[译]利用 gopackage 进行包的捕获、注入和分析
2015 年 4 月 11 日
原文: Packet Capture, Injection, and Analysis with Gopacket
by John Leon。
最近准备使用 gopackage 写个工具，搜索了相关的资料，这篇文章是值的推荐的一篇介绍 gopackage的文档，所以动手翻译了一下。
gopacket包为用C编写的libpcap提供了一个go包装器(译者按: 不仅如此，现在gopackage有多种捕获包的binding方式，比如PF_RING，AF_PACKET等)，但它不仅仅是一个简单的包装器。它提供了额外的功能，并利用了Go之类的接口，这使得它非常强大。
作者在写这篇文章的时候，在Denver GopherCon 2016 大会上分享了相关的技术，有兴趣的同学可以观看: GopherCon 2016: John Leon – Packet Capture, Analysis, and Injection with Go
开始之前
安装必需的库。你需要 go
， libpcap
和 gopacket
包。因为gopacket是在libpcap之上构建的，所以我强烈建议您了解该libpcap库是如何工作的。你可以学习如何在C语言中使用libpcap来加深理解。这些示例应该在使用libpcap的Linux/Mac和使用winpcap的Windows上工作。如果出现错误（如 cc1.exe: sorry, unimplemented: 64-bit mode not compiled in
），可能需要设置 GOARCH=386
。
# Get the gopacket package from GitHub go get github.com/google/gopacket # Pcap dev headers might be necessary sudo apt-get install libpcap-dev
你可能还想签出 gopacket project
， 查看它的 godoc文档
。
查找设备
package main
import (
"fmt"
"log"
"github.com/google/gopacket/pcap"
)
func main() {
// 得到所有的(网络)设备
devices, err := pcap.FindAllDevs()
if err != nil {
log.Fatal(err)
}
// 打印设备信息
fmt.Println("Devices found:")
for _, device := range devices {
fmt.Println("\nName: ", device.Name)
fmt.Println("Description: ", device.Description)
fmt.Println("Devices addresses: ", device.Description)
for _, address := range device.Addresses {
fmt.Println("- IP address: ", address.IP)
fmt.Println("- Subnet mask: ", address.Netmask)
}
}
}
打开一个设备进行实时捕获
package main
import (
"fmt"
"github.com/google/gopacket"
"github.com/google/gopacket/pcap"
"log"
"time"
)
var (
device string = "eth0"
snapshot_len int32 =1024
promiscuous bool = false
err error
timeout time.Duration =30 * time.Second
handle *pcap.Handle
)
func main() {
// Open device
handle, err = pcap.OpenLive(device, snapshot_len, promiscuous, timeout)
if err != nil {log.Fatal(err) }
defer handle.Close()
// Use the handle as a packet source to process all packets
packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
for packet := range packetSource.Packets() {
// Process packet here
fmt.Println(packet)
}
}
写入到pcap文件
为了写入到pcap格式的文件中，我们需要 gopacket/pcapgo
,它包含一个 Writer
,还有两个有用的辅助函数: WriteFileHeader()
和 WritePacket()
。
package main
import (
"fmt"
"os"
"time"
"github.com/google/gopacket"
"github.com/google/gopacket/layers"
"github.com/google/gopacket/pcap"
"github.com/google/gopacket/pcapgo"
)
var (
deviceName string = "eth0"
snapshotLen int32 =1024
promiscuous bool = false
err error
timeout time.Duration =-1 * time.Second
handle *pcap.Handle
packetCount int =0
)
func main() {
// Open output pcap file and write header
f, _ := os.Create("test.pcap")
w := pcapgo.NewWriter(f)
w.WriteFileHeader(snapshotLen, layers.LinkTypeEthernet)
defer f.Close()
// Open the device for capturing
handle, err = pcap.OpenLive(deviceName, snapshotLen, promiscuous, timeout)
if err != nil {
fmt.Printf("Error opening device %s: %v", deviceName, err)
os.Exit(1)
}
defer handle.Close()
// Start processing packets
packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
for packet := range packetSource.Packets() {
// Process packet here
fmt.Println(packet)
w.WritePacket(packet.Metadata().CaptureInfo, packet.Data())
packetCount++
// Only capture 100 and then stop
if packetCount >100 {
break
}
}
}
打开pcap文件
除了打开一个设备实时捕获以外，我们还可以读取pcap文件进行离线分析。你可以通过tcpdump捕获一个文件来测试。
# Capture packets to test.pcap file sudo tcpdump -w test.pcap
打开这个文件，遍历其中的packet:
package main
// Use tcpdump to create a test file
// tcpdump -w test.pcap
// or use the example above for writing pcap files
import (
"fmt"
"github.com/google/gopacket"
"github.com/google/gopacket/pcap"
"log"
)
var (
pcapFile string = "test.pcap"
handle *pcap.Handle
err error
)
func main() {
// Open file instead of device
handle, err = pcap.OpenOffline(pcapFile)
if err != nil { log.Fatal(err) }
defer handle.Close()
// Loop through packets in file
packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
for packet := range packetSource.Packets() {
fmt.Println(packet)
}
}
设置过滤器
下面的代码仅仅返回端口80上的packet:
package main
import (
"fmt"
"github.com/google/gopacket"
"github.com/google/gopacket/pcap"
"log"
"time"
)
var (
device string = "eth0"
snapshot_len int32 =1024
promiscuous bool = false
err error
timeout time.Duration =30 * time.Second
handle *pcap.Handle
)
func main() {
// Open device
handle, err = pcap.OpenLive(device, snapshot_len, promiscuous, timeout)
if err != nil {
log.Fatal(err)
}
defer handle.Close()
// Set filter
var filter string = "tcp and port 80"
err = handle.SetBPFFilter(filter)
if err != nil {
log.Fatal(err)
}
fmt.Println("Only capturing TCP port 80 packets.")
packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
for packet := range packetSource.Packets() {
// Do something with a packet here.
fmt.Println(packet)
}
}
解码packet的各层
我们可以获取原始数据包，并尝试将其强制转换为已知格式。它与不同的层兼容，因此我们可以轻松访问ethernet、IP和TCP层。 layers
包是gopacket的Go库中的新功能，在底层libpcap库中不存在。它是gopacket库的非常有用的一部分。它允许我们轻松地识别数据包是否包含特定类型的层。这个代码示例将演示如何使用layers包来查看包是否是ethernet、IP和TCP，以及如何轻松访问这些头中的元素。
找到 payload
(有效载荷)取决于涉及的所有层。每个协议都是不同的，必须相应地进行处理。这就是layers包的强大之处。gopacket的作者花了很多时间为许多已知层（ethernet、IP、UDP和TCP）创建layer类型。其中 payload
(有效负载)是应用程序层的一部分。
package main
import (
"fmt"
"github.com/google/gopacket"
"github.com/google/gopacket/layers"
"github.com/google/gopacket/pcap"
"log"
"strings"
"time"
)
var (
device string = "eth0"
snapshotLen int32 =1024
promiscuous bool = false
err error
timeout time.Duration =30 * time.Second
handle *pcap.Handle
)
func main() {
// Open device
handle, err = pcap.OpenLive(device, snapshotLen, promiscuous, timeout)
if err != nil {log.Fatal(err) }
defer handle.Close()
packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
for packet := range packetSource.Packets() {
printPacketInfo(packet)
}
}
func printPacketInfo(packet gopacket.Packet) {
// Let's see if the packet is an ethernet packet
ethernetLayer := packet.Layer(layers.LayerTypeEthernet)
if ethernetLayer != nil {
fmt.Println("Ethernet layer detected.")
ethernetPacket, _ := ethernetLayer.(*layers.Ethernet)
fmt.Println("Source MAC: ", ethernetPacket.SrcMAC)
fmt.Println("Destination MAC: ", ethernetPacket.DstMAC)
// Ethernet type is typically IPv4 but could be ARP or other
fmt.Println("Ethernet type: ", ethernetPacket.EthernetType)
fmt.Println()
}
// Let's see if the packet is IP (even though the ether type told us)
ipLayer := packet.Layer(layers.LayerTypeIPv4)
if ipLayer != nil {
fmt.Println("IPv4 layer detected.")
ip, _ := ipLayer.(*layers.IPv4)
// IP layer variables:
// Version (Either 4 or 6)
// IHL (IP Header Length in 32-bit words)
// TOS, Length, Id, Flags, FragOffset, TTL, Protocol (TCP?),
// Checksum, SrcIP, DstIP
fmt.Printf("From %s to %s\n", ip.SrcIP, ip.DstIP)
fmt.Println("Protocol: ", ip.Protocol)
fmt.Println()
}
// Let's see if the packet is TCP
tcpLayer := packet.Layer(layers.LayerTypeTCP)
if tcpLayer != nil {
fmt.Println("TCP layer detected.")
tcp, _ := tcpLayer.(*layers.TCP)
// TCP layer variables:
// SrcPort, DstPort, Seq, Ack, DataOffset, Window, Checksum, Urgent
// Bool flags: FIN, SYN, RST, PSH, ACK, URG, ECE, CWR, NS
fmt.Printf("From port %d to %d\n", tcp.SrcPort, tcp.DstPort)
fmt.Println("Sequence number: ", tcp.Seq)
fmt.Println()
}
// Iterate over all layers, printing out each layer type
fmt.Println("All packet layers:")
for _, layer := range packet.Layers() {
fmt.Println("- ", layer.LayerType())
}
// When iterating through packet.Layers() above,
// if it lists Payload layer then that is the same as
// this applicationLayer. applicationLayer contains the payload
applicationLayer := packet.ApplicationLayer()
if applicationLayer != nil {
fmt.Println("Application layer/Payload found.")
fmt.Printf("%s\n", applicationLayer.Payload())
// Search for a string inside the payload
if strings.Contains(string(applicationLayer.Payload()), "HTTP") {
fmt.Println("HTTP found!")
}
}
// Check for errors
if err := packet.ErrorLayer(); err != nil {
fmt.Println("Error decoding some part of the packet:", err)
}
}
创建和发送packet
下面这个例子做了几个事情。首先，它将演示如何使用网络设备发送原始字节。这样，您就可以像串行连接( serial connection
)一样使用它来发送数据。这对于真正的低层的数据传输很有用，但是如果你想与一个应用程序交互，你可能想建立硬件和软件都能识别的包。
接下来，它将演示如何使用ethernet、IP和TCP层创建数据包。所有的东西都是默认的和空的，所以它实际上不做任何事情。
为了完成它，我们创建了另一个数据包，但实际上为ethernet层填充了一些MAC地址，为IPv4填充了一些IP地址，为TCP层填充了一些端口号。您应该看到如何用它伪造数据包和模拟设备。
TCP层结构具有可读取或设置的SYN, FIN, and ACK 布尔标志。这有利于控制和模糊TCP握手、会话和端口扫描。
pcap库提供了一个发送字节的简单方法，但是gopacket中的layers包帮助我们为各个层创建字节结构。
package main
import (
"github.com/google/gopacket"
"github.com/google/gopacket/layers"
"github.com/google/gopacket/pcap"
"log"
"net"
"time"
)
var (
device string = "eth0"
snapshot_len int32 =1024
promiscuous bool = false
err error
timeout time.Duration =30 * time.Second
handle *pcap.Handle
buffer gopacket.SerializeBuffer
options gopacket.SerializeOptions
)
func main() {
// Open device
handle, err = pcap.OpenLive(device, snapshot_len, promiscuous, timeout)
if err != nil {log.Fatal(err) }
defer handle.Close()
// Send raw bytes over wire
rawBytes := []byte{10,20,30}
err = handle.WritePacketData(rawBytes)
if err != nil {
log.Fatal(err)
}
// Create a properly formed packet, just with
// empty details. Should fill out MAC addresses,
// IP addresses, etc.
buffer = gopacket.NewSerializeBuffer()
gopacket.SerializeLayers(buffer, options,
&layers.Ethernet{},
&layers.IPv4{},
&layers.TCP{},
gopacket.Payload(rawBytes),
)
outgoingPacket := buffer.Bytes()
// Send our packet
err = handle.WritePacketData(outgoingPacket)
if err != nil {
log.Fatal(err)
}
// This time lets fill out some information
ipLayer := &layers.IPv4{
SrcIP: net.IP{127,0,0,1},
DstIP: net.IP{8,8,8,8},
}
ethernetLayer := &layers.Ethernet{
SrcMAC: net.HardwareAddr{0xFF,0xAA,0xFA,0xAA,0xFF,0xAA},
DstMAC: net.HardwareAddr{0xBD,0xBD,0xBD,0xBD,0xBD,0xBD},
}
tcpLayer := &layers.TCP{
SrcPort: layers.TCPPort(4321),
DstPort: layers.TCPPort(80),
}
// And create the packet with the layers
buffer = gopacket.NewSerializeBuffer()
gopacket.SerializeLayers(buffer, options,
ethernetLayer,
ipLayer,
tcpLayer,
gopacket.Payload(rawBytes),
)
outgoingPacket = buffer.Bytes()
}
更多创建和解码packet的例子
package main
import (
"fmt"
"github.com/google/gopacket"
"github.com/google/gopacket/layers"
)
func main() {
// If we don't have a handle to a device or a file, but we have a bunch
// of raw bytes, we can try to decode them in to packet information
// NewPacket() takes the raw bytes that make up the packet as the first parameter
// The second parameter is the lowest level layer you want to decode. It will
// decode that layer and all layers on top of it. The third layer
// is the type of decoding: default(all at once), lazy(on demand), and NoCopy
// which will not create a copy of the buffer
// Create an packet with ethernet, IP, TCP, and payload layers
// We are creating one we know will be decoded properly but
// your byte source could be anything. If any of the packets
// come back as nil, that means it could not decode it in to
// the proper layer (malformed or incorrect packet type)
payload := []byte{2,4,6}
options := gopacket.SerializeOptions{}
buffer := gopacket.NewSerializeBuffer()
gopacket.SerializeLayers(buffer, options,
&layers.Ethernet{},
&layers.IPv4{},
&layers.TCP{},
gopacket.Payload(payload),
)
rawBytes := buffer.Bytes()
// Decode an ethernet packet
ethPacket :=
gopacket.NewPacket(
rawBytes,
layers.LayerTypeEthernet,
gopacket.Default,
)
// with Lazy decoding it will only decode what it needs when it needs it
// This is not concurrency safe. If using concurrency, use default
ipPacket :=
gopacket.NewPacket(
rawBytes,
layers.LayerTypeIPv4,
gopacket.Lazy,
)
// With the NoCopy option, the underlying slices are referenced
// directly and not copied. If the underlying bytes change so will
// the packet
tcpPacket :=
gopacket.NewPacket(
rawBytes,
layers.LayerTypeTCP,
gopacket.NoCopy,
)
fmt.Println(ethPacket)
fmt.Println(ipPacket)
fmt.Println(tcpPacket)
}
定制层
下一个程序将演示如何创建您自己的层。这有助于实现当前不包含在gopacket layers包中的协议。如果您想创建自己的 l33t
协议，甚至不使用TCP/IP或ethernet，那么它也很有用。
package main
import (
"fmt"
"github.com/google/gopacket"
)
// Create custom layer structure
type CustomLayer struct {
// This layer just has two bytes at the front
SomeByte byte
AnotherByte byte
restOfData []byte
}
// Register the layer type so we can use it
// The first argument is an ID. Use negative
// or 2000+ for custom layers. It must be unique
var CustomLayerType = gopacket.RegisterLayerType(
2001,
gopacket.LayerTypeMetadata{
"CustomLayerType",
gopacket.DecodeFunc(decodeCustomLayer),
},
)
// When we inquire about the type, what type of layer should
// we say it is? We want it to return our custom layer type
func (l CustomLayer) LayerType() gopacket.LayerType {
return CustomLayerType
}
// LayerContents returns the information that our layer
// provides. In this case it is a header layer so
// we return the header information
func (l CustomLayer) LayerContents() []byte {
return []byte{l.SomeByte, l.AnotherByte}
}
// LayerPayload returns the subsequent layer built
// on top of our layer or raw payload
func (l CustomLayer) LayerPayload() []byte {
return l.restOfData
}
// Custom decode function. We can name it whatever we want
// but it should have the same arguments and return value
// When the layer is registered we tell it to use this decode function
func decodeCustomLayer(data []byte, p gopacket.PacketBuilder) error {
// AddLayer appends to the list of layers that the packet has
p.AddLayer(&CustomLayer{data[0], data[1], data[2:]})
// The return value tells the packet what layer to expect
// with the rest of the data. It could be another header layer,
// nothing, or a payload layer.
// nil means this is the last layer. No more decoding
// return nil
// Returning another layer type tells it to decode
// the next layer with that layer's decoder function
// return p.NextDecoder(layers.LayerTypeEthernet)
// Returning payload type means the rest of the data
// is raw payload. It will set the application layer
// contents with the payload
return p.NextDecoder(gopacket.LayerTypePayload)
}
func main() {
// If you create your own encoding and decoding you can essentially
// create your own protocol or implement a protocol that is not
// already defined in the layers package. In our example we are just
// wrapping a normal ethernet packet with our own layer.
// Creating your own protocol is good if you want to create
// some obfuscated binary data type that was difficult for others
// to decode
// Finally, decode your packets:
rawBytes := []byte{0xF0,0x0F,65,65,66,67,68}
packet := gopacket.NewPacket(
rawBytes,
CustomLayerType,
gopacket.Default,
)
fmt.Println("Created packet out of raw bytes.")
fmt.Println(packet)
// Decode the packet as our custom layer
customLayer := packet.Layer(CustomLayerType)
if customLayer != nil {
fmt.Println("Packet was successfully decoded with custom layer decoder.")
customLayerContent, _ := customLayer.(*CustomLayer)
// Now we can access the elements of the custom struct
fmt.Println("Payload: ", customLayerContent.LayerPayload())
fmt.Println("SomeByte element:", customLayerContent.SomeByte)
fmt.Println("AnotherByte element:", customLayerContent.AnotherByte)
}
}
更快的解码packet
如果我们知道需要什么层，我们可以使用已有的结构来存储packet信息，而不是为每个packet创建新的结构,既浪费内存又浪费时间。使用 DecodingLayerParser
可以更快一点。这就像marshalling/unmarshalling数据一样。
package main
import (
"fmt"
"github.com/google/gopacket"
"github.com/google/gopacket/layers"
"github.com/google/gopacket/pcap"
"log"
"time"
)
var (
device string = "eth0"
snapshot_len int32 =1024
promiscuous bool = false
err error
timeout time.Duration =30 * time.Second
handle *pcap.Handle
// Will reuse these for each packet
ethLayer layers.Ethernet
ipLayer layers.IPv4
tcpLayer layers.TCP
)
func main() {
// Open device
handle, err = pcap.OpenLive(device, snapshot_len, promiscuous, timeout)
if err != nil {
log.Fatal(err)
}
defer handle.Close()
packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
for packet := range packetSource.Packets() {
parser := gopacket.NewDecodingLayerParser(
layers.LayerTypeEthernet,
ðLayer,
&ipLayer,
&tcpLayer,
)
foundLayerTypes := []gopacket.LayerType{}
err := parser.DecodeLayers(packet.Data(), &foundLayerTypes)
if err != nil {
fmt.Println("Trouble decoding layers: ", err)
}
for _, layerType := range foundLayerTypes {
if layerType == layers.LayerTypeIPv4 {
fmt.Println("IPv4: ", ipLayer.SrcIP, "->", ipLayer.DstIP)
}
if layerType == layers.LayerTypeTCP {
fmt.Println("TCP Port: ", tcpLayer.SrcPort, "->", tcpLayer.DstPort)
fmt.Println("TCP SYN:", tcpLayer.SYN, " | ACK:", tcpLayer.ACK)
}
}
}
}
TCP流重组
gopacket提供了 Flow
、 Endpoint
类型，我没有深入了解这些类型，但是官方文档使用它发送特定的TCP流到一个channel中，参看 very deeply but the documentation has an example of using it to send specific TCP streams to a channel of packets. GoDoc gopacket basic usage
更多的信息可以查看官方项目和godoc。
About The Author
