UDP校验和（checksum）原理

07-09 1326阅读

**思考：**为什么发送方校验数据（UDP首部校验和）为0，而接收方校验数据（UDP首部校验和）已填充校验和，双方校验数据不一样，不会出错吗？

****其实接收方是把计算累加和和校验和验证两个步骤合成了一个步骤，可以理解为接收方先计算校验数据（UDP首部校验和为0）的累加和，再把累加和UDP校验和相加。

图 1

3.UDP伪首部

**作用：**增加对IP地址等信息的校验，确保信息不出错。

图 2

字段分析：

源地址：源IP地址（4字节），网络字节序

目的地址：目的IP地址（4字节），网络字节序

保留：填0

协议名：UDP协议（17）

UDP报文长度：UDP数据包长度（包含UDP头部），网络字节序

伪头部自定义：

struct pseudo_udphdr {
    uint32_t saddr;
    uint32_t daddr;
    uint8_t fill;
    uint8_t protocol;
    uint16_t len;
};

4.UDP校验和示例代码

4.1 发送方示例代码

struct pseudo_udphdr {
    uint32_t saddr;
    uint32_t daddr;
    uint8_t fill;
    uint8_t protocol;
    uint16_t len;
};
uint16_t checksum(uint16_t *buf, int size) {
    register uint32_t sum = 0;
    while(size > 1) {
        sum += *(buf++);
        size -= 2;
    }
    while(sum >> 16) {
        sum = (sum >> 16) + (sum & 0xffff);
    }
    return ~sum;
}
uint32_t create_pack(char *buf, const char *payload, uint32_t payload_len) {
    struct ethhdr *eh = (struct ethhdr *)buf;
    memcpy(eh->h_dest, nexthop_mac, ETH_ALEN);
    memcpy(eh->h_source, src_mac, ETH_ALEN);
    eh->h_proto = htons(ETH_P_IP);
    struct iphdr *iph = (struct iphdr *)(buf + ETH_HLEN);
    iph->ihl = IP_HDRLEN / sizeof(uint32_t);
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len = htons(IP_HDRLEN + UDP_HDRLEN + payload_len);
    iph->id = htons(0);
    iph->frag_off = htons(0);
    iph->ttl = 255;
    iph->protocol = IPPROTO_UDP;
    iph->saddr = inet_addr(SRC_IP);
    iph->daddr = inet_addr(DST_IP);
    iph->check = 0;
    iph->check = checksum((uint16_t *)iph, IP_HDRLEN);
    struct udphdr *uh = (struct udphdr *)(buf + ETH_HLEN + sizeof(struct iphdr));
    uh->uh_sport = htons(SPORT);
    uh->uh_dport = htons(DPORT);
    uh->uh_ulen = htons(UDP_HDRLEN + payload_len);
    uint32_t udp_len = UDP_HDRLEN + payload_len;
    char checksum_buf[MAX_BUF_SIZE] = {0};
    uint32_t checksum_len = sizeof(struct pseudo_udphdr) + udp_len;
    if (checksum_len % 2 != 0) checksum_len += 1;
    struct pseudo_udphdr puh = {
        .saddr = iph->saddr,
        .daddr = iph->daddr,
        .fill = 0,
        .protocol = IPPROTO_UDP,
        .len = uh->uh_ulen,
    };
    uh->uh_sum = 0;
    memcpy(checksum_buf, &puh, sizeof(struct pseudo_udphdr));
    memcpy(checksum_buf + sizeof(struct pseudo_udphdr), uh, UDP_HDRLEN);
    memcpy(checksum_buf + sizeof(struct pseudo_udphdr) + UDP_HDRLEN, payload, payload_len);
    uh->uh_sum = checksum((uint16_t *)checksum_buf, checksum_len);
    printf("udp checksum:%04x\n", uh->uh_sum);
    memcpy(buf + ETH_HLEN + sizeof(struct iphdr) + sizeof(struct udphdr), payload, payload_len);
    return ETH_HLEN + IP_HDRLEN + UDP_HDRLEN + payload_len;
}

4.2 接收方示例代码

struct pseudo_udphdr {
    uint32_t saddr;
    uint32_t daddr;
    uint8_t fill;
    uint8_t protocol;
    uint16_t len;
};
uint16_t checksum_nofold(uint16_t *buf, int size) {
    register uint32_t sum = 0;
    while(size > 1) {
        sum += *(buf++);
        size -= 2;
    }
    while(sum >> 16) {
        sum = (sum >> 16) + (sum & 0xffff);
    }
    return sum;
}
int isvalid(const char *buf) {
    struct iphdr *iph = (struct iphdr *)(buf + ETH_HLEN);
    struct udphdr *uh = (struct udphdr *)(buf + ETH_HLEN + sizeof(struct iphdr));
    char checksum_buf[MAX_BUF_SIZE] = {0};
    uint32_t checksum_len = sizeof(struct pseudo_udphdr) + ntohs(uh->uh_ulen);
    if (checksum_len % 2 != 0) checksum_len += 1;
    struct pseudo_udphdr puh = {
        .saddr = iph->saddr,
        .daddr = iph->daddr,
        .fill = 0,
        .protocol = IPPROTO_UDP,
        .len = uh->uh_ulen,
    };
    memcpy(checksum_buf, &puh, sizeof(struct pseudo_udphdr));
    memcpy(checksum_buf + sizeof(struct pseudo_udphdr), (char *)uh, ntohs(uh->uh_ulen));
    uint16_t csum = checksum_nofold((uint16_t *)checksum_buf, checksum_len);
    printf("udp csum:%04x\n", csum);
    if (csum != 0xffff) return -1;
    return 0;
}