C语言ip分片程序

以下是一个简单的C语言IP分片程序的示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>

#define MAX_PACKET_SIZE 65535
#define MAX_FRAGMENT_SIZE 1480

struct ipheader {
    unsigned char version_and_header_length;
    unsigned char differentiated_services;
    unsigned short total_length;
    unsigned short identification;
    unsigned short flags_and_fragment_offset;
    unsigned char time_to_live;
    unsigned char protocol;
    unsigned short header_checksum;
    unsigned int source_address;
    unsigned int destination_address;
};

int main(int argc, char *argv[]) {
    if (argc != 3) {
        printf("Usage: %s <source file> <destination file>\n", argv[0]);
        return 0;
    }

    FILE *input_file = fopen(argv[1], "rb");
    if (input_file == NULL) {
        printf("Unable to open input file %s\n", argv[1]);
        return 0;
    }

    FILE *output_file = fopen(argv[2], "wb");
    if (output_file == NULL) {
        printf("Unable to open output file %s\n", argv[2]);
        fclose(input_file);
        return 0;
    }

    unsigned char packet[MAX_PACKET_SIZE];
    int packet_size = fread(packet, 1, MAX_PACKET_SIZE, input_file);
    if (packet_size <= 0) {
        printf("Error reading input file %s\n", argv[1]);
        fclose(input_file);
        fclose(output_file);
        return 0;
    }

    struct ipheader *ip_header = (struct ipheader *)packet;
    if (ip_header->version_and_header_length >> 4 != 4) {
        printf("Input file %s does not contain an IPv4 packet\n", argv[1]);
        fclose(input_file);
        fclose(output_file);
        return 0;
    }

    int total_length = ntohs(ip_header->total_length);
    if (total_length <= MAX_FRAGMENT_SIZE) {
        fwrite(packet, 1, packet_size, output_file);
    } else {
        int fragment_offset = 0;
        while (fragment_offset < total_length) {
            int fragment_size = total_length - fragment_offset;
            if (fragment_size > MAX_FRAGMENT_SIZE) {
                fragment_size = MAX_FRAGMENT_SIZE;
            }

            unsigned char *fragment = packet + fragment_offset;
            struct ipheader *fragment_header = (struct ipheader *)fragment;
            fragment_header->total_length = htons(fragment_size + sizeof(struct ipheader));
            fragment_header->flags_and_fragment_offset = htons((fragment_offset >> 3) & 0x1fff);
            if (fragment_offset + fragment_size >= total_length) {
                fragment_header->flags_and_fragment_offset |= htons(1 << 13);
            }

            unsigned short old_checksum = fragment_header->header_checksum;
            fragment_header->header_checksum = 0;
            unsigned int sum = 0;
            unsigned short *data = (unsigned short *)fragment;
            for (int i = 0; i < sizeof(struct ipheader) / 2; i++) {
                sum += ntohs(data[i]);
            }
            while (sum >> 16) {
                sum = (sum & 0xffff) + (sum >> 16);
            }
            fragment_header->header_checksum = htons(~sum);

            fwrite(fragment, 1, fragment_size + sizeof(struct ipheader), output_file);
            fragment_offset += fragment_size;
        }
    }

    fclose(input_file);
    fclose(output_file);
    return 0;
}

这个程序接受两个参数，一个是输入文件名，另一个是输出文件名。它从输入文件中读取一个IPv4数据包，如果数据包长度小于等于最大分片大小（1480字节），则直接将整个数据包写入输出文件；否则，将数据包分成多个分片，并将每个分片写入输出文件。在分片过程中，程序会设置每个分片的总长度、分片偏移和标志位，并重新计算IP头部校验和。