1 /* 2 * Copyright (C) 2009 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 /* A simple implementation of L2TP Access Concentrator (RFC 2661) which only 18 * creates a single session. The following code only handles control packets. 19 * Data packets are handled by PPPoLAC driver which can be found in Android 20 * kernel tree. */ 21 22 #include <stdio.h> 23 #include <stdlib.h> 24 #include <string.h> 25 #include <errno.h> 26 #include <fcntl.h> 27 #include <sys/types.h> 28 #include <sys/socket.h> 29 #include <sys/stat.h> 30 #include <arpa/inet.h> 31 #include <linux/netdevice.h> 32 #include <linux/if_pppox.h> 33 #include <openssl/md5.h> 34 35 #include "mtpd.h" 36 37 /* To avoid unnecessary endianness conversions, tunnels, sessions, attributes, 38 * and values are all accessed in network order. */ 39 40 /* 0 is reserved. We put ACK here just for convenience. */ 41 enum l2tp_message { 42 ACK = 0, 43 SCCRQ = 1, 44 SCCRP = 2, 45 SCCCN = 3, 46 STOPCCN = 4, 47 HELLO = 6, 48 OCRQ = 7, 49 OCRP = 8, 50 OCCN = 9, 51 ICRQ = 10, 52 ICRP = 11, 53 ICCN = 12, 54 CDN = 14, 55 WEN = 15, 56 SLI = 16, 57 MESSAGE_MAX = 16, 58 }; 59 60 static char *messages[] = { 61 "ACK", "SCCRQ", "SCCRP", "SCCCN", "STOPCCN", NULL, "HELLO", "OCRQ", 62 "OCRP", "OCCN", "ICRQ", "ICRP", "ICCN", NULL, "CDN", "WEN", "SLI", 63 }; 64 65 /* This is incomplete. Only those we used are listed here. */ 66 #define RESULT_CODE htons(1) 67 #define PROTOCOL_VERSION htons(2) 68 #define FRAMING_CAPABILITIES htons(3) 69 #define HOST_NAME htons(7) 70 #define ASSIGNED_TUNNEL htons(9) 71 #define WINDOW_SIZE htons(10) 72 #define CHALLENGE htons(11) 73 #define CHALLENGE_RESPONSE htons(13) 74 #define ASSIGNED_SESSION htons(14) 75 #define CALL_SERIAL_NUMBER htons(15) 76 #define FRAMING_TYPE htons(19) 77 #define CONNECT_SPEED htons(24) 78 #define RANDOM_VECTOR htons(36) 79 80 #define MESSAGE_FLAG 0xC802 81 #define MESSAGE_MASK 0xCB0F 82 #define ATTRIBUTE_FLAG(length) (0x8006 + (length)) 83 #define ATTRIBUTE_LENGTH(flag) (0x03FF & (flag)) 84 #define ATTRIBUTE_HIDDEN(flag) (0x4000 & (flag)) 85 86 #define ACK_SIZE 12 87 #define MESSAGE_HEADER_SIZE 20 88 #define ATTRIBUTE_HEADER_SIZE 6 89 #define MAX_ATTRIBUTE_SIZE 1024 90 91 static uint16_t local_tunnel; 92 static uint16_t local_session; 93 static uint16_t local_sequence; 94 static uint16_t remote_tunnel; 95 static uint16_t remote_session; 96 static uint16_t remote_sequence; 97 98 static uint16_t state; 99 static int acknowledged; 100 101 #define RANDOM_DEVICE "/dev/urandom" 102 #define CHALLENGE_SIZE 32 103 104 static char *secret; 105 static int secret_length; 106 static uint8_t challenge[CHALLENGE_SIZE]; 107 108 /* According to RFC 2661 page 46, an exponential backoff strategy is required 109 * for retransmission. However, it might waste too much time waiting for IPsec 110 * negotiation. Here we use the same interval to keep things simple. */ 111 #define TIMEOUT_INTERVAL 2000 112 113 #define MAX_PACKET_LENGTH 2048 114 115 static struct packet { 116 int message; 117 int length; 118 uint8_t buffer[MAX_PACKET_LENGTH] __attribute__((aligned(4))); 119 } incoming, outgoing; 120 121 struct attribute { 122 uint16_t flag; 123 uint16_t vendor; 124 uint16_t type; 125 uint8_t value[1]; 126 } __attribute__((packed)); 127 128 static void set_message(uint16_t session, uint16_t message) 129 { 130 uint16_t *p = (uint16_t *)outgoing.buffer; 131 p[0] = htons(MESSAGE_FLAG); 132 /* p[1] will be filled in send_packet(). */ 133 p[2] = remote_tunnel; 134 p[3] = session; 135 p[4] = htons(local_sequence); 136 p[5] = htons(remote_sequence); 137 p[6] = htons(ATTRIBUTE_FLAG(2)); 138 p[7] = 0; 139 p[8] = 0; 140 p[9] = htons(message); 141 outgoing.message = message; 142 outgoing.length = MESSAGE_HEADER_SIZE; 143 ++local_sequence; 144 } 145 146 static void add_attribute_raw(uint16_t type, void *value, int size) 147 { 148 struct attribute *p = (struct attribute *)&outgoing.buffer[outgoing.length]; 149 p->flag = htons(ATTRIBUTE_FLAG(size)); 150 p->vendor = 0; 151 p->type = type; 152 memcpy(&p->value, value, size); 153 outgoing.length += ATTRIBUTE_HEADER_SIZE + size; 154 } 155 156 static void add_attribute_u16(uint16_t attribute, uint16_t value) 157 { 158 add_attribute_raw(attribute, &value, sizeof(uint16_t)); 159 } 160 161 static void add_attribute_u32(uint16_t attribute, uint32_t value) 162 { 163 add_attribute_raw(attribute, &value, sizeof(uint32_t)); 164 } 165 166 static void send_packet() 167 { 168 uint16_t *p = (uint16_t *)outgoing.buffer; 169 p[1] = htons(outgoing.length); 170 send(the_socket, outgoing.buffer, outgoing.length, 0); 171 acknowledged = 0; 172 } 173 174 static void send_ack() 175 { 176 uint16_t buffer[6] = { 177 htons(MESSAGE_FLAG), htons(ACK_SIZE), remote_tunnel, 0, 178 htons(local_sequence), htons(remote_sequence), 179 }; 180 send(the_socket, buffer, ACK_SIZE, 0); 181 } 182 183 static int recv_packet(uint16_t *session) 184 { 185 uint16_t *p = (uint16_t *)incoming.buffer; 186 187 incoming.length = recv(the_socket, incoming.buffer, MAX_PACKET_LENGTH, 0); 188 if (incoming.length == -1) { 189 if (errno == EINTR) { 190 return 0; 191 } 192 log_print(FATAL, "Recv() %s", strerror(errno)); 193 exit(NETWORK_ERROR); 194 } 195 196 /* We only handle packets in our tunnel. */ 197 if ((incoming.length != ACK_SIZE && incoming.length < MESSAGE_HEADER_SIZE) 198 || (p[0] & htons(MESSAGE_MASK)) != htons(MESSAGE_FLAG) || 199 ntohs(p[1]) != incoming.length || p[2] != local_tunnel) { 200 return 0; 201 } 202 203 if (incoming.length == ACK_SIZE) { 204 incoming.message = ACK; 205 } else if (p[6] == htons(ATTRIBUTE_FLAG(2)) && !p[7] && !p[8]) { 206 incoming.message = ntohs(p[9]); 207 } else { 208 return 0; 209 } 210 211 /* Check if the packet is duplicated and send ACK if necessary. */ 212 if ((uint16_t)(ntohs(p[4]) - remote_sequence) > 32767) { 213 if (incoming.message != ACK) { 214 send_ack(); 215 } 216 return 0; 217 } 218 219 if (ntohs(p[5]) == local_sequence) { 220 acknowledged = 1; 221 } 222 223 /* Our sending and receiving window sizes are both 1. Thus we only handle 224 * this packet if it is their next one and they received our last one. */ 225 if (ntohs(p[4]) != remote_sequence || !acknowledged) { 226 return 0; 227 } 228 *session = p[3]; 229 if (incoming.message != ACK) { 230 ++remote_sequence; 231 } 232 return 1; 233 } 234 235 static int get_attribute_raw(uint16_t type, void *value, int size) 236 { 237 int offset = MESSAGE_HEADER_SIZE; 238 uint8_t *vector = NULL; 239 int vector_length = 0; 240 241 while (incoming.length >= offset + ATTRIBUTE_HEADER_SIZE) { 242 struct attribute *p = (struct attribute *)&incoming.buffer[offset]; 243 uint16_t flag = ntohs(p->flag); 244 int length = ATTRIBUTE_LENGTH(flag); 245 246 offset += length; 247 length -= ATTRIBUTE_HEADER_SIZE; 248 if (length < 0 || offset > incoming.length) { 249 break; 250 } 251 if (p->vendor) { 252 continue; 253 } 254 if (p->type != type) { 255 if (p->type == RANDOM_VECTOR && !ATTRIBUTE_HIDDEN(flag)) { 256 vector = p->value; 257 vector_length = length; 258 } 259 continue; 260 } 261 262 if (!ATTRIBUTE_HIDDEN(flag)) { 263 if (size > length) { 264 size = length; 265 } 266 memcpy(value, p->value, size); 267 return size; 268 } 269 270 if (!secret || !vector || length < 2) { 271 return 0; 272 } else { 273 uint8_t buffer[MAX_ATTRIBUTE_SIZE]; 274 uint8_t hash[MD5_DIGEST_LENGTH]; 275 MD5_CTX ctx; 276 int i; 277 278 MD5_Init(&ctx); 279 MD5_Update(&ctx, &type, sizeof(uint16_t)); 280 MD5_Update(&ctx, secret, secret_length); 281 MD5_Update(&ctx, vector, vector_length); 282 MD5_Final(hash, &ctx); 283 284 for (i = 0; i < length; ++i) { 285 int j = i % MD5_DIGEST_LENGTH; 286 if (i && !j) { 287 MD5_Init(&ctx); 288 MD5_Update(&ctx, secret, secret_length); 289 MD5_Update(&ctx, &p->value[i - MD5_DIGEST_LENGTH], 290 MD5_DIGEST_LENGTH); 291 MD5_Final(hash, &ctx); 292 } 293 buffer[i] = p->value[i] ^ hash[j]; 294 } 295 296 length = buffer[0] << 8 | buffer[1]; 297 if (length > i - 2) { 298 return 0; 299 } 300 if (size > length) { 301 size = length; 302 } 303 memcpy(value, &buffer[2], size); 304 return size; 305 } 306 } 307 return 0; 308 } 309 310 static int get_attribute_u16(uint16_t type, uint16_t *value) 311 { 312 return get_attribute_raw(type, value, sizeof(uint16_t)) == sizeof(uint16_t); 313 } 314 315 static int l2tp_connect(char **arguments) 316 { 317 create_socket(AF_INET, SOCK_DGRAM, arguments[0], arguments[1]); 318 319 while (!local_tunnel) { 320 local_tunnel = random(); 321 } 322 323 log_print(DEBUG, "Sending SCCRQ (local_tunnel = %d)", local_tunnel); 324 state = SCCRQ; 325 set_message(0, SCCRQ); 326 add_attribute_u16(PROTOCOL_VERSION, htons(0x0100)); 327 add_attribute_raw(HOST_NAME, "anonymous", 9); 328 add_attribute_u32(FRAMING_CAPABILITIES, htonl(3)); 329 add_attribute_u16(ASSIGNED_TUNNEL, local_tunnel); 330 add_attribute_u16(WINDOW_SIZE, htons(1)); 331 332 if (arguments[2][0]) { 333 int fd = open(RANDOM_DEVICE, O_RDONLY); 334 if (fd == -1 || read(fd, challenge, CHALLENGE_SIZE) != CHALLENGE_SIZE) { 335 log_print(FATAL, "Cannot read %s", RANDOM_DEVICE); 336 exit(SYSTEM_ERROR); 337 } 338 close(fd); 339 340 add_attribute_raw(CHALLENGE, challenge, CHALLENGE_SIZE); 341 secret = arguments[2]; 342 secret_length = strlen(arguments[2]); 343 } 344 345 send_packet(); 346 return TIMEOUT_INTERVAL; 347 } 348 349 static int create_pppox() 350 { 351 int pppox = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OLAC); 352 log_print(INFO, "Creating PPPoX socket"); 353 354 if (pppox == -1) { 355 log_print(FATAL, "Socket() %s", strerror(errno)); 356 exit(SYSTEM_ERROR); 357 } else { 358 struct sockaddr_pppolac address = { 359 .sa_family = AF_PPPOX, 360 .sa_protocol = PX_PROTO_OLAC, 361 .udp_socket = the_socket, 362 .local = {.tunnel = local_tunnel, .session = local_session}, 363 .remote = {.tunnel = remote_tunnel, .session = remote_session}, 364 }; 365 if (connect(pppox, (struct sockaddr *)&address, sizeof(address))) { 366 log_print(FATAL, "Connect() %s", strerror(errno)); 367 exit(SYSTEM_ERROR); 368 } 369 } 370 return pppox; 371 } 372 373 static uint8_t *compute_response(uint8_t type, void *challenge, int size) 374 { 375 static uint8_t response[MD5_DIGEST_LENGTH]; 376 MD5_CTX ctx; 377 MD5_Init(&ctx); 378 MD5_Update(&ctx, &type, sizeof(uint8_t)); 379 MD5_Update(&ctx, secret, secret_length); 380 MD5_Update(&ctx, challenge, size); 381 MD5_Final(response, &ctx); 382 return response; 383 } 384 385 static int verify_challenge() 386 { 387 if (secret) { 388 uint8_t response[MD5_DIGEST_LENGTH]; 389 if (get_attribute_raw(CHALLENGE_RESPONSE, response, MD5_DIGEST_LENGTH) 390 != MD5_DIGEST_LENGTH) { 391 return 0; 392 } 393 return !memcmp(compute_response(SCCRP, challenge, CHALLENGE_SIZE), 394 response, MD5_DIGEST_LENGTH); 395 } 396 return 1; 397 } 398 399 static void answer_challenge() 400 { 401 if (secret) { 402 uint8_t challenge[MAX_ATTRIBUTE_SIZE]; 403 int size = get_attribute_raw(CHALLENGE, challenge, MAX_ATTRIBUTE_SIZE); 404 if (size > 0) { 405 uint8_t *response = compute_response(SCCCN, challenge, size); 406 add_attribute_raw(CHALLENGE_RESPONSE, response, MD5_DIGEST_LENGTH); 407 } 408 } 409 } 410 411 static int l2tp_process() 412 { 413 uint16_t sequence = local_sequence; 414 uint16_t tunnel = 0; 415 uint16_t session = 0; 416 417 if (!recv_packet(&session)) { 418 return acknowledged ? 0 : TIMEOUT_INTERVAL; 419 } 420 421 /* Here is the fun part. We always try to protect our tunnel and session 422 * from being closed even if we received unexpected messages. */ 423 switch(incoming.message) { 424 case SCCRP: 425 if (state == SCCRQ) { 426 if (get_attribute_u16(ASSIGNED_TUNNEL, &tunnel) && tunnel && 427 verify_challenge()) { 428 remote_tunnel = tunnel; 429 log_print(DEBUG, "Received SCCRP (remote_tunnel = %d) -> " 430 "Sending SCCCN", remote_tunnel); 431 state = SCCCN; 432 answer_challenge(); 433 set_message(0, SCCCN); 434 break; 435 } 436 log_print(DEBUG, "Received SCCRP without %s", tunnel ? 437 "valid challenge response" : "assigned tunnel"); 438 log_print(ERROR, "Protocol error"); 439 return tunnel ? -CHALLENGE_FAILED : -PROTOCOL_ERROR; 440 } 441 break; 442 443 case ICRP: 444 if (state == ICRQ && session == local_session) { 445 if (get_attribute_u16(ASSIGNED_SESSION, &session) && session) { 446 remote_session = session; 447 log_print(DEBUG, "Received ICRP (remote_session = %d) -> " 448 "Sending ICCN", remote_session); 449 state = ICCN; 450 set_message(remote_session, ICCN); 451 add_attribute_u32(CONNECT_SPEED, htonl(100000000)); 452 add_attribute_u32(FRAMING_TYPE, htonl(3)); 453 break; 454 } 455 log_print(DEBUG, "Received ICRP without assigned session"); 456 log_print(ERROR, "Protocol error"); 457 return -PROTOCOL_ERROR; 458 } 459 break; 460 461 case STOPCCN: 462 log_print(DEBUG, "Received STOPCCN"); 463 log_print(INFO, "Remote server hung up"); 464 state = STOPCCN; 465 return -REMOTE_REQUESTED; 466 467 case CDN: 468 if (session && session == local_session) { 469 log_print(DEBUG, "Received CDN (local_session = %d)", 470 local_session); 471 log_print(INFO, "Remote server hung up"); 472 return -REMOTE_REQUESTED; 473 } 474 break; 475 476 case ACK: 477 case HELLO: 478 case WEN: 479 case SLI: 480 /* These are harmless, so we just treat them in the same way. */ 481 if (state == SCCCN) { 482 while (!local_session) { 483 local_session = random(); 484 } 485 log_print(DEBUG, "Received %s -> Sending ICRQ (local_session = " 486 "%d)", messages[incoming.message], local_session); 487 log_print(INFO, "Tunnel established"); 488 state = ICRQ; 489 set_message(0, ICRQ); 490 add_attribute_u16(ASSIGNED_SESSION, local_session); 491 add_attribute_u32(CALL_SERIAL_NUMBER, random()); 492 break; 493 } 494 495 if (incoming.message == ACK) { 496 log_print(DEBUG, "Received ACK"); 497 } else { 498 log_print(DEBUG, "Received %s -> Sending ACK", 499 messages[incoming.message]); 500 send_ack(); 501 } 502 503 if (state == ICCN) { 504 log_print(INFO, "Session established"); 505 state = ACK; 506 start_pppd(create_pppox()); 507 } 508 return 0; 509 510 case ICRQ: 511 case OCRQ: 512 /* Since we run pppd as a client, it does not makes sense to 513 * accept ICRQ or OCRQ. Always send CDN with a proper error. */ 514 if (get_attribute_u16(ASSIGNED_SESSION, &session) && session) { 515 log_print(DEBUG, "Received %s (remote_session = %d) -> " 516 "Sending CDN", messages[incoming.message], session); 517 set_message(session, CDN); 518 add_attribute_u32(RESULT_CODE, htonl(0x00020006)); 519 add_attribute_u16(ASSIGNED_SESSION, 0); 520 } 521 break; 522 } 523 524 if (sequence != local_sequence) { 525 send_packet(); 526 return TIMEOUT_INTERVAL; 527 } 528 529 /* We reach here if we got an unexpected message. Log it and send ACK. */ 530 if (incoming.message > MESSAGE_MAX || !messages[incoming.message]) { 531 log_print(DEBUG, "Received UNKNOWN %d -> Sending ACK anyway", 532 incoming.message); 533 } else { 534 log_print(DEBUG, "Received UNEXPECTED %s -> Sending ACK anyway", 535 messages[incoming.message]); 536 } 537 send_ack(); 538 return 0; 539 } 540 541 static int l2tp_timeout() 542 { 543 if (acknowledged) { 544 return 0; 545 } 546 log_print(DEBUG, "Timeout -> Sending %s", messages[outgoing.message]); 547 send(the_socket, outgoing.buffer, outgoing.length, 0); 548 return TIMEOUT_INTERVAL; 549 } 550 551 static void l2tp_shutdown() 552 { 553 if (state != STOPCCN) { 554 log_print(DEBUG, "Sending STOPCCN"); 555 set_message(0, STOPCCN); 556 add_attribute_u16(ASSIGNED_TUNNEL, local_tunnel); 557 add_attribute_u16(RESULT_CODE, htons(6)); 558 send_packet(); 559 } 560 } 561 562 struct protocol l2tp = { 563 .name = "l2tp", 564 .arguments = 3, 565 .usage = "<server> <port> <secret>", 566 .connect = l2tp_connect, 567 .process = l2tp_process, 568 .timeout = l2tp_timeout, 569 .shutdown = l2tp_shutdown, 570 }; 571
