c - TCP校验和C代码返回错误结果

Question

我得到一个很大的 TCP 校验和结果，而它本应为 0。我通过将 TCP psuedoheader 复制到数组的前 12 个字节，然后将 TCP header 和数据复制到接下来的几个字节来解决这个问题数组的字节，并将该数组及其长度传递给校验和函数。我无法弄清楚为什么校验和与正确值有很大不同(即其中一个数据包的 9180 本应为 0)。

这是我的 IP、TCP 和伪 TCP header :

typedef struct __attribute__((__packed__)) IPHeader {
  #if __BYTE_ORDER__ == __LITTLE_ENDIAN__
  uint8_t hdrLen:4;
  uint8_t version:4;
  #else
  uint8_t version:4;
  uint8_t hdrLen:4;
  #endif
  uint8_t TOS;
  uint16_t totLen;
  uint16_t id;
  uint16_t offset;
  #define DF 0x4            
  #define MF 0x2           
  #define OFF 0 
  uint8_t TTL;
  uint8_t protocol;
  uint16_t checksum;
  struct in_addr srcIP;
  struct in_addr destIP; 
}IPHeader;

typedef struct __attribute__((__packed__)) TCPHeader {
  uint16_t srcPort;
  uint16_t destPort;
  uint32_t seqNum;
  uint32_t ackNum;
  uint8_t offset:4;
  uint8_t res:4;
  uint8_t flags;
  uint16_t window;
  uint16_t checksum;
  uint16_t urg;
}TCPHeader;

typedef struct __attribute__((__packed__)) TCPPseudo {
  struct in_addr srcAddr;
  struct in_addr destAddr;
  uint8_t zeroes;
  uint8_t protocol;
  uint16_t len;
}TCPPseudo;

下面是我如何将数据复制到数组中:

IPHeader *iph
TCPPseudo ts;
TCPHeader *tcp;
u_char checkData[1000];
uint16_t checkLen = 0;
---
tcp = (TCPHeader *)(packet + ETHER_SIZE + iph->hdrLen * 4);
ts.srcAddr = iph->srcIP;
ts.destAddr = iph->destIP;
ts.zeroes = 0;
ts.protocol = iph->protocol;
checkLen = ntohs(iph->totLen) - (iph->hdrLen * 4);
ts.len = checkLen;
memcpy(checkData, &ts, 12);
memcpy(checkData + 12, tcp, checkLen);
getTCPInfo(tcp, iph, checkData, checkLen);
---
unsigned short checksum = in_cksum((unsigned short *)checkData, len + 12);
printf("\t\tChecksum: %d, (0x%x)", checksum, htons(tcp->checksum)); 

这是我计算校验和的方法:

unsigned short in_cksum(unsigned short *addr,int len)
{
    register int sum = 0;
    u_short answer = 0;
    register u_short *w = addr;
    register int nleft = len;

    /*
     * Our algorithm is simple, using a 32 bit accumulator (sum), we add
     * sequential 16 bit words to it, and at the end, fold back all the
     * carry bits from the top 16 bits into the lower 16 bits.
     */
    while (nleft > 1)  {
            sum += *w++;
            nleft -= 2;
    }

    /* mop up an odd byte, if necessary */
    if (nleft == 1) {
            *(u_char *)(&answer) = *(u_char *)w ;
            sum += answer;
    }

    /* add back carry outs from top 16 bits to low 16 bits */
    sum = (sum >> 16) + (sum & 0xffff);     /* add hi 16 to low 16 */
    sum += (sum >> 16);                     /* add carry */
    answer = ~sum;                          /* truncate to 16 bits */
    return(answer);
}

Answer 1

您应该保存接收到的数据包的 TCP 校验和，然后在计算伪报头和 TCP 段的校验和之前，将 tcp header 中的校验和字段设置为零。然后将计算出的校验和与之前保存的值进行匹配。如果网络传输没有错误，两者应该相等。这样做

recvPktChkSum = tcp->checksum
tcp->checksum = 0;

同样在计算校验和之前你应该确保长度是网络字节顺序

ts.len = htons(checkLen);

之前

memcpy(checkData + 12, tcp, checkLen);

之后

checksum = in_cksum((unsigned short *)checkData, len + 12);

比较

if(htons(recvPktChkSum) == htons(checksum))
   printf("checksum valid");