复制通过网络接收的数据（数据包有效负载）

Question

我有一个通过网络接收数据包的应用程序。我能够解释除有效负载（数据）部分之外的所有字段值。

考虑一个数据包结构： 源地址 |源端口 |目标地址 |目的港 |数据 |校验和

数据包本身存储为 char*，我可以获得所有值 src/des addr, port。但是有效载荷部分让我感到困惑。我能够以十六进制格式打印它，但以 char 打印它会给出非 ascii 值。当我将有效负载部分复制到新字符串并打印新字符串时，我什么也没有得到，即它不会打印任何内容并且它的长度为 0。我没有收到任何错误或警告。有人可以告诉我哪里出错了吗？我想我在这里遗漏了一些东西。

下面是这段代码：

  typedef struct pcap_802_15_4_s{
   guint16 fcf;
   unsigned char seqno;
   guint16 dpan;
   guint16 daddr;
   guint16 saddr;
   char *payload_data;  
  } pcap_802_15_4_t;


char *packet = read_serial_packet(src, &len);


      if (!packet)
    exit(0);
      else if (packet[0] != TOS_SERIAL_802_15_4_ID) {
    printf("bad packet (serial type is %02x, not %02x)\n", packet[0], TOS_SERIAL_802_15_4_ID);
      }

      plen = packet[1];
      printf("Received packet of length %i: \n", plen);
      if (plen != len) {
    printf("Packet format error: read packet length (%hhx) is different than expected from frame (%hhx).\n", plen, len);
      }

      struct timeval tv;    
      gettimeofday(&tv, NULL);


      i = 2;
      // Read in FCF and i+=2
      fcf = packet[i+1] << 8 | packet[i];
      packet_802_15_4.fcf = fcf;
      i += 2;


      {
    if ((fcf & 0x7) == 0x01) {
      printf("  Frame type: data\n");
    }
    else if ((fcf & 0x7) == 0x02) {
      printf("  Frame type: acknowledgement\n");
    }
    else {
      printf("  Frame type: other\n");
    }

    printf("  Security: %s\n", (fcf & (1 << 3)) ? "enabled":"disabled");
    printf("  Frame pending: %s\n", (fcf & (1 << 4)) ? "yes":"no");
    printf("  Ack request: %s\n", (fcf & (1 << 5)) ? "yes":"no");
    printf("  Intra-PAN: %s\n", (fcf & (1 << 6)) ? "yes":"no");
    intraPan = (fcf & (1 << 6));
      }


      {
    unsigned char seqno = packet[i++];
    packet_802_15_4.seqno = seqno;
    printf("  Sequence number: 0x%hhx\n", seqno);
    printf("  Sequence number(dump): 0x%hhx\n", packet_802_15_4.seqno);

      }

      {
    char addrLen = (fcf >> 10) & 0x3;
    short saddr = 0;
    long long laddr = 0;

    // 16- and 64-bit destinations have a PAN ID
    if (addrLen == 2 || addrLen == 3) { 
      short destPan = packet[i++] << 8 | packet[i++];
      packet_802_15_4.dpan = destPan;
      printf("  Destination PAN: 0x%02hx\n", destPan);
      printf("  Destination PAN (dump): 0x%02hx\n", packet_802_15_4.dpan);
    }

    switch (addrLen) {
    case 0:
      printf("  Destination address: none\n");
      break;
    case 1:
      printf("  Destination address: invalid? (0x01)\n");
      break;
    case 2:
      saddr =  (packet[i] << 8 | packet[i+1]);
      i += 2;
      packet_802_15_4.daddr = saddr;
      printf("  Destination address: 0x%04hx\n", saddr);
      printf("  case 2: Destination address(dump): 0x%04hx\n", packet_802_15_4.daddr);
      break;
    case 3: {
      int j;
      for (j = 0; j < 8; j++) {
        laddr = laddr << 8;
        laddr |= packet[i++];
      }
      packet_802_15_4.daddr = saddr;
      printf("  Destination address: 0x%016llx\n", laddr);
      printf("  case 3: Destination address(dump): 0x%016llx\n", (long long unsigned int)packet_802_15_4.daddr);
      break;
    }
    default:
      printf("  Destination address: parse serror\n");
    }
      }


      {
    char addrLen = (fcf >> 14) & 0x3;
    short saddr = 0;
    long long laddr = 0;

    if (!intraPan) { // Intra-PAN packet
      short srcPan = packet[i] << 8 | packet[i+1];
      //packet_802_15_4.span = srcPan;
      i += 2;
      printf("  Source PAN: 0x%02hx\n", srcPan);
    }

    switch (addrLen) {
    case 0:
      printf("  Source address: none\n");
      break;
    case 1:
      printf("  Source address: invalid? (0x01)\n");
      break;
    case 2:
      saddr =  (packet[i] << 8 | packet[i + 1]);
      packet_802_15_4.saddr = saddr;
      i += 2;
      printf("  case(2): Source address: 0x%04hx\n", saddr);
      break;
    case 3: {
      int j;
      for (j = 0; j < 8; j++) {
        laddr = laddr << 8;
        laddr |= packet[i++];
      }
      printf("  case(3): Source address: 0x%016llx\n", laddr);
      packet_802_15_4.saddr = saddr;
      break;
    }
    default:
      printf("  Source address: parse serror\n");
    }
      }

      if (iframes) {
    printf("  I-Frame: %s\n", (packet[i++] == 0x3f)? "yes":"no");
      }

      printf("  AM type: 0x%02hhx\n", packet[i++]);

      if (i >= plen) {
    printf("Packet format error: read packet is shorter than expected.\n");
      }
      else {
        int j=0;
    packet_802_15_4.payload_data = (char *)malloc(sizeof(char) * (plen - i));
    for (; i < plen; i++) {
      printf("Payload byte %d: 0x%02hhx \n",i,packet[i]);
      printf("Payload byte %d: %c \n",i,packet[i]);
      packet_802_15_4.payload_data[j] = (char) packet[i];
      j++;  
    }

有效载荷的十六进制输出： 0x00 0x01 0x10 0xdd

Answer 1

1) 如果您有 Wireshark，请捕获几个数据包并检查那里的有效负载表示。如果 Wireshark 正确显示您的有效负载，则代码的接收器部分肯定有问题。

2) 您确定您没有任何 IP 标头或 TCP 标头吗？您在传输原始以太网吗？

3) 你确定有效载荷只是字符吗？