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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>
41 #include <netinet/ip.h>
42 #include <netinet/ip_icmp.h>
43 #include <netinet/in.h>
48 * Number of UDP ports to keep open (typically >= 256).
50 #define NUM_UDP_PORTS 256
53 * Number of ICMP replies to send per message received (typically >= 1024)
55 #define NUM_ICMP_REPLIES 1024
58 * How often do we send our UDP messages to keep ports open? (typically < 100ms)
60 #define UDP_SEND_FREQUENCY_MS 50
63 * Port we use for the dummy target.
65 #define NAT_TRAV_PORT 2222
68 * How often do we retry to open and bind a UDP socket before giving up?
80 uint16_t flags_frag_offset;
93 uint16_t mlen_aka_reply_port_magic;
94 uint16_t checksum_aka_my_magic;
105 struct udp_packet udp;
110 * Structure of the data we tack on to the fake ICMP reply
111 * (last 4 bytes of the 64 bytes).
116 * if this is a reply to an icmp, what was the 'my_magic'
117 * value from the original icmp?
119 uint16_t reply_port_magic;
122 * magic value of the sender of this icmp message.
127 static int udpsocks[NUM_UDP_PORTS];
129 static uint16_t udpports[NUM_UDP_PORTS];
135 static struct in_addr dummy;
139 * create a random port number that is not totally
140 * unlikely to be chosen by the nat box.
142 static uint16_t make_port ()
144 return 1024 + ( (unsigned int)rand ()) % (63 * 1024 - 2);
149 * create a fresh udp socket bound to a random local port.
152 make_udp_socket (uint16_t *port)
156 struct sockaddr_in src;
158 for (tries=0;tries<MAX_TRIES;tries++)
160 ret = socket (AF_INET, SOCK_DGRAM, 0);
164 "Error opening udp socket: %s\n",
168 if (ret >= FD_SETSIZE)
171 "Socket number too large (%d > %u)\n",
173 (unsigned int) FD_SETSIZE);
177 memset (&src, 0, sizeof (src));
178 src.sin_family = AF_INET;
179 src.sin_port = htons (make_port ());
180 if (0 != bind (ret, (struct sockaddr*) &src, sizeof (src)))
185 *port = ntohs (src.sin_port);
189 "Error binding udp socket: %s\n",
196 calc_checksum(const uint16_t *data,
203 for (i=0;i<bytes/2;i++)
205 sum = (sum & 0xffff) + (sum >> 16);
206 sum = htons(0xffff - sum);
212 * send an icmp message to the target.
214 * @param my_ip source address (our ip address)
215 * @param other target address
216 * @param target_port_number fake port number to put into icmp response
217 * as well as the icmpextradata as 'my_magic'
218 * @param source_port_number magic_number that enables the other peer to
219 * identify our port number ('reply in response to') to
220 * put in the data portion; 0 if we are initiating;
221 * goes into 'reply_port_magic' of the icmpextradata
224 send_icmp (const struct in_addr *my_ip,
225 const struct in_addr *other,
226 uint16_t target_port_number,
227 uint16_t source_port_number)
229 struct ip_packet ip_pkt;
230 struct icmp_packet icmp_pkt;
231 struct sockaddr_in dst;
232 char packet[sizeof (ip_pkt) + sizeof (icmp_pkt)];
236 /* ip header: send to (known) ip address */
238 memset(&ip_pkt, 0, sizeof(ip_pkt));
239 ip_pkt.vers_ihl = 0x45;//|(pkt_len>>2);//5;//(ipversion << 4) | (iphdr_size >> 2);
241 ip_pkt.pkt_len = sizeof (packet); /* huh? */
242 ip_pkt.id = 1; /* kernel will change anyway!? */
243 ip_pkt.flags_frag_offset = 0;
244 ip_pkt.ttl = IPDEFTTL;
245 ip_pkt.proto = IPPROTO_ICMP;
246 ip_pkt.checksum = 0; /* maybe the kernel helps us out..? */
247 ip_pkt.src_ip = my_ip->s_addr;
248 ip_pkt.dst_ip = other->s_addr;
249 ip_pkt.checksum = htons(calc_checksum((uint16_t*)&ip_pkt, sizeof (ip_pkt)));
250 memcpy (packet, &ip_pkt, sizeof (ip_pkt));
251 off += sizeof (ip_pkt);
253 /* icmp reply: time exceeded */
254 memset(&icmp_pkt, 0, sizeof(icmp_pkt));
255 icmp_pkt.type = ICMP_TIME_EXCEEDED;
256 icmp_pkt.code = ICMP_NET_UNREACH;
257 icmp_pkt.reserved = 0;
258 icmp_pkt.checksum = 0;
260 /* ip header of the presumably 'lost' udp packet */
261 icmp_pkt.ip.vers_ihl = 0x45;
263 /* no idea why i need to shift the bits here, but not on ip_pkt->pkt_len... */
264 icmp_pkt.ip.pkt_len = (sizeof (ip_pkt) + sizeof (icmp_pkt)) << 8;
265 icmp_pkt.ip.id = 1; /* kernel sets proper value htons(ip_id_counter); */
266 icmp_pkt.ip.flags_frag_offset = 0;
267 icmp_pkt.ip.ttl = 1; /* real TTL would be 1 on a time exceeded packet */
268 icmp_pkt.ip.proto = IPPROTO_UDP;
269 icmp_pkt.ip.src_ip = other->s_addr;
270 icmp_pkt.ip.dst_ip = dummy.s_addr;
271 icmp_pkt.ip.checksum = 0;
272 icmp_pkt.ip.checksum = htons(calc_checksum((uint16_t*)&icmp_pkt.ip, sizeof (icmp_pkt.ip)));
273 icmp_pkt.udp.source_port = htons (target_port_number);
274 icmp_pkt.udp.dst_port = htons (NAT_TRAV_PORT);
275 icmp_pkt.udp.mlen_aka_reply_port_magic = htons (source_port_number);
276 icmp_pkt.udp.checksum_aka_my_magic = htons (target_port_number);
277 icmp_pkt.checksum = htons(calc_checksum((uint16_t*)&icmp_pkt, sizeof (icmp_pkt)));
278 memcpy (&packet[off], &icmp_pkt, sizeof (icmp_pkt));
279 off += sizeof (icmp_pkt);
281 memset (&dst, 0, sizeof (dst));
282 dst.sin_family = AF_INET;
283 dst.sin_addr = *other;
284 err = sendto(rawsock,
287 (struct sockaddr*)&dst,
288 sizeof(dst)); /* or sizeof 'struct sockaddr'? */
291 "sendto failed: %s\n", strerror(errno));
292 } else if (err != off)
294 "Error: partial send of ICMP message\n");
299 * We discovered the IP address of the other peer.
300 * Try to connect back to it.
303 try_connect (const struct in_addr *my_ip,
304 const struct in_addr *other,
309 char sbuf [INET_ADDRSTRLEN];
312 "Sending %u ICMPs to `%s' with reply magic %u\n",
320 for (i=0;i<NUM_ICMP_REPLIES;i++)
321 send_icmp (my_ip, other, make_port(), port_magic);
326 process_icmp_response (const struct in_addr *my_ip,
333 uint16_t reply_magic;
335 struct ip_packet ip_pkt;
336 struct icmp_packet icmp_pkt;
339 have = read (s, buf, sizeof (buf));
343 "Error reading raw socket: %s\n",
348 if (have != sizeof (struct ip_packet) + sizeof (struct icmp_packet))
351 "Received ICMP message of unexpected size: %u bytes\n",
352 (unsigned int) have);
356 memcpy (&ip_pkt, &buf[off], sizeof (ip_pkt));
357 off += sizeof (ip_pkt);
358 memcpy (&icmp_pkt, &buf[off], sizeof (icmp_pkt));
359 off += sizeof (icmp_pkt);
361 if ( (ip_pkt.proto == IPPROTO_ICMP) &&
362 (icmp_pkt.type == ICMP_DEST_UNREACH) &&
363 (icmp_pkt.code == ICMP_HOST_UNREACH) )
365 /* this is what is normal due to our UDP traffic */
368 if ( (ip_pkt.proto == IPPROTO_ICMP) &&
369 (icmp_pkt.type == ICMP_TIME_EXCEEDED) &&
370 (icmp_pkt.code == ICMP_NET_UNREACH) )
372 /* this is what we might see on loopback: this is the format
373 we as the server send out (the client uses 'ICMP_HOST_UNREACH');
378 if ( (ip_pkt.proto != IPPROTO_ICMP) ||
379 (icmp_pkt.type != ICMP_TIME_EXCEEDED) ||
380 (icmp_pkt.code != ICMP_HOST_UNREACH) )
382 /* Note the expected client response and not the normal network response */
384 "Received unexpected ICMP message contents (%u, %u, %u), ignoring\n",
390 memcpy(&sip, &ip_pkt.src_ip, sizeof (sip));
391 reply_magic = ntohs (icmp_pkt.udp.checksum_aka_my_magic);
392 my_magic = ntohs (icmp_pkt.udp.mlen_aka_reply_port_magic);
393 local_port = ntohs (icmp_pkt.udp.source_port);
396 "Received ICMP from `%s' with outgoing port %u, listen port %u and incoming port hint for other peer %u\n",
407 try_connect (my_ip, &sip, reply_magic);
411 /* FIXME: should close 'local_port' */
412 printf ("%s:%u listen on %u\n",
428 ret = socket (AF_INET, SOCK_RAW, IPPROTO_ICMP);
432 "Error opening RAW socket: %s\n",
436 if (ret >= FD_SETSIZE)
439 "Socket number too large (%d > %u)\n",
441 (unsigned int) FD_SETSIZE);
455 ret = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
459 "Error opening RAW socket: %s\n",
463 if (ret >= FD_SETSIZE)
466 "Socket number too large (%d > %u)\n",
468 (unsigned int) FD_SETSIZE);
472 if (setsockopt(ret, SOL_SOCKET, SO_BROADCAST,
473 (char *)&one, sizeof(one)) == -1)
475 "setsockopt failed: %s\n",
477 if (setsockopt(ret, IPPROTO_IP, IP_HDRINCL,
478 (char *)&one, sizeof(one)) == -1)
480 "setsockopt failed: %s\n",
487 main (int argc, char *const *argv)
489 struct in_addr external;
494 struct sockaddr_in dst;
499 "This program must be started with our external IP and the dummy IP address as arguments.\n");
502 if ( (1 != inet_pton (AF_INET, argv[1], &external)) ||
503 (1 != inet_pton (AF_INET, argv[2], &dummy)) )
506 "Error parsing IPv4 address: %s\n",
511 memset (&dst, 0, sizeof (dst));
512 dst.sin_family = AF_INET;
513 dst.sin_port = htons (NAT_TRAV_PORT);
514 dst.sin_addr = dummy;
516 if (-1 == (icmpsock = make_icmp_socket()))
518 if (-1 == (rawsock = make_raw_socket()))
523 for (i=0;i<NUM_UDP_PORTS;i++)
524 udpsocks[i] = make_udp_socket (&udpports[i]);
529 FD_SET (icmpsock, &rs);
531 tv.tv_usec = UDP_SEND_FREQUENCY_MS * 1000;
532 select (icmpsock + 1, &rs, NULL, NULL, &tv);
533 /* FIXME: do I need my external IP here? */
534 if (FD_ISSET (icmpsock, &rs))
536 process_icmp_response (&external, icmpsock);
541 "Sending UDP message to %s:%u\n",
545 if (-1 == sendto (udpsocks[pos],
547 (struct sockaddr*) &dst, sizeof (dst)))
550 "sendto failed: %s\n",
552 close (udpsocks[pos]);
553 udpsocks[pos] = make_udp_socket (&udpports[pos]);
555 pos = (pos+1) % NUM_UDP_PORTS;
561 /* end of gnunet-nat-server.c */