/* 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 General Public License as published by the Free Software Foundation; either version 3, 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 General Public License for more details. You should have received a copy of the GNU General Public License along with GNUnet; see the file COPYING. If not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ /** * @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 #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include /* The following constant is missing from FreeBSD 9.2 */ #ifndef ICMP_TIME_EXCEEDED #define ICMP_TIME_EXCEEDED 11 #endif /** * 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))); 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))); 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; memcpy (&ip_pkt, &buf[off], sizeof (struct ip_header)); off += sizeof (struct ip_header); 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 */ 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 */ 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 */ 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 (); 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 */