/* PostSRSd - Sender Rewriting Scheme daemon for Postfix
 * Copyright (c) 2012 Timo Röhling <timo.roehling@gmx.de>
 *
 * 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 2 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/>.
 */

/* This program uses the libsrs2 library. The relevant source
 * files have been added to this distribution. */

#include "srs2.h"
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <errno.h>
#include <unistd.h>
#include <fcntl.h>
#include <pwd.h>
#include <string.h>
#include <poll.h>
#include <wait.h>
#include <syslog.h>

#ifndef VERSION
#define VERSION "1.1"
#endif

static char *self = NULL;

static int bind_service (const char *service, int family)
{
  struct addrinfo *addr, *it;
  struct addrinfo hints;
  int err, sock, flags;
  static const int one = 1;

  memset (&hints, 0, sizeof(hints));
  hints.ai_family = family;
  hints.ai_socktype = SOCK_STREAM;
  hints.ai_flags = AI_ADDRCONFIG | AI_V4MAPPED;
  err = getaddrinfo(NULL, service, &hints, &addr);
  if (err != 0) {
    fprintf(stderr, "%s: bind_service(%s): %s\n", self, service, gai_strerror(err));
    return -1;
  }
  sock = -1;
  for (it = addr; it; it = it->ai_next) {
    sock = socket(it->ai_family, it->ai_socktype, it->ai_protocol);
    if (sock < 0) goto fail;
    if (setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)) < 0) goto fail;
    if (bind(sock, it->ai_addr, it->ai_addrlen) < 0) goto fail;
    if (listen(sock, 10) < 0) goto fail;
    flags = fcntl (sock, F_GETFL, 0);
    if (flags < 0) goto fail;
    if (fcntl(sock, F_SETFL, flags | O_NONBLOCK) < 0) goto fail;
    break;
  fail:
    err = errno;
    if (sock >= 0) close (sock);
    sock = -1;
  }
  freeaddrinfo (addr);
  if (sock < 0)
    fprintf (stderr, "%s: bind_service(%s): %s\n", self, service, strerror(err));
  return sock;
}

static int is_hexdigit (char c)
{
  return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F');
}

static char hex2num (char c)
{
  if (c >= '0' && c <= '9') return c - '0';
  if (c >= 'a' && c <= 'f') return c - 'a' + 10;
  if (c >= 'A' && c <= 'F') return c - 'A' + 10;
  return 0;
}

static char num2hex (char c)
{
  if (c < 10) return '0' + c;
  return 'a' + c - 10;
}

static char hex2char (const char *s)
{
  return 16 * hex2num(s[0]) + hex2num(s[1]);
}

static void char2hex (char c, char *buf)
{
  buf[0] = num2hex((c >> 4) & 0xf);
  buf[1] = num2hex((c     ) & 0xf);
}

static char* url_decode (char *buf, size_t len, const char *input)
{
  char *output = buf;
  if (!input || !output || len == 0) return NULL;

  while (*input && --len) {
    if (*input == '%' && is_hexdigit(input[1]) && is_hexdigit(input[2])) {
      *output++ = hex2char(++input);
      input += 2;
    } else {
      *output++ = *input++;
    }
  }
  *output = 0;
  return buf;
}

static char* url_encode (char* buf, size_t len, const char *input)
{
  char *output = buf;
  if (!input || !output || len == 0) return NULL;
  while (*input && --len) {
    if (!isascii(*input) || !isgraph(*input) || *input == '%') {
      if (len <= 2) break;
      *output++ = '%';
      char2hex(*input++, output);
      output += 2; len -= 2;
    } else {
      *output++ = *input++;
    }
  }
  *output = 0;
  return buf;
}

static void handle_forward (srs_t *srs, FILE *fp, const char *address, const char *domain, const char **excludes)
{
  int result;
  size_t addrlen;
  char value[1024];
  char outputbuf[1024], *output;

  addrlen = strlen(address);
  for(; *excludes; excludes++) {
    size_t len;
    len = strlen(*excludes);
    if (len >= addrlen) continue;
    if (strcasecmp(*excludes, &address[addrlen - len]) == 0 && (**excludes == '.' || address[addrlen - len - 1] == '@')) {
      fputs ("500 Domain excluded from SRS\n", fp);
      fflush (fp);
      return;
    }
  }
  result = srs_forward(srs, value, sizeof(value), address, domain);
  if (result == SRS_SUCCESS) {
    output = url_encode(outputbuf, sizeof(outputbuf), value);
    fprintf (fp, "200 %s\n", output);
    if (strcmp(address, value) != 0) syslog (LOG_MAIL | LOG_INFO, "srs_forward: <%s> rewritten as <%s>", address, value);
  } else {
    fprintf (fp, "500 %s\n", srs_strerror(result));
    if (result != SRS_ENOTREWRITTEN)
      syslog (LOG_MAIL | LOG_INFO, "srs_forward: <%s> not rewritten: %s", address, srs_strerror(result));
  }
  fflush (fp);
}

static void handle_reverse (srs_t *srs, FILE *fp, const char *address, const char *domain, const char **excludes)
{
  int result;
  char value[1024];
  char outputbuf[1024], *output;
  result = srs_reverse(srs, value, sizeof(value), address);
  if (result == SRS_SUCCESS) {
    output = url_encode(outputbuf, sizeof(outputbuf), value);
    fprintf (fp, "200 %s\n", output);
    syslog (LOG_MAIL | LOG_INFO, "srs_reverse: <%s> rewritten as <%s>", address, value); 
  } else {
    fprintf (fp, "500 %s\n", srs_strerror(result));
    if (result != SRS_ENOTREWRITTEN && result != SRS_ENOTSRSADDRESS)
      syslog (LOG_MAIL | LOG_INFO, "srs_reverse: <%s> not rewritten: %s", address, srs_strerror(result));
  }
  fflush (fp);
}

static void show_help ()
{
  fprintf (stdout,
    "Sender Rewriting Scheme implementation for Postfix.\n\n"
    "Implements two TCP lookup tables to rewrite mail addresses\n"
    "as needed. The forward SRS is for sender envelope addresses\n"
    "to prevent SPF-related bounces. The reverse SRS is for\n"
    "recipient envelope addresses so that bounced mails can be\n"
    "routed back to their original sender.\n"
    "\n"
    "Usage: %s -s<file> -d<domain> [other options]\n"
    "Options:\n"
    "   -s<file>       read secrets from file (required)\n"
    "   -d<domain>     set domain name for rewrite (required)\n"
    "   -f<port>       set port for the forward SRS lookup (default: 10001)\n"
    "   -r<port>       set port for the reverse SRS lookup (default: 10002)\n"
    "   -p<pidfile>    write process ID to pidfile (default: none)\n"
    "   -c<dir>        chroot to <dir> (default: none)\n"
    "   -u<user>       switch user id after port bind (default: none)\n"
    "   -t<seconds>    timeout for idle client connections (default: 1800)\n"
    "   -X<domain>     exclude additional domain from address rewriting\n"
    "   -D             fork into background\n"
    "   -4             force IPv4 socket (default: any)\n"
    "   -6             force IPv6 socket (default: any)\n"
    "   -h             show this help\n"
    "   -v             show version\n"
    "\n",
    self
  );
}

typedef void(*handle_t)(srs_t*, FILE*, const char*, const char*, const char**);

int main (int argc, char **argv)
{
  int opt, timeout = 1800, family = AF_UNSPEC;
  int daemonize = FALSE;
  char *forward_service = NULL, *reverse_service = NULL,
       *user = NULL, *domain = NULL, *chroot_dir = NULL;
  int forward_sock, reverse_sock;
  char *secret_file = NULL, *pid_file = NULL;
  FILE *pf = NULL, *sf = NULL;
  struct passwd *pwd = NULL;
  char secretbuf[1024], *secret = NULL;
  char *tmp;
  srs_t *srs;
  struct pollfd fds[3];
  const char **excludes;
  size_t s1 = 0, s2 = 1;
  handle_t handler[2] = { handle_forward, handle_reverse };

  excludes = (const char**)calloc(1, sizeof(char*));
  tmp = strrchr(argv[0], '/');
  if (tmp) self = strdup(tmp + 1); else self = strdup(argv[0]);

  while ((opt = getopt(argc, argv, "46d:f:r:s:u:t:p:c:X::Dhv")) != -1) {
    switch (opt) {
      case '?':
        return EXIT_FAILURE;
      case '4':
        family = AF_INET;
        break;
      case '6':
        family = AF_INET6;
        break;
      case 'd':
        domain = strdup(optarg);
        break;
      case 'f':
        forward_service = strdup(optarg);
        break;
      case 'r':
        reverse_service = strdup(optarg);
        break;
      case 't':
        timeout = atoi(optarg);
        break;
      case 's':
        secret_file = strdup(optarg);
        break;
      case 'p':
        pid_file = strdup(optarg);
        break;
      case 'u':
        user = strdup(optarg);
        break;
      case 'c':
        chroot_dir = strdup(optarg);
        break;
      case 'D':
        daemonize = TRUE;
        break;
      case 'h':
        show_help();
        return EXIT_SUCCESS;
      case 'X':
        if (optarg != NULL) {
          tmp = strtok(optarg, ",; \t\r\n");
          while (tmp) {
            if (s1 + 1 >= s2) {
              s2 *= 2;
              excludes = (const char **)realloc(excludes, s2 * sizeof(char*));
              if (excludes == NULL) {
                fprintf (stderr, "%s: Out of memory\n\n", self);
                return EXIT_FAILURE;
              }
            }
            excludes[s1++] = strdup(tmp);
            tmp = strtok(NULL, ",; \t\r\n");
          }
          excludes[s1] = NULL;
        }
        break;
      case 'v':
        fprintf (stdout, "%s\n", VERSION);
        return EXIT_SUCCESS;
    }
  }
  if (optind < argc) {
    fprintf (stderr, "%s: extra argument on command line: %s\n", self, argv[optind]);
    return EXIT_FAILURE;
  }
  if (domain == NULL) {
    fprintf (stderr, "%s: You must set a home domain (-d)\n", self);
    return EXIT_FAILURE;
  }

  /* The stuff we do first may not be possible from within chroot or without privileges */

  /* Open pid file for writing (the actual process ID is filled in later) */
  if (pid_file) {
    pf = fopen (pid_file, "w");
    if (pf == NULL) {
      fprintf (stderr, "%s: Cannot write PID: %s\n\n", self, pid_file);
      return EXIT_FAILURE;
    }
  }
  /* Read secret. The default installation makes this root accessible only. */
  if (secret_file != NULL) {
    size_t len;
    sf = fopen(secret_file, "rb");
    if (sf == NULL) {
      fprintf (stderr, "%s: Cannot open file with secret: %s\n", self, secret_file);
      return EXIT_FAILURE;
    }
  } else {
    fprintf (stderr, "%s: You must set a secret (-s)\n", self);
    return EXIT_FAILURE;
  }
  /* Bind ports. May require privileges if the config specifies ports below 1024 */
  if (forward_service != NULL) {
    forward_sock = bind_service(forward_service, family);
    free (forward_service);
  } else {
    forward_sock = bind_service("10001", family);
  }
  if (forward_sock < 0) return EXIT_FAILURE;
  if (reverse_service != NULL) {
    reverse_sock = bind_service(reverse_service, family);
    free (reverse_service);
  } else {
    reverse_sock = bind_service("10002", family);
  }
  if (reverse_sock < 0) return EXIT_FAILURE;

  /* Open syslog now (NDELAY), because it may no longer reachable from chroot */
  openlog (self, LOG_PID | LOG_NDELAY, LOG_MAIL);
  /* We also have to lookup the uid of the unprivileged user for the same reason. */
  if (user) {
    errno = 0;
    pwd = getpwnam(user);
    if (pwd == NULL) {
      if (errno != 0)
        fprintf (stderr, "%s: Failed to lookup user: %s\n", self, strerror(errno));
      else
        fprintf (stderr, "%s: No such user: %s\n", self, user);
      return EXIT_FAILURE;
    }
  }
  /* Now we can chroot, which again requires root privileges */
  if (chroot_dir) {
    if (chdir(chroot_dir) < 0) {
      fprintf (stderr, "%s: Cannot change to chroot: %s\n", self, strerror(errno));
      return EXIT_FAILURE;
    }
    if (chroot(chroot_dir) < 0) {
      fprintf (stderr, "%s: Failed to enable chroot: %s\n", self, strerror(errno));
      return EXIT_FAILURE;
    }
  }
  /* Finally, we revert to the unprivileged user */
  if (pwd) {
    if (setuid(pwd->pw_uid) < 0) {
      fprintf (stderr, "%s: Failed to switch user id: %s\n", self, strerror(errno));
      return EXIT_FAILURE;
    }
  }
  /* Standard double fork technique to disavow all knowledge about the controlling terminal */
  if (daemonize) {
    close(0); close(1); close(2);
    if (fork() != 0) return EXIT_SUCCESS;
    setsid();
    if (fork() != 0) return EXIT_SUCCESS;
  }
  /* Make note of our actual process ID */
  if (pf) {
    fprintf (pf, "%d", (int)getpid());
    fclose (pf);
  }

  srs = srs_new();
  while ((secret = fgets(secretbuf, sizeof(secretbuf), sf))) {
    secret = strtok(secret, "\r\n");
    if (secret)
      srs_add_secret (srs, secret);
  }
  fclose (sf);
  srs_set_separator (srs, '+');

  fds[0].fd = forward_sock;
  fds[0].events = POLLIN;
  fds[1].fd = reverse_sock;
  fds[1].events = POLLIN;

  while(TRUE) {
    int i, conn;
    FILE *fp;
    char linebuf[1024], *line;
    char keybuf[1024], *key;

    if (poll(fds, 2, 1000) < 0) {
      if (errno == EINTR)
        continue;
      if (daemonize)
        syslog (LOG_MAIL | LOG_ERR, "Poll failure: %s", strerror(errno));
      else
        fprintf (stderr, "%s: Poll failure: %s\n", self, strerror(errno));
      return EXIT_FAILURE;
    }
    for (i = 0; i < 2; ++i) {
      if (fds[i].revents) {
        conn = accept(fds[i].fd, NULL, NULL);
        if (conn < 0) continue;
        if (fork() == 0) {
          // close listen sockets so that we don't stop the main daemon process from restarting
          close(forward_sock);
          close(reverse_sock);

          fp = fdopen(conn, "r+");
          if (fp == NULL) exit(EXIT_FAILURE);
          fds[2].fd = conn;
          fds[2].events = POLLIN;
          if (poll(&fds[2], 1, timeout * 1000) <= 0) return EXIT_FAILURE;
          line = fgets(linebuf, sizeof(linebuf), fp);
          while (line) {
            char* token;
            token = strtok(line, " \r\n");
            if (token == NULL || strcmp(token, "get") != 0) {
              fprintf (fp, "500 Invalid request\n");
              return EXIT_FAILURE;
            }
            token = strtok(NULL, "\r\n");
            if (!token) {
              fprintf (fp, "500 Invalid request\n");
              return EXIT_FAILURE;
            }
            key = url_decode(keybuf, sizeof(keybuf), token);
            if (!key) break;
            handler[i](srs, fp, key, domain, excludes);
            if (poll(&fds[2], 1, timeout * 1000) <= 0) break;
            line = fgets(linebuf, sizeof(linebuf), fp);
          }
          fclose (fp);
          return EXIT_SUCCESS;
        }
        close (conn);
      }
    }
    waitpid(-1, NULL, WNOHANG);
  }
  return EXIT_SUCCESS;
}