/*
     This file is part of GNUnet.
     Copyright (C) 2010 GNUnet e.V.

     GNUnet is free software: you can redistribute it and/or modify it
     under the terms of the GNU Affero General Public License as published
     by the Free Software Foundation, either version 3 of the License,
     or (at your option) any later version.

     GNUnet 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
     Affero General Public License for more details.

     You should have received a copy of the GNU Affero General Public License
     along with this program.  If not, see <http://www.gnu.org/licenses/>.

     SPDX-License-Identifier: AGPL3.0-or-later
 */

/**
 * @file src/nat/gnunet-helper-nat-server.c
 * @brief Tool to help bypass NATs using ICMP method; must run as root (SUID will do)
 *        This code will work under GNU/Linux only (or maybe BSDs, but never W32)
 * @author Christian Grothoff
 *
 * This program will send ONE ICMP message every 500 ms RAW sockets
 * to a DUMMY IP address and also listens for ICMP replies.  Since
 * it uses RAW sockets, it must be installed SUID or run as 'root'.
 * In order to keep the security risk of the resulting SUID binary
 * minimal, the program ONLY opens the two RAW sockets with root
 * privileges, then drops them and only then starts to process
 * command line arguments.  The code also does not link against
 * any shared libraries (except libc) and is strictly minimal
 * (except for checking for errors).  The following list of people
 * have reviewed this code and considered it safe since the last
 * modification (if you reviewed it, please have your name added
 * to the list):
 *
 * - Christian Grothoff
 * - Nathan Evans
 * - Benjamin Kuperman (22 Aug 2010)
 * - Jacob Appelbaum (19 Dec 2011)
 */
#if HAVE_CONFIG_H
/* Just needed for HAVE_SOCKADDR_IN_SIN_LEN test macro! */
#include "gnunet_config.h"
#else
#define _GNU_SOURCE
#endif
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <sys/select.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <stdint.h>
#include <time.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <netinet/in.h>

/* The following constant is missing from FreeBSD 9.2 */
#ifndef ICMP_TIME_EXCEEDED
#define ICMP_TIME_EXCEEDED 11
#endif

/**
 * Call memcpy() but check for @a n being 0 first. In the latter
 * case, it is now safe to pass NULL for @a src or @a dst.
 * Unlike traditional memcpy(), returns nothing.
 *
 * @param dst destination of the copy, may be NULL if @a n is zero
 * @param src source of the copy, may be NULL if @a n is zero
 * @param n number of bytes to copy
 */
#define GNUNET_memcpy(dst, src, n) do { if (0 != n) { (void)memcpy(dst, src, n); } } while (0)

/**
 * Should we print some debug output?
 */
#define VERBOSE 0

/**
 * Must match packet ID used by gnunet-helper-nat-client.c
 */
#define PACKET_ID 256

/**
 * Must match IP given in the client.
 */
#define DUMMY_IP "192.0.2.86"

/**
 * Port for UDP
 */
#define NAT_TRAV_PORT 22225

/**
 * How often do we send our ICMP messages to receive replies?
 */
#define ICMP_SEND_FREQUENCY_MS 500

/**
 * IPv4 header.
 */
struct ip_header {
  /**
   * Version (4 bits) + Internet header length (4 bits)
   */
  uint8_t vers_ihl;

  /**
   * Type of service
   */
  uint8_t tos;

  /**
   * Total length
   */
  uint16_t pkt_len;

  /**
   * Identification
   */
  uint16_t id;

  /**
   * Flags (3 bits) + Fragment offset (13 bits)
   */
  uint16_t flags_frag_offset;

  /**
   * Time to live
   */
  uint8_t ttl;

  /**
   * Protocol
   */
  uint8_t proto;

  /**
   * Header checksum
   */
  uint16_t checksum;

  /**
   * Source address
   */
  uint32_t src_ip;

  /**
   * Destination address
   */
  uint32_t dst_ip;
};

/**
 * Format of ICMP packet.
 */
struct icmp_ttl_exceeded_header {
  uint8_t type;

  uint8_t code;

  uint16_t checksum;

  uint32_t unused;

  /* followed by original payload */
};

struct icmp_echo_header {
  uint8_t type;

  uint8_t code;

  uint16_t checksum;

  uint32_t reserved;
};


/**
 * Beginning of UDP packet.
 */
struct udp_header {
  uint16_t src_port;

  uint16_t dst_port;

  uint16_t length;

  uint16_t crc;
};

/**
 * Socket we use to receive "fake" ICMP replies.
 */
static int icmpsock;

/**
 * Socket we use to send our ICMP requests.
 */
static int rawsock;

/**
 * Socket we use to send our UDP requests.
 */
static int udpsock;

/**
 * Target "dummy" address.
 */
static struct in_addr dummy;


/**
 * CRC-16 for IP/ICMP headers.
 *
 * @param data what to calculate the CRC over
 * @param bytes number of bytes in data (must be multiple of 2)
 * @return the CRC 16.
 */
static uint16_t
calc_checksum(const uint16_t * data, unsigned int bytes)
{
  uint32_t sum;
  unsigned int i;

  sum = 0;
  for (i = 0; i < bytes / 2; i++)
    sum += data[i];
  sum = (sum & 0xffff) + (sum >> 16);
  sum = htons(0xffff - sum);
  return sum;
}


/**
 * Send an ICMP message to the dummy IP.
 *
 * @param my_ip source address (our ip address)
 */
static void
send_icmp_echo(const struct in_addr *my_ip)
{
  char packet[sizeof(struct ip_header) + sizeof(struct icmp_echo_header)];
  struct icmp_echo_header icmp_echo;
  struct ip_header ip_pkt;
  struct sockaddr_in dst;
  size_t off;
  int err;

  off = 0;
  ip_pkt.vers_ihl = 0x45;
  ip_pkt.tos = 0;
  ip_pkt.pkt_len = htons(sizeof(packet));
  ip_pkt.id = htons(PACKET_ID);
  ip_pkt.flags_frag_offset = 0;
  ip_pkt.ttl = IPDEFTTL;
  ip_pkt.proto = IPPROTO_ICMP;
  ip_pkt.checksum = 0;
  ip_pkt.src_ip = my_ip->s_addr;
  ip_pkt.dst_ip = dummy.s_addr;
  ip_pkt.checksum =
    htons(calc_checksum((uint16_t *)&ip_pkt,
                        sizeof(struct ip_header)));
  GNUNET_memcpy(&packet[off],
                &ip_pkt,
                sizeof(struct ip_header));
  off += sizeof(struct ip_header);

  icmp_echo.type = ICMP_ECHO;
  icmp_echo.code = 0;
  icmp_echo.checksum = 0;
  icmp_echo.reserved = 0;
  icmp_echo.checksum =
    htons(calc_checksum
            ((uint16_t *)&icmp_echo,
            sizeof(struct icmp_echo_header)));
  GNUNET_memcpy(&packet[off],
                &icmp_echo,
                sizeof(struct icmp_echo_header));
  off += sizeof(struct icmp_echo_header);

  memset(&dst, 0, sizeof(dst));
  dst.sin_family = AF_INET;
#if HAVE_SOCKADDR_IN_SIN_LEN
  dst.sin_len = sizeof(struct sockaddr_in);
#endif
  dst.sin_addr = dummy;
  err = sendto(rawsock,
               packet,
               off,
               0,
               (struct sockaddr *)&dst,
               sizeof(dst));
  if (err < 0)
    {
#if VERBOSE
      fprintf(stderr,
              "sendto failed: %s\n",
              strerror(errno));
#endif
    }
  else if (sizeof(packet) != err)
    {
      fprintf(stderr,
              "Error: partial send of ICMP message\n");
    }
}


/**
 * Send a UDP message to the dummy IP.
 */
static void
send_udp()
{
  struct sockaddr_in dst;
  ssize_t err;

  memset(&dst, 0, sizeof(dst));
  dst.sin_family = AF_INET;
#if HAVE_SOCKADDR_IN_SIN_LEN
  dst.sin_len = sizeof(struct sockaddr_in);
#endif
  dst.sin_addr = dummy;
  dst.sin_port = htons(NAT_TRAV_PORT);
  err = sendto(udpsock,
               NULL,
               0,
               0,
               (struct sockaddr *)&dst,
               sizeof(dst));
  if (err < 0)
    {
#if VERBOSE
      fprintf(stderr,
              "sendto failed: %s\n",
              strerror(errno));
#endif
    }
  else if (0 != err)
    {
      fprintf(stderr,
              "Error: partial send of ICMP message\n");
    }
}


/**
 * We've received an ICMP response.  Process it.
 */
static void
process_icmp_response()
{
  char buf[65536];
  ssize_t have;
  struct in_addr source_ip;
  struct ip_header ip_pkt;
  struct icmp_ttl_exceeded_header icmp_ttl;
  struct icmp_echo_header icmp_echo;
  struct udp_header udp_pkt;
  size_t off;
  uint16_t port;

  have = read(icmpsock, buf, sizeof(buf));
  if (-1 == have)
    {
      fprintf(stderr,
              "Error reading raw socket: %s\n",
              strerror(errno));
      return;
    }
#if VERBOSE
  fprintf(stderr,
          "Received message of %u bytes\n",
          (unsigned int)have);
#endif
  if (have <
      (ssize_t)(sizeof(struct ip_header) +
                sizeof(struct icmp_ttl_exceeded_header) +
                sizeof(struct ip_header)))
    {
      /* malformed */
      return;
    }
  off = 0;
  GNUNET_memcpy(&ip_pkt,
                &buf[off],
                sizeof(struct ip_header));
  off += sizeof(struct ip_header);
  GNUNET_memcpy(&icmp_ttl,
                &buf[off],
                sizeof(struct icmp_ttl_exceeded_header));
  off += sizeof(struct icmp_ttl_exceeded_header);
  if ((ICMP_TIME_EXCEEDED != icmp_ttl.type) || (0 != icmp_ttl.code))
    {
      /* different type than what we want */
      return;
    }
  /* grab source IP of 1st IP header */
  source_ip.s_addr = ip_pkt.src_ip;

  /* skip 2nd IP header */
  GNUNET_memcpy(&ip_pkt,
                &buf[off],
                sizeof(struct ip_header));
  off += sizeof(struct ip_header);

  switch (ip_pkt.proto)
    {
    case IPPROTO_ICMP:
      if (have !=
          (sizeof(struct ip_header) * 2 +
           sizeof(struct icmp_ttl_exceeded_header) +
           sizeof(struct icmp_echo_header)))
        {
          /* malformed */
          return;
        }
      /* grab ICMP ECHO content */
      GNUNET_memcpy(&icmp_echo,
                    &buf[off],
                    sizeof(struct icmp_echo_header));
      port = (uint16_t)ntohl(icmp_echo.reserved);
      break;

    case IPPROTO_UDP:
      if (have !=
          (sizeof(struct ip_header) * 2 +
           sizeof(struct icmp_ttl_exceeded_header) + sizeof(struct udp_header)))
        {
          /* malformed */
          return;
        }
      /* grab UDP content */
      GNUNET_memcpy(&udp_pkt,
                    &buf[off],
                    sizeof(struct udp_header));
      port = ntohs(udp_pkt.length);
      break;

    default:
      /* different type than what we want */
      return;
    }

  if (port == 0)
    fprintf(stdout, "%s\n",
            inet_ntop(AF_INET, &source_ip, buf, sizeof(buf)));
  else
    fprintf(stdout, "%s:%u\n",
            inet_ntop(AF_INET, &source_ip, buf, sizeof(buf)),
            (unsigned int)port);
  fflush(stdout);
}


/**
 * Fully initialize the raw socket.
 *
 * @return -1 on error, 0 on success
 */
static int
setup_raw_socket()
{
  const int one = 1;

  if (-1 ==
      setsockopt(rawsock,
                 SOL_SOCKET,
                 SO_BROADCAST,
                 (char *)&one,
                 sizeof(one)))
    {
      fprintf(stderr,
              "setsockopt failed: %s\n",
              strerror(errno));
      return -1;
    }
  if (-1 ==
      setsockopt(rawsock,
                 IPPROTO_IP,
                 IP_HDRINCL,
                 (char *)&one,
                 sizeof(one)))
    {
      fprintf(stderr,
              "setsockopt failed: %s\n",
              strerror(errno));
      return -1;
    }
  return 0;
}


/**
 * Create a UDP socket for writing.
 *
 * @param my_ip source address (our ip address)
 * @return -1 on error
 */
static int
make_udp_socket(const struct in_addr *my_ip)
{
  int ret;
  struct sockaddr_in addr;

  ret = socket(AF_INET, SOCK_DGRAM, 0);
  if (-1 == ret)
    {
      fprintf(stderr,
              "Error opening UDP socket: %s\n",
              strerror(errno));
      return -1;
    }
  memset(&addr, 0, sizeof(addr));
  addr.sin_family = AF_INET;
#if HAVE_SOCKADDR_IN_SIN_LEN
  addr.sin_len = sizeof(struct sockaddr_in);
#endif
  addr.sin_addr = *my_ip;
  addr.sin_port = htons(NAT_TRAV_PORT);

  if (0 != bind(ret,
                (struct sockaddr *)&addr,
                sizeof(addr)))
    {
      fprintf(stderr,
              "Error binding UDP socket to port %u: %s\n",
              NAT_TRAV_PORT,
              strerror(errno));
      (void)close(ret);
      return -1;
    }
  return ret;
}


int
main(int argc,
     char *const *argv)
{
  struct in_addr external;
  fd_set rs;
  struct timeval tv;
  uid_t uid;
  unsigned int alt;
  int icmp_eno;
  int raw_eno;
  int global_ret;

  /* Create an ICMP raw socket for reading (we'll check errors later) */
  icmpsock = socket(AF_INET,
                    SOCK_RAW,
                    IPPROTO_ICMP);
  icmp_eno = errno;

  /* Create an (ICMP) raw socket for writing (we'll check errors later) */
  rawsock = socket(AF_INET,
                   SOCK_RAW,
                   IPPROTO_RAW);
  raw_eno = errno;
  udpsock = -1;

  /* drop root rights */
  uid = getuid();
#ifdef HAVE_SETRESUID
  if (0 != setresuid(uid, uid, uid))
    {
      fprintf(stderr,
              "Failed to setresuid: %s\n",
              strerror(errno));
      global_ret = 1;
      goto error_exit;
    }
#else
  if (0 != (setuid(uid) | seteuid(uid)))
    {
      fprintf(stderr,
              "Failed to setuid: %s\n",
              strerror(errno));
      global_ret = 2;
      goto error_exit;
    }
#endif

  /* Now that we run without root rights, we can do error checking... */
  if (2 != argc)
    {
      fprintf(stderr,
              "This program must be started with our (internal NAT) IP as the only argument.\n");
      global_ret = 3;
      goto error_exit;
    }
  if (1 != inet_pton(AF_INET, argv[1], &external))
    {
      fprintf(stderr,
              "Error parsing IPv4 address: %s\n",
              strerror(errno));
      global_ret = 4;
      goto error_exit;
    }
  if (1 != inet_pton(AF_INET, DUMMY_IP, &dummy))
    {
      fprintf(stderr,
              "Internal error converting dummy IP to binary.\n");
      global_ret = 5;
      goto error_exit;
    }

  /* error checking icmpsock */
  if (-1 == icmpsock)
    {
      fprintf(stderr,
              "Error opening RAW socket: %s\n",
              strerror(icmp_eno));
      global_ret = 6;
      goto error_exit;
    }
  if (icmpsock >= FD_SETSIZE)
    {
      /* this could happen if we were started with a large number of already-open
         file descriptors... */
      fprintf(stderr,
              "Socket number too large (%d > %u)\n",
              icmpsock,
              (unsigned int)FD_SETSIZE);
      global_ret = 7;
      goto error_exit;
    }

  /* error checking rawsock */
  if (-1 == rawsock)
    {
      fprintf(stderr,
              "Error opening RAW socket: %s\n",
              strerror(raw_eno));
      global_ret = 8;
      goto error_exit;
    }
  /* no need to check 'rawsock' against FD_SETSIZE as it is never used
     with 'select' */

  if (0 != setup_raw_socket())
    {
      global_ret = 9;
      goto error_exit;
    }

  if (-1 == (udpsock = make_udp_socket(&external)))
    {
      global_ret = 10;
      goto error_exit;
    }

  alt = 0;
  while (1)
    {
      FD_ZERO(&rs);
      FD_SET(icmpsock, &rs);
      tv.tv_sec = 0;
      tv.tv_usec = ICMP_SEND_FREQUENCY_MS * 1000;
      if (-1 == select(icmpsock + 1, &rs, NULL, NULL, &tv))
        {
          if (errno == EINTR)
            continue;
          fprintf(stderr,
                  "select failed: %s\n",
                  strerror(errno));
          break;
        }
      if (1 == getppid())       /* Check the parent process id, if 1 the parent has died, so we should die too */
        break;
      if (FD_ISSET(icmpsock, &rs))
        {
          process_icmp_response();
          continue;
        }
      if (0 == (++alt % 2))
        send_icmp_echo(&external);
      else
        send_udp();
    }

  /* select failed (internal error or OS out of resources) */
  global_ret = 11;
error_exit:
  if (-1 != icmpsock)
    (void)close(icmpsock);
  if (-1 != rawsock)
    (void)close(rawsock);
  if (-1 != udpsock)
    (void)close(udpsock);
  return global_ret;
}


/* end of gnunet-helper-nat-server.c */