TCP校验和C代码返回错误结果

TCP checksum C code returns wrong result

在假定应该为0的情况下，我得到了一个较大的TCP校验和结果。我正在通过将TCP psuedoheader复制到数组的前12个字节中，然后将TCP标头和数据复制到接下来的几个方法中来解决此问题。数组的字节，然后将该数组及其长度传递给校验和函数。我无法弄清为什么校验和与正确值显着不同(例如，其中一个数据包数据包应为0时为9180)。

这是我的IP，TCP和伪TCP标头：

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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;

这是我将数据复制到数组中的方式：

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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式：

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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);
}

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，在tcp报头中将校验和字段设置为零。然后将计算出的校验和与先前保存的值进行匹配。如果网络传输中没有错误，则两者应该相等。
这样做

1 2	recvPktChkSum = tcp->checksum tcp->checksum = 0;

在计算校验和之前，还应确保长度以网络字节顺序排列

1	ts.len = htons(checkLen);

之前

1	memcpy(checkData + 12, tcp, checkLen);

然后在

之后

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

比较

1 2	if(htons(recvPktChkSum) == htons(checksum)) printf("checksum valid");