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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>
68 /* The following constant is missing from FreeBSD 9.2 */
69 #ifndef ICMP_TIME_EXCEEDED
70 #define ICMP_TIME_EXCEEDED 11
74 * Should we print some debug output?
79 * Must match packet ID used by gnunet-helper-nat-client.c
84 * Must match IP given in the client.
86 #define DUMMY_IP "192.0.2.86"
91 #define NAT_TRAV_PORT 22225
94 * How often do we send our ICMP messages to receive replies?
96 #define ICMP_SEND_FREQUENCY_MS 500
105 * Version (4 bits) + Internet header length (4 bits)
125 * Flags (3 bits) + Fragment offset (13 bits)
127 uint16_t flags_frag_offset;
150 * Destination address
156 * Format of ICMP packet.
158 struct icmp_ttl_exceeded_header
168 /* followed by original payload */
171 struct icmp_echo_header
184 * Beginning of UDP packet.
198 * Socket we use to receive "fake" ICMP replies.
203 * Socket we use to send our ICMP requests.
208 * Socket we use to send our UDP requests.
213 * Target "dummy" address.
215 static struct in_addr dummy;
219 * CRC-16 for IP/ICMP headers.
221 * @param data what to calculate the CRC over
222 * @param bytes number of bytes in data (must be multiple of 2)
223 * @return the CRC 16.
226 calc_checksum (const uint16_t * data, unsigned int bytes)
232 for (i = 0; i < bytes / 2; i++)
234 sum = (sum & 0xffff) + (sum >> 16);
235 sum = htons (0xffff - sum);
241 * Send an ICMP message to the dummy IP.
243 * @param my_ip source address (our ip address)
246 send_icmp_echo (const struct in_addr *my_ip)
248 char packet[sizeof (struct ip_header) + sizeof (struct icmp_echo_header)];
249 struct icmp_echo_header icmp_echo;
250 struct ip_header ip_pkt;
251 struct sockaddr_in dst;
256 ip_pkt.vers_ihl = 0x45;
258 ip_pkt.pkt_len = htons (sizeof (packet));
259 ip_pkt.id = htons (PACKET_ID);
260 ip_pkt.flags_frag_offset = 0;
261 ip_pkt.ttl = IPDEFTTL;
262 ip_pkt.proto = IPPROTO_ICMP;
264 ip_pkt.src_ip = my_ip->s_addr;
265 ip_pkt.dst_ip = dummy.s_addr;
267 htons (calc_checksum ((uint16_t *) & ip_pkt,
268 sizeof (struct ip_header)));
269 memcpy (&packet[off],
271 sizeof (struct ip_header));
272 off += sizeof (struct ip_header);
274 icmp_echo.type = ICMP_ECHO;
276 icmp_echo.checksum = 0;
277 icmp_echo.reserved = 0;
280 ((uint16_t *) & icmp_echo,
281 sizeof (struct icmp_echo_header)));
282 memcpy (&packet[off],
284 sizeof (struct icmp_echo_header));
285 off += sizeof (struct icmp_echo_header);
287 memset (&dst, 0, sizeof (dst));
288 dst.sin_family = AF_INET;
289 #if HAVE_SOCKADDR_IN_SIN_LEN
290 dst.sin_len = sizeof (struct sockaddr_in);
292 dst.sin_addr = dummy;
293 err = sendto (rawsock,
297 (struct sockaddr *) &dst,
303 "sendto failed: %s\n",
307 else if (sizeof (packet) != err)
310 "Error: partial send of ICMP message\n");
316 * Send a UDP message to the dummy IP.
321 struct sockaddr_in dst;
324 memset (&dst, 0, sizeof (dst));
325 dst.sin_family = AF_INET;
326 #if HAVE_SOCKADDR_IN_SIN_LEN
327 dst.sin_len = sizeof (struct sockaddr_in);
329 dst.sin_addr = dummy;
330 dst.sin_port = htons (NAT_TRAV_PORT);
331 err = sendto (udpsock,
335 (struct sockaddr *) &dst,
341 "sendto failed: %s\n",
348 "Error: partial send of ICMP message\n");
354 * We've received an ICMP response. Process it.
357 process_icmp_response ()
361 struct in_addr source_ip;
362 struct ip_header ip_pkt;
363 struct icmp_ttl_exceeded_header icmp_ttl;
364 struct icmp_echo_header icmp_echo;
365 struct udp_header udp_pkt;
369 have = read (icmpsock, buf, sizeof (buf));
373 "Error reading raw socket: %s\n",
379 "Received message of %u bytes\n",
380 (unsigned int) have);
383 (ssize_t) (sizeof (struct ip_header) +
384 sizeof (struct icmp_ttl_exceeded_header) +
385 sizeof (struct ip_header)))
391 memcpy (&ip_pkt, &buf[off], sizeof (struct ip_header));
392 off += sizeof (struct ip_header);
393 memcpy (&icmp_ttl, &buf[off], sizeof (struct icmp_ttl_exceeded_header));
394 off += sizeof (struct icmp_ttl_exceeded_header);
395 if ((ICMP_TIME_EXCEEDED != icmp_ttl.type) || (0 != icmp_ttl.code))
397 /* different type than what we want */
400 /* grab source IP of 1st IP header */
401 source_ip.s_addr = ip_pkt.src_ip;
403 /* skip 2nd IP header */
404 memcpy (&ip_pkt, &buf[off], sizeof (struct ip_header));
405 off += sizeof (struct ip_header);
407 switch (ip_pkt.proto)
411 (sizeof (struct ip_header) * 2 +
412 sizeof (struct icmp_ttl_exceeded_header) +
413 sizeof (struct icmp_echo_header)))
418 /* grab ICMP ECHO content */
419 memcpy (&icmp_echo, &buf[off], sizeof (struct icmp_echo_header));
420 port = (uint16_t) ntohl (icmp_echo.reserved);
424 (sizeof (struct ip_header) * 2 +
425 sizeof (struct icmp_ttl_exceeded_header) + sizeof (struct udp_header)))
430 /* grab UDP content */
431 memcpy (&udp_pkt, &buf[off], sizeof (struct udp_header));
432 port = ntohs (udp_pkt.length);
435 /* different type than what we want */
440 fprintf (stdout, "%s\n",
441 inet_ntop (AF_INET, &source_ip, buf, sizeof (buf)));
443 fprintf (stdout, "%s:%u\n",
444 inet_ntop (AF_INET, &source_ip, buf, sizeof (buf)),
445 (unsigned int) port);
451 * Fully initialize the raw socket.
453 * @return -1 on error, 0 on success
468 "setsockopt failed: %s\n",
480 "setsockopt failed: %s\n",
489 * Create a UDP socket for writing.
491 * @param my_ip source address (our ip address)
492 * @return -1 on error
495 make_udp_socket (const struct in_addr *my_ip)
498 struct sockaddr_in addr;
500 ret = socket (AF_INET, SOCK_DGRAM, 0);
504 "Error opening UDP socket: %s\n",
508 memset (&addr, 0, sizeof (addr));
509 addr.sin_family = AF_INET;
510 #if HAVE_SOCKADDR_IN_SIN_LEN
511 addr.sin_len = sizeof (struct sockaddr_in);
513 addr.sin_addr = *my_ip;
514 addr.sin_port = htons (NAT_TRAV_PORT);
517 (struct sockaddr *) &addr,
521 "Error binding UDP socket to port %u: %s\n",
535 struct in_addr external;
544 /* Create an ICMP raw socket for reading (we'll check errors later) */
545 icmpsock = socket (AF_INET,
550 /* Create an (ICMP) raw socket for writing (we'll check errors later) */
551 rawsock = socket (AF_INET,
557 /* drop root rights */
559 #ifdef HAVE_SETRESUID
560 if (0 != setresuid (uid, uid, uid))
563 "Failed to setresuid: %s\n",
569 if (0 != (setuid (uid) | seteuid (uid)))
572 "Failed to setuid: %s\n",
579 /* Now that we run without root rights, we can do error checking... */
583 "This program must be started with our (internal NAT) IP as the only argument.\n");
587 if (1 != inet_pton (AF_INET, argv[1], &external))
590 "Error parsing IPv4 address: %s\n",
595 if (1 != inet_pton (AF_INET, DUMMY_IP, &dummy))
598 "Internal error converting dummy IP to binary.\n");
603 /* error checking icmpsock */
607 "Error opening RAW socket: %s\n",
608 strerror (icmp_eno));
612 if (icmpsock >= FD_SETSIZE)
614 /* this could happen if we were started with a large number of already-open
615 file descriptors... */
617 "Socket number too large (%d > %u)\n",
619 (unsigned int) FD_SETSIZE);
624 /* error checking rawsock */
628 "Error opening RAW socket: %s\n",
633 /* no need to check 'rawsock' against FD_SETSIZE as it is never used
636 if (0 != setup_raw_socket ())
642 if (-1 == (udpsock = make_udp_socket (&external)))
652 FD_SET (icmpsock, &rs);
654 tv.tv_usec = ICMP_SEND_FREQUENCY_MS * 1000;
655 if (-1 == select (icmpsock + 1, &rs, NULL, NULL, &tv))
660 "select failed: %s\n",
664 if (1 == getppid ()) /* Check the parent process id, if 1 the parent has died, so we should die too */
666 if (FD_ISSET (icmpsock, &rs))
667 process_icmp_response ();
668 if (0 == (++alt % 2))
669 send_icmp_echo (&external);
674 /* select failed (internal error or OS out of resources) */
678 (void) close (icmpsock);
680 (void) close (rawsock);
682 (void) close (udpsock);
687 /* end of gnunet-helper-nat-server.c */