2 This file is part of GNUnet.
3 (C) 2010 Christian Grothoff (and other contributing authors)
5 GNUnet is free software; you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published
7 by the Free Software Foundation; either version 3, or (at your
8 option) any later version.
10 GNUnet is distributed in the hope that it will be useful, but
11 WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with GNUnet; see the file COPYING. If not, write to the
17 Free Software Foundation, Inc., 59 Temple Place - Suite 330,
18 Boston, MA 02111-1307, USA.
22 * @file src/transport/gnunet-nat-server.c
23 * @brief Tool to help bypass NATs using ICMP method; must run as root (SUID will do)
24 * This code will work under GNU/Linux only (or maybe BSDs, but never W32)
25 * @author Christian Grothoff
27 * This program will send ONE ICMP message every 500 ms RAW sockets
28 * to a DUMMY IP address and also listens for ICMP replies. Since
29 * it uses RAW sockets, it must be installed SUID or run as 'root'.
30 * In order to keep the security risk of the resulting SUID binary
31 * minimal, the program ONLY opens the two RAW sockets with root
32 * privileges, then drops them and only then starts to process
33 * command line arguments. The code also does not link against
34 * any shared libraries (except libc) and is strictly minimal
35 * (except for checking for errors). The following list of people
36 * have reviewed this code and considered it safe since the last
37 * modification (if you reviewed it, please have your name added
40 * - Christian Grothoff
44 /* Just needed for HAVE_SOCKADDR_IN_SIN_LEN test macro! */
45 #include "gnunet_config.h"
49 #include <sys/types.h>
50 #include <sys/socket.h>
51 #include <arpa/inet.h>
52 #include <sys/select.h>
54 #include <sys/types.h>
62 #include <netinet/ip.h>
63 #include <netinet/ip_icmp.h>
64 #include <netinet/in.h>
67 * Should we print some debug output?
72 * Must match IP given in the client.
74 #define DUMMY_IP "192.0.2.86"
77 * How often do we send our ICMP messages to receive replies?
79 #define ICMP_SEND_FREQUENCY_MS 500
88 * Version (4 bits) + Internet header length (4 bits)
108 * Flags (3 bits) + Fragment offset (13 bits)
110 uint16_t flags_frag_offset;
133 * Destination address
139 * Format of ICMP packet.
153 * Beginning of UDP packet.
165 * Socket we use to receive "fake" ICMP replies.
170 * Socket we use to send our ICMP requests.
175 * Target "dummy" address.
177 static struct in_addr dummy;
181 * CRC-16 for IP/ICMP headers.
183 * @param data what to calculate the CRC over
184 * @param bytes number of bytes in data (must be multiple of 2)
185 * @return the CRC 16.
188 calc_checksum(const uint16_t *data,
195 for (i=0;i<bytes/2;i++)
197 sum = (sum & 0xffff) + (sum >> 16);
198 sum = htons(0xffff - sum);
204 * Send an ICMP message to the dummy IP.
206 * @param my_ip source address (our ip address)
209 send_icmp_echo (const struct in_addr *my_ip)
211 struct icmp_packet icmp_echo;
212 struct sockaddr_in dst;
215 struct ip_packet ip_pkt;
216 struct icmp_packet icmp_pkt;
217 char packet[sizeof (ip_pkt) + sizeof (icmp_pkt)];
220 memset(&ip_pkt, 0, sizeof(ip_pkt));
221 ip_pkt.vers_ihl = 0x45;
223 ip_pkt.pkt_len = sizeof (packet);
225 ip_pkt.flags_frag_offset = 0;
226 ip_pkt.ttl = IPDEFTTL;
227 ip_pkt.proto = IPPROTO_ICMP;
229 ip_pkt.src_ip = my_ip->s_addr;
230 ip_pkt.dst_ip = dummy.s_addr;
231 ip_pkt.checksum = htons(calc_checksum((uint16_t*)&ip_pkt, sizeof (ip_pkt)));
232 memcpy (packet, &ip_pkt, sizeof (ip_pkt));
233 off += sizeof (ip_pkt);
235 icmp_echo.type = ICMP_ECHO;
237 icmp_echo.reserved = 0;
238 icmp_echo.checksum = 0;
239 icmp_echo.checksum = htons(calc_checksum((uint16_t*)&icmp_echo,
240 sizeof (struct icmp_packet)));
241 memcpy (&packet[off], &icmp_echo, sizeof (icmp_echo));
242 off += sizeof (icmp_echo);
244 memset (&dst, 0, sizeof (dst));
245 dst.sin_family = AF_INET;
246 #if HAVE_SOCKADDR_IN_SIN_LEN
247 dst.sin_len = sizeof (struct sockaddr_in);
249 dst.sin_addr = dummy;
250 err = sendto(rawsock,
252 (struct sockaddr*)&dst,
258 "sendto failed: %s\n", strerror(errno));
264 "Error: partial send of ICMP message\n");
270 * We've received an ICMP response. Process it.
273 process_icmp_response ()
278 struct ip_packet ip_pkt;
279 struct icmp_packet icmp_pkt;
280 struct udp_packet udp_pkt;
285 have = read (icmpsock, buf, sizeof (buf));
289 "Error reading raw socket: %s\n",
296 "Received message of %u bytes\n",
297 (unsigned int) have);
299 if (have == sizeof (struct ip_packet) *2 + sizeof (struct icmp_packet) * 2 + sizeof(uint32_t))
303 else if (have != sizeof (struct ip_packet) *2 + sizeof (struct icmp_packet) * 2)
307 "Received ICMP message of unexpected size: %u bytes\n",
308 (unsigned int) have);
313 memcpy (&ip_pkt, &buf[off], sizeof (ip_pkt));
314 off += sizeof (ip_pkt);
315 memcpy (&icmp_pkt, &buf[off], sizeof (icmp_pkt));
316 off += sizeof (icmp_pkt);
317 if ( ((ip_pkt.proto != IPPROTO_ICMP) && (ip_pkt.proto != IPPROTO_UDP)) ||
318 (icmp_pkt.type != ICMP_TIME_EXCEEDED) ||
319 (icmp_pkt.code != 0) )
321 /* maybe we got an actual reply back... */
327 memcpy (&ip_pkt, &buf[off], sizeof (ip_pkt));
328 off += sizeof (ip_pkt);
333 &buf[sizeof (struct ip_packet) *2 + sizeof (struct icmp_packet) * 2],
344 else if (ip_pkt.proto == IPPROTO_UDP)
355 ntohs((uint16_t) udp_pkt.length));
371 * Create an ICMP raw socket for reading.
373 * @return -1 on error
380 ret = socket (AF_INET, SOCK_RAW, IPPROTO_ICMP);
384 "Error opening RAW socket: %s\n",
388 if (ret >= FD_SETSIZE)
391 "Socket number too large (%d > %u)\n",
393 (unsigned int) FD_SETSIZE);
402 * Create an ICMP raw socket for writing.
404 * @return -1 on error
412 ret = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
416 "Error opening RAW socket: %s\n",
427 "setsockopt failed: %s\n",
435 (char *)&one, sizeof(one)) == -1)
438 "setsockopt failed: %s\n",
451 struct in_addr external;
459 "This program must be started with our (internal NAT) IP as the only argument.\n");
462 if (1 != inet_pton (AF_INET, argv[1], &external))
465 "Error parsing IPv4 address: %s\n",
469 if (1 != inet_pton (AF_INET, DUMMY_IP, &dummy))
472 "Internal error converting dummy IP to binary.\n");
475 if (-1 == (icmpsock = make_icmp_socket()))
479 if (-1 == (rawsock = make_raw_socket()))
485 if (0 != setresuid (uid, uid, uid))
488 "Failed to setresuid: %s\n",
490 /* not critical, continue anyway */
495 FD_SET (icmpsock, &rs);
497 tv.tv_usec = ICMP_SEND_FREQUENCY_MS * 1000;
498 if (-1 == select (icmpsock + 1, &rs, NULL, NULL, &tv))
503 "select failed: %s\n",
507 if (FD_ISSET (icmpsock, &rs))
508 process_icmp_response ();
509 send_icmp_echo (&external);
511 /* select failed (internal error or OS out of resources) */
518 /* end of gnunet-nat-server.c */