c ping request using raw sockets fails

Question

My program sends an icmp request to the specified host.

#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define ICMP_DATALEN    4
#define IP4_HDRLEN              20
#define ICMP_HDRLEN             8

uint16_t                                checksum (uint16_t *addr, int len)
{
    int nleft = len;
    int sum = 0;
    uint16_t *w = addr;
    uint16_t answer = 0;

    while (nleft > 1) {
            sum += *w++;
            nleft -= sizeof (uint16_t);
    }

    if (nleft == 1) {
            *(uint8_t *) (&answer) = *(uint8_t *) w;
            sum += answer;
    }

    sum = (sum >> 16) + (sum & 0xFFFF);
    sum += (sum >> 16);
    answer = ~sum;
    return (answer);
}

int                                             main(int ac, char **av)
{
    int                              icmp_socket;
    uint8_t                          data[ICMP_DATALEN];
    unsigned char                    datagram[IP_MAXPACKET];
    int                              ip_flags[4];
    struct ip                        *iph = (struct ip *) datagram;
    struct icmp                      *icmph = (struct icmp *) (datagram + IP4_HDRLEN);
    struct sockaddr_in               dst;
    int                              one = 1;
    int                              *val = &one;

    if (ac != 3)
    {
            fprintf(stderr, "usage: %s <source adress> <target adress>\n", av[0]);
            return (1);
    }

    if ((icmp_socket = socket(AF_INET, SOCK_RAW, 1)) == -1)
    {
            perror("socket");
            exit(1);
    }
    if (setsockopt(icmp_socket, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0)
    {
            perror("setsockopt");
            exit(1);
    }

    memset(datagram, 0, IP_MAXPACKET);

    //prepare ipv4 header data
    memset((void*)&dst, 0, sizeof(dst));
    dst.sin_family = AF_INET;
    inet_pton(AF_INET, av[2], &(dst.sin_addr));
    ip_flags[0] = 0;
    ip_flags[1] = 0;
    ip_flags[2] = 0;
    ip_flags[3] = 0;

    //fill in ipv4 header
    iph->ip_hl = IP4_HDRLEN / sizeof(uint32_t);
    iph->ip_v = 4;
    iph->ip_tos = 0;
    iph->ip_len = (IP4_HDRLEN + ICMP_HDRLEN + ICMP_DATALEN);
    iph->ip_id = 666;
    iph->ip_off = (ip_flags[0] << 15)
                                            + (ip_flags[1] << 14)
                                            + (ip_flags[2] << 13)
                                            + ip_flags[3];
    iph->ip_ttl = 255;
    iph->ip_p = IPPROTO_ICMP;
    iph->ip_src.s_addr = inet_addr(av[1]);
    iph->ip_dst.s_addr = dst.sin_addr.s_addr;
    iph->ip_sum = 0;
    iph->ip_sum = checksum((uint16_t *)datagram, IP4_HDRLEN);

    //prepare icmp data
    data[0] = '4';
    data[1] = '2';
    data[2] = '4';
    data[3] = '2';

    //fill in icmp header
    icmph->icmp_type = ICMP_ECHO;
    icmph->icmp_code = 0;
    icmph->icmp_id = htons(666);
    icmph->icmp_seq = htons(0);
    memcpy(datagram + IP4_HDRLEN + ICMP_HDRLEN, data, ICMP_DATALEN);
    icmph->icmp_cksum = 0;
    icmph->icmp_cksum = checksum((uint16_t *)datagram + IP4_HDRLEN, (ICMP_HDRLEN + ICMP_DATALEN));

    int             i = 0;

    printf("sending packet:\n\n");
    while (i < (IP4_HDRLEN + ICMP_HDRLEN + ICMP_DATALEN))
    {
            printf("%02x", datagram[i++]);
            if (i % 2 == 0)
                    printf(" ");
            if (i % 16 == 0)
                    printf("\n");
    }
    printf("ip header checksump: %hx\n", iph->ip_sum);
    printf("icmp header checksump: %hx\n", icmph->icmp_cksum);

    if (sendto(icmp_socket, datagram, (IP4_HDRLEN + ICMP_HDRLEN + ICMP_DATALEN), 0, (struct sockaddr *)&dst, sizeof(dst)) < 0)
    {
            perror("sendto");
            exit(1);
    }
    close(icmp_socket);
    return (0);
}

I watched the network with tcpdump and the packet is sent, but I never recieve an answer, as I would with the ping command. I hope you can help me !

Answer 1

The checksum is in network byte order.

You need to read the bytes being added to the checksum using ntohs, and write back the checksum with htons.