ftp_server.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <dirent.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <pthread.h>

#define CONTROL_PORT 2121
#define BUF_SIZE 1024
#define MAX_USERS 3
#define MAX_CLIENTS 10

typedef struct {
    const char *username;
    const char *password;
} User;

User valid_users[MAX_USERS] = {
    {"user1", "pass1"},
    {"user2", "pass2"},
    {"user3", "pass3"}
};

// 启动监听数据端口（被动模式），返回监听socket和端口号
int start_data_port(int *port) {
    int data_sock = socket(AF_INET, SOCK_STREAM, 0);
    if (data_sock < 0) {
        perror("data socket");
        return -1;
    }

    // ✅ 设置 SO_REUSEADDR，防止端口占用问题
    int opt = 1;
    setsockopt(data_sock, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));

    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = INADDR_ANY;
    addr.sin_port = 0;

    if (bind(data_sock, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
        perror("bind data port");
        close(data_sock);
        return -1;
    }

    socklen_t len = sizeof(addr);
    if (getsockname(data_sock, (struct sockaddr*)&addr, &len) < 0) {
        perror("getsockname");
        close(data_sock);
        return -1;
    }

    *port = ntohs(addr.sin_port);

    if (listen(data_sock, 1) < 0) {
        perror("listen data port");
        close(data_sock);
        return -1;
    }
    return data_sock;
}

void send_pasv_response(int ctrl_sock, int port) {
    unsigned char p1 = port / 256;
    unsigned char p2 = port % 256;
    char resp[100];
    snprintf(resp, sizeof(resp), "227 Entering Passive Mode (127,0,0,1,%d,%d).\r\n", p1, p2);
    send(ctrl_sock, resp, strlen(resp), 0);
}

void send_dir(int data_sock) {
    DIR *d = opendir(".");
    if (!d) {
        perror("opendir");
        return;
    }
    struct dirent *entry;
    char buf[BUF_SIZE];
    while ((entry = readdir(d)) != NULL) {
        snprintf(buf, sizeof(buf), "%s\r\n", entry->d_name);
        send(data_sock, buf, strlen(buf), 0);
    }
    closedir(d);
}

void *handle_client(void *arg) {
    int client_sock = *(int*)arg;
    free(arg);
    send(client_sock, "220 FTP Server Ready\r\n", 22, 0);

    char cmd_buf[BUF_SIZE];
    char cmd[16], arg_val[256];
    int data_listen_sock = -1;
    int logged_in = 0;
    int user_index = -1;

    while (1) {
        memset(cmd_buf, 0, sizeof(cmd_buf));
        int n = recv(client_sock, cmd_buf, sizeof(cmd_buf) - 1, 0);
        if (n <= 0) break;

        sscanf(cmd_buf, "%s %255[^\r\n]", cmd, arg_val);

        if (strcasecmp(cmd, "USER") == 0) {
            user_index = -1;
            for (int i = 0; i < MAX_USERS; ++i) {
                if (strcmp(arg_val, valid_users[i].username) == 0) {
                    user_index = i;
                    break;
                }
            }
            if (user_index != -1) {
                send(client_sock, "331 Username OK, need password\r\n", 33, 0);
            } else {
                send(client_sock, "530 Invalid username\r\n", 23, 0);
            }
        } else if (strcasecmp(cmd, "PASS") == 0) {
            if (user_index != -1 && strcmp(arg_val, valid_users[user_index].password) == 0) {
                logged_in = 1;
                send(client_sock, "230 User logged in\r\n", 21, 0);
            } else {
                send(client_sock, "530 Login incorrect\r\n", 22, 0);
            }
        } else if (!logged_in) {
            send(client_sock, "530 Please login with USER and PASS\r\n", 38, 0);
        } else if (strcasecmp(cmd, "QUIT") == 0) {
            send(client_sock, "221 Goodbye\r\n", 13, 0);
            break;
        } else if (strcasecmp(cmd, "PWD") == 0) {
            char cwd[512];
            getcwd(cwd, sizeof(cwd));
            char resp[600];
            snprintf(resp, sizeof(resp), "257 \"%s\"\r\n", cwd);
            send(client_sock, resp, strlen(resp), 0);
        } else if (strcasecmp(cmd, "CWD") == 0) {
            if (chdir(arg_val) == 0) {
                send(client_sock, "250 Directory changed\r\n", 23, 0);
            } else {
                send(client_sock, "550 Failed to change directory\r\n", 32, 0);
            }
        } else if (strcasecmp(cmd, "MKD") == 0) {
            if (mkdir(arg_val, 0755) == 0) {
                send(client_sock, "257 Directory created\r\n", 24, 0);
            } else {
                send(client_sock, "550 Failed to create directory\r\n", 32, 0);
            }       
        } else if (strcasecmp(cmd, "RMD") == 0) {
            if (rmdir(arg_val) == 0) {
                send(client_sock, "250 Directory removed\r\n", 24, 0);
            } else {
                send(client_sock, "550 Failed to remove directory\r\n", 32, 0);
            }
        } else if (strcasecmp(cmd, "PASV") == 0) {
            if (data_listen_sock > 0) {
                close(data_listen_sock);
                data_listen_sock = -1;
            }
            int data_port;
            data_listen_sock = start_data_port(&data_port);
            if (data_listen_sock < 0) {
                send(client_sock, "425 Can't open data connection\r\n", 32, 0);
            } else {
                send_pasv_response(client_sock, data_port);
            }
        } else if (strcasecmp(cmd, "LIST") == 0 || strcasecmp(cmd, "DIR") == 0) {
            if (data_listen_sock < 0) {
                send(client_sock, "425 Use PASV first\r\n", 20, 0);
                continue;
            }
            send(client_sock, "150 Here comes the directory listing\r\n", 39, 0);
            int data_sock = accept(data_listen_sock, NULL, NULL);
            if (data_sock < 0) {
                send(client_sock, "425 Can't open data connection\r\n", 32, 0);
                close(data_listen_sock);
                data_listen_sock = -1;
                continue;
            }
            send_dir(data_sock);
            close(data_sock);
            close(data_listen_sock);
            data_listen_sock = -1;
            send(client_sock, "226 Directory send OK\r\n", 24, 0);
        } else if (strcasecmp(cmd, "RETR") == 0) {
            if (data_listen_sock < 0) {
                send(client_sock, "425 Use PASV first\r\n", 20, 0);
                continue;
            }
            FILE *fp = fopen(arg_val, "rb");
            if (!fp) {
                send(client_sock, "550 Failed to open file\r\n", 25, 0);
                continue;
            }
            send(client_sock, "150 Opening data connection\r\n", 29, 0);
            int data_sock = accept(data_listen_sock, NULL, NULL);
            if (data_sock < 0) {
                send(client_sock, "425 Can't open data connection\r\n", 32, 0);
                fclose(fp);
                close(data_listen_sock);
                data_listen_sock = -1;
                continue;
            }
            char file_buf[BUF_SIZE];
            size_t read_bytes;
            while ((read_bytes = fread(file_buf, 1, BUF_SIZE, fp)) > 0) {
                send(data_sock, file_buf, read_bytes, 0);
            }
            fclose(fp);
            close(data_sock);
            close(data_listen_sock);
            data_listen_sock = -1;
            send(client_sock, "226 Transfer complete\r\n", 23, 0);
        } else if (strcasecmp(cmd, "STOR") == 0) {
            if (data_listen_sock < 0) {
                send(client_sock, "425 Use PASV first\r\n", 20, 0);
                continue;
            }
            FILE *fp = fopen(arg_val, "wb");
            if (!fp) {
                send(client_sock, "550 Failed to open file\r\n", 25, 0);
                continue;
            }
            send(client_sock, "150 Opening data connection\r\n", 29, 0);
            int data_sock = accept(data_listen_sock, NULL, NULL);
            if (data_sock < 0) {
                send(client_sock, "425 Can't open data connection\r\n", 32, 0);
                fclose(fp);
                close(data_listen_sock);
                data_listen_sock = -1;
                continue;
            }
            ssize_t recvd;
            while ((recvd = recv(data_sock, cmd_buf, sizeof(cmd_buf), 0)) > 0) {
                fwrite(cmd_buf, 1, recvd, fp);
            }
            fclose(fp);
            close(data_sock);
            close(data_listen_sock);
            data_listen_sock = -1;
            send(client_sock, "226 Transfer complete\r\n", 23, 0);
        } else {
            send(client_sock, "502 Command not implemented\r\n", 29, 0);
        }
    }

    close(client_sock);
    return NULL;
}

int main() {
    int ctrl_sock = socket(AF_INET, SOCK_STREAM, 0);
    if (ctrl_sock < 0) {
        perror("control socket");
        exit(1);
    }

    // ✅ 设置 SO_REUSEADDR 防止 "Address already in use"
    int opt = 1;
    setsockopt(ctrl_sock, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));

    struct sockaddr_in serv_addr;
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(CONTROL_PORT);
    serv_addr.sin_addr.s_addr = INADDR_ANY;

    if (bind(ctrl_sock, (struct sockaddr*)&serv_addr, sizeof(serv_addr)) < 0) {
        perror("bind");
        exit(1);
    }

    if (listen(ctrl_sock, MAX_CLIENTS) < 0) {
        perror("listen");
        exit(1);
    }

    printf("FTP server started on port %d, waiting for clients...\n", CONTROL_PORT);

    while (1) {
        struct sockaddr_in client_addr;
        socklen_t client_len = sizeof(client_addr);
        int *client_sock = malloc(sizeof(int));
        *client_sock = accept(ctrl_sock, (struct sockaddr*)&client_addr, &client_len);
        if (*client_sock < 0) {
            perror("accept");
            free(client_sock);
            continue;
        }

        pthread_t tid;
        if (pthread_create(&tid, NULL, handle_client, client_sock) != 0) {
            perror("pthread_create");
            close(*client_sock);
            free(client_sock);
        } else {
            pthread_detach(tid);
        }
    }

    close(ctrl_sock);
    return 0;
}