TCP校验和为0

我有tcp头的以下代码：

struct tcp_header {
    uint16_t tcp_sport;
    uint16_t tcp_dport;
    uint32_t tcp_th_seq;
    uint32_t tcp_ack;
    uint8_t tcp_off:4;
    uint8_t tcp_res:6;
    uint8_t tcp_uf:1, tcp_af:1, tcp_pf:1, tcp_rf:1, tcp_sf:1, tcp_ff:1;
    uint16_t tcp_win;
    uint16_t tcp_sum;
    uint16_t tcp_urp;
};

...

void decode_tcp(const unsigned char *header_start) {

    const struct tcp_header *tcp_hdr;

    tcp_hdr = (const struct tcp_header *)(header_start + ETHERNET_HEADER_SIZE + IP_HEADER_SIZE);

    printf("\n    TCP Header\n");

    printf("\tSource Port       : %u\n", ntohs(tcp_hdr->tcp_sport));
    printf("\tDestination Port  : %u\n", ntohs(tcp_hdr->tcp_dport));
    printf("\tSequence number   : %u\n", ntohl(tcp_hdr->tcp_th_seq));
    printf("\tAcknowledge number: %u\n", ntohl(tcp_hdr->tcp_ack));
    printf("\tOffset            : %d\n", tcp_hdr->tcp_off);
    printf("\tReserved          : %d\n", (unsigned int)tcp_hdr->tcp_res);
    printf("\tUrgent Flag       : %d\n", (unsigned int)tcp_hdr->tcp_uf);
    printf("\tAcknoledge Flag   : %d\n", (unsigned int)tcp_hdr->tcp_af);
    printf("\tPush Flag         : %d\n", (unsigned int)tcp_hdr->tcp_pf);
    printf("\tReset Flag        : %d\n", (unsigned int)tcp_hdr->tcp_rf);
    printf("\tSynchronise Flag  : %d\n", (unsigned int)tcp_hdr->tcp_sf);
    printf("\tFinish Flag       : %d\n", (unsigned int)tcp_hdr->tcp_ff);
    printf("\tWindow            : %d\n", ntohs(tcp_hdr->tcp_win));
    printf("\tChecksum          : %d\n", ntohs(tcp_hdr->tcp_sum));
    printf("\tUrgent Pointer    : %d", ntohs(tcp_hdr->tcp_urp));
}

我得到的输出是

校验和是0，所以我认为有些东西是错的。你们能发现错误吗？ 或者canchecksum应该是0

---

sourceport也可以是22吗？

此处使用的大多数转换说明符都是错误的：

printf("\tSource Port       : %u\n", ntohs(tcp_hdr->tcp_sport));
printf("\tDestination Port  : %u\n", ntohs(tcp_hdr->tcp_dport));
printf("\tSequence number   : %u\n", ntohl(tcp_hdr->tcp_th_seq));
printf("\tAcknowledge number: %u\n", ntohl(tcp_hdr->tcp_ack));
printf("\tOffset            : %d\n", tcp_hdr->tcp_off);
printf("\tReserved          : %d\n", (unsigned int)tcp_hdr->tcp_res);
printf("\tUrgent Flag       : %d\n", (unsigned int)tcp_hdr->tcp_uf);
printf("\tAcknoledge Flag   : %d\n", (unsigned int)tcp_hdr->tcp_af);
printf("\tPush Flag         : %d\n", (unsigned int)tcp_hdr->tcp_pf);
printf("\tReset Flag        : %d\n", (unsigned int)tcp_hdr->tcp_rf);
printf("\tSynchronise Flag  : %d\n", (unsigned int)tcp_hdr->tcp_sf);
printf("\tFinish Flag       : %d\n", (unsigned int)tcp_hdr->tcp_ff);
printf("\tWindow            : %d\n", ntohs(tcp_hdr->tcp_win));
printf("\tChecksum          : %d\n", ntohs(tcp_hdr->tcp_sum));
printf("\tUrgent Pointer    : %d", ntohs(tcp_hdr->tcp_urp));

应该是：

printf("\tSource Port       : %hu\n", ntohs(tcp_hdr->tcp_sport));
printf("\tDestination Port  : %hu\n", ntohs(tcp_hdr->tcp_dport));
printf("\tSequence number   : %u\n", ntohl(tcp_hdr->tcp_th_seq));
printf("\tAcknowledge number: %u\n", ntohl(tcp_hdr->tcp_ack));
printf("\tOffset            : %hhu\n", tcp_hdr->tcp_off);
printf("\tReserved          : %u\n", (unsigned int)tcp_hdr->tcp_res);
printf("\tUrgent Flag       : %u\n", (unsigned int)tcp_hdr->tcp_uf);
printf("\tAcknoledge Flag   : %u\n", (unsigned int)tcp_hdr->tcp_af);
printf("\tPush Flag         : %u\n", (unsigned int)tcp_hdr->tcp_pf);
printf("\tReset Flag        : %u\n", (unsigned int)tcp_hdr->tcp_rf);
printf("\tSynchronise Flag  : %u\n", (unsigned int)tcp_hdr->tcp_sf);
printf("\tFinish Flag       : %u\n", (unsigned int)tcp_hdr->tcp_ff);
printf("\tWindow            : %hu\n", ntohs(tcp_hdr->tcp_win));
printf("\tChecksum          : %hu\n", ntohs(tcp_hdr->tcp_sum));
printf("\tUrgent Pointer    : %hu", ntohs(tcp_hdr->tcp_urp));

---

如果您已将所有这些强制转换删除为

unsigned int

，则可以使用

hhu

作为强制转换的转换说明符，因为所有这些变量都定义为

unsigned

8位值。

您的代码存在一些问题：

• 不要使用位域！它们不能跨不同的ABI进行移植
• 确保结构已打包（例如，通过

  \uuuuu属性（（\uuuu packed\uuuu））

  注释）。ABI可以在其他属性之间添加填充

• tcp\u hdr=（const struct tcp\u header*）（header\u start

  在读取/写入

  tcp\u hdr

  字段时可能会导致访问不对齐。上面的

  \u packed\u

  注释可以防止这种情况；或者，您可以复制（正确对齐的）局部变量中的内容

我会添加一些健全的检查，如

BUILD_BUG_ON(offsetof(struct tcp_header, tcp_sum) != 0x10));

---

（请在tcp rfc中检查tcp_sum是否真的位于位置0x10！）

我认为您的tcp标头定义有问题（24字节，但标准标头为20）。在系统上使用标准声明，同时考虑对齐：

#include <netinet/tcp.h>

---

入站还是出站？如果启用了向NIC卸载校验和，则出站校验和可以为0…

没有任何更改

struct tcphdr {
    u_short th_sport;               /* source port */
    u_short th_dport;               /* destination port */
    tcp_seq th_seq;                 /* sequence number */
    tcp_seq th_ack;                 /* acknowledgement number */
#if __BYTE_ORDER == __LITTLE_ENDIAN
    u_int   th_x2:4,                /* (unused) */
        th_off:4;               /* data offset */
#endif
#if __BYTE_ORDER == __BIG_ENDIAN
    u_int   th_off:4,               /* data offset */
        th_x2:4;                /* (unused) */
#endif
    u_char  th_flags;
#define TH_FIN  0x01
#define TH_SYN  0x02
#define TH_RST  0x04
#define TH_PUSH 0x08
#define TH_ACK  0x10
#define TH_URG  0x20
#define TH_FLAGS (TH_FIN|TH_SYN|TH_RST|TH_ACK|TH_URG)

    u_short th_win;                 /* window */
    u_short th_sum;                 /* checksum */
    u_short th_urp;                 /* urgent pointer */
};