用c写一个MTU测试代码

好的，以下是一个简单的MTU测试代码的示例，使用C语言编写：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <errno.h>

#define PACKET_SIZE 4096
#define MAX_WAIT_TIME 5
#define MAX_NO_PACKETS 3

char sendpacket[PACKET_SIZE];
char recvpacket[PACKET_SIZE];
int sockfd, datalen = 56;
int nsend = 0, nreceived = 0;
struct sockaddr_in dest_addr;
pid_t pid;

void send_packet();
void recv_packet();
void tv_sub(struct timeval *out, struct timeval *in);
unsigned short cal_chksum(unsigned short *addr, int len);

int main(int argc, char *argv[])
{
    if (argc != 2) {
        printf("usage: %s <IP address>\n", argv[0]);
        return 1;
    }

    memset(&dest_addr, 0, sizeof(dest_addr));
    dest_addr.sin_family = AF_INET;
    dest_addr.sin_addr.s_addr = inet_addr(argv[1]);

    pid = getpid();

    printf("MTU test begin...\n");

    sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
    if (sockfd < 0) {
        printf("socket error: %s\n", strerror(errno));
        return 1;
    }

    send_packet();
    recv_packet();

    close(sockfd);

    return 0;
}

void send_packet()
{
    int packetsize;
    struct icmp *icmp;
    struct timeval *tval;

    icmp = (struct icmp *)sendpacket;
    icmp->icmp_type = ICMP_ECHO;
    icmp->icmp_code = 0;
    icmp->icmp_cksum = 0;
    icmp->icmp_id = pid;

    tval = (struct timeval *)icmp->icmp_data;
    gettimeofday(tval, NULL);

    packetsize = 8 + datalen;
    icmp->icmp_cksum = cal_chksum((unsigned short *)icmp, packetsize);

    nsend++;
    sendto(sockfd, sendpacket, packetsize, 0, (struct sockaddr *)&dest_addr, sizeof(dest_addr));
}

void recv_packet()
{
    int n, fromlen;
    struct timeval tvrecv;
    struct ip *ip;
    struct icmp *icmp;
    struct timeval *tvsend;
    double rtt;

    fromlen = sizeof(dest_addr);
    while (nreceived < MAX_NO_PACKETS) {
        fd_set rfds;
        struct timeval tv;
        int retval;

        FD_ZERO(&rfds);
        FD_SET(sockfd, &rfds);

        tv.tv_sec = MAX_WAIT_TIME;
        tv.tv_usec = 0;

        retval = select(sockfd + 1, &rfds, NULL, NULL, &tv);
        if (retval == -1) {
            printf("select error: %s\n", strerror(errno));
            return;
        } else if (retval == 0) {
            printf("timeout\n");
            return;
        }

        n = recvfrom(sockfd, recvpacket, sizeof(recvpacket), 0, (struct sockaddr *)&dest_addr, &fromlen);
        if (n < 0) {
            printf("recvfrom error: %s\n", strerror(errno));
            return;
        }

        ip = (struct ip *)recvpacket;
        icmp = (struct icmp *)(recvpacket + (ip->ip_hl << 2));
        tvsend = (struct timeval *)icmp->icmp_data;
        tv_sub(&tvrecv, tvsend);

        rtt = tvrecv.tv_sec * 1000.0 + tvrecv.tv_usec / 1000.0;

        printf("%d bytes from %s: icmp_seq=%u ttl=%d time=%.3f ms\n",
               n, inet_ntoa(dest_addr.sin_addr), icmp->icmp_seq, ip->ip_ttl, rtt);

        nreceived++;
    }
}

void tv_sub(struct timeval *out, struct timeval *in)
{
    if ((out->tv_usec -= in->tv_usec) < 0) {
        --out->tv_sec;
        out->tv_usec += 1000000;
    }
    out->tv_sec -= in->tv_sec;
}

unsigned short cal_chksum(unsigned short *addr, int len)
{
    int nleft = len;
    int sum = 0;
    unsigned short *w = addr;
    unsigned short answer = 0;

    while (nleft > 1) {
        sum += *w++;
        nleft -= 2;
    }

    if (nleft == 1) {
        *(unsigned char *)(&answer) = *(unsigned char *)w;
        sum += answer;
    }

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

    return answer;
}

这个代码可以用来测试给定IP地址的最大传输单元（MTU）。它使用ICMP协议发送一个带有不同大小负载的数据包，并在每个数据包的响应中计算往返时间（RTT）。通过逐步减小负载大小，可以确定最大的负载大小，这就是MTU。