Question

我正在尝试在C中创建一个ICMP ping测试程序，但是在成功发送数据包时遇到了困难。 sendto函数返回字节数和所有内容但实际上没有发送数据包。我已经在目标计算机上使用WireShark验证了这一点。主机上的常规ping工作正常，但在WireShark中显示。

这是我的代码：

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <string.h>
#include <arpa/inet.h>
#include <sys/select.h>
#include <sys/time.h>

unsigned short cksum(unsigned short *addr, int len);

int main(int argc, char *argv[])
{
    int sock;
    char send_buf[400], recv_buf[400], src_name[256], src_ip[15], dst_ip[15];
    struct ip *ip = (struct ip *)send_buf;
    struct icmp *icmp = (struct icmp *)(ip + 1);
    struct hostent *src_hp, *dst_hp;
    struct sockaddr_in src, dst;
    struct timeval t;
    int on;
    int num = 10;
    int failed_count = 0;
    int bytes_sent, bytes_recv;
    int dst_addr_len;
    int result;
    fd_set socks;

    /* Initialize variables */
    on = 1;
    memset(send_buf, 0, sizeof(send_buf));

    /* Check for valid args */
    if(argc < 2)
    {
        printf("\nUsage: %s <dst_server>\n", argv[0]);
        printf("- dst_server is the target\n");
        exit(EXIT_FAILURE);
    }

    /* Check for root access */
    if (getuid() != 0)
    {
        fprintf(stderr, "%s: This program requires root privileges!\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    /* Get source IP address */
    if(gethostname(src_name, sizeof(src_name)) < 0)
    {
        perror("gethostname() error");
        exit(EXIT_FAILURE);
    }
    else
    {
        if((src_hp = gethostbyname(src_name)) == NULL)
        {
            fprintf(stderr, "%s: Can't resolve, unknown source.\n", src_name);
            exit(EXIT_FAILURE);
        }
        else
            ip->ip_src = (*(struct in_addr *)src_hp->h_addr);
    }

    /* Get destination IP address */
    if((dst_hp = gethostbyname(argv[1])) == NULL)
    {
        if((ip->ip_dst.s_addr = inet_addr(argv[1])) == -1)
        {
            fprintf(stderr, "%s: Can't resolve, unknown destination.\n", argv[1]);
            exit(EXIT_FAILURE);
        }
    }
    else
    {
        ip->ip_dst = (*(struct in_addr *)dst_hp->h_addr);
        dst.sin_addr = (*(struct in_addr *)dst_hp->h_addr);
    }

    sprintf(src_ip, "%s", inet_ntoa(ip->ip_src));
    sprintf(dst_ip, "%s", inet_ntoa(ip->ip_dst));
    printf("Source IP: '%s' -- Destination IP: '%s'\n", src_ip, dst_ip);

    /* Create RAW socket */
    if((sock = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0)
    {
        perror("socket() error");

        /* If something wrong, just exit */
        exit(EXIT_FAILURE);
    }

    /* Socket options, tell the kernel we provide the IP structure */
    if(setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &on, sizeof(on)) < 0)
    {
        perror("setsockopt() for IP_HDRINCL error");
        exit(EXIT_FAILURE);
    }

    /* IP structure, check the ip.h */
    ip->ip_v = 4;
    ip->ip_hl = 5;
    ip->ip_tos = 0;
    ip->ip_len = htons(sizeof(send_buf));
    ip->ip_id = htons(321);
    ip->ip_off = htons(0);
    ip->ip_ttl = 255;
    ip->ip_p = IPPROTO_ICMP;
    ip->ip_sum = 0;

    /* ICMP structure, check ip_icmp.h */
    icmp->icmp_type = ICMP_ECHO;
    icmp->icmp_code = 0;
    icmp->icmp_id = 123;
    icmp->icmp_seq = 0;

    /* Set up destination address family */
    dst.sin_family = AF_INET;

    /* Loop based on the packet number */
    for(int i = 1; i <= num; i++)
    {
        /* Header checksums */
        icmp->icmp_cksum = 0;
        ip->ip_sum = cksum((unsigned short *)send_buf, ip->ip_hl);
        icmp->icmp_cksum = cksum((unsigned short *)icmp, sizeof(send_buf) - sizeof(struct icmp));

        /* Get destination address length */
        dst_addr_len = sizeof(dst);

        /* Set listening timeout */
        t.tv_sec = 5;
        t.tv_usec = 0;

        /* Set socket listening descriptors */
        FD_ZERO(&socks);
        FD_SET(sock, &socks);

        /* Send packet */
        if((bytes_sent = sendto(sock, send_buf, sizeof(send_buf), 0, (struct sockaddr *)&dst, dst_addr_len)) < 0)
        {
            perror("sendto() error");
            failed_count++;
            printf("Failed to send packet.\n");
            fflush(stdout);
        }
        else
        {
            printf("Sent %d byte packet... ", bytes_sent);

            fflush(stdout);

            /* Listen for the response or timeout */
            if((result = select(sock + 1, &socks, NULL, NULL, &t)) < 0)
            {
                perror("select() error");
                failed_count++;
                printf("Error receiving packet!\n");
            }
            else if (result > 0)
            {
                printf("Waiting for packet... ");
                fflush(stdout);

                if((bytes_recv = recvfrom(sock, recv_buf, sizeof(ip) + sizeof(icmp) + sizeof(recv_buf), 0, (struct sockaddr *)&dst, (socklen_t *)&dst_addr_len)) < 0)
                {
                    perror("recvfrom() error");
                    failed_count++;
                    fflush(stdout);
                }
                else
                    printf("Received %d byte packet!\n", bytes_recv);
            }
            else
            {
                printf("Failed to receive packet!\n");
                failed_count++;
            }

            fflush(stdout);

            icmp->icmp_seq++;
        }
    }

    /* Display success rate */
    printf("Ping test completed with %d%% success rate.\n", (((num - failed_count) / num) * 100));

    /* close socket */
    close(sock);

    return 0;
}

/* One's Complement checksum algorithm */
unsigned short cksum(unsigned short *addr, int len)
{
    int nleft = len;
    int sum = 0;
    unsigned short *w = addr;
    unsigned short answer = 0;

    while (nleft > 1)
    {
      sum += *w++;
      nleft -= 2;
    }

    if (nleft == 1)
    {
      *(unsigned char *)(&answer) = *(unsigned char *)w;
      sum += answer;
    }

    sum = (sum >> 16) + (sum & 0xffff);
    sum += (sum >> 16);
    answer = ~sum;

    return (answer);
}

对我做错的任何想法？校验和还好吗？两者都使用0可以吗？

编辑：好的，所以我设法正确地发送了正确的数据包，感谢下面的人们！目标计算机将发回回信回复。但是，现在，我的程序没有收到答复。 select()函数总是超时。我已经更新了上面的代码。

编辑2：好吧，我已经开始工作了！我需要将套接字协议设置为IPPROTO_ICMP而不是IPPROTO_RAW，并且它工作得很好！再次感谢你们的评论！真有帮助！看起来我只能将一个答案标记为正确，但你们都帮助解决这个问题。：）

Answer 1

我注意到的一件事......

你有这个：

 struct ip *ip = (struct ip *)send_buf;

然后，您正在分配目标字段：

ip->ip_dst = (*(struct in_addr *)dst_hp->h_addr)

然后你用memset擦除它（因为send_buff指向同一个东西）：

memset(send_buf, 0, sizeof(send_buf));

所以，当你想在这里获得ip_dst：

dst.sin_addr = ip->ip_dst;

你得到0而不是你之前设定的。

Answer 2

乍一看：在select（）之后你不能依赖时间结构。你还应该设置usec。

所以在代码中，在for循环中包含t值的规范：

for (i = 1; i <= num; i++) {
    t.tv_sec = 5;
    t.tv_usec = 0;
    ...

否则当你进行第二次迭代时t（可以）已经改变。

在您的sprintf（src_ip，...）和dst_ip中，您省略了格式。

Answer 3

除了ebutusov的回复：

ip->ip_sum = 0;
icmp->icmp_cksum = htons(~(ICMP_ECHO << 8));

两者都不正确。
您需要正确计算校验和（两者的算法相同，但涵盖不同的字段）。

在C程序中发送ICMP数据包

3 个答案: