Go语言linux下构造tcp数据包(不使用net包和Cgo)

Golang可以通过syscall包中的相关socket函数(Windows和Linux中syscall中网络编程的函数不太一样)来进行网络编

程，可以用我们熟悉的方法使用raw socket，因为最近在搞一些协议的学习，所以特意试了试，和普通的linux+C的

socket编程基本没什么区别。

代码如下：

package main

import (
    "bytes"
    "encoding/binary"
    . "fmt"
    "strconv"
    "strings"
    "syscall"
    "unsafe"
)

type TCPHeader struct {
    SrcPort   uint16
    DstPort   uint16
    SeqNum    uint32
    AckNum    uint32
    Offset    uint8
    Flag      uint8
    Window    uint16
    Checksum  uint16
    UrgentPtr uint16
}

type PsdHeader struct {
    SrcAddr   uint32
    DstAddr   uint32
    Zero      uint8
    ProtoType uint8
    TcpLength uint16
}

func inet_addr(ipaddr string) uint32 {
    var (
        segments []string = strings.Split(ipaddr, ".")
        ip       [4]uint64
        ret      uint64
    )
    for i := 0; i < 4; i++ {
        ip[i], _ = strconv.ParseUint(segments[i], 10, 64)
    }
    ret = ip[3]<<24 + ip[2]<<16 + ip[1]<<8 + ip[0]
    return uint32(ret)
}

func htons(port uint16) uint16 {
    var (
        high uint16 = port >> 8
        ret  uint16 = port<<8 + high
    )
    return ret
}

func CheckSum(data []byte) uint16 {
	var (
		sum    uint32
		length int = len(data)
		index  int
	)
	for length > 1 {
		sum += uint32(data[index])<<8 + uint32(data[index+1])
		index += 2
		length -= 2
	}
	if length > 0 {
		sum += uint32(data[index])
	}
	sum += (sum >> 16)

	return uint16(^sum)
}

func main() {
    var (
        msg       string
        psdheader PsdHeader
        tcpheader TCPHeader
    )

    Printf("Input the content: ")
    Scanf("%s", &msg)

    /*填充TCP伪首部*/
    psdheader.SrcAddr = inet_addr("127.0.0.1")
    psdheader.DstAddr = inet_addr("127.0.0.1")
    psdheader.Zero = 0
    psdheader.ProtoType = syscall.IPPROTO_TCP
    psdheader.TcpLength = uint16(unsafe.Sizeof(TCPHeader{})) + uint16(len(msg))

    /*填充TCP首部*/
    tcpheader.SrcPort = htons(3000)
    tcpheader.DstPort = htons(8080)
    tcpheader.SeqNum = 0
    tcpheader.AckNum = 0
    tcpheader.Offset = uint8(uint16(unsafe.Sizeof(TCPHeader{}))/4) << 4
    tcpheader.Flag = 2 //SYN
    tcpheader.Window = 60000
    tcpheader.Checksum = 0

    /*buffer用来写入两种首部来求得校验和*/
    var (
        buffer bytes.Buffer
    )
    binary.Write(&buffer, binary.BigEndian, psdheader)
    binary.Write(&buffer, binary.BigEndian, tcpheader)
    tcpheader.Checksum = CheckSum(buffer.Bytes())

    /*接下来清空buffer，填充实际要发送的部分*/
    buffer.Reset()
    binary.Write(&buffer, binary.BigEndian, tcpheader)
    binary.Write(&buffer, binary.BigEndian, msg)

    /*下面的操作都是raw socket操作，大家都看得懂*/
    var (
        sockfd int
        addr   syscall.SockaddrInet4
        err    error
    )
    if sockfd, err = syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, syscall.IPPROTO_TCP); err != nil {
        Println("Socket() error: ", err.Error())
        return
    }
    defer syscall.Shutdown(sockfd, syscall.SHUT_RDWR)
    addr.Addr[0], addr.Addr[1], addr.Addr[2], addr.Addr[3] = 127, 0, 0, 1
    addr.Port = 8080
    if err = syscall.Sendto(sockfd, buffer.Bytes(), 0, &addr); err != nil {
        Println("Sendto() error: ", err.Error())
        return
    }
    Println("Send success!")
}

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