/*
* Copyright (C) 2009 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/* A simple implementation of L2TP Access Concentrator (RFC 2661) which only
* creates a single session. The following code only handles control packets.
* Data packets are handled by PPPoLAC driver which can be found in Android
* kernel tree. */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <arpa/inet.h>
#include <linux/netdevice.h>
#include <linux/if_pppox.h>
#include <openssl/md5.h>
#include "mtpd.h"
/* To avoid unnecessary endianness conversions, tunnels, sessions, attributes,
* and values are all accessed in network order. */
/* 0 is reserved. We put ACK here just for convenience. */
enum l2tp_message {
ACK = 0,
SCCRQ = 1,
SCCRP = 2,
SCCCN = 3,
STOPCCN = 4,
HELLO = 6,
OCRQ = 7,
OCRP = 8,
OCCN = 9,
ICRQ = 10,
ICRP = 11,
ICCN = 12,
CDN = 14,
WEN = 15,
SLI = 16,
MESSAGE_MAX = 16,
};
static char *messages[] = {
"ACK", "SCCRQ", "SCCRP", "SCCCN", "STOPCCN", NULL, "HELLO", "OCRQ",
"OCRP", "OCCN", "ICRQ", "ICRP", "ICCN", NULL, "CDN", "WEN", "SLI",
};
/* This is incomplete. Only those we used are listed here. */
#define RESULT_CODE htons(1)
#define PROTOCOL_VERSION htons(2)
#define FRAMING_CAPABILITIES htons(3)
#define HOST_NAME htons(7)
#define ASSIGNED_TUNNEL htons(9)
#define WINDOW_SIZE htons(10)
#define CHALLENGE htons(11)
#define CHALLENGE_RESPONSE htons(13)
#define ASSIGNED_SESSION htons(14)
#define CALL_SERIAL_NUMBER htons(15)
#define FRAMING_TYPE htons(19)
#define CONNECT_SPEED htons(24)
#define RANDOM_VECTOR htons(36)
#define MESSAGE_FLAG 0xC802
#define MESSAGE_MASK 0xCB0F
#define ATTRIBUTE_FLAG(length) (0x8006 + (length))
#define ATTRIBUTE_LENGTH(flag) (0x03FF & (flag))
#define ATTRIBUTE_HIDDEN(flag) (0x4000 & (flag))
#define ACK_SIZE 12
#define MESSAGE_HEADER_SIZE 20
#define ATTRIBUTE_HEADER_SIZE 6
#define MAX_ATTRIBUTE_SIZE 1024
static uint16_t local_tunnel;
static uint16_t local_session;
static uint16_t local_sequence;
static uint16_t remote_tunnel;
static uint16_t remote_session;
static uint16_t remote_sequence;
static uint16_t state;
static int acknowledged;
#define RANDOM_DEVICE "/dev/urandom"
#define CHALLENGE_SIZE 32
static char *secret;
static int secret_length;
static uint8_t challenge[CHALLENGE_SIZE];
/* According to RFC 2661 page 46, an exponential backoff strategy is required
* for retransmission. However, it might waste too much time waiting for IPsec
* negotiation. Here we use the same interval to keep things simple. */
#define TIMEOUT_INTERVAL 2000
#define MAX_PACKET_LENGTH 2048
static struct packet {
int message;
int length;
uint8_t buffer[MAX_PACKET_LENGTH] __attribute__((aligned(4)));
} incoming, outgoing;
struct attribute {
uint16_t flag;
uint16_t vendor;
uint16_t type;
uint8_t value[1];
} __attribute__((packed));
static void set_message(uint16_t session, uint16_t message)
{
uint16_t *p = (uint16_t *)outgoing.buffer;
p[0] = htons(MESSAGE_FLAG);
/* p[1] will be filled in send_packet(). */
p[2] = remote_tunnel;
p[3] = session;
p[4] = htons(local_sequence);
p[5] = htons(remote_sequence);
p[6] = htons(ATTRIBUTE_FLAG(2));
p[7] = 0;
p[8] = 0;
p[9] = htons(message);
outgoing.message = message;
outgoing.length = MESSAGE_HEADER_SIZE;
++local_sequence;
}
static void add_attribute_raw(uint16_t type, void *value, int size)
{
struct attribute *p = (struct attribute *)&outgoing.buffer[outgoing.length];
p->flag = htons(ATTRIBUTE_FLAG(size));
p->vendor = 0;
p->type = type;
memcpy(&p->value, value, size);
outgoing.length += ATTRIBUTE_HEADER_SIZE + size;
}
static void add_attribute_u16(uint16_t attribute, uint16_t value)
{
add_attribute_raw(attribute, &value, sizeof(uint16_t));
}
static void add_attribute_u32(uint16_t attribute, uint32_t value)
{
add_attribute_raw(attribute, &value, sizeof(uint32_t));
}
static void send_packet()
{
uint16_t *p = (uint16_t *)outgoing.buffer;
p[1] = htons(outgoing.length);
send(the_socket, outgoing.buffer, outgoing.length, 0);
acknowledged = 0;
}
static void send_ack()
{
uint16_t buffer[6] = {
htons(MESSAGE_FLAG), htons(ACK_SIZE), remote_tunnel, 0,
htons(local_sequence), htons(remote_sequence),
};
send(the_socket, buffer, ACK_SIZE, 0);
}
static int recv_packet(uint16_t *session)
{
uint16_t *p = (uint16_t *)incoming.buffer;
incoming.length = recv(the_socket, incoming.buffer, MAX_PACKET_LENGTH, 0);
if (incoming.length == -1) {
if (errno == EINTR) {
return 0;
}
log_print(FATAL, "Recv() %s", strerror(errno));
exit(NETWORK_ERROR);
}
/* We only handle packets in our tunnel. */
if ((incoming.length != ACK_SIZE && incoming.length < MESSAGE_HEADER_SIZE)
|| (p[0] & htons(MESSAGE_MASK)) != htons(MESSAGE_FLAG) ||
ntohs(p[1]) != incoming.length || p[2] != local_tunnel) {
return 0;
}
if (incoming.length == ACK_SIZE) {
incoming.message = ACK;
} else if (p[6] == htons(ATTRIBUTE_FLAG(2)) && !p[7] && !p[8]) {
incoming.message = ntohs(p[9]);
} else {
return 0;
}
/* Check if the packet is duplicated and send ACK if necessary. */
if ((uint16_t)(ntohs(p[4]) - remote_sequence) > 32767) {
if (incoming.message != ACK) {
send_ack();
}
return 0;
}
if (ntohs(p[5]) == local_sequence) {
acknowledged = 1;
}
/* Our sending and receiving window sizes are both 1. Thus we only handle
* this packet if it is their next one and they received our last one. */
if (ntohs(p[4]) != remote_sequence || !acknowledged) {
return 0;
}
*session = p[3];
if (incoming.message != ACK) {
++remote_sequence;
}
return 1;
}
static int get_attribute_raw(uint16_t type, void *value, int size)
{
int offset = MESSAGE_HEADER_SIZE;
uint8_t *vector = NULL;
int vector_length = 0;
while (incoming.length >= offset + ATTRIBUTE_HEADER_SIZE) {
struct attribute *p = (struct attribute *)&incoming.buffer[offset];
uint16_t flag = ntohs(p->flag);
int length = ATTRIBUTE_LENGTH(flag);
offset += length;
length -= ATTRIBUTE_HEADER_SIZE;
if (length < 0 || offset > incoming.length) {
break;
}
if (p->vendor) {
continue;
}
if (p->type != type) {
if (p->type == RANDOM_VECTOR && !ATTRIBUTE_HIDDEN(flag)) {
vector = p->value;
vector_length = length;
}
continue;
}
if (!ATTRIBUTE_HIDDEN(flag)) {
if (size > length) {
size = length;
}
memcpy(value, p->value, size);
return size;
}
if (!secret || !vector || length < 2) {
return 0;
} else {
uint8_t buffer[MAX_ATTRIBUTE_SIZE];
uint8_t hash[MD5_DIGEST_LENGTH];
MD5_CTX ctx;
int i;
MD5_Init(&ctx);
MD5_Update(&ctx, &type, sizeof(uint16_t));
MD5_Update(&ctx, secret, secret_length);
MD5_Update(&ctx, vector, vector_length);
MD5_Final(hash, &ctx);
for (i = 0; i < length; ++i) {
int j = i % MD5_DIGEST_LENGTH;
if (i && !j) {
MD5_Init(&ctx);
MD5_Update(&ctx, secret, secret_length);
MD5_Update(&ctx, &p->value[i - MD5_DIGEST_LENGTH],
MD5_DIGEST_LENGTH);
MD5_Final(hash, &ctx);
}
buffer[i] = p->value[i] ^ hash[j];
}
length = buffer[0] << 8 | buffer[1];
if (length > i - 2) {
return 0;
}
if (size > length) {
size = length;
}
memcpy(value, &buffer[2], size);
return size;
}
}
return 0;
}
static int get_attribute_u16(uint16_t type, uint16_t *value)
{
return get_attribute_raw(type, value, sizeof(uint16_t)) == sizeof(uint16_t);
}
static int l2tp_connect(char **arguments)
{
create_socket(AF_INET, SOCK_DGRAM, arguments[0], arguments[1]);
while (!local_tunnel) {
local_tunnel = random();
}
log_print(DEBUG, "Sending SCCRQ (local_tunnel = %d)", local_tunnel);
state = SCCRQ;
set_message(0, SCCRQ);
add_attribute_u16(PROTOCOL_VERSION, htons(0x0100));
add_attribute_raw(HOST_NAME, "anonymous", 9);
add_attribute_u32(FRAMING_CAPABILITIES, htonl(3));
add_attribute_u16(ASSIGNED_TUNNEL, local_tunnel);
add_attribute_u16(WINDOW_SIZE, htons(1));
if (arguments[2][0]) {
int fd = open(RANDOM_DEVICE, O_RDONLY);
if (fd == -1 || read(fd, challenge, CHALLENGE_SIZE) != CHALLENGE_SIZE) {
log_print(FATAL, "Cannot read %s", RANDOM_DEVICE);
exit(SYSTEM_ERROR);
}
close(fd);
add_attribute_raw(CHALLENGE, challenge, CHALLENGE_SIZE);
secret = arguments[2];
secret_length = strlen(arguments[2]);
}
send_packet();
return TIMEOUT_INTERVAL;
}
static int create_pppox()
{
int pppox = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OLAC);
log_print(INFO, "Creating PPPoX socket");
if (pppox == -1) {
log_print(FATAL, "Socket() %s", strerror(errno));
exit(SYSTEM_ERROR);
} else {
struct sockaddr_pppolac address = {
.sa_family = AF_PPPOX,
.sa_protocol = PX_PROTO_OLAC,
.udp_socket = the_socket,
.local = {.tunnel = local_tunnel, .session = local_session},
.remote = {.tunnel = remote_tunnel, .session = remote_session},
};
if (connect(pppox, (struct sockaddr *)&address, sizeof(address))) {
log_print(FATAL, "Connect() %s", strerror(errno));
exit(SYSTEM_ERROR);
}
}
return pppox;
}
static uint8_t *compute_response(uint8_t type, void *challenge, int size)
{
static uint8_t response[MD5_DIGEST_LENGTH];
MD5_CTX ctx;
MD5_Init(&ctx);
MD5_Update(&ctx, &type, sizeof(uint8_t));
MD5_Update(&ctx, secret, secret_length);
MD5_Update(&ctx, challenge, size);
MD5_Final(response, &ctx);
return response;
}
static int verify_challenge()
{
if (secret) {
uint8_t response[MD5_DIGEST_LENGTH];
if (get_attribute_raw(CHALLENGE_RESPONSE, response, MD5_DIGEST_LENGTH)
!= MD5_DIGEST_LENGTH) {
return 0;
}
return !memcmp(compute_response(SCCRP, challenge, CHALLENGE_SIZE),
response, MD5_DIGEST_LENGTH);
}
return 1;
}
static void answer_challenge()
{
if (secret) {
uint8_t challenge[MAX_ATTRIBUTE_SIZE];
int size = get_attribute_raw(CHALLENGE, challenge, MAX_ATTRIBUTE_SIZE);
if (size > 0) {
uint8_t *response = compute_response(SCCCN, challenge, size);
add_attribute_raw(CHALLENGE_RESPONSE, response, MD5_DIGEST_LENGTH);
}
}
}
static int l2tp_process()
{
uint16_t sequence = local_sequence;
uint16_t tunnel = 0;
uint16_t session = 0;
if (!recv_packet(&session)) {
return acknowledged ? 0 : TIMEOUT_INTERVAL;
}
/* Here is the fun part. We always try to protect our tunnel and session
* from being closed even if we received unexpected messages. */
switch(incoming.message) {
case SCCRP:
if (state == SCCRQ) {
if (get_attribute_u16(ASSIGNED_TUNNEL, &tunnel) && tunnel &&
verify_challenge()) {
remote_tunnel = tunnel;
log_print(DEBUG, "Received SCCRP (remote_tunnel = %d) -> "
"Sending SCCCN", remote_tunnel);
state = SCCCN;
answer_challenge();
set_message(0, SCCCN);
break;
}
log_print(DEBUG, "Received SCCRP without %s", tunnel ?
"valid challenge response" : "assigned tunnel");
log_print(ERROR, "Protocol error");
return tunnel ? -CHALLENGE_FAILED : -PROTOCOL_ERROR;
}
break;
case ICRP:
if (state == ICRQ && session == local_session) {
if (get_attribute_u16(ASSIGNED_SESSION, &session) && session) {
remote_session = session;
log_print(DEBUG, "Received ICRP (remote_session = %d) -> "
"Sending ICCN", remote_session);
state = ICCN;
set_message(remote_session, ICCN);
add_attribute_u32(CONNECT_SPEED, htonl(100000000));
add_attribute_u32(FRAMING_TYPE, htonl(3));
break;
}
log_print(DEBUG, "Received ICRP without assigned session");
log_print(ERROR, "Protocol error");
return -PROTOCOL_ERROR;
}
break;
case STOPCCN:
log_print(DEBUG, "Received STOPCCN");
log_print(INFO, "Remote server hung up");
state = STOPCCN;
return -REMOTE_REQUESTED;
case CDN:
if (session && session == local_session) {
log_print(DEBUG, "Received CDN (local_session = %d)",
local_session);
log_print(INFO, "Remote server hung up");
return -REMOTE_REQUESTED;
}
break;
case ACK:
case HELLO:
case WEN:
case SLI:
/* These are harmless, so we just treat them in the same way. */
if (state == SCCCN) {
while (!local_session) {
local_session = random();
}
log_print(DEBUG, "Received %s -> Sending ICRQ (local_session = "
"%d)", messages[incoming.message], local_session);
log_print(INFO, "Tunnel established");
state = ICRQ;
set_message(0, ICRQ);
add_attribute_u16(ASSIGNED_SESSION, local_session);
add_attribute_u32(CALL_SERIAL_NUMBER, random());
break;
}
if (incoming.message == ACK) {
log_print(DEBUG, "Received ACK");
} else {
log_print(DEBUG, "Received %s -> Sending ACK",
messages[incoming.message]);
send_ack();
}
if (state == ICCN) {
log_print(INFO, "Session established");
state = ACK;
start_pppd(create_pppox());
}
return 0;
case ICRQ:
case OCRQ:
/* Since we run pppd as a client, it does not makes sense to
* accept ICRQ or OCRQ. Always send CDN with a proper error. */
if (get_attribute_u16(ASSIGNED_SESSION, &session) && session) {
log_print(DEBUG, "Received %s (remote_session = %d) -> "
"Sending CDN", messages[incoming.message], session);
set_message(session, CDN);
add_attribute_u32(RESULT_CODE, htonl(0x00020006));
add_attribute_u16(ASSIGNED_SESSION, 0);
}
break;
}
if (sequence != local_sequence) {
send_packet();
return TIMEOUT_INTERVAL;
}
/* We reach here if we got an unexpected message. Log it and send ACK. */
if (incoming.message > MESSAGE_MAX || !messages[incoming.message]) {
log_print(DEBUG, "Received UNKNOWN %d -> Sending ACK anyway",
incoming.message);
} else {
log_print(DEBUG, "Received UNEXPECTED %s -> Sending ACK anyway",
messages[incoming.message]);
}
send_ack();
return 0;
}
static int l2tp_timeout()
{
if (acknowledged) {
return 0;
}
log_print(DEBUG, "Timeout -> Sending %s", messages[outgoing.message]);
send(the_socket, outgoing.buffer, outgoing.length, 0);
return TIMEOUT_INTERVAL;
}
static void l2tp_shutdown()
{
if (state != STOPCCN) {
log_print(DEBUG, "Sending STOPCCN");
set_message(0, STOPCCN);
add_attribute_u16(ASSIGNED_TUNNEL, local_tunnel);
add_attribute_u16(RESULT_CODE, htons(6));
send_packet();
}
}
struct protocol l2tp = {
.name = "l2tp",
.arguments = 3,
.usage = "<server> <port> <secret>",
.connect = l2tp_connect,
.process = l2tp_process,
.timeout = l2tp_timeout,
.shutdown = l2tp_shutdown,
};