為甚麼tcp檢驗和老是算不對？
本帖最后由 laxend 于 2014-08-10 15:29 编辑10.10.99.100.43333 > www2635.sakura.ne.jp.http: Flags , cksum 0xa6c2 (incorrect -> 0xa6b0), seq 123456789, win 8192, options , length 0
上面是tcpdump的輸出結果
總是a6c2，而不是a6b0，yun
下面是計算檢驗和的代碼（ip頭部，tcp選項什麼的就不貼出來了，沒問題的）
struct tcp_psd_hdr {
struct in_addr p_src;
struct in_addrp_dst;
uint8_t p_zero;
uint8_t p_proto;
uint16_t p_off;
};
int build_tcp_raw(struct tcphdr *tcp, int sport, int dport, struct in_addr *src, struct in_addr *dst) {
char *ptr;
struct tcp_opt *opt;
char sumbuf;
struct tcp_psd_hdr *tph;
tcp->th_sport = htons(43333);
tcp->th_dport = htons(dport);
tcp->th_seq = htonl(123456789);
tcp->th_ack = htonl(0);
tcp->th_off = 6;
tcp->th_flags = TH_SYN;
tcp->th_win = htons(8192);
tcp->th_sum = 0;
tcp->th_urp = 0;
ptr = (char *)tcp + sizeof(struct tcphdr);
opt = (struct tcp_opt*)ptr;
build_tcp_opt_mss(opt);
tph = (struct tcp_psd_hdr*)sumbuf;
if (src != NULL)
tph->p_src = *src;
else
tph->p_src.s_addr = inet_addr("10.10.99.100");
tph->p_dst = *dst;
tph->p_proto = IPPROTO_TCP;
tph->p_zero = 0;
tph->p_off = htons(6);
memcpy(sumbuf+sizeof(*tph), tcp, sizeof(*tcp));
memcpy(sumbuf+sizeof(*tph)+sizeof(*tcp), opt, sizeof(*opt));
tcp->th_sum = in_cksum((uint16_t *)sumbuf, sizeof(sumbuf));
//tcp->th_sum = 0xb0a6;
printf("TCP: %x\n", tcp->th_sum);
return 0;
}
到底是哪裏有問題？求教，感激不盡。
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