icmp的校验和计算代码

以下是一个简单的 ICMP 校验和计算代码示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netinet/ip_icmp.h>
#include <netinet/ip.h>

unsigned short checksum(unsigned short *ptr, int nbytes) {
    register long sum;
    unsigned short oddbyte;
    register short answer;

    sum = 0;
    while (nbytes > 1) {
        sum += *ptr++;
        nbytes -= 2;
    }

    if (nbytes == 1) {
        oddbyte = 0;
        *((unsigned char *) &oddbyte) = *(unsigned char *) ptr;
        sum += oddbyte;
    }

    sum = (sum >> 16) + (sum & 0xffff);
    sum += (sum >> 16);
    answer = (short) ~sum;

    return answer;
}

int main() {
    char buffer[1024];
    memset(buffer, 0, sizeof(buffer));

    struct iphdr *ip_header = (struct iphdr *) buffer;
    struct icmphdr *icmp_header = (struct icmphdr *) (buffer + sizeof(struct iphdr));

    ip_header->ihl = 5;
    ip_header->version = 4;
    ip_header->tos = 0;
    ip_header->tot_len = sizeof(struct iphdr) + sizeof(struct icmphdr);
    ip_header->id = htons(54321);
    ip_header->frag_off = 0;
    ip_header->ttl = 255;
    ip_header->protocol = IPPROTO_ICMP;
    ip_header->saddr = inet_addr("192.168.1.1");
    ip_header->daddr = inet_addr("192.168.1.2");

    icmp_header->type = ICMP_ECHO;
    icmp_header->code = 0;
    icmp_header->un.echo.id = 0;
    icmp_header->un.echo.sequence = 0;
    icmp_header->checksum = 0;

    icmp_header->checksum = checksum((unsigned short *) icmp_header, sizeof(struct icmphdr));

    printf("ICMP checksum: %d\n", icmp_header->checksum);

    return 0;
}

这段代码演示了如何计算 ICMP 校验和。它首先构造了一个 IP 头和一个 ICMP 头，然后调用 `checksum` 函数计算 ICMP 校验和。