Question

我在C中写了一个小ping程序。我已经成功构建了ip header和icmp header。我发送的数据长度为64个字节。但是，我的ICMP标头的校验和值有问题。 IP正在发送正确的校验和，但ICMP中的校验和不断变化，在wireshark中，我收到一条消息，告诉我它需要相同的值。

这是我的代码：

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/time.h>
#include <stdlib.h>
#include <memory.h>
#include <ifaddrs.h>
#include <net/if.h>
#include <stdarg.h>
#include <math.h>
#include <sys/termios.h>

#define PCKT_LEN 8192
struct ipheader {
 unsigned char      iph_ihl:4, iph_ver:4;
 unsigned char      iph_tos;
 unsigned short int iph_len;
 unsigned short int iph_ident;
 //unsigned char      iph_flag;
 unsigned short int iph_offset;
 unsigned char      iph_ttl;
 unsigned char      iph_protocol;
 unsigned short int iph_chksum;
 unsigned int       iph_sourceip;
 unsigned int       iph_destip;
};
struct icmpheader{
    char type;
    char code;
    short int checksum; 
    char content[4];
    char data[64];

};

unsigned short checksum(unsigned short *buf, int nwords)
{       //
        unsigned long sum;
        for(sum=0; nwords>0; nwords--)
                sum += *buf++;
        sum = (sum >> 16) + (sum &0xffff);
        sum += (sum >> 16);
        return (unsigned short)(~sum);
}

int main(int argc, char *argv[]){
    int sd;
    char buffer[92]; // ICMP +IP_HEADER + DATA(64 BYTES)
    int i=0;
    //Definiendo estructura de header IP
    struct ipheader *ip = (struct ipheader *) buffer;
    struct icmpheader *icmp = (struct icmpheader *) (buffer + sizeof(struct ipheader));
    //Direcciones host y dest
    struct sockaddr_in sin, din;

    int one = 1;
    const int *val = &one;

    memset(buffer, 0, 92);

    if(argc != 3)
    {
        printf("- Parametros invalidos\n");
        printf("- Usage %s <source hostname/IP> <source port> <target hostname/IP> <target port>\n", argv[0]);

        exit(-1);
    }
    sd = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);

    if(sd < 0){

        perror("socket() error");
        // If something wrong just exit
        exit(-1);
    }
    printf("socket() Creado exitosamente.");
    // The source is redundant, may be used later if needed
    // The address family
    sin.sin_family = AF_INET;
    din.sin_family = AF_INET;
    // IP addresses
    sin.sin_addr.s_addr = inet_addr(argv[2]);
    din.sin_addr.s_addr = inet_addr(argv[1]);   
    // Creando el header IP
    ip->iph_ihl = 5;
    ip->iph_ver = 4;
    ip->iph_tos = 16; 
    ip->iph_len = sizeof(struct ipheader) + 8+64;
    ip->iph_ident = htons(54321);
    ip->iph_ttl = 64; // hops
    ip->iph_protocol = 1; // ICMP
    // Source IP 
    ip->iph_sourceip = inet_addr(argv[2]);
    //DEST IP
    ip->iph_destip = inet_addr(argv[1]);

    //Creamos header icmp
    icmp->type=8;
    icmp->code=0;
    for(i=0;i<4;i++){
        icmp->content[i]=0;

    }
    for(i=0;i<64;i++){
        icmp->data[i]=(char)i;

    }
    printf("ICMP HEADER: %d\n",sizeof(struct icmpheader));
    printf("IP HEADER: %d\n",sizeof(struct ipheader));
    // Calcular checksum IP
    ip->iph_chksum = checksum((unsigned short *)buffer, sizeof(struct ipheader) +sizeof(struct icmpheader));    
    //ICMP checksum
    icmp->checksum = 0;
    icmp->checksum = checksum((unsigned short *)buffer+sizeof(struct ipheader) , sizeof(struct icmpheader));
    //icmp->checksum = 0xfc13;
    // Inform the kernel do not fill up the packet structure. we will build our own...
    if(setsockopt(sd, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0)
    {
        perror("setsockopt() error");
        exit(-1);
    }
    else{
        printf("setsockopt() OK.\n");   

    }
    // Send loop, enviar cada 2 segundos por 100
    printf("Trying...\n");  
    printf("Using raw socket and ICMP protocol\n");
    printf("Using Source IP: %s Destination IP: %s .\n", argv[2], argv[1]); 
    int count;
    for(count = 1; count <=20; count++)
    {
        if(sendto(sd, buffer, ip->iph_len, 0, (struct sockaddr *)&sin, sizeof(sin)) < 0)
        // Verify
        {
            perror("sendto() error");
            exit(-1);
        }
        else
        {
            printf("Count #%u - sendto() is OK.\n", count);
            sleep(2);
        }


    }   
    close(sd);
    return 0;

}

有人能告诉我计算ICMP标头校验和的正确方法吗？

Answer 1

int ip_headerlen = ip->iph_ihl * 4;
ip->iph_chksum = in_cksum ((unsigned short *) ip, 
                                ip_headerlen);//only need ipheaderlen
icmp->icmp_chksum = in_cksum((unsigned short *)icmp,
                                //sizeof(struct icmpheader));
                                    ntohs(ip->iph_len)-ip_headerlen   );
//need icmp header + icmp data

ping程序中的ICMP校验和错误

1 个答案: