2 This file is part of GNUnet.
3 (C) 2010 Christian Grothoff (and other contributing authors)
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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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22 * @file src/transport/gnunet-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
44 #include <sys/types.h>
45 #include <sys/socket.h>
46 #include <arpa/inet.h>
47 #include <sys/select.h>
49 #include <sys/types.h>
57 #include <netinet/ip.h>
58 #include <netinet/ip_icmp.h>
59 #include <netinet/in.h>
62 * Should we print some debug output?
67 * Must match IP given in the client.
69 #define DUMMY_IP "192.0.2.86"
72 * How often do we send our ICMP messages to receive replies?
74 #define ICMP_SEND_FREQUENCY_MS 500
83 * Version (4 bits) + Internet header length (4 bits)
103 * Flags (3 bits) + Fragment offset (13 bits)
105 uint16_t flags_frag_offset;
128 * Destination address
154 * Socket we use to receive "fake" ICMP replies.
159 * Socket we use to send our ICMP requests.
164 * Target "dummy" address.
166 static struct in_addr dummy;
170 * CRC-16 for IP/ICMP headers.
172 * @param data what to calculate the CRC over
173 * @param bytes number of bytes in data (must be multiple of 2)
174 * @return the CRC 16.
177 calc_checksum(const uint16_t *data,
184 for (i=0;i<bytes/2;i++)
186 sum = (sum & 0xffff) + (sum >> 16);
187 sum = htons(0xffff - sum);
193 * Send an ICMP message to the dummy IP.
195 * @param my_ip source address (our ip address)
198 send_icmp_echo (const struct in_addr *my_ip)
200 struct icmp_packet icmp_echo;
201 struct sockaddr_in dst;
204 struct ip_packet ip_pkt;
205 struct icmp_packet icmp_pkt;
206 char packet[sizeof (ip_pkt) + sizeof (icmp_pkt)];
209 memset(&ip_pkt, 0, sizeof(ip_pkt));
210 ip_pkt.vers_ihl = 0x45;
212 ip_pkt.pkt_len = sizeof (packet);
214 ip_pkt.flags_frag_offset = 0;
215 ip_pkt.ttl = IPDEFTTL;
216 ip_pkt.proto = IPPROTO_ICMP;
218 ip_pkt.src_ip = my_ip->s_addr;
219 ip_pkt.dst_ip = dummy.s_addr;
220 ip_pkt.checksum = htons(calc_checksum((uint16_t*)&ip_pkt, sizeof (ip_pkt)));
221 memcpy (packet, &ip_pkt, sizeof (ip_pkt));
222 off += sizeof (ip_pkt);
224 icmp_echo.type = ICMP_ECHO;
226 icmp_echo.reserved = 0;
227 icmp_echo.checksum = 0;
228 icmp_echo.checksum = htons(calc_checksum((uint16_t*)&icmp_echo,
229 sizeof (struct icmp_packet)));
231 make_echo (my_ip, &icmp_echo);
233 memcpy (&packet[off], &icmp_echo, sizeof (icmp_echo));
234 off += sizeof (icmp_echo);
236 memset (&dst, 0, sizeof (dst));
237 dst.sin_family = AF_INET;
238 dst.sin_addr = dummy;
239 err = sendto(rawsock,
241 (struct sockaddr*)&dst,
247 "sendto failed: %s\n", strerror(errno));
253 "Error: partial send of ICMP message\n");
259 * We've received an ICMP response. Process it.
262 process_icmp_response ()
267 struct ip_packet ip_pkt;
268 struct icmp_packet icmp_pkt;
269 struct udp_packet udp_pkt;
274 have = read (icmpsock, buf, sizeof (buf));
278 "Error reading raw socket: %s\n",
285 "Received message of %u bytes\n",
286 (unsigned int) have);
288 if (have == sizeof (struct ip_packet) *2 + sizeof (struct icmp_packet) * 2 + sizeof(uint32_t))
292 else if (have != sizeof (struct ip_packet) *2 + sizeof (struct icmp_packet) * 2)
296 "Received ICMP message of unexpected size: %u bytes\n",
297 (unsigned int) have);
302 memcpy (&ip_pkt, &buf[off], sizeof (ip_pkt));
303 off += sizeof (ip_pkt);
304 memcpy (&icmp_pkt, &buf[off], sizeof (icmp_pkt));
305 off += sizeof (icmp_pkt);
306 if ( ((ip_pkt.proto != IPPROTO_ICMP) && (ip_pkt.proto != IPPROTO_UDP)) ||
307 (icmp_pkt.type != ICMP_TIME_EXCEEDED) ||
308 (icmp_pkt.code != 0) )
310 /* maybe we got an actual reply back... */
316 memcpy (&ip_pkt, &buf[off], sizeof (ip_pkt));
317 off += sizeof (ip_pkt);
322 &buf[sizeof (struct ip_packet) *2 + sizeof (struct icmp_packet) * 2],
333 else if (ip_pkt.proto == IPPROTO_UDP)
344 ntohs((uint16_t) udp_pkt.length));
360 * Create an ICMP raw socket for reading.
362 * @return -1 on error
369 ret = socket (AF_INET, SOCK_RAW, IPPROTO_ICMP);
373 "Error opening RAW socket: %s\n",
377 if (ret >= FD_SETSIZE)
380 "Socket number too large (%d > %u)\n",
382 (unsigned int) FD_SETSIZE);
391 * Create an ICMP raw socket for writing.
393 * @return -1 on error
401 ret = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
405 "Error opening RAW socket: %s\n",
416 "setsockopt failed: %s\n",
424 (char *)&one, sizeof(one)) == -1)
427 "setsockopt failed: %s\n",
440 struct in_addr external;
448 "This program must be started with our (internal NAT) IP as the only argument.\n");
451 if (1 != inet_pton (AF_INET, argv[1], &external))
454 "Error parsing IPv4 address: %s\n",
458 if (1 != inet_pton (AF_INET, DUMMY_IP, &dummy))
461 "Internal error converting dummy IP to binary.\n");
464 if (-1 == (icmpsock = make_icmp_socket()))
468 if (-1 == (rawsock = make_raw_socket()))
474 if (0 != setresuid (uid, uid, uid))
477 "Failed to setresuid: %s\n",
479 /* not critical, continue anyway */
484 FD_SET (icmpsock, &rs);
486 tv.tv_usec = ICMP_SEND_FREQUENCY_MS * 1000;
487 if (-1 == select (icmpsock + 1, &rs, NULL, NULL, &tv))
492 "select failed: %s\n",
496 if (FD_ISSET (icmpsock, &rs))
497 process_icmp_response ();
498 send_icmp_echo (&external);
500 /* select failed (internal error or OS out of resources) */
507 /* end of gnunet-nat-server.c */