/*
 * tlsproxy is a transparent TLS proxy for HTTPS connections.
 *
 * Copyright (C) 2011  Simon Ruderich
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <stdlib.h>
#include <stdio.h>
/* socket(), bind(), accept(), listen() */
#include <sys/types.h>
#include <sys/socket.h>
/* close() */
#include <unistd.h>
/* htons() */
#include <arpa/inet.h>
/* getaddrinfo() */
#include <netdb.h>
/* strncmp() */
#include <string.h>
/* sigaction() */
#include <signal.h>
/* poll() */
#include <poll.h>

#include <config.h>


/* Maximum line of the request line. Longer request lines are aborted with an
 * error. The standard doesn't specify a maximum line length but this should
 * be a good limit to make processing simpler. */
#define MAX_REQUEST_LINE 4096


/* Server should shut down. Set by SIGINT handler. */
static volatile int done;

/* Proxy hostname and port if specified on the command line. */
static char *use_proxy_host;
static char *use_proxy_port;


static void sigint_handler(int signal);

static void parse_arguments(int argc, char **argv);
static void print_usage(const char *argv);

static void handle_connection(int socket);
static int read_http_request(FILE *client_fd, char *request, size_t length);
static void send_close_bad_request(FILE *client_fd);
static void send_close_forwarding_failure(FILE *client_fd);

static void transfer_data(int client, int server);
static int read_from_write_to(int from, int to);

static int connect_to_host(const char *hostname, const char *port);

static int parse_request(const char *buffer, char *host, char *port,
                                             int *version_minor);


int main(int argc, char **argv) {
    int port;
    int client_socket, server_socket;
    struct sockaddr_in6 server_in;

    struct sigaction action;

    parse_arguments(argc, argv);

    port = atoi(argv[argc - 1]);
    if (0 >= port || 0xffff < port) {
        print_usage(argv[0]);
        fprintf(stderr, "\ninvalid port");
        return EXIT_FAILURE;
    }

    /* Setup our SIGINT signal handler which allows a "normal" termination of
     * the server. */
    sigemptyset(&action.sa_mask);
    action.sa_handler = sigint_handler;
    action.sa_flags   = 0;
    sigaction(SIGINT, &action, NULL);

    server_socket = socket(PF_INET6, SOCK_STREAM, 0);
    if (-1 == server_socket) {
        perror("socket()");
        return EXIT_FAILURE;
    }

#ifdef DEBUG
    /* Fast rebinding for debug mode, could cause invalid packets. */
    {
        int socket_option = 1;
        setsockopt(server_socket, SOL_SOCKET, SO_REUSEADDR,
                   &socket_option, sizeof(socket_option));
    }
#endif

    /* Bind to the listen socket. */
    memset(&server_in, 0, sizeof(server_in));
    server_in.sin6_family = AF_INET6;              /* IPv6 (and IPv4) */
    server_in.sin6_addr   = in6addr_any;           /* bind to any address */
    server_in.sin6_port   = htons((uint16_t)port); /* port to bind to */
    if (-1 == bind(server_socket, (struct sockaddr *)&server_in,
                                  sizeof(server_in))) {
        perror("bind()");
        return EXIT_FAILURE;
    }
    /* And accept connections. */
    if (-1 == listen(server_socket, 5)) {
        perror("listen()");
        return EXIT_FAILURE;
    }

#ifdef DEBUG
    printf("Listening for connections on port %d.\n", port);

    if (NULL != use_proxy_host && NULL != use_proxy_port) {
        printf("Using proxy: %s:%s.\n", use_proxy_host, use_proxy_port);
    }
#endif

    while (!done) {
        /* Accept new connection. */
        client_socket = accept(server_socket, NULL, NULL);
        if (-1 == client_socket) {
            perror("accept()");
            break;
        }

        handle_connection(client_socket);
    }

    close(server_socket);

    free(use_proxy_host);
    free(use_proxy_port);

    return EXIT_FAILURE;
}

static void sigint_handler(int signal_number) {
    (void)signal_number;

    done = 1;
}

static void parse_arguments(int argc, char **argv) {
    int option;

    while (-1 != (option = getopt(argc, argv, "p:h?"))) {
        switch (option) {
            case 'p': {
                char *position;

                /* -p must have the format host:port. */
                if (NULL == (position = strchr(optarg, ':'))
                        || position == optarg
                        || 0 == strlen(position + 1)
                        || 0 >= atoi(position + 1)
                        || 0xffff < atoi(position + 1)) {
                    fprintf(stderr, "-p host:port\n");
                    exit(EXIT_FAILURE);
                }

                use_proxy_host = malloc((size_t)(position - optarg) + 1);
                if (NULL == use_proxy_host) {
                    perror("malloc()");
                    exit(EXIT_FAILURE);
                }
                memcpy(use_proxy_host, optarg, (size_t)(position - optarg));
                use_proxy_host[position - optarg] = '\0';

                use_proxy_port = malloc(strlen(position + 1) + 1);
                if (NULL == use_proxy_port) {
                    perror("malloc()");
                    exit(EXIT_FAILURE);
                }
                strcpy(use_proxy_port, position + 1);

                break;
            }
            case 'h':
            default: /* '?' */
                print_usage(argv[0]);
                exit(EXIT_FAILURE);
        }
    }

    if (optind >= argc) {
        print_usage(argv[0]);
        exit(EXIT_FAILURE);
    }
}
static void print_usage(const char *argv) {
    fprintf(stderr, "Usage: %s [-p host:port] port\n", argv);
    fprintf(stderr, "\n");
    fprintf(stderr, "-p proxy hostname and port\n");
}

static void handle_connection(int client_socket) {
    int server_socket;
    FILE *client_fd, *server_fd;

    char buffer[MAX_REQUEST_LINE];
    char host[MAX_REQUEST_LINE];
    char port[5 + 1];

    int version_minor;
    int result;

    client_fd = fdopen(client_socket, "a+");
    if (NULL == client_fd) {
        perror("fdopen()");
        close(client_socket);
        return;
    }

#ifdef DEBUG
    printf("New connection:\n");
#endif

    /* Read request line (CONNECT ..) and headers (they are discarded). */
    result = read_http_request(client_fd, buffer, sizeof(buffer));
    if (result == -1) {
        /* Read error. */
        return;
    } else if (result == -2) {
        /* EOF */
        send_close_bad_request(client_fd);
        return;
    }

#ifdef DEBUG
    printf("  request: %s", buffer);
#endif

    if (0 != parse_request(buffer, host, port, &version_minor)) {
        send_close_bad_request(client_fd);
#ifdef DEBUG
        printf("  bad request\n");
#endif
        return;
    }

#ifdef DEBUG
    printf("  %s:%s (HTTP 1.%d)\n", host, port, version_minor);
#endif

    /* Connect to proxy server or directly to server. */
    if (NULL != use_proxy_host && NULL != use_proxy_port) {
        server_socket = connect_to_host(use_proxy_host, use_proxy_port);
    } else {
        server_socket = connect_to_host(host, port);
    }

    if (-1 == server_socket) {
        send_close_forwarding_failure(client_fd);
        return;
    }
    server_fd = fdopen(server_socket, "a+");
    if (NULL == server_fd) {
        send_close_forwarding_failure(client_fd);
        return;
    }

    /* Connect to proxy if requested (command line option). */
    if (NULL != use_proxy_host && NULL != use_proxy_port) {
        fprintf(server_fd, "CONNECT %s:%s HTTP/1.0\r\n", host, port);
        fprintf(server_fd, "\r\n");

        /* Read response line from proxy server. */
        result = read_http_request(server_fd, buffer, sizeof(buffer));
        if (result == -1) {
            /* Read error. */
            send_close_forwarding_failure(client_fd);
            return;
        } else if (result == -2) {
            /* EOF */
            fclose(server_fd);
            send_close_forwarding_failure(client_fd);
            return;
        }

        /* Check response of proxy server. */
        if (0 != strncmp(buffer, "HTTP/1.0 200", 12)) {
#ifdef DEBUG
            printf("  bad proxy response\n");
#endif
            fclose(server_fd);
            send_close_forwarding_failure(client_fd);
            return;
        }
    }

#ifdef DEBUG
    printf("  connection to server established\n");
#endif

    /* We've established a connection, tell the client. */
    fprintf(client_fd, "HTTP/1.0 200 Connection established\r\n");
    fprintf(client_fd, "\r\n");
    fflush(client_fd);

    /* And transfer all data between client and server transparently. */
    transfer_data(client_socket, server_socket);

    fclose(client_fd);
    fclose(server_fd);
}

/* Read HTTP request line and headers (ignored).
 *
 * On success 0 is returned, -1 on client error (we close client descriptor in
 * this case), -2 on unexpected EOF.
 */
static int read_http_request(FILE *client_fd, char *request, size_t length) {
    char buffer[MAX_REQUEST_LINE];

    if (NULL == fgets(request, (int)length, client_fd)) {
        if (ferror(client_fd)) {
            perror("fgets(), request");
            fclose(client_fd);
            return -1;
        }

        return -2;
    }

    while (NULL != fgets(buffer, MAX_REQUEST_LINE, client_fd)) {
        /* End of header. */
        if (0 == strcmp(buffer, "\n") || 0 == strcmp(buffer, "\r\n")) {
            break;
        }
    }
    if (ferror(client_fd)) {
        perror("fgets(), header");
        fclose(client_fd);
        return -1;
    }

    return 0;
}

static void send_close_bad_request(FILE *client_fd) {
    fprintf(client_fd, "HTTP/1.0 400 Bad Request\r\n");
    fprintf(client_fd, "\r\n");
    fclose(client_fd);
}
static void send_close_forwarding_failure(FILE *client_fd) {
    fprintf(client_fd, "HTTP/1.0 503 Forwarding failure\r\n");
    fprintf(client_fd, "\r\n");
    fclose(client_fd);
}


/* Transfer data between client and server sockets until one closes the
 * connection. */
static void transfer_data(int client, int server) {
    struct pollfd fds[2];
    fds[0].fd      = client;
    fds[0].events  = POLLIN | POLLPRI | POLLHUP | POLLERR;
    fds[0].revents = 0;
    fds[1].fd      = server;
    fds[1].events  = POLLIN | POLLPRI | POLLHUP | POLLERR;
    fds[1].revents = 0;

    for (;;) {
        int result = poll(fds, 2, 0);
        if (result < 0) {
            perror("poll()");
            return;
        }

        /* Data available from client. */
        if (fds[0].revents & POLLIN || fds[0].revents & POLLPRI) {
            if (0 != read_from_write_to(client, server)) {
                /* EOF (or other error) */
                break;
            }
        }
        /* Data available from server. */
        if (fds[1].revents & POLLIN || fds[1].revents & POLLPRI) {
            if (0 != read_from_write_to(server, client)) {
                /* EOF (or other error) */
                break;
            }
        }

        /* Client closed connection. */
        if (fds[0].revents & POLLERR || fds[0].revents & POLLHUP) {
            break;
        }
        /* Server closed connection. */
        if (fds[1].revents & POLLERR || fds[1].revents & POLLHUP) {
            break;
        }
    }
}

/* Read available data from socket from and write it to socket to. At maximum
 * 4096 bytes are read/written. */
static int read_from_write_to(int from, int to) {
    ssize_t size_read;
    ssize_t size_written;
    char buffer[4096];

    size_read = read(from, buffer, sizeof(buffer));
    if (0 > size_read) {
        perror("read()");
        return -1;
    }
    /* EOF */
    if (0 == size_read) {
        return -1;
    }

    size_written = write(to, buffer, (size_t)size_read);
    if (0 > size_written) {
        perror("write()");
        return -1;
    }
    if (size_read != size_written) {
        printf("only written %ld of %ld bytes!\n", size_read, size_written);
        return -1;
    }

    return 0;
}


static int connect_to_host(const char *hostname, const char *port) {
    struct addrinfo gai_hints;
    struct addrinfo *gai_result;
    int gai_return;

    int server_socket;
    struct addrinfo *server;

    if (NULL == hostname || NULL == port) {
        return -1;
    }

    /* Get IP of hostname server. */
    memset(&gai_hints, 0, sizeof(gai_hints));
    gai_hints.ai_family   = AF_UNSPEC;
    gai_hints.ai_socktype = SOCK_STREAM;
    gai_hints.ai_protocol = 0;
    gai_hints.ai_flags    = AI_NUMERICSERV /* given port is numeric */
                          | AI_ADDRCONFIG  /* supported by this computer */
                          | AI_V4MAPPED;   /* support IPv4 through IPv6 */
    gai_return = getaddrinfo(hostname, port, &gai_hints, &gai_result);
    if (0 != gai_return) {
        perror("getaddrinfo()");
        return -1;
    }

    /* Now try to connect to each server returned by getaddrinfo(), use the
     * first successful connect. */
    for (server = gai_result; NULL != server; server = server->ai_next) {
        server_socket = socket(server->ai_family,
                               server->ai_socktype,
                               server->ai_protocol);
        if (-1 == server_socket) {
            perror("socket(), trying next");
            continue;
        }

        if (-1 != connect(server_socket, server->ai_addr,
                                         server->ai_addrlen)) {
            break;
        }
        perror("connect(), trying next");

        close(server_socket);
    }
    /* Make sure we free the result from getaddrinfo(). */
    freeaddrinfo(gai_result);

    if (NULL == server) {
        fprintf(stderr, "no server found, aborting\n");
        return -1;
    }

    return server_socket;
}


/* Parse HTTP CONNECT request string and save its parameters.
 *
 * The following format is expected: "CONNECT host:port HTTP/1.y".
 *
 * request and host must have the same size! port must be at least 6 bytes
 * long (5 + '\0').
 */
static int parse_request(const char *request, char *host, char *port,
                                              int *version_minor) {
    int port_unused; /* just used to verify the port is numeric */
    char *position;

    /* scanf() doesn't check spaces. */
    if (0 != strncmp(request, "CONNECT ", 8)) {
        return -1;
    }
    /* Check request and extract data, "host:port" is not yet separated. */
    if (2 != sscanf(request, "CONNECT %s HTTP/1.%d",
                             host, version_minor)) {
        return -1;
    }
    /* Make sure ":port" is there. */
    if (NULL == (position = strchr(host, ':'))) {
        return -1;
    }
    /* Make sure port is numeric. */
    if (1 != sscanf(position + 1, "%d", &port_unused)) {
        return -1;
    }
    /* Store it in *port. */
    strncpy(port, position + 1, 5);
    port[5] = '\0';
    /* And remove port from host. */
    *position = '\0';

    return 0;
}