使用C语言实现tftp协议

TFTP（Trivial File Transfer Protocol）是一种简单的文件传输协议，常用于将文件从服务器传输到客户端。下面是一个使用C语言实现TFTP协议的例子：

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

#define TFTP_OPCODE_RRQ 1
#define TFTP_OPCODE_DATA 3
#define TFTP_OPCODE_ACK 4
#define TFTP_OPCODE_ERROR 5

#define TFTP_ERROR_UNDEFINED 0
#define TFTP_ERROR_FILE_NOT_FOUND 1
#define TFTP_ERROR_ACCESS_VIOLATION 2
#define TFTP_ERROR_DISK_FULL 3
#define TFTP_ERROR_ILLEGAL_OPERATION 4
#define TFTP_ERROR_UNKNOWN_TID 5
#define TFTP_ERROR_FILE_EXISTS 6
#define TFTP_ERROR_NO_SUCH_USER 7

#define TFTP_BLOCK_SIZE 512

struct tftp_packet {
    unsigned short opcode;
    union {
        struct {
            char filename[1];
            char mode[1];
        } rrq;
        struct {
            unsigned short block_num;
            char data[TFTP_BLOCK_SIZE];
        } data;
        struct {
            unsigned short block_num;
        } ack;
        struct {
            unsigned short error_code;
            char error_msg[1];
        } error;
    } data;
};

int main(int argc, char *argv[]) {
    if (argc != 4) {
        fprintf(stderr, "Usage: %s <ip> <port> <file>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    int sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock_fd < 0) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = inet_addr(argv[1]);
    server_addr.sin_port = htons(atoi(argv[2]));

    char *filename = argv[3];
    char mode[] = "octet";
    int filename_len = strlen(filename);
    int mode_len = strlen(mode);
    int packet_len = 2 + filename_len + 1 + mode_len + 1;

    char *packet_buf = malloc(packet_len);
    if (!packet_buf) {
        perror("malloc");
        exit(EXIT_FAILURE);
    }

    struct tftp_packet *packet = (struct tftp_packet *)packet_buf;
    packet->opcode = htons(TFTP_OPCODE_RRQ);
    memcpy(packet->data.rrq.filename, filename, filename_len);
    memcpy(packet->data.rrq.filename + filename_len, "\0", 1);
    memcpy(packet->data.rrq.mode, mode, mode_len);
    memcpy(packet->data.rrq.mode + mode_len, "\0", 1);

    int ret = sendto(sock_fd, packet_buf, packet_len, 0, (struct sockaddr *)&server_addr, sizeof(server_addr));
    if (ret < 0) {
        perror("sendto");
        exit(EXIT_FAILURE);
    }

    int block_num = 1;
    FILE *file = fopen(filename, "wb");
    if (!file) {
        perror("fopen");
        exit(EXIT_FAILURE);
    }

    while (1) {
        char recv_buf[TFTP_BLOCK_SIZE + 4];
        struct sockaddr_in client_addr;
        socklen_t client_len = sizeof(client_addr);
        ret = recvfrom(sock_fd, recv_buf, sizeof(recv_buf), 0, (struct sockaddr *)&client_addr, &client_len);
        if (ret < 0) {
            perror("recvfrom");
            exit(EXIT_FAILURE);
        }

        struct tftp_packet *recv_packet = (struct tftp_packet *)recv_buf;
        unsigned short opcode = ntohs(recv_packet->opcode);
        if (opcode == TFTP_OPCODE_DATA) {
            unsigned short recv_block_num = ntohs(recv_packet->data.data.block_num);
            if (recv_block_num == block_num) {
                int data_len = ret - 4;
                fwrite(recv_packet->data.data.data, 1, data_len, file);
                block_num++;

                char ack_buf[4];
                struct tftp_packet *ack_packet = (struct tftp_packet *)ack_buf;
                ack_packet->opcode = htons(TFTP_OPCODE_ACK);
                ack_packet->data.ack.block_num = htons(recv_block_num);
                ret = sendto(sock_fd, ack_buf, sizeof(ack_buf), 0, (struct sockaddr *)&client_addr, client_len);
                if (ret < 0) {
                    perror("sendto");
                    exit(EXIT_FAILURE);
                }

                if (data_len < TFTP_BLOCK_SIZE) {
                    break;
                }
            }
        } else if (opcode == TFTP_OPCODE_ERROR) {
            unsigned short error_code = ntohs(recv_packet->data.error.error_code);
            fprintf(stderr, "TFTP error: %d\n", error_code);
            exit(EXIT_FAILURE);
        } else {
            fprintf(stderr, "Unexpected TFTP opcode: %d\n", opcode);
            exit(EXIT_FAILURE);
        }
    }

    fclose(file);
    close(sock_fd);
    free(packet_buf);

    return 0;
}

这个例子实现了TFTP的读取请求（RRQ）和数据（DATA）包的处理，以及确认（ACK）包的发送。它会将服务器上的文件传输到本地，并保存为和服务器上的文件名相同的文件。