dpvs 调整tcp mss

修改tcp options中mss值

src/ipvs/ip_vs_proto_tcp.c

因为tcp头部options中不同kind顺序是随机的，所以需要遍历找到kind值是mss 2和length值是4，再修改后面的mss value。

static void tcp_out_adjust_mss(int af, struct tcphdr *tcph)
{
    unsigned char *ptr;
    int length;

    if (unlikely(af != AF_INET && af != AF_INET6)) {
        RTE_LOG(DEBUG, IPVS, "adjust mss: unknow af, af : %d.\n",
                af);
        return ;
    }

    ptr = (unsigned char *)(tcph + 1);
    length = (tcph->doff << 2) - sizeof(struct tcphdr);

    while (length > 0) {
        int opcode = *ptr++;
        int opsize;

        switch (opcode) {
        case TCP_OPT_EOL:
            return;
        case TCP_OPT_NOP:
            length--;
            continue;
        default:
            opsize = *ptr++;
            if (opsize < 2)    /* "silly options" */
                return;
            if (opsize > length)
                return;    /* partial options */
            if ((opcode == TCP_OPT_MSS) && (opsize == TCP_OLEN_MSS)) {
                uint16_t in_mss = ntohs(*(__be16 *) ptr);

                in_mss -= (af == AF_INET ? TCP_OLEN_IP4_ADDR : TCP_OLEN_IP6_ADDR);

                /* set mss, 16bit */
                *((uint16_t *) ptr) = htons(in_mss);
                /* re-calc csum later */
                return;
            }

            ptr += opsize - 2;
            length -= opsize;
            break;
        }
    }
}