ping 实现设计---ICMP

发送ICMP报文时，必须程序自己计算校验和，将它填入ICMP头部对应的域中。

校验和的计算方法:

　　将数据以字为单位累加到一个双字中，如果数据长度为奇数，最后一个字节将被扩展到字，累加的结果是一个双字，最后将这个双字的高16位，低16位相加后取反，便得到了校验和。

下面是checksum的计算校验和的代码：

USHORT checksum(USHORT* buff, int size)
{
    unsigned long cksum = 0;
    while(size>1)
    {
        cksum += *buff++;
        size -= sizeof(USHORT);
    }
    // 是奇数
    if(size)
    {
        cksum += *(UCHAR*)buff;
    }
    // 将32位的chsum高16位和低16位相加，然后取反
    cksum = (cksum >> 16) + (cksum & 0xffff);
    cksum += (cksum >> 16);            
    return (USHORT)(~cksum);
}

Ping程序实例：

Ping用来检查主机是否存在，是否可达。

下面是Ping的执行步骤：

1 创建协议类型为IPPROTO_ICMP的原始套接字

2 创建并初始化ICMP封包

3 调用sendto函数向远程主机发送ICMP请求

4 调用recvfrom函数接收ICMP响应

完整代码如下：

///////////////////////////////////////////
// ping.cpp文件

#include "../common/initsock.h"
#include "../common/protoinfo.h"
#include <stdio.h>
#include <winsock2.h>
#include <windows.h>
#include "Ws2tcpip.h"
CInitSock theSock;

typedef struct icmp_hdr
{
    unsigned char   icmp_type;        // 消息类型
    unsigned char   icmp_code;        // 代码
    unsigned short  icmp_checksum;    // 校验和
    // 下面是回显头
    unsigned short  icmp_id;        // 用来惟一标识此请求的ID号，通常设置为进程ID
    unsigned short  icmp_sequence;    // 序列号
    unsigned long   icmp_timestamp; // 时间戳
} ICMP_HDR, *PICMP_HDR;

USHORT checksum(USHORT* buff, int size);
BOOL SetTTL(SOCKET s, int nValue);
BOOL SetTimeout(SOCKET s, int nTime, BOOL bRecv = TRUE);

USHORT checksum(USHORT* buff, int size)
{
    unsigned long cksum = 0;
    while(size>1)
    {
        cksum += *buff++;
        size -= sizeof(USHORT);
    }
    // 是奇数
    if(size)
    {
        cksum += *(UCHAR*)buff;
    }
    // 将32位的chsum高16位和低16位相加，然后取反
    cksum = (cksum >> 16) + (cksum & 0xffff);
    cksum += (cksum >> 16);            
    return (USHORT)(~cksum);
}

BOOL SetTTL(SOCKET s, int nValue)
{
    int ret = ::setsockopt(s, IPPROTO_IP, IP_TTL, (char*)&nValue, sizeof(nValue));
    return ret != SOCKET_ERROR;
}

BOOL SetTimeout(SOCKET s, int nTime, BOOL bRecv)
{
    int ret = ::setsockopt(s, SOL_SOCKET, 
        bRecv ? SO_RCVTIMEO : SO_SNDTIMEO, (char*)&nTime, sizeof(nTime));
    return ret != SOCKET_ERROR;
}




int main()
{
    // 目的IP地址，即要Ping的IP地址
    char szDestIp[] = "127.0.0.1";    // 127.0.0.1

    // 创建原始套节字
    SOCKET sRaw = ::socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);

    // 设置接收超时
    SetTimeout(sRaw, 1000, TRUE);

    // 设置目的地址
    SOCKADDR_IN dest;
    dest.sin_family = AF_INET;
    dest.sin_port = htons(0);
    dest.sin_addr.S_un.S_addr = inet_addr(szDestIp);

    // 创建ICMP封包
    char buff[sizeof(ICMP_HDR) + 32];
    ICMP_HDR* pIcmp = (ICMP_HDR*)buff;
    // 填写ICMP封包数据
    pIcmp->icmp_type = 8;    // 请求一个ICMP回显
    pIcmp->icmp_code = 0;
    pIcmp->icmp_id = (USHORT)::GetCurrentProcessId();
    pIcmp->icmp_checksum = 0;
    pIcmp->icmp_sequence = 0;
    // 填充数据部分，可以为任意
    memset(&buff[sizeof(ICMP_HDR)], 'E', 32);
    
    // 开始发送和接收ICMP封包
    USHORT    nSeq = 0;
    char recvBuf[1024];
    SOCKADDR_IN from;
    int nLen = sizeof(from);
    while(TRUE)
    {
        static int nCount = 0;
        int nRet;
        if(nCount++ == 4)
            break;
        pIcmp->icmp_checksum = 0;
        pIcmp->icmp_timestamp = ::GetTickCount();
        pIcmp->icmp_sequence = nSeq++;
        pIcmp->icmp_checksum = checksum((USHORT*)buff, sizeof(ICMP_HDR) + 32);
        nRet = ::sendto(sRaw, buff, sizeof(ICMP_HDR) + 32, 0, (SOCKADDR *)&dest, sizeof(dest));
        if(nRet == SOCKET_ERROR)
        {
            printf(" sendto() failed: %d \n", ::WSAGetLastError());
            return -1;
        }
        nRet = ::recvfrom(sRaw, recvBuf, 1024, 0, (sockaddr*)&from, &nLen);
        if(nRet == SOCKET_ERROR)
        {
            if(::WSAGetLastError() == WSAETIMEDOUT)
            {
                printf(" timed out\n");
                continue;
            }
            printf(" recvfrom() failed: %d\n", ::WSAGetLastError());
            return -1;
        }

        // 下面开始解析接收到的ICMP封包
        int nTick = ::GetTickCount();
        if(nRet < sizeof(IPHeader) + sizeof(ICMP_HDR))
        {
            printf(" Too few bytes from %s \n", ::inet_ntoa(from.sin_addr));
        }
        // 接收到的数据中包含IP头，IP头大小为20个字节，所以加20得到ICMP头
        ICMP_HDR* pRecvIcmp = (ICMP_HDR*)(recvBuf + 20); // (ICMP_HDR*)(recvBuf + sizeof(IPHeader));
        if(pRecvIcmp->icmp_type != 0)    // 回显
        {
            printf(" nonecho type %d recvd \n", pRecvIcmp->icmp_type);
            return -1;
        }

        if(pRecvIcmp->icmp_id != ::GetCurrentProcessId())
        {
            printf(" someone else's packet! \n");
            return -1;
        }
        
        printf(" %d bytes from %s:", nRet, inet_ntoa(from.sin_addr));
        printf(" icmp_seq = %d. ", pRecvIcmp->icmp_sequence);
        printf(" time: %d ms", nTick - pRecvIcmp->icmp_timestamp);
        printf(" \n");

        ::Sleep(1000);
    }

    return 0;
}

执行结果：