Question

我正在开发一个通过 ICMP协议发送和接收数据的脚本。我在base64中剪切，加密，编码，伪造数据包。我的代码工作正常，但如果我尝试发送大文件，ICMP数据包不再发送（大约在第1021个数据包，似乎我已达到最大段大小65495字节。

以下是事件出现时wireshark捕获的屏幕截图：

如果这可能有帮助，那么我的 C实现伪造和发送数据包（我从文件中读取要发送的数据）：

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/socket.h>
#include <resolv.h>
#include <netdb.h>
#include <string.h>
#include <netinet/in.h>
#include <netinet/ip_icmp.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <dirent.h>
#include <errno.h>
#include <limits.h>
#include <time.h>

#define PACKET_SIZE 128
struct packet
{
    struct icmphdr hdr;
    char msg[PACKET_SIZE-sizeof(struct icmphdr)];
};

int pid=-1;
struct protoent *proto=NULL;

/*--------------------------------------------------------------------*/
/*--- checksum - standard 1s complement checksum                   ---*/
/*--------------------------------------------------------------------*/
unsigned short checksum(void *b, int len)
{   unsigned short *buf = b;
    unsigned int sum=0;
    unsigned short result;

    for ( sum = 0; len > 1; len -= 2 )
        sum += *buf++;
    if ( len == 1 )
        sum += *(unsigned char*)buf;
    sum = (sum >> 16) + (sum & 0xFFFF);
    sum += (sum >> 16);
    result = ~sum;
    return result;
}

/*--------------------------------------------------------------------*/
/*--- listener - separate process to listen for and collect messages--*/
/*--------------------------------------------------------------------*/
void listener(void)
{   int sd;
    struct sockaddr_in addr;
    unsigned char buf[1024];

    sd = socket(AF_INET, SOCK_RAW, proto->p_proto);
    if ( sd < 0 )
    {
        perror("socket");
        exit(0);
    }
    for (;;)
        {   int bytes, len=sizeof(addr);

            bzero(buf, sizeof(buf));
            bytes = recvfrom(sd, buf, sizeof(buf), 0, (struct sockaddr*)&addr, &len);
            if ( bytes > 0 )
                printf("***Got Reply***\n");
            else
                perror("recvfrom");
        }
        exit(0);
}



/*--------------------------------------------------------------------*/
/*--- ping - Create message and send it.                           ---*/
/*--------------------------------------------------------------------*/
int my_ping(struct sockaddr_in *addr, char *pkt, int cnt)
{   
    const int val=255;
    int i, u, sd;
    struct packet pckt;
    struct sockaddr_in r_addr;

    sd = socket(AF_INET, SOCK_RAW, proto->p_proto);


    if ( sd < 0 )
        return 1;
    if ( setsockopt(sd, SOL_IP, IP_TTL, &val, sizeof(val)) != 0)
        return 2;
    if ( fcntl(sd, F_SETFL, O_NONBLOCK) != 0 )
        return 3;
    int len=sizeof(r_addr);
    bzero(&pckt, sizeof(pckt));
    pckt.hdr.type = ICMP_ECHO;
    pckt.hdr.un.echo.id = pid;
     for(i = 0; pkt[i]; i++){
        pckt.msg[i] = pkt[i];
    }
    u = i;
    for ( i = u; i < sizeof(pckt.msg); i++ )
        pckt.msg[i] = '0';
    //pckt.msg[i] = 0;
    pckt.hdr.un.echo.sequence = cnt++;
    pckt.hdr.checksum = checksum(&pckt, sizeof(pckt));
    if ( sendto(sd, &pckt, sizeof(pckt), 0, (struct sockaddr*)addr, sizeof(*addr)) <= 0 )
        return 4;
    return 0;
}

/*--------------------------------------------------------------------*/
/*--- ping - Create message and send it.                           ---*/
/*--------------------------------------------------------------------*/
int sending(struct sockaddr_in *addr, char* folderPath)
{   
    int status, packetSent = 0, nbTries = 0;
    FILE *entry_file;
    char buffer[BUFSIZ];
    struct dirent **namelist;
    char pathFile[PATH_MAX + 1];
    int i,n;
    char pkt[2048];
    char *pktTemp;
    int nbPkts = 1;


    /* Scanning the in directory */
    n = scandir(folderPath, &namelist, 0, alphasort);
    printf("n == %d \n", n);
    if (n < 0){
        fprintf(stderr, "Scan directory can not be performed\n");
        return(3);
    }else {
        for (i = 0; i < n; i++) {


            /* On linux/Unix we don't want current and parent directories
            *          * On windows machine too, thanks Greg Hewgill
            *                   */
            if (!strcmp (namelist[i]->d_name, ".") || !strcmp (namelist[i]->d_name, ".."))
                continue;
            pathFile[0] = '\0';
            strncat(pathFile, folderPath,PATH_MAX);
            strncat(pathFile, "/",PATH_MAX);
            strncat(pathFile, namelist[i]->d_name, PATH_MAX);
            printf("\n----------------\n");
            printf("File path read: %s \n", pathFile);
            entry_file = fopen(pathFile, "r");
            if (entry_file == NULL)
            {
                fprintf(stderr, "Failed to open entry file \n");
                return(3);
            }

            pkt[0] = '\0';
            packetSent = 0;
            while((pktTemp = fgets(buffer, BUFSIZ, entry_file)) != NULL){
                strncat(pkt,pktTemp, 2048);
            }
            printf("Sending Packet %d with message : \"%s\" \n", nbPkts, pkt);
            printf("----------------\n");
            while(packetSent == 0 && nbTries < 3){
                if((status = my_ping(addr, pkt, nbPkts)) == 0){
                    packetSent = 1;
                    nbPkts++;
                }else {
                    nbTries++;
                }
                usleep(50000);
            }
            if(packetSent == 0){
                fprintf(stderr, "Connection has been lost, check if the host is alived\n");
            }
            /* When you finish with the file, close it */
            fclose(entry_file);
            free(namelist[i]);
        }
        free(namelist);
    }   
    return 0;
}

/*--------------------------------------------------------------------*/
/*--- main - look up host and start ping processes.                ---*/
/*--------------------------------------------------------------------*/
int main(int argc, char *argv[])
{   
    struct hostent *hname;
    struct sockaddr_in addr;
    if ( argc != 3 )
    {
        printf("usage: %s <addr>\n", argv[0]);
        exit(0);
    }
    if ( argc > 2 )
    {
        pid = getpid();
        proto = getprotobyname("ICMP");
        hname = gethostbyname(argv[1]);
        bzero(&addr, sizeof(addr));
        addr.sin_family = hname->h_addrtype;
        addr.sin_port = 0;
        addr.sin_addr.s_addr = *(long*)hname->h_addr;
        if ( fork() == 0 )
            listener();
        else
            sending(&addr, argv[2]);
        wait(0);
    }
    else
        printf("Usage: ./bin <hostname> <folderPath>\n");
    return 0;
}

发送ICMP报文后的最大段大小

0 个答案: