decoded
+a number of layers successfully before running into our erroneous layer.  You
+may still be able to get your Ethernet and IPv4 layers correctly, even if
+your TCP layer is malformed.
+
+
+Flow And Endpoint
+
+gopacket has two useful objects, Flow and Endpoint, for communicating in a protocol
+independent manner the fact that a packet is coming from A and going to B.
+The general layer types LinkLayer, NetworkLayer, and TransportLayer all provide
+methods for extracting their flow information, without worrying about the type
+of the underlying Layer.
+
+A Flow is a simple object made up of a set of two Endpoints, one source and one
+destination.  It details the sender and receiver of the Layer of the Packet.
+
+An Endpoint is a hashable representation of a source or destination.  For
+example, for LayerTypeIPv4, an Endpoint contains the IP address bytes for a v4
+IP packet.  A Flow can be broken into Endpoints, and Endpoints can be combined
+into Flows:
+
+ packet := gopacket.NewPacket(myPacketData, layers.LayerTypeEthernet, gopacket.Lazy)
+ netFlow := packet.NetworkLayer().NetworkFlow()
+ src, dst := netFlow.Endpoints()
+ reverseFlow := gopacket.NewFlow(dst, src)
+
+Both Endpoint and Flow objects can be used as map keys, and the equality
+operator can compare them, so you can easily group together all packets
+based on endpoint criteria:
+
+ flows := map[gopacket.Endpoint]chan gopacket.Packet
+ packet := gopacket.NewPacket(myPacketData, layers.LayerTypeEthernet, gopacket.Lazy)
+ // Send all TCP packets to channels based on their destination port.
+ if tcp := packet.Layer(layers.LayerTypeTCP); tcp != nil {
+   flows[tcp.TransportFlow().Dst()] <- packet
+ }
+ // Look for all packets with the same source and destination network address
+ if net := packet.NetworkLayer(); net != nil {
+   src, dst := net.NetworkFlow().Endpoints()
+   if src == dst {
+     fmt.Println("Fishy packet has same network source and dst: %s", src)
+   }
+ }
+ // Find all packets coming from UDP port 1000 to UDP port 500
+ interestingFlow := gopacket.FlowFromEndpoints(layers.NewUDPPortEndpoint(1000), layers.NewUDPPortEndpoint(500))
+ if t := packet.NetworkLayer(); t != nil && t.TransportFlow() == interestingFlow {
+   fmt.Println("Found that UDP flow I was looking for!")
+ }
+
+For load-balancing purposes, both Flow and Endpoint have FastHash() functions,
+which provide quick, non-cryptographic hashes of their contents.  Of particular
+importance is the fact that Flow FastHash() is symmetric: A->B will have the same
+hash as B->A.  An example usage could be:
+
+ channels := [8]chan gopacket.Packet
+ for i := 0; i < 8; i++ {
+   channels[i] = make(chan gopacket.Packet)
+   go packetHandler(channels[i])
+ }
+ for packet := range getPackets() {
+   if net := packet.NetworkLayer(); net != nil {
+     channels[int(net.NetworkFlow().FastHash()) & 0x7] <- packet
+   }
+ }
+
+This allows us to split up a packet stream while still making sure that each
+stream sees all packets for a flow (and its bidirectional opposite).
+
+
+Implementing Your Own Decoder
+
+If your network has some strange encapsulation, you can implement your own
+decoder.  In this example, we handle Ethernet packets which are encapsulated
+in a 4-byte header.
+
+ // Create a layer type, should be unique and high, so it doesn't conflict,
+ // giving it a name and a decoder to use.
+ var MyLayerType = gopacket.RegisterLayerType(12345, gopacket.LayerTypeMetadata{Name: "MyLayerType", Decoder: gopacket.DecodeFunc(decodeMyLayer)})
+
+ // Implement my layer
+ type MyLayer struct {
+   StrangeHeader []byte
+   payload []byte
+ }
+ func (m MyLayer) LayerType() gopacket.LayerType { return MyLayerType }
+ func (m MyLayer) LayerContents() []byte { return m.StrangeHeader }
+ func (m MyLayer) LayerPayload() []byte { return m.payload }
+
+ // Now implement a decoder... this one strips off the first 4 bytes of the
+ // packet.
+ func decodeMyLayer(data []byte, p gopacket.PacketBuilder) error {
+   // Create my layer
+   p.AddLayer(&MyLayer{data[:4], data[4:]})
+   // Determine how to handle the rest of the packet
+   return p.NextDecoder(layers.LayerTypeEthernet)
+ }
+
+ // Finally, decode your packets:
+ p := gopacket.NewPacket(data, MyLayerType, gopacket.Lazy)
+
+See the docs for Decoder and PacketBuilder for more details on how coding
+decoders works, or look at RegisterLayerType and RegisterEndpointType to see how
+to add layer/endpoint types to gopacket.
+
+
+Fast Decoding With DecodingLayerParser
+
+TLDR:  DecodingLayerParser takes about 10% of the time as NewPacket to decode
+packet data, but only for known packet stacks.
+
+Basic decoding using gopacket.NewPacket or PacketSource.Packets is somewhat slow
+due to its need to allocate a new packet and every respective layer.  It's very
+versatile and can handle all known layer types, but sometimes you really only
+care about a specific set of layers regardless, so that versatility is wasted.
+
+DecodingLayerParser avoids memory allocation altogether by decoding packet
+layers directly into preallocated objects, which you can then reference to get
+the packet's information.  A quick example:
+
+ func main() {
+   var eth layers.Ethernet
+   var ip4 layers.IPv4
+   var ip6 layers.IPv6
+   var tcp layers.TCP
+   parser := gopacket.NewDecodingLayerParser(layers.LayerTypeEthernet, &eth, &ip4, &ip6, &tcp)
+   decoded := []gopacket.LayerType{}
+   for packetData := range somehowGetPacketData() {
+     if err := parser.DecodeLayers(packetData, &decoded); err != nil {
+       fmt.Fprintf(os.Stderr, "Could not decode layers: %v\n", err)
+       continue
+     }
+     for _, layerType := range decoded {
+       switch layerType {
+         case layers.LayerTypeIPv6:
+           fmt.Println("    IP6 ", ip6.SrcIP, ip6.DstIP)
+         case layers.LayerTypeIPv4:
+           fmt.Println("    IP4 ", ip4.SrcIP, ip4.DstIP)
+       }
+     }
+   }
+ }
+
+The important thing to note here is that the parser is modifying the passed in
+layers (eth, ip4, ip6, tcp) instead of allocating new ones, thus greatly
+speeding up the decoding process.  It's even branching based on layer type...
+it'll handle an (eth, ip4, tcp) or (eth, ip6, tcp) stack.  However, it won't
+handle any other type... since no other decoders were passed in, an (eth, ip4,
+udp) stack will stop decoding after ip4, and only pass back [LayerTypeEthernet,
+LayerTypeIPv4] through the 'decoded' slice (along with an error saying it can't
+decode a UDP packet).
+
+Unfortunately, not all layers can be used by DecodingLayerParser... only those
+implementing the DecodingLayer interface are usable.  Also, it's possible to
+create DecodingLayers that are not themselves Layers... see
+layers.IPv6ExtensionSkipper for an example of this.
+
+Faster And Customized Decoding with DecodingLayerContainer
+
+By default, DecodingLayerParser uses native map to store and search for a layer
+to decode. Though being versatile, in some cases this solution may be not so
+optimal. For example, if you have only few layers faster operations may be
+provided by sparse array indexing or linear array scan.
+
+To accomodate these scenarios, DecodingLayerContainer interface is introduced
+along with its implementations: DecodingLayerSparse, DecodingLayerArray and
+DecodingLayerMap. You can specify a container implementation to
+DecodingLayerParser with SetDecodingLayerContainer method. Example:
+
+ dlp := gopacket.NewDecodingLayerParser(LayerTypeEthernet)
+ dlp.SetDecodingLayerContainer(gopacket.DecodingLayerSparse(nil))
+ var eth layers.Ethernet
+ dlp.AddDecodingLayer(&eth)
+ // ... add layers and use DecodingLayerParser as usual...
+
+To skip one level of indirection (though sacrificing some capabilities) you may
+also use DecodingLayerContainer as a decoding tool as it is. In this case you have to
+handle unknown layer types and layer panics by yourself. Example:
+
+ func main() {
+   var eth layers.Ethernet
+   var ip4 layers.IPv4
+   var ip6 layers.IPv6
+   var tcp layers.TCP
+   dlc := gopacket.DecodingLayerContainer(gopacket.DecodingLayerArray(nil))
+   dlc = dlc.Put(&eth)
+   dlc = dlc.Put(&ip4)
+   dlc = dlc.Put(&ip6)
+   dlc = dlc.Put(&tcp)
+   // you may specify some meaningful DecodeFeedback
+   decoder := dlc.LayersDecoder(LayerTypeEthernet, gopacket.NilDecodeFeedback)
+   decoded := make([]gopacket.LayerType, 0, 20)
+   for packetData := range somehowGetPacketData() {
+     lt, err := decoder(packetData, &decoded)
+     if err != nil {
+       fmt.Fprintf(os.Stderr, "Could not decode layers: %v\n", err)
+       continue
+     }
+     if lt != gopacket.LayerTypeZero {
+       fmt.Fprintf(os.Stderr, "unknown layer type: %v\n", lt)
+       continue
+     }
+     for _, layerType := range decoded {
+       // examine decoded layertypes just as already shown above
+     }
+   }
+ }
+
+DecodingLayerSparse is the fastest but most effective when LayerType values
+that layers in use can decode are not large because otherwise that would lead
+to bigger memory footprint. DecodingLayerArray is very compact and primarily
+usable if the number of decoding layers is not big (up to ~10-15, but please do
+your own benchmarks). DecodingLayerMap is the most versatile one and used by
+DecodingLayerParser by default. Please refer to tests and benchmarks in layers
+subpackage to further examine usage examples and performance measurements.
+
+You may also choose to implement your own DecodingLayerContainer if you want to
+make use of your own internal packet decoding logic.
+
+Creating Packet Data
+
+As well as offering the ability to decode packet data, gopacket will allow you
+to create packets from scratch, as well.  A number of gopacket layers implement
+the SerializableLayer interface; these layers can be serialized to a []byte in
+the following manner:
+
+  ip := &layers.IPv4{
+    SrcIP: net.IP{1, 2, 3, 4},
+    DstIP: net.IP{5, 6, 7, 8},
+    // etc...
+  }
+  buf := gopacket.NewSerializeBuffer()
+  opts := gopacket.SerializeOptions{}  // See SerializeOptions for more details.
+  err := ip.SerializeTo(buf, opts)
+  if err != nil { panic(err) }
+  fmt.Println(buf.Bytes())  // prints out a byte slice containing the serialized IPv4 layer.
+
+SerializeTo PREPENDS the given layer onto the SerializeBuffer, and they treat
+the current buffer's Bytes() slice as the payload of the serializing layer.
+Therefore, you can serialize an entire packet by serializing a set of layers in
+reverse order (Payload, then TCP, then IP, then Ethernet, for example).  The
+SerializeBuffer's SerializeLayers function is a helper that does exactly that.
+
+To generate a (empty and useless, because no fields are set)
+Ethernet(IPv4(TCP(Payload))) packet, for example, you can run:
+
+  buf := gopacket.NewSerializeBuffer()
+  opts := gopacket.SerializeOptions{}
+  gopacket.SerializeLayers(buf, opts,
+    &layers.Ethernet{},
+    &layers.IPv4{},
+    &layers.TCP{},
+    gopacket.Payload([]byte{1, 2, 3, 4}))
+  packetData := buf.Bytes()
+
+A Final Note
+
+If you use gopacket, you'll almost definitely want to make sure gopacket/layers
+is imported, since when imported it sets all the LayerType variables and fills
+in a lot of interesting variables/maps (DecodersByLayerName, etc).  Therefore,
+it's recommended that even if you don't use any layers functions directly, you still import with:
+
+  import (
+    _ "github.com/google/gopacket/layers"
+  )
+*/
+package gopacket