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3 Copyright (C) 2010 GNUnet e.V.
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6 under the terms of the GNU Affero General Public License as published
7 by the Free Software Foundation, either version 3 of the License,
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11 WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Affero General Public License for more details.
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16 along with this program. If not, see <http://www.gnu.org/licenses/>.
18 SPDX-License-Identifier: AGPL3.0-or-later
22 * @file src/nat/gnunet-helper-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
42 * - Benjamin Kuperman (22 Aug 2010)
43 * - Jacob Appelbaum (19 Dec 2011)
46 /* Just needed for HAVE_SOCKADDR_IN_SIN_LEN test macro! */
47 #include "gnunet_config.h"
51 #include <sys/types.h>
52 #include <sys/socket.h>
53 #include <arpa/inet.h>
54 #include <sys/select.h>
56 #include <sys/types.h>
64 #include <netinet/ip.h>
65 #include <netinet/ip_icmp.h>
66 #include <netinet/in.h>
68 /* The following constant is missing from FreeBSD 9.2 */
69 #ifndef ICMP_TIME_EXCEEDED
70 #define ICMP_TIME_EXCEEDED 11
74 * Call memcpy() but check for @a n being 0 first. In the latter
75 * case, it is now safe to pass NULL for @a src or @a dst.
76 * Unlike traditional memcpy(), returns nothing.
78 * @param dst destination of the copy, may be NULL if @a n is zero
79 * @param src source of the copy, may be NULL if @a n is zero
80 * @param n number of bytes to copy
82 #define GNUNET_memcpy(dst, src, n) do { if (0 != n) { (void) memcpy (dst, src, \
87 * Should we print some debug output?
92 * Must match packet ID used by gnunet-helper-nat-client.c
97 * Must match IP given in the client.
99 #define DUMMY_IP "192.0.2.86"
104 #define NAT_TRAV_PORT 22225
107 * How often do we send our ICMP messages to receive replies?
109 #define ICMP_SEND_FREQUENCY_MS 500
117 * Version (4 bits) + Internet header length (4 bits)
137 * Flags (3 bits) + Fragment offset (13 bits)
139 uint16_t flags_frag_offset;
162 * Destination address
168 * Format of ICMP packet.
170 struct icmp_ttl_exceeded_header
180 /* followed by original payload */
183 struct icmp_echo_header
196 * Beginning of UDP packet.
210 * Socket we use to receive "fake" ICMP replies.
215 * Socket we use to send our ICMP requests.
220 * Socket we use to send our UDP requests.
225 * Target "dummy" address.
227 static struct in_addr dummy;
231 * CRC-16 for IP/ICMP headers.
233 * @param data what to calculate the CRC over
234 * @param bytes number of bytes in data (must be multiple of 2)
235 * @return the CRC 16.
238 calc_checksum (const uint16_t *data, unsigned int bytes)
244 for (i = 0; i < bytes / 2; i++)
246 sum = (sum & 0xffff) + (sum >> 16);
247 sum = htons (0xffff - sum);
253 * Send an ICMP message to the dummy IP.
255 * @param my_ip source address (our ip address)
258 send_icmp_echo (const struct in_addr *my_ip)
260 char packet[sizeof(struct ip_header) + sizeof(struct icmp_echo_header)];
261 struct icmp_echo_header icmp_echo;
262 struct ip_header ip_pkt;
263 struct sockaddr_in dst;
268 ip_pkt.vers_ihl = 0x45;
270 ip_pkt.pkt_len = htons (sizeof(packet));
271 ip_pkt.id = htons (PACKET_ID);
272 ip_pkt.flags_frag_offset = 0;
273 ip_pkt.ttl = IPDEFTTL;
274 ip_pkt.proto = IPPROTO_ICMP;
276 ip_pkt.src_ip = my_ip->s_addr;
277 ip_pkt.dst_ip = dummy.s_addr;
279 htons (calc_checksum ((uint16_t *) &ip_pkt,
280 sizeof(struct ip_header)));
281 GNUNET_memcpy (&packet[off],
283 sizeof(struct ip_header));
284 off += sizeof(struct ip_header);
286 icmp_echo.type = ICMP_ECHO;
288 icmp_echo.checksum = 0;
289 icmp_echo.reserved = 0;
292 ((uint16_t *) &icmp_echo,
293 sizeof(struct icmp_echo_header)));
294 GNUNET_memcpy (&packet[off],
296 sizeof(struct icmp_echo_header));
297 off += sizeof(struct icmp_echo_header);
299 memset (&dst, 0, sizeof(dst));
300 dst.sin_family = AF_INET;
301 #if HAVE_SOCKADDR_IN_SIN_LEN
302 dst.sin_len = sizeof(struct sockaddr_in);
304 dst.sin_addr = dummy;
305 err = sendto (rawsock,
309 (struct sockaddr *) &dst,
315 "sendto failed: %s\n",
319 else if (sizeof(packet) != err)
322 "Error: partial send of ICMP message\n");
328 * Send a UDP message to the dummy IP.
333 struct sockaddr_in dst;
336 memset (&dst, 0, sizeof(dst));
337 dst.sin_family = AF_INET;
338 #if HAVE_SOCKADDR_IN_SIN_LEN
339 dst.sin_len = sizeof(struct sockaddr_in);
341 dst.sin_addr = dummy;
342 dst.sin_port = htons (NAT_TRAV_PORT);
343 err = sendto (udpsock,
347 (struct sockaddr *) &dst,
353 "sendto failed: %s\n",
360 "Error: partial send of ICMP message\n");
366 * We've received an ICMP response. Process it.
369 process_icmp_response ()
373 struct in_addr source_ip;
374 struct ip_header ip_pkt;
375 struct icmp_ttl_exceeded_header icmp_ttl;
376 struct icmp_echo_header icmp_echo;
377 struct udp_header udp_pkt;
381 have = read (icmpsock, buf, sizeof(buf));
385 "Error reading raw socket: %s\n",
391 "Received message of %u bytes\n",
392 (unsigned int) have);
395 (ssize_t) (sizeof(struct ip_header)
396 + sizeof(struct icmp_ttl_exceeded_header)
397 + sizeof(struct ip_header)))
403 GNUNET_memcpy (&ip_pkt,
405 sizeof(struct ip_header));
406 off += sizeof(struct ip_header);
407 GNUNET_memcpy (&icmp_ttl,
409 sizeof(struct icmp_ttl_exceeded_header));
410 off += sizeof(struct icmp_ttl_exceeded_header);
411 if ((ICMP_TIME_EXCEEDED != icmp_ttl.type) || (0 != icmp_ttl.code))
413 /* different type than what we want */
416 /* grab source IP of 1st IP header */
417 source_ip.s_addr = ip_pkt.src_ip;
419 /* skip 2nd IP header */
420 GNUNET_memcpy (&ip_pkt,
422 sizeof(struct ip_header));
423 off += sizeof(struct ip_header);
425 switch (ip_pkt.proto)
429 (sizeof(struct ip_header) * 2
430 + sizeof(struct icmp_ttl_exceeded_header)
431 + sizeof(struct icmp_echo_header)))
436 /* grab ICMP ECHO content */
437 GNUNET_memcpy (&icmp_echo,
439 sizeof(struct icmp_echo_header));
440 port = (uint16_t) ntohl (icmp_echo.reserved);
445 (sizeof(struct ip_header) * 2
446 + sizeof(struct icmp_ttl_exceeded_header) + sizeof(struct udp_header)))
451 /* grab UDP content */
452 GNUNET_memcpy (&udp_pkt,
454 sizeof(struct udp_header));
455 port = ntohs (udp_pkt.length);
459 /* different type than what we want */
464 fprintf (stdout, "%s\n",
465 inet_ntop (AF_INET, &source_ip, buf, sizeof(buf)));
467 fprintf (stdout, "%s:%u\n",
468 inet_ntop (AF_INET, &source_ip, buf, sizeof(buf)),
469 (unsigned int) port);
475 * Fully initialize the raw socket.
477 * @return -1 on error, 0 on success
492 "setsockopt failed: %s\n",
504 "setsockopt failed: %s\n",
513 * Create a UDP socket for writing.
515 * @param my_ip source address (our ip address)
516 * @return -1 on error
519 make_udp_socket (const struct in_addr *my_ip)
522 struct sockaddr_in addr;
524 ret = socket (AF_INET, SOCK_DGRAM, 0);
528 "Error opening UDP socket: %s\n",
532 memset (&addr, 0, sizeof(addr));
533 addr.sin_family = AF_INET;
534 #if HAVE_SOCKADDR_IN_SIN_LEN
535 addr.sin_len = sizeof(struct sockaddr_in);
537 addr.sin_addr = *my_ip;
538 addr.sin_port = htons (NAT_TRAV_PORT);
541 (struct sockaddr *) &addr,
545 "Error binding UDP socket to port %u: %s\n",
559 struct in_addr external;
568 /* Create an ICMP raw socket for reading (we'll check errors later) */
569 icmpsock = socket (AF_INET,
574 /* Create an (ICMP) raw socket for writing (we'll check errors later) */
575 rawsock = socket (AF_INET,
581 /* drop root rights */
583 #ifdef HAVE_SETRESUID
584 if (0 != setresuid (uid, uid, uid))
587 "Failed to setresuid: %s\n",
593 if (0 != (setuid (uid) | seteuid (uid)))
596 "Failed to setuid: %s\n",
603 /* Now that we run without root rights, we can do error checking... */
607 "This program must be started with our (internal NAT) IP as the only argument.\n");
611 if (1 != inet_pton (AF_INET, argv[1], &external))
614 "Error parsing IPv4 address: %s\n",
619 if (1 != inet_pton (AF_INET, DUMMY_IP, &dummy))
622 "Internal error converting dummy IP to binary.\n");
627 /* error checking icmpsock */
631 "Error opening RAW socket: %s\n",
632 strerror (icmp_eno));
636 if (icmpsock >= FD_SETSIZE)
638 /* this could happen if we were started with a large number of already-open
639 file descriptors... */
641 "Socket number too large (%d > %u)\n",
643 (unsigned int) FD_SETSIZE);
648 /* error checking rawsock */
652 "Error opening RAW socket: %s\n",
657 /* no need to check 'rawsock' against FD_SETSIZE as it is never used
660 if (0 != setup_raw_socket ())
666 if (-1 == (udpsock = make_udp_socket (&external)))
676 FD_SET (icmpsock, &rs);
678 tv.tv_usec = ICMP_SEND_FREQUENCY_MS * 1000;
679 if (-1 == select (icmpsock + 1, &rs, NULL, NULL, &tv))
684 "select failed: %s\n",
688 if (1 == getppid ()) /* Check the parent process id, if 1 the parent has died, so we should die too */
690 if (FD_ISSET (icmpsock, &rs))
692 process_icmp_response ();
695 if (0 == (++alt % 2))
696 send_icmp_echo (&external);
701 /* select failed (internal error or OS out of resources) */
705 (void) close (icmpsock);
707 (void) close (rawsock);
709 (void) close (udpsock);
714 /* end of gnunet-helper-nat-server.c */