2 This file is part of GNUnet.
3 (C) 2010 Christian Grothoff (and other contributing authors)
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22 * @file src/transport/gnunet-nat-server.c
23 * @brief Tool to help bypass NATs using ICMP method; must run as root (for now, later SUID will do)
24 * This code will work under GNU/Linux only (or maybe BSDs, but never W32)
25 * @author Christian Grothoff
28 #include <sys/types.h>
29 #include <sys/socket.h>
30 #include <arpa/inet.h>
31 #include <sys/select.h>
33 #include <sys/types.h>
40 #include <netinet/ip.h>
41 #include <netinet/ip_icmp.h>
42 #include <netinet/in.h>
45 * Number of UDP ports to keep open.
47 #define NUM_UDP_PORTS 512
50 * How often do we send our UDP messages to keep ports open?
52 #define UDP_SEND_FREQUENCY_MS 50
55 * Port we use for the dummy target.
57 #define NAT_TRAV_PORT 2222
60 * How often do we retry to open and bind a UDP socket before giving up?
72 uint16_t flags_frag_offset;
92 uint16_t mlen_aka_reply_port_magic;
93 uint16_t checksum_aka_my_magic;
98 * Structure of the data we tack on to the fake ICMP reply
99 * (last 4 bytes of the 64 bytes).
104 * if this is a reply to an icmp, what was the 'my_magic'
105 * value from the original icmp?
107 uint16_t reply_port_magic;
110 * magic value of the sender of this icmp message.
115 static int udpsocks[NUM_UDP_PORTS];
117 static uint16_t udpports[NUM_UDP_PORTS];
121 static struct in_addr dummy;
125 * create a random port number that is not totally
126 * unlikely to be chosen by the nat box.
128 static uint16_t make_port ()
130 return 1024 + ( (unsigned int)rand ()) % (63 * 1024 - 2);
135 * create a fresh udp socket bound to a random local port.
138 make_udp_socket (uint16_t *port)
142 struct sockaddr_in src;
144 for (tries=0;tries<MAX_TRIES;tries++)
146 ret = socket (AF_INET, SOCK_DGRAM, 0);
150 "Error opening udp socket: %s\n",
154 if (ret >= FD_SETSIZE)
157 "Socket number too large (%d > %u)\n",
159 (unsigned int) FD_SETSIZE);
163 memset (&src, 0, sizeof (src));
164 src.sin_family = AF_INET;
165 src.sin_port = htons (make_port ());
166 if (0 != bind (ret, (struct sockaddr*) &src, sizeof (src)))
171 *port = ntohs (src.sin_port);
175 "Error binding udp socket: %s\n",
182 calc_checksum(const uint16_t *data,
189 for (i=0;i<bytes/2;i++)
191 sum = (sum & 0xffff) + (sum >> 16);
192 sum = htons(0xffff - sum);
198 * send an icmp message to the target.
200 * @param my_ip source address (our ip address)
201 * @param other target address
202 * @param target_port_number fake port number to put into icmp response
203 * as well as the icmpextradata as 'my_magic'
204 * @param source_port_number magic_number that enables the other peer to
205 * identify our port number ('reply in response to') to
206 * put in the data portion; 0 if we are initiating;
207 * goes into 'reply_port_magic' of the icmpextradata
210 send_icmp (const struct in_addr *my_ip,
211 const struct in_addr *other,
212 uint16_t target_port_number,
213 uint16_t source_port_number)
215 struct ip_packet ip_pkt;
216 struct icmp_packet icmp_pkt;
217 struct udp_packet udp_pkt;
218 struct sockaddr_in dst;
219 char packet[sizeof (ip_pkt) + sizeof (icmp_pkt) + sizeof (udp_pkt)];
220 char sbuf [INET_ADDRSTRLEN];
225 "Sending ICMP to `%s' at %u with reply magic %u\n",
232 /* ip header: send to (known) ip address */
234 memset(&ip_pkt, 0, sizeof(ip_pkt));
235 ip_pkt.vers_ihl = 0x45;//|(pkt_len>>2);//5;//(ipversion << 4) | (iphdr_size >> 2);
237 ip_pkt.pkt_len = sizeof (packet); /* huh? */
238 ip_pkt.id = 1; /* kernel will change anyway!? */
239 ip_pkt.flags_frag_offset = 0;
240 ip_pkt.ttl = IPDEFTTL;
241 ip_pkt.proto = IPPROTO_ICMP;
242 ip_pkt.checksum = 0; /* maybe the kernel helps us out..? */
243 ip_pkt.src_ip = my_ip->s_addr;
244 ip_pkt.dst_ip = other->s_addr;
245 ip_pkt.checksum = htons(calc_checksum((uint16_t*)&ip_pkt, sizeof (ip_pkt)));
246 memcpy (packet, &ip_pkt, sizeof (ip_pkt));
247 off += sizeof (ip_pkt);
249 /* icmp reply: time exceeded */
250 memset(&icmp_pkt, 0, sizeof(icmp_pkt));
251 icmp_pkt.type = ICMP_TIME_EXCEEDED;
252 icmp_pkt.code = ICMP_NET_UNREACH;
253 icmp_pkt.reserved = 0;
254 icmp_pkt.checksum = 0;
255 icmp_pkt.checksum = htons(calc_checksum((uint16_t*)&icmp_pkt, sizeof (icmp_pkt)));
256 memcpy (&packet[off], &icmp_pkt, sizeof (icmp_pkt));
257 off += sizeof (icmp_pkt);
259 /* ip header of the presumably 'lost' udp packet */
260 memset(&ip_pkt, 0, sizeof (ip_pkt));
261 ip_pkt.vers_ihl = 0x45;
263 /* no idea why i need to shift the bits here, but not on ip_pkt->pkt_len... */
264 ip_pkt.pkt_len = (sizeof (ip_pkt) + sizeof (icmp_pkt)) << 8;
265 ip_pkt.id = 1; /* kernel sets proper value htons(ip_id_counter); */
266 ip_pkt.flags_frag_offset = 0;
267 ip_pkt.ttl = 1; /* real TTL would be 1 on a time exceeded packet */
268 ip_pkt.proto = IPPROTO_UDP;
269 ip_pkt.src_ip = other->s_addr;
270 ip_pkt.dst_ip = dummy.s_addr;
272 ip_pkt.checksum = htons(calc_checksum((uint16_t*)&ip_pkt, sizeof (ip_pkt)));
273 memcpy (&packet[off], &ip_pkt, sizeof (ip_pkt));
274 off += sizeof (ip_pkt);
276 memset(&udp_pkt, 0, sizeof (udp_pkt));
277 udp_pkt.source_port = htons (target_port_number);
278 udp_pkt.dst_port = htons (NAT_TRAV_PORT);
279 udp_pkt.mlen_aka_reply_port_magic = htons (source_port_number);
280 udp_pkt.checksum_aka_my_magic = htons (target_port_number);
281 memcpy (&packet[off], &udp_pkt, sizeof (udp_pkt));
282 off += sizeof (udp_pkt);
284 memset (&dst, 0, sizeof (dst));
285 dst.sin_family = AF_INET;
286 dst.sin_addr = *other;
287 err = sendto(icmpsock,
290 (struct sockaddr*)&dst,
291 sizeof(dst)); /* or sizeof 'struct sockaddr'? */
294 "sendto failed: %s\n", strerror(errno));
295 } else if (err != off)
297 "Error: partial send of ICMP message\n");
302 * We discovered the IP address of the other peer.
303 * Try to connect back to it.
306 try_connect (const struct in_addr *my_ip,
307 const struct in_addr *other,
312 for (i=0;i<NUM_UDP_PORTS;i++)
313 send_icmp (my_ip, other, make_port(), port_magic);
318 process_icmp_response (const struct in_addr *my_ip,
325 uint16_t reply_magic;
326 struct ip_packet ip_pkt;
327 struct icmp_packet icmp_pkt;
328 struct udp_packet udp_pkt;
331 have = read (s, buf, sizeof (buf));
335 "Error reading raw socket: %s\n",
340 if (have < sizeof (struct ip_packet) + sizeof (struct icmp_packet) +
341 sizeof (struct udp_packet))
344 "Received invalid ICMP message: only %u bytes\n",
345 (unsigned int) have);
349 memcpy (&ip_pkt, &buf[off], sizeof (ip_pkt));
350 off += sizeof (ip_pkt);
351 memcpy (&icmp_pkt, &buf[off], sizeof (icmp_pkt));
352 off += sizeof (icmp_pkt);
353 memcpy (&udp_pkt, &buf[off], sizeof (udp_pkt));
354 off += sizeof (struct udp_packet);
356 /* If not ICMP and not TTL exceeded */
357 if ( (ip_pkt.proto != IPPROTO_ICMP) ||
358 (icmp_pkt.type != ICMP_TIME_EXCEEDED) ||
359 (icmp_pkt.code != ICMP_NET_UNREACH) )
362 "Received unexpected ICMP message contents, ignoring\n");
365 memcpy(&sip, &ip_pkt.src_ip, sizeof (sip));
366 my_magic = ntohs (udp_pkt.checksum_aka_my_magic);
367 reply_magic = ntohs (udp_pkt.mlen_aka_reply_port_magic);
369 "Received ICMP from `%s' with hints %u and %u\n",
378 try_connect (my_ip, &sip, reply_magic);
398 ret = socket (AF_INET, SOCK_RAW, 0);
402 "Error opening RAW socket: %s\n",
406 if (ret >= FD_SETSIZE)
409 "Socket number too large (%d > %u)\n",
411 (unsigned int) FD_SETSIZE);
415 if (setsockopt(ret, SOL_SOCKET, SO_BROADCAST,
416 (char *)&one, sizeof(one)) == -1)
418 "setsockopt failed: %s\n",
420 if (setsockopt(ret, IPPROTO_IP, IP_HDRINCL,
421 (char *)&one, sizeof(one)) == -1)
423 "setsockopt failed: %s\n",
430 main (int argc, char *const *argv)
432 struct in_addr external;
437 struct sockaddr_in dst;
442 "This program must be started with our external IP and the dummy IP address as arguments.\n");
445 if ( (1 != inet_pton (AF_INET, argv[1], &external)) ||
446 (1 != inet_pton (AF_INET, argv[2], &dummy)) )
449 "Error parsing IPv4 address: %s\n",
453 memset (&dst, 0, sizeof (dst));
454 dst.sin_family = AF_INET;
455 dst.sin_port = htons (NAT_TRAV_PORT);
456 dst.sin_addr = dummy;
457 if (-1 == (icmpsock = make_icmp_socket()))
459 for (i=0;i<NUM_UDP_PORTS;i++)
460 udpsocks[i] = make_udp_socket (&udpports[i]);
465 FD_SET (icmpsock, &rs);
467 tv.tv_usec = UDP_SEND_FREQUENCY_MS * 1000 * 1000;
468 select (icmpsock + 1, &rs, NULL, NULL, &tv);
469 /* FIXME: do I need my external IP here? */
470 if (FD_ISSET (icmpsock, &rs))
471 process_icmp_response (&external, icmpsock);
472 if (-1 == sendto (udpsocks[pos],
474 (struct sockaddr*) &dst, sizeof (dst)))
477 "sendto failed: %s\n",
479 close (udpsocks[pos]);
480 udpsocks[pos] = make_udp_socket (&udpports[pos]);
482 pos = (pos+1) % NUM_UDP_PORTS;
488 /* end of gnunet-nat-server.c */