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VPN 隧道抓的报文, 没有带上 完整的 MAC 头部信息.
不便于 某些专用程序的处理.
写了一个小方法, 实现转换功能. MAC数据为自己造的.

ip_p2p.pcap

padding_ETH_MAC.pcap

pcap_ip_to_eth.c

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#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>

#define DLT_NULL    0   /* BSD loopback encapsulation */
#define DLT_EN10MB  1   /* Ethernet (10Mb) */
#define DLT_PPP     9   /* Point-to-point Protocol */
#define PCAP_VERSION_MAJOR 2
#define PCAP_VERSION_MINOR 4

const char *ETH_SMAC  = "\x00\x21\x45\x00\x01\x13";
const char *ETH_DMAC  = "\x7c\x69\xf6\x71\x8b\x10";
const char *ETH_PROTO = "\x08\00";

struct pcap_file_header {
    uint32_t     magic;
    uint16_t     version_major;
    uint16_t     version_minor;
    uint32_t     thiszone;   /* gmt to local correction */
    uint32_t     sigfigs;    /* accuracy of timestamps */
    uint32_t     snaplen;    /* max length saved portion of each pkt */
    uint32_t     linktype;   /* data link type (LINKTYPE_*) */
};

struct pcap_pkthdr {
    uint32_t       sec;    /* time stamp */
    uint32_t       usec;   /* time stamp */
    uint32_t       caplen; /* length of portion present */
    uint32_t       len;    /* length this packet (off wire) */
};

int copy_header(FILE *src, FILE*dst)
{
    uint64_t rc;
    struct pcap_file_header fh;

    fread(&fh, 1, sizeof(struct pcap_file_header), stdin);
    if(0xA1B2C3D4 != (uint64_t)fh.magic)
    {
        fprintf(stderr, "Bad file Magic %08X\n", fh.magic);
        abort();
    }

    fh.version_major = PCAP_VERSION_MAJOR;
    fh.version_minor = PCAP_VERSION_MINOR;
    fh.snaplen       = ntohl(1024);
    fh.linktype      = DLT_EN10MB;

    rc = fwrite(&fh, 1, sizeof(struct pcap_file_header), dst);
    if(sizeof(struct pcap_file_header) != rc)
    {
        perror("write");
        abort();
    }

    return 0;
}


int main(int argc, char**args)
{
    int      rc;
    int      len;
    struct   pcap_pkthdr *pkt;
    char     buff[1024*1024];

    pkt = (struct pcap_pkthdr*)buff;

    copy_header(stdin, stdout);

    while(1)
    {
        rc = fread (buff, 1, sizeof(struct pcap_pkthdr), stdin);
        if(rc != sizeof(struct pcap_pkthdr))
        {
            fprintf(stderr,"END OF FILE\n");
            break;
        }

        len          = pkt->len;
        pkt->caplen += 14;
        pkt->len    += 14;

        rc = fwrite(buff,      1, sizeof(struct pcap_pkthdr), stdout);
        if(rc != sizeof(struct pcap_pkthdr))
        {
            fprintf(stderr, "ERROR write pkt_hdr\n");
            abort();
        }

        rc = fwrite(ETH_DMAC,  1, 6, stdout);
        if(rc != 6)
        {
            fprintf(stderr, "ERROR write DMAC\n");
            abort();
        }

        rc = fwrite(ETH_SMAC,  1, 6, stdout);
        if(rc != 6)
        {
            fprintf(stderr, "ERROR write SMAC\n");
            abort();
        }

        rc = fwrite(ETH_PROTO, 1, 2, stdout);
        if(rc != 2)
        {
            fprintf(stderr, "ERROR write Proto\n");
            abort();
        }

        rc = fread (buff, 1, len, stdin);
        if(rc != len)
        {
            fprintf(stderr, "ERROR fread rc=%d\n", rc);
            abort();
        }

        fwrite(buff, 1, len, stdout);
    }
    return 0;
}

使用方法：

gcc pcap_ip_to_eth.c
cat ip_p2p.pcap | ./a.out > padding_ETH_MAC.pcap