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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 500
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];
123 static struct in_addr dummy;
127 * create a random port number that is not totally
128 * unlikely to be chosen by the nat box.
130 static uint16_t make_port ()
132 return 1024 + ( (unsigned int)rand ()) % (63 * 1024 - 2);
137 * create a fresh udp socket bound to a random local port.
140 make_udp_socket (uint16_t *port)
144 struct sockaddr_in src;
146 for (tries=0;tries<MAX_TRIES;tries++)
148 ret = socket (AF_INET, SOCK_DGRAM, 0);
152 "Error opening udp socket: %s\n",
156 if (ret >= FD_SETSIZE)
159 "Socket number too large (%d > %u)\n",
161 (unsigned int) FD_SETSIZE);
165 memset (&src, 0, sizeof (src));
166 src.sin_family = AF_INET;
167 src.sin_port = htons (make_port ());
168 if (0 != bind (ret, (struct sockaddr*) &src, sizeof (src)))
173 *port = ntohs (src.sin_port);
177 "Error binding udp socket: %s\n",
184 calc_checksum(const uint16_t *data,
191 for (i=0;i<bytes/2;i++)
193 sum = (sum & 0xffff) + (sum >> 16);
194 sum = htons(0xffff - sum);
200 * send an icmp message to the target.
202 * @param my_ip source address (our ip address)
203 * @param other target address
204 * @param target_port_number fake port number to put into icmp response
205 * as well as the icmpextradata as 'my_magic'
206 * @param source_port_number magic_number that enables the other peer to
207 * identify our port number ('reply in response to') to
208 * put in the data portion; 0 if we are initiating;
209 * goes into 'reply_port_magic' of the icmpextradata
212 send_icmp (const struct in_addr *my_ip,
213 const struct in_addr *other,
214 uint16_t target_port_number,
215 uint16_t source_port_number)
217 struct ip_packet ip_pkt;
218 struct icmp_packet icmp_pkt;
219 struct udp_packet udp_pkt;
220 struct sockaddr_in dst;
221 char packet[sizeof (ip_pkt) + sizeof (icmp_pkt) + sizeof (udp_pkt)];
225 /* ip header: send to (known) ip address */
227 memset(&ip_pkt, 0, sizeof(ip_pkt));
228 ip_pkt.vers_ihl = 0x45;//|(pkt_len>>2);//5;//(ipversion << 4) | (iphdr_size >> 2);
230 ip_pkt.pkt_len = sizeof (packet); /* huh? */
231 ip_pkt.id = 1; /* kernel will change anyway!? */
232 ip_pkt.flags_frag_offset = 0;
233 ip_pkt.ttl = IPDEFTTL;
234 ip_pkt.proto = IPPROTO_ICMP;
235 ip_pkt.checksum = 0; /* maybe the kernel helps us out..? */
236 ip_pkt.src_ip = my_ip->s_addr;
237 ip_pkt.dst_ip = other->s_addr;
238 ip_pkt.checksum = htons(calc_checksum((uint16_t*)&ip_pkt, sizeof (ip_pkt)));
239 memcpy (packet, &ip_pkt, sizeof (ip_pkt));
240 off += sizeof (ip_pkt);
242 /* icmp reply: time exceeded */
243 memset(&icmp_pkt, 0, sizeof(icmp_pkt));
244 icmp_pkt.type = ICMP_TIME_EXCEEDED;
245 icmp_pkt.code = ICMP_NET_UNREACH;
246 icmp_pkt.reserved = 0;
247 icmp_pkt.checksum = 0;
248 icmp_pkt.checksum = htons(calc_checksum((uint16_t*)&icmp_pkt, sizeof (icmp_pkt)));
249 memcpy (&packet[off], &icmp_pkt, sizeof (icmp_pkt));
250 off += sizeof (icmp_pkt);
252 /* ip header of the presumably 'lost' udp packet */
253 memset(&ip_pkt, 0, sizeof (ip_pkt));
254 ip_pkt.vers_ihl = 0x45;
256 /* no idea why i need to shift the bits here, but not on ip_pkt->pkt_len... */
257 ip_pkt.pkt_len = (sizeof (ip_pkt) + sizeof (icmp_pkt)) << 8;
258 ip_pkt.id = 1; /* kernel sets proper value htons(ip_id_counter); */
259 ip_pkt.flags_frag_offset = 0;
260 ip_pkt.ttl = 1; /* real TTL would be 1 on a time exceeded packet */
261 ip_pkt.proto = IPPROTO_UDP;
262 ip_pkt.src_ip = other->s_addr;
263 ip_pkt.dst_ip = dummy.s_addr;
265 ip_pkt.checksum = htons(calc_checksum((uint16_t*)&ip_pkt, sizeof (ip_pkt)));
266 memcpy (&packet[off], &ip_pkt, sizeof (ip_pkt));
267 off += sizeof (ip_pkt);
269 memset(&udp_pkt, 0, sizeof (udp_pkt));
270 udp_pkt.source_port = htons (target_port_number); /* this one will be re-written by NAT */
271 udp_pkt.dst_port = htons (NAT_TRAV_PORT);
272 udp_pkt.mlen_aka_reply_port_magic = htons (source_port_number);
273 udp_pkt.checksum_aka_my_magic = htons (target_port_number); /* this one should be bounced back to me as 'reply_port_magic' */
274 memcpy (&packet[off], &udp_pkt, sizeof (udp_pkt));
275 off += sizeof (udp_pkt);
277 memset (&dst, 0, sizeof (dst));
278 dst.sin_family = AF_INET;
279 dst.sin_addr = *other;
280 err = sendto(rawsock,
283 (struct sockaddr*)&dst,
284 sizeof(dst)); /* or sizeof 'struct sockaddr'? */
287 "sendto failed: %s\n", strerror(errno));
288 } else if (err != off)
290 "Error: partial send of ICMP message\n");
295 * We discovered the IP address of the other peer.
296 * Try to connect back to it.
299 try_connect (const struct in_addr *my_ip,
300 const struct in_addr *other,
304 char sbuf [INET_ADDRSTRLEN];
307 "Sending %u ICMPs to `%s' with reply magic %u\n",
314 for (i=0;i<NUM_UDP_PORTS;i++)
315 send_icmp (my_ip, other, make_port(), port_magic);
320 process_icmp_response (const struct in_addr *my_ip,
327 uint16_t reply_magic;
329 struct ip_packet ip_pkt;
330 struct icmp_packet icmp_pkt;
331 struct udp_packet udp_pkt;
334 have = read (s, buf, sizeof (buf));
338 "Error reading raw socket: %s\n",
343 if (have != 2 * sizeof (struct ip_packet) + sizeof (struct icmp_packet) +
344 sizeof (struct udp_packet))
347 "Received ICMP message of unexpected size: %u bytes\n",
348 (unsigned int) have);
352 memcpy (&ip_pkt, &buf[off], sizeof (ip_pkt));
353 off += sizeof (ip_pkt);
354 memcpy (&icmp_pkt, &buf[off], sizeof (icmp_pkt));
355 off += sizeof (icmp_pkt);
356 off += sizeof (ip_pkt);
357 memcpy (&udp_pkt, &buf[off], sizeof (udp_pkt));
358 off += sizeof (struct udp_packet);
360 if ( (ip_pkt.proto == IPPROTO_ICMP) &&
361 (icmp_pkt.type == ICMP_DEST_UNREACH) &&
362 (icmp_pkt.code == ICMP_HOST_UNREACH) )
364 /* this is what is normal due to our UDP traffic */
367 if ( (ip_pkt.proto != IPPROTO_ICMP) ||
368 (icmp_pkt.type != ICMP_TIME_EXCEEDED) ||
369 (icmp_pkt.code != ICMP_NET_UNREACH) )
371 /* Note the expected client response and not the normal network response */
373 "Received unexpected ICMP message contents (%u, %u, %u), ignoring\n",
379 memcpy(&sip, &ip_pkt.src_ip, sizeof (sip));
380 reply_magic = ntohs (udp_pkt.checksum_aka_my_magic);
381 my_magic = ntohs (udp_pkt.mlen_aka_reply_port_magic);
382 local_port = ntohs (udp_pkt.source_port);
384 "Received ICMP from `%s' with outgoing port %u, listen port %u and incoming port hint for other peer %u\n",
394 try_connect (my_ip, &sip, reply_magic);
398 /* FIXME: should close 'local_port' */
399 printf ("%s:%u listen on %u\n",
415 ret = socket (AF_INET, SOCK_RAW, IPPROTO_ICMP);
419 "Error opening RAW socket: %s\n",
423 if (ret >= FD_SETSIZE)
426 "Socket number too large (%d > %u)\n",
428 (unsigned int) FD_SETSIZE);
442 ret = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
446 "Error opening RAW socket: %s\n",
450 if (ret >= FD_SETSIZE)
453 "Socket number too large (%d > %u)\n",
455 (unsigned int) FD_SETSIZE);
459 if (setsockopt(ret, SOL_SOCKET, SO_BROADCAST,
460 (char *)&one, sizeof(one)) == -1)
462 "setsockopt failed: %s\n",
464 if (setsockopt(ret, IPPROTO_IP, IP_HDRINCL,
465 (char *)&one, sizeof(one)) == -1)
467 "setsockopt failed: %s\n",
474 main (int argc, char *const *argv)
476 struct in_addr external;
481 struct sockaddr_in dst;
486 "This program must be started with our external IP and the dummy IP address as arguments.\n");
489 if ( (1 != inet_pton (AF_INET, argv[1], &external)) ||
490 (1 != inet_pton (AF_INET, argv[2], &dummy)) )
493 "Error parsing IPv4 address: %s\n",
497 memset (&dst, 0, sizeof (dst));
498 dst.sin_family = AF_INET;
499 dst.sin_port = htons (NAT_TRAV_PORT);
500 dst.sin_addr = dummy;
502 if (-1 == (icmpsock = make_icmp_socket()))
504 if (-1 == (rawsock = make_raw_socket()))
509 for (i=0;i<NUM_UDP_PORTS;i++)
510 udpsocks[i] = make_udp_socket (&udpports[i]);
515 FD_SET (icmpsock, &rs);
517 tv.tv_usec = UDP_SEND_FREQUENCY_MS * 1000;
518 select (icmpsock + 1, &rs, NULL, NULL, &tv);
519 /* FIXME: do I need my external IP here? */
520 if (FD_ISSET (icmpsock, &rs))
522 process_icmp_response (&external, icmpsock);
526 "Sending UDP message to %s:%u\n",
530 if (-1 == sendto (udpsocks[pos],
532 (struct sockaddr*) &dst, sizeof (dst)))
535 "sendto failed: %s\n",
537 close (udpsocks[pos]);
538 udpsocks[pos] = make_udp_socket (&udpports[pos]);
540 pos = (pos+1) % NUM_UDP_PORTS;
546 /* end of gnunet-nat-server.c */