TCP校驗和為0

Question

我有tcp標頭的以下代碼：

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));
}

我得到的輸出是

CHecksum是0，所以我認為有些事情是錯誤的。 你們能發現錯誤嗎？ 或者canchecksum應該是0？

sourceport也可以22？

Answer 1

這里使用的大多數轉換說明符都是錯誤的：

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));

它應該是：

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));

如果你將所有這些轉換刪除為unsigned int你可以使用hhu作為printf()轉換說明符，因為所有這些變量都被定義為unsigned 8位值。

Answer 2

您的代碼有一些問題：

• 不要使用位域！ 它們不能在不同的ABI中移植。
• 確保結構被打包（例如通過__attribute__((__packed__))注釋）。 ABI可以在其他屬性之間添加填充。
• tcp_hdr = (const struct tcp_header *)(header_start在讀取/寫入tcp_hdr字段時可能導致錯位訪問。上面的__packed__注釋可以防止這種情況;或者，您可以復制（正確對齊的）局部變量中的內容。

我會添加一些健全性檢查

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

（請檢查tcp rfc tcp_sum是否確實位於0x10位置！）

Answer 3

我認為你的TCP頭定義有問題（這是24字節，但標准頭是20）。 在系統上使用標准聲明，也考慮對齊：

#include <netinet/tcp.h>

tcp頭定義：

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 */
};

Answer 4

入境或出境？ 如果啟用了校驗和卸載到NIC，則出站校驗和可以為0 ...