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3 Copyright (C) 2010 GNUnet e.V.
5 GNUnet is free software: you can redistribute it and/or modify it
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,
8 or (at your 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 Affero General Public License for more details.
15 You should have received a copy of the GNU Affero General Public License
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-client.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.
25 * @author Christian Grothoff
27 * This program will send ONE ICMP message using RAW sockets
28 * to the IP address specified as the second argument. 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 RAW socket 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)
45 /* Just needed for HAVE_SOCKADDR_IN_SIN_LEN test macro! */
46 #include "gnunet_config.h"
50 #include <sys/types.h>
51 #include <sys/socket.h>
52 #include <arpa/inet.h>
53 #include <sys/types.h>
60 #include <netinet/ip.h>
61 #include <netinet/ip_icmp.h>
62 #include <netinet/in.h>
64 /* The following constant is missing from FreeBSD 9.2 */
65 #ifndef ICMP_TIME_EXCEEDED
66 #define ICMP_TIME_EXCEEDED 11
70 * Call memcpy() but check for @a n being 0 first. In the latter
71 * case, it is now safe to pass NULL for @a src or @a dst.
72 * Unlike traditional memcpy(), returns nothing.
74 * @param dst destination of the copy, may be NULL if @a n is zero
75 * @param src source of the copy, may be NULL if @a n is zero
76 * @param n number of bytes to copy
78 #define GNUNET_memcpy(dst, src, n) do { if (0 != n) { (void) memcpy (dst, src, \
83 * Must match IP given in the server.
85 #define DUMMY_IP "192.0.2.86"
87 #define NAT_TRAV_PORT 22225
90 * Must match packet ID used by gnunet-helper-nat-server.c
100 * Version (4 bits) + Internet header length (4 bits)
120 * Flags (3 bits) + Fragment offset (13 bits)
122 uint16_t flags_frag_offset;
145 * Destination address
151 * Format of ICMP packet.
153 struct icmp_ttl_exceeded_header
163 /* followed by original payload */
166 struct icmp_echo_header
178 * Beginning of UDP packet.
192 * Socket we use to send our fake ICMP replies.
197 * Target "dummy" address of the packet we pretend to respond to.
199 static struct in_addr dummy;
204 static uint16_t port;
208 * CRC-16 for IP/ICMP headers.
210 * @param data what to calculate the CRC over
211 * @param bytes number of bytes in data (must be multiple of 2)
212 * @return the CRC 16.
215 calc_checksum (const uint16_t *data, unsigned int bytes)
221 for (i = 0; i < bytes / 2; i++)
223 sum = (sum & 0xffff) + (sum >> 16);
224 sum = htons (0xffff - sum);
230 * Send an ICMP message to the target.
232 * @param my_ip source address
233 * @param other target address
236 send_icmp_udp (const struct in_addr *my_ip, const struct in_addr *other)
238 char packet[sizeof(struct ip_header) * 2
239 + sizeof(struct icmp_ttl_exceeded_header)
240 + sizeof(struct udp_header)];
241 struct ip_header ip_pkt;
242 struct icmp_ttl_exceeded_header icmp_pkt;
243 struct udp_header udp_pkt;
244 struct sockaddr_in dst;
248 /* ip header: send to (known) ip address */
250 ip_pkt.vers_ihl = 0x45;
253 ip_pkt.pkt_len = sizeof(packet); /* Workaround PR kern/21737 */
255 ip_pkt.pkt_len = htons (sizeof(packet));
257 ip_pkt.id = htons (PACKET_ID);
258 ip_pkt.flags_frag_offset = 0;
260 ip_pkt.proto = IPPROTO_ICMP;
262 ip_pkt.src_ip = my_ip->s_addr;
263 ip_pkt.dst_ip = other->s_addr;
265 htons (calc_checksum ((uint16_t *) &ip_pkt, sizeof(struct ip_header)));
266 GNUNET_memcpy (&packet[off],
268 sizeof(struct ip_header));
269 off += sizeof(struct ip_header);
271 icmp_pkt.type = ICMP_TIME_EXCEEDED;
273 icmp_pkt.checksum = 0;
275 GNUNET_memcpy (&packet[off],
277 sizeof(struct icmp_ttl_exceeded_header));
278 off += sizeof(struct icmp_ttl_exceeded_header);
280 /* ip header of the presumably 'lost' udp packet */
281 ip_pkt.vers_ihl = 0x45;
284 htons (sizeof(struct ip_header) + sizeof(struct udp_header));
285 ip_pkt.id = htons (0);
286 ip_pkt.flags_frag_offset = 0;
288 ip_pkt.proto = IPPROTO_UDP;
290 ip_pkt.src_ip = other->s_addr;
291 ip_pkt.dst_ip = dummy.s_addr;
293 htons (calc_checksum ((uint16_t *) &ip_pkt, sizeof(struct ip_header)));
294 GNUNET_memcpy (&packet[off],
296 sizeof(struct ip_header));
297 off += sizeof(struct ip_header);
299 /* build UDP header */
300 udp_pkt.src_port = htons (NAT_TRAV_PORT);
301 udp_pkt.dst_port = htons (NAT_TRAV_PORT);
302 udp_pkt.length = htons (port);
304 GNUNET_memcpy (&packet[off],
306 sizeof(struct udp_header));
307 off += sizeof(struct udp_header);
309 /* set ICMP checksum */
312 ((uint16_t *) &packet[sizeof(struct ip_header)],
313 sizeof(struct icmp_ttl_exceeded_header)
314 + sizeof(struct ip_header) + sizeof(struct udp_header)));
315 GNUNET_memcpy (&packet[sizeof(struct ip_header)],
317 sizeof(struct icmp_ttl_exceeded_header));
319 memset (&dst, 0, sizeof(dst));
320 dst.sin_family = AF_INET;
321 #if HAVE_SOCKADDR_IN_SIN_LEN
322 dst.sin_len = sizeof(struct sockaddr_in);
324 dst.sin_addr = *other;
326 sendto (rawsock, packet, sizeof(packet), 0, (struct sockaddr *) &dst,
330 fprintf (stderr, "sendto failed: %s\n", strerror (errno));
332 else if (sizeof(packet) != (size_t) err)
334 fprintf (stderr, "Error: partial send of ICMP message with size %lu\n",
335 (unsigned long) off);
341 * Send an ICMP message to the target.
343 * @param my_ip source address
344 * @param other target address
347 send_icmp (const struct in_addr *my_ip, const struct in_addr *other)
349 struct ip_header ip_pkt;
350 struct icmp_ttl_exceeded_header icmp_ttl;
351 struct icmp_echo_header icmp_echo;
352 struct sockaddr_in dst;
353 char packet[sizeof(struct ip_header) * 2
354 + sizeof(struct icmp_ttl_exceeded_header)
355 + sizeof(struct icmp_echo_header)];
359 /* ip header: send to (known) ip address */
361 ip_pkt.vers_ihl = 0x45;
364 ip_pkt.pkt_len = sizeof(packet); /* Workaround PR kern/21737 */
366 ip_pkt.pkt_len = htons (sizeof(packet));
368 ip_pkt.id = htons (PACKET_ID);
369 ip_pkt.flags_frag_offset = 0;
370 ip_pkt.ttl = IPDEFTTL;
371 ip_pkt.proto = IPPROTO_ICMP;
373 ip_pkt.src_ip = my_ip->s_addr;
374 ip_pkt.dst_ip = other->s_addr;
376 htons (calc_checksum ((uint16_t *) &ip_pkt, sizeof(struct ip_header)));
377 GNUNET_memcpy (&packet[off],
379 sizeof(struct ip_header));
380 off = sizeof(ip_pkt);
382 /* icmp reply: time exceeded */
383 icmp_ttl.type = ICMP_TIME_EXCEEDED;
385 icmp_ttl.checksum = 0;
387 GNUNET_memcpy (&packet[off],
389 sizeof(struct icmp_ttl_exceeded_header));
390 off += sizeof(struct icmp_ttl_exceeded_header);
392 /* ip header of the presumably 'lost' udp packet */
393 ip_pkt.vers_ihl = 0x45;
396 htons (sizeof(struct ip_header) + sizeof(struct icmp_echo_header));
397 ip_pkt.id = htons (PACKET_ID);
398 ip_pkt.flags_frag_offset = 0;
399 ip_pkt.ttl = 1; /* real TTL would be 1 on a time exceeded packet */
400 ip_pkt.proto = IPPROTO_ICMP;
401 ip_pkt.src_ip = other->s_addr;
402 ip_pkt.dst_ip = dummy.s_addr;
405 htons (calc_checksum ((uint16_t *) &ip_pkt, sizeof(struct ip_header)));
406 GNUNET_memcpy (&packet[off],
408 sizeof(struct ip_header));
409 off += sizeof(struct ip_header);
411 icmp_echo.type = ICMP_ECHO;
413 icmp_echo.reserved = htonl (port);
414 icmp_echo.checksum = 0;
417 ((uint16_t *) &icmp_echo, sizeof(struct icmp_echo_header)));
418 GNUNET_memcpy (&packet[off],
420 sizeof(struct icmp_echo_header));
422 /* no go back to calculate ICMP packet checksum */
423 off = sizeof(struct ip_header);
426 ((uint16_t *) &packet[off],
427 sizeof(struct icmp_ttl_exceeded_header)
428 + sizeof(struct ip_header) + sizeof(struct icmp_echo_header)));
429 GNUNET_memcpy (&packet[off],
431 sizeof(struct icmp_ttl_exceeded_header));
433 /* prepare for transmission */
434 memset (&dst, 0, sizeof(dst));
435 dst.sin_family = AF_INET;
436 #if HAVE_SOCKADDR_IN_SIN_LEN
437 dst.sin_len = sizeof(struct sockaddr_in);
439 dst.sin_addr = *other;
441 sendto (rawsock, packet, sizeof(packet), 0, (struct sockaddr *) &dst,
445 fprintf (stderr, "sendto failed: %s\n", strerror (errno));
447 else if (sizeof(packet) != (size_t) err)
449 fprintf (stderr, "Error: partial send of ICMP message\n");
455 main (int argc, char *const *argv)
458 struct in_addr external;
459 struct in_addr target;
465 /* Create an ICMP raw socket for writing (only operation that requires root) */
466 rawsock = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
467 raw_eno = errno; /* for later error checking */
469 /* now drop root privileges */
471 #ifdef HAVE_SETRESUID
472 if (0 != setresuid (uid, uid, uid))
474 fprintf (stderr, "Failed to setresuid: %s\n", strerror (errno));
479 if (0 != (setuid (uid) | seteuid (uid)))
481 fprintf (stderr, "Failed to setuid: %s\n", strerror (errno));
488 fprintf (stderr, "Error opening RAW socket: %s\n", strerror (raw_eno));
493 setsockopt (rawsock, SOL_SOCKET, SO_BROADCAST, (char *) &one,
496 fprintf (stderr, "setsockopt failed: %s\n", strerror (errno));
501 setsockopt (rawsock, IPPROTO_IP, IP_HDRINCL, (char *) &one, sizeof(one)))
503 fprintf (stderr, "setsockopt failed: %s\n", strerror (errno));
511 "This program must be started with our IP, the targets external IP, and our port as arguments.\n");
515 if ((1 != inet_pton (AF_INET, argv[1], &external)) ||
516 (1 != inet_pton (AF_INET, argv[2], &target)))
518 fprintf (stderr, "Error parsing IPv4 address: %s\n", strerror (errno));
522 if ((1 != sscanf (argv[3], "%u", &p)) || (0 == p) || (0xFFFF < p))
524 fprintf (stderr, "Error parsing port value `%s'\n", argv[3]);
529 if (1 != inet_pton (AF_INET, DUMMY_IP, &dummy))
531 fprintf (stderr, "Internal error converting dummy IP to binary.\n");
535 send_icmp (&external, &target);
536 send_icmp_udp (&external, &target);
540 (void) close (rawsock);
545 /* end of gnunet-helper-nat-client.c */