2 This file is part of GNUnet.
3 (C) 2010 Christian Grothoff (and other contributing authors)
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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
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12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 General Public License for more details.
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17 Free Software Foundation, Inc., 59 Temple Place - Suite 330,
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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>
69 * Should we print some debug output?
74 * Must match packet ID used by gnunet-helper-nat-client.c
79 * Must match IP given in the client.
81 #define DUMMY_IP "192.0.2.86"
86 #define NAT_TRAV_PORT 22225
89 * How often do we send our ICMP messages to receive replies?
91 #define ICMP_SEND_FREQUENCY_MS 500
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
179 * Beginning of UDP packet.
193 * Socket we use to receive "fake" ICMP replies.
198 * Socket we use to send our ICMP requests.
203 * Socket we use to send our UDP requests.
208 * Target "dummy" address.
210 static struct in_addr dummy;
214 * CRC-16 for IP/ICMP headers.
216 * @param data what to calculate the CRC over
217 * @param bytes number of bytes in data (must be multiple of 2)
218 * @return the CRC 16.
221 calc_checksum (const uint16_t * data, unsigned int bytes)
227 for (i = 0; i < bytes / 2; i++)
229 sum = (sum & 0xffff) + (sum >> 16);
230 sum = htons (0xffff - sum);
236 * Send an ICMP message to the dummy IP.
238 * @param my_ip source address (our ip address)
241 send_icmp_echo (const struct in_addr *my_ip)
243 char packet[sizeof (struct ip_header) + sizeof (struct icmp_echo_header)];
244 struct icmp_echo_header icmp_echo;
245 struct ip_header ip_pkt;
246 struct sockaddr_in dst;
251 ip_pkt.vers_ihl = 0x45;
253 ip_pkt.pkt_len = htons (sizeof (packet));
254 ip_pkt.id = htons (PACKET_ID);
255 ip_pkt.flags_frag_offset = 0;
256 ip_pkt.ttl = IPDEFTTL;
257 ip_pkt.proto = IPPROTO_ICMP;
259 ip_pkt.src_ip = my_ip->s_addr;
260 ip_pkt.dst_ip = dummy.s_addr;
262 htons (calc_checksum ((uint16_t *) & ip_pkt, sizeof (struct ip_header)));
263 memcpy (&packet[off], &ip_pkt, sizeof (struct ip_header));
264 off += sizeof (struct ip_header);
266 icmp_echo.type = ICMP_ECHO;
268 icmp_echo.checksum = 0;
269 icmp_echo.reserved = 0;
272 ((uint16_t *) & icmp_echo, sizeof (struct icmp_echo_header)));
273 memcpy (&packet[off], &icmp_echo, sizeof (struct icmp_echo_header));
274 off += sizeof (struct icmp_echo_header);
276 memset (&dst, 0, sizeof (dst));
277 dst.sin_family = AF_INET;
278 #if HAVE_SOCKADDR_IN_SIN_LEN
279 dst.sin_len = sizeof (struct sockaddr_in);
281 dst.sin_addr = dummy;
283 sendto (rawsock, packet, off, 0, (struct sockaddr *) &dst, sizeof (dst));
287 fprintf (stderr, "sendto failed: %s\n", strerror (errno));
290 else if (sizeof (packet) != err)
292 fprintf (stderr, "Error: partial send of ICMP message\n");
298 * Send a UDP message to the dummy IP.
303 struct sockaddr_in dst;
306 memset (&dst, 0, sizeof (dst));
307 dst.sin_family = AF_INET;
308 #if HAVE_SOCKADDR_IN_SIN_LEN
309 dst.sin_len = sizeof (struct sockaddr_in);
311 dst.sin_addr = dummy;
312 dst.sin_port = htons (NAT_TRAV_PORT);
313 err = sendto (udpsock, NULL, 0, 0, (struct sockaddr *) &dst, sizeof (dst));
317 fprintf (stderr, "sendto failed: %s\n", strerror (errno));
322 fprintf (stderr, "Error: partial send of ICMP message\n");
328 * We've received an ICMP response. Process it.
331 process_icmp_response ()
335 struct in_addr source_ip;
336 struct ip_header ip_pkt;
337 struct icmp_ttl_exceeded_header icmp_ttl;
338 struct icmp_echo_header icmp_echo;
339 struct udp_header udp_pkt;
343 have = read (icmpsock, buf, sizeof (buf));
346 fprintf (stderr, "Error reading raw socket: %s\n", strerror (errno));
350 fprintf (stderr, "Received message of %u bytes\n", (unsigned int) have);
353 (ssize_t) (sizeof (struct ip_header) +
354 sizeof (struct icmp_ttl_exceeded_header) +
355 sizeof (struct ip_header)))
361 memcpy (&ip_pkt, &buf[off], sizeof (struct ip_header));
362 off += sizeof (struct ip_header);
363 memcpy (&icmp_ttl, &buf[off], sizeof (struct icmp_ttl_exceeded_header));
364 off += sizeof (struct icmp_ttl_exceeded_header);
365 if ((ICMP_TIME_EXCEEDED != icmp_ttl.type) || (0 != icmp_ttl.code))
367 /* different type than what we want */
370 /* skip 2nd IP header */
371 memcpy (&ip_pkt, &buf[off], sizeof (struct ip_header));
372 off += sizeof (struct ip_header);
374 switch (ip_pkt.proto)
378 (sizeof (struct ip_header) * 2 +
379 sizeof (struct icmp_ttl_exceeded_header) +
380 sizeof (struct icmp_echo_header)))
385 /* grab ICMP ECHO content */
386 memcpy (&icmp_echo, &buf[off], sizeof (struct icmp_echo_header));
387 port = (uint16_t) ntohl (icmp_echo.reserved);
391 (sizeof (struct ip_header) * 2 +
392 sizeof (struct icmp_ttl_exceeded_header) + sizeof (struct udp_header)))
397 /* grab UDP content */
398 memcpy (&udp_pkt, &buf[off], sizeof (struct udp_header));
399 port = ntohs (udp_pkt.length);
402 /* different type than what we want */
406 source_ip.s_addr = ip_pkt.src_ip;
408 fprintf (stdout, "%s\n",
409 inet_ntop (AF_INET, &source_ip, buf, sizeof (buf)));
411 fprintf (stdout, "%s:%u\n",
412 inet_ntop (AF_INET, &source_ip, buf, sizeof (buf)),
413 (unsigned int) port);
419 * Fully initialize the raw socket.
421 * @return -1 on error, 0 on success
429 setsockopt (rawsock, SOL_SOCKET, SO_BROADCAST, (char *) &one, sizeof (one)))
431 fprintf (stderr, "setsockopt failed: %s\n", strerror (errno));
435 setsockopt (rawsock, IPPROTO_IP, IP_HDRINCL, (char *) &one, sizeof (one)))
437 fprintf (stderr, "setsockopt failed: %s\n", strerror (errno));
445 * Create a UDP socket for writing.
447 * @param my_ip source address (our ip address)
448 * @return -1 on error
451 make_udp_socket (const struct in_addr *my_ip)
454 struct sockaddr_in addr;
456 ret = socket (AF_INET, SOCK_DGRAM, 0);
459 fprintf (stderr, "Error opening UDP socket: %s\n", strerror (errno));
462 memset (&addr, 0, sizeof (addr));
463 addr.sin_family = AF_INET;
464 #if HAVE_SOCKADDR_IN_SIN_LEN
465 addr.sin_len = sizeof (struct sockaddr_in);
467 addr.sin_addr = *my_ip;
468 addr.sin_port = htons (NAT_TRAV_PORT);
470 if (0 != bind (ret, &addr, sizeof (addr)))
472 fprintf (stderr, "Error binding UDP socket to port %u: %s\n", NAT_TRAV_PORT,
482 main (int argc, char *const *argv)
484 struct in_addr external;
493 /* Create an ICMP raw socket for reading (we'll check errors later) */
494 icmpsock = socket (AF_INET, SOCK_RAW, IPPROTO_ICMP);
497 /* Create an (ICMP) raw socket for writing (we'll check errors later) */
498 rawsock = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
502 /* drop root rights */
504 #ifdef HAVE_SETRESUID
505 if (0 != setresuid (uid, uid, uid))
507 fprintf (stderr, "Failed to setresuid: %s\n", strerror (errno));
512 if (0 != (setuid (uid) | seteuid (uid)))
514 fprintf (stderr, "Failed to setuid: %s\n", strerror (errno));
520 /* Now that we run without root rights, we can do error checking... */
524 "This program must be started with our (internal NAT) IP as the only argument.\n");
528 if (1 != inet_pton (AF_INET, argv[1], &external))
530 fprintf (stderr, "Error parsing IPv4 address: %s\n", strerror (errno));
534 if (1 != inet_pton (AF_INET, DUMMY_IP, &dummy))
536 fprintf (stderr, "Internal error converting dummy IP to binary.\n");
541 /* error checking icmpsock */
544 fprintf (stderr, "Error opening RAW socket: %s\n", strerror (icmp_eno));
548 if (icmpsock >= FD_SETSIZE)
550 /* this could happen if we were started with a large number of already-open
551 file descriptors... */
552 fprintf (stderr, "Socket number too large (%d > %u)\n", icmpsock,
553 (unsigned int) FD_SETSIZE);
558 /* error checking rawsock */
561 fprintf (stderr, "Error opening RAW socket: %s\n", strerror (raw_eno));
565 /* no need to check 'rawsock' against FD_SETSIZE as it is never used
568 if (0 != setup_raw_socket ())
574 if (-1 == (udpsock = make_udp_socket (&external)))
584 FD_SET (icmpsock, &rs);
586 tv.tv_usec = ICMP_SEND_FREQUENCY_MS * 1000;
587 if (-1 == select (icmpsock + 1, &rs, NULL, NULL, &tv))
591 fprintf (stderr, "select failed: %s\n", strerror (errno));
594 if (1 == getppid ()) /* Check the parent process id, if 1 the parent has died, so we should die too */
596 if (FD_ISSET (icmpsock, &rs))
597 process_icmp_response ();
598 if (0 == (++alt % 2))
599 send_icmp_echo (&external);
604 /* select failed (internal error or OS out of resources) */
608 (void) close (icmpsock);
610 (void) close (rawsock);
612 (void) close (udpsock);
617 /* end of gnunet-helper-nat-server.c */