c语言实现ip分片重组

以下是一个简单的C语言实现IP分片和重组的示例代码，仅供参考：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

#define IP_HEADER_LEN 20 // IP header length
#define MAX_PACKET_LEN 65535 // maximum packet length

typedef struct ip_header {
    uint8_t ver_ihl; // Version (4 bits) + Internet header length (4 bits)
    uint8_t tos; // Type of service 
    uint16_t total_len; // Total length 
    uint16_t id; // Identification
    uint16_t frag_off; // Fragment offset field
    uint8_t ttl; // Time to live
    uint8_t protocol; // Protocol
    uint16_t checksum; // Header checksum
    uint32_t src_addr; // Source address
    uint32_t dest_addr; // Destination address
} ip_header_t;

void fragment_packet(uint8_t *packet, uint32_t len, uint32_t mtu) {
    // check packet length
    if (len > MAX_PACKET_LEN) {
        printf("Packet is too long!\n");
        return;
    }

    // check MTU
    if (mtu < IP_HEADER_LEN + 8) {
        printf("MTU is too small!\n");
        return;
    }

    // calculate number of fragments
    uint32_t num_frags = (len + mtu - 1) / mtu;
    if (num_frags > 65535) {
        printf("Too many fragments!\n");
        return;
    }

    // create IP header for each fragment
    ip_header_t *headers[num_frags];
    for (uint32_t i = 0; i < num_frags; i++) {
        headers[i] = malloc(IP_HEADER_LEN);
        memset(headers[i], 0, IP_HEADER_LEN);
        ip_header_t *header = headers[i];
        header->ver_ihl = 0x45; // IPv4, 5 * 4 = 20 bytes header length
        header->total_len = htons(mtu);
        header->id = htons((uint16_t) rand());
        header->frag_off = htons(i * (mtu - IP_HEADER_LEN) / 8);
        if (i == num_frags - 1) {
            header->frag_off |= htons(0x2000); // last fragment
        }
        header->ttl = 64;
        header->protocol = 17; // UDP
        header->src_addr = 0x0100007f; // localhost
        header->dest_addr = 0x0100007f; // localhost
        // calculate header checksum
        uint32_t sum = 0;
        for (int j = 0; j < IP_HEADER_LEN; j += 2) {
            sum += ((uint16_t) header[j]) << 8 | header[j + 1];
        }
        while (sum >> 16) {
            sum = (sum & 0xffff) + (sum >> 16);
        }
        header->checksum = htons(~sum);
    }

    // fragment packet
    uint32_t offset = 0;
    for (uint32_t i = 0; i < num_frags; i++) {
        uint32_t frag_len = mtu - IP_HEADER_LEN;
        if (offset + frag_len > len) {
            frag_len = len - offset;
        }
        memcpy(headers[i] + IP_HEADER_LEN, packet + offset, frag_len);
        offset += frag_len;
    }

    // send fragments
    for (uint32_t i = 0; i < num_frags; i++) {
        ip_header_t *header = headers[i];
        uint32_t frag_len = ntohs(header->total_len);
        uint8_t *fragment = malloc(frag_len);
        memcpy(fragment, header, IP_HEADER_LEN);
        memcpy(fragment + IP_HEADER_LEN, header + IP_HEADER_LEN, frag_len - IP_HEADER_LEN);
        printf("Sending fragment %d...\n", i);
        // send fragment over network
        free(fragment);
    }

    // free headers
    for (uint32_t i = 0; i < num_frags; i++) {
        free(headers[i]);
    }
}

void reassemble_packet(ip_header_t **headers, uint32_t num_frags) {
    uint32_t packet_len = 0;
    for (uint32_t i = 0; i < num_frags; i++) {
        packet_len += ntohs(headers[i]->total_len) - IP_HEADER_LEN;
    }
    uint8_t *packet = malloc(packet_len);
    uint32_t offset = 0;
    for (uint32_t i = 0; i < num_frags; i++) {
        uint32_t frag_len = ntohs(headers[i]->total_len) - IP_HEADER_LEN;
        memcpy(packet + offset, headers[i] + IP_HEADER_LEN, frag_len);
        offset += frag_len;
    }
    printf("Reassembled packet:\n");
    // process reassembled packet
    free(packet);
}

int main() {
    srand(time(NULL));
    uint8_t packet[] = {0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa};
    uint32_t len = sizeof(packet);
    uint32_t mtu = 1472;
    fragment_packet(packet, len, mtu);
    // assume fragments are received in order and all belong to the same packet
    uint32_t num_frags = 3;
    ip_header_t *headers[num_frags];
    uint16_t ids[] = {12345, 12345, 12345};
    uint16_t offsets[] = {0, 185, 370};
    for (uint32_t i = 0; i < num_frags; i++) {
        headers[i] = malloc(IP_HEADER_LEN);
        memset(headers[i], 0, IP_HEADER_LEN);
        ip_header_t *header = headers[i];
        header->ver_ihl = 0x45;
        header->total_len = htons(1480);
        header->id = htons(ids[i]);
        header->frag_off = htons(offsets[i] / 8);
        if (i == num_frags - 1) {
            header->frag_off |= htons(0x2000);
        }
        header->ttl = 64;
        header->protocol = 17;
        header->src_addr = 0x0100007f;
        header->dest_addr = 0x0100007f;
    }
    reassemble_packet(headers, num_frags);
    // free headers
    for (uint32_t i = 0; i < num_frags; i++) {
        free(headers[i]);
    }
    return 0;
}

该示例代码实现了IP分片和重组的基本功能，可以按照指定的MTU将一个IP数据包分成多个片段，并将这些片段发送到网络上；同时，也可以接收多个IP分片并将它们重组成原始数据包。这个示例代码可以作为学习IP分片和重组的入门教程，但实际应用中还需要考虑更多的细节和安全问题。