stun.c

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/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 1999 - 2008, Digium, Inc.
 *
 * Mark Spencer <[email protected]>
 *
 * See http://www.asterisk.org for more information about
 * the Asterisk project. Please do not directly contact
 * any of the maintainers of this project for assistance;
 * the project provides a web site, mailing lists and IRC
 * channels for your use.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License Version 2. See the LICENSE file
 * at the top of the source tree.
 */

/*!
 * \file
 *
 * \brief STUN Support
 *
 * \author Mark Spencer <[email protected]>
 *
 * \note STUN is defined in RFC 3489.
 */

/*** MODULEINFO
 <support_level>core</support_level>
 ***/

#include "asterisk.h"

#include "asterisk/logger.h"
#include "asterisk/logger_category.h"
#include "asterisk/_private.h"
#include "asterisk/stun.h"
#include "asterisk/cli.h"
#include "asterisk/utils.h"
#include "asterisk/channel.h"

/*!
 * \brief STUN support code
 *
 * This code provides some support for doing STUN transactions.
 * Eventually it should be moved elsewhere as other protocols
 * than RTP can benefit from it - e.g. SIP.
 * STUN is described in RFC3489 and it is based on the exchange
 * of UDP packets between a client and one or more servers to
 * determine the externally visible address (and port) of the client
 * once it has gone through the NAT boxes that connect it to the
 * outside.
 * The simplest request packet is just the header defined in
 * struct stun_header, and from the response we may just look at
 * one attribute, STUN_MAPPED_ADDRESS, that we find in the response.
 * By doing more transactions with different server addresses we
 * may determine more about the behaviour of the NAT boxes, of
 * course - the details are in the RFC.
 *
 * All STUN packets start with a simple header made of a type,
 * length (excluding the header) and a 16-byte random transaction id.
 * Following the header we may have zero or more attributes, each
 * structured as a type, length and a value (whose format depends
 * on the type, but often contains addresses).
 * Of course all fields are in network format.
 */

typedef struct { unsigned int id[4]; } __attribute__((packed)) stun_trans_id;

struct stun_header {
 unsigned short msgtype;
 unsigned short msglen;
 stun_trans_id id;
 unsigned char ies[0];
} __attribute__((packed));

struct stun_attr {
 unsigned short attr;
 unsigned short len;
 unsigned char value[0];
} __attribute__((packed));

/*
 * The format normally used for addresses carried by STUN messages.
 */
struct stun_addr {
 unsigned char unused;
 unsigned char family;
 unsigned short port;
 unsigned int addr;
} __attribute__((packed));

/*! \brief STUN message types
 * 'BIND' refers to transactions used to determine the externally
 * visible addresses. 'SEC' refers to transactions used to establish
 * a session key for subsequent requests.
 * 'SEC' functionality is not supported here.
 */

#define STUN_BINDREQ 0x0001
#define STUN_BINDRESP 0x0101
#define STUN_BINDERR 0x0111
#define STUN_SECREQ 0x0002
#define STUN_SECRESP 0x0102
#define STUN_SECERR 0x0112

/*! \brief Basic attribute types in stun messages.
 * Messages can also contain custom attributes (codes above 0x7fff)
 */
#define STUN_MAPPED_ADDRESS 0x0001
#define STUN_RESPONSE_ADDRESS 0x0002
#define STUN_CHANGE_REQUEST 0x0003
#define STUN_SOURCE_ADDRESS 0x0004
#define STUN_CHANGED_ADDRESS 0x0005
#define STUN_USERNAME 0x0006
#define STUN_PASSWORD 0x0007
#define STUN_MESSAGE_INTEGRITY 0x0008
#define STUN_ERROR_CODE 0x0009
#define STUN_UNKNOWN_ATTRIBUTES 0x000a
#define STUN_REFLECTED_FROM 0x000b

/*! \brief helper function to print message names */
static const char *stun_msg2str(int msg)
{
 switch (msg) {
 case STUN_BINDREQ:
 return "Binding Request";
 case STUN_BINDRESP:
 return "Binding Response";
 case STUN_BINDERR:
 return "Binding Error Response";
 case STUN_SECREQ:
 return "Shared Secret Request";
 case STUN_SECRESP:
 return "Shared Secret Response";
 case STUN_SECERR:
 return "Shared Secret Error Response";
 }
 return "Non-RFC3489 Message";
}

/*! \brief helper function to print attribute names */
static const char *stun_attr2str(int msg)
{
 switch (msg) {
 case STUN_MAPPED_ADDRESS:
 return "Mapped Address";
 case STUN_RESPONSE_ADDRESS:
 return "Response Address";
 case STUN_CHANGE_REQUEST:
 return "Change Request";
 case STUN_SOURCE_ADDRESS:
 return "Source Address";
 case STUN_CHANGED_ADDRESS:
 return "Changed Address";
 case STUN_USERNAME:
 return "Username";
 case STUN_PASSWORD:
 return "Password";
 case STUN_MESSAGE_INTEGRITY:
 return "Message Integrity";
 case STUN_ERROR_CODE:
 return "Error Code";
 case STUN_UNKNOWN_ATTRIBUTES:
 return "Unknown Attributes";
 case STUN_REFLECTED_FROM:
 return "Reflected From";
 }
 return "Non-RFC3489 Attribute";
}

/*! \brief here we store credentials extracted from a message */
struct stun_state {
 const char *username;
 const char *password;
};

static int stun_process_attr(struct stun_state *state, struct stun_attr *attr)
{
 if (ast_debug_stun_packet_is_allowed) {
 ast_verbose("Found STUN Attribute %s (%04x), length %d\n",
 stun_attr2str(ntohs(attr->attr)), (unsigned)ntohs(attr->attr), ntohs(attr->len));
 }
 switch (ntohs(attr->attr)) {
 case STUN_USERNAME:
 state->username = (const char *) (attr->value);
 break;
 case STUN_PASSWORD:
 state->password = (const char *) (attr->value);
 break;
 default:
 if (ast_debug_stun_packet_is_allowed) {
 ast_verbose("Ignoring STUN attribute %s (%04x), length %d\n",
 stun_attr2str(ntohs(attr->attr)), (unsigned)ntohs(attr->attr), ntohs(attr->len));
 }
 }

 return 0;
}

/*! \brief append a string to an STUN message */
static void append_attr_string(struct stun_attr **attr, int attrval, const char *s, int *len, int *left)
{
 int str_length = strlen(s);
 int attr_length = str_length + ((~(str_length - 1)) & 0x3);
 int size = sizeof(**attr) + attr_length;
 if (*left > size) {
 (*attr)->attr = htons(attrval);
 (*attr)->len = htons(attr_length);
 memcpy((*attr)->value, s, str_length);
 memset((*attr)->value + str_length, 0, attr_length - str_length);
 (*attr) = (struct stun_attr *)((*attr)->value + attr_length);
 *len += size;
 *left -= size;
 }
}

/*! \brief append an address to an STUN message */
static void append_attr_address(struct stun_attr **attr, int attrval, struct sockaddr_in *sin, int *len, int *left)
{
 int size = sizeof(**attr) + 8;
 struct stun_addr *addr;
 if (*left > size) {
 (*attr)->attr = htons(attrval);
 (*attr)->len = htons(8);
 addr = (struct stun_addr *)((*attr)->value);
 addr->unused = 0;
 addr->family = 0x01;
 addr->port = sin->sin_port;
 addr->addr = sin->sin_addr.s_addr;
 (*attr) = (struct stun_attr *)((*attr)->value + 8);
 *len += size;
 *left -= size;
 }
}

/*! \brief wrapper to send an STUN message */
static int stun_send(int s, struct sockaddr_in *dst, struct stun_header *resp)
{
 return sendto(s, resp, ntohs(resp->msglen) + sizeof(*resp), 0,
 (struct sockaddr *)dst, sizeof(*dst));
}

/*!
 * \internal
 * \brief Compare the STUN tranaction IDs.
 *
 * \param left Transaction ID.
 * \param right Transaction ID.
 *
 * \retval 0 if match.
 * \retval non-zero if not match.
 */
static int stun_id_cmp(stun_trans_id *left, stun_trans_id *right)
{
 return memcmp(left, right, sizeof(*left));
}

/*! \brief helper function to generate a random request id */
static void stun_req_id(struct stun_header *req)
{
 int x;
 for (x = 0; x < 4; x++)
 req->id.id[x] = ast_random();
}

int ast_stun_handle_packet(int s, struct sockaddr_in *src, unsigned char *data, size_t len, stun_cb_f *stun_cb, void *arg)
{
 struct stun_header *hdr = (struct stun_header *)data;
 struct stun_attr *attr;
 struct stun_state st;
 int ret = AST_STUN_IGNORE;
 int x;

 /* On entry, 'len' is the length of the udp payload. After the
 * initial checks it becomes the size of unprocessed options,
 * while 'data' is advanced accordingly.
 */
 if (len < sizeof(struct stun_header)) {
 ast_debug_stun(1, "Runt STUN packet (only %d, wanting at least %d)\n", (int) len, (int) sizeof(struct stun_header));
 return -1;
 }
 len -= sizeof(struct stun_header);
 data += sizeof(struct stun_header);
 x = ntohs(hdr->msglen); /* len as advertised in the message */
 if (ast_debug_stun_packet_is_allowed)
 ast_verbose("STUN Packet, msg %s (%04x), length: %d\n", stun_msg2str(ntohs(hdr->msgtype)), (unsigned)ntohs(hdr->msgtype), x);
 if (x > len) {
 ast_debug_stun(1, "Scrambled STUN packet length (got %d, expecting %d)\n", x, (int)len);
 } else
 len = x;
 memset(&st, 0, sizeof(st));
 while (len) {
 if (len < sizeof(struct stun_attr)) {
 ast_debug_stun(1, "Runt Attribute (got %d, expecting %d)\n", (int)len, (int) sizeof(struct stun_attr));
 break;
 }
 attr = (struct stun_attr *)data;
 /* compute total attribute length */
 x = ntohs(attr->len) + sizeof(struct stun_attr);
 if (x > len) {
 ast_debug_stun(1, "Inconsistent Attribute (length %d exceeds remaining msg len %d)\n", x, (int)len);
 break;
 }
 if (stun_cb)
 stun_cb(attr, arg);
 if (stun_process_attr(&st, attr)) {
 ast_debug_stun(1, "Failed to handle attribute %s (%04x)\n", stun_attr2str(ntohs(attr->attr)), (unsigned)ntohs(attr->attr));
 break;
 }
 /* Clear attribute id: in case previous entry was a string,
 * this will act as the terminator for the string.
 */
 attr->attr = 0;
 data += x;
 len -= x;
 }
 /* Null terminate any string.
 * XXX NOTE, we write past the size of the buffer passed by the
 * caller, so this is potentially dangerous. The only thing that
 * saves us is that usually we read the incoming message in a
 * much larger buffer in the struct ast_rtp
 */
 *data = '\0';

 /* Now prepare to generate a reply, which at the moment is done
 * only for properly formed (len == 0) STUN_BINDREQ messages.
 */
 if (len == 0) {
 unsigned char respdata[1024];
 struct stun_header *resp = (struct stun_header *)respdata;
 int resplen = 0; /* len excluding header */
 int respleft = sizeof(respdata) - sizeof(struct stun_header);
 char combined[33];

 resp->id = hdr->id;
 resp->msgtype = 0;
 resp->msglen = 0;
 attr = (struct stun_attr *)resp->ies;
 switch (ntohs(hdr->msgtype)) {
 case STUN_BINDREQ:
 if (ast_debug_stun_packet_is_allowed)
 ast_verbose("STUN Bind Request, username: %s\n",
 st.username ? st.username : "<none>");
 if (st.username) {
 append_attr_string(&attr, STUN_USERNAME, st.username, &resplen, &respleft);
 snprintf(combined, sizeof(combined), "%16s%16s", st.username + 16, st.username);
 } else {
 combined[0] = '\0';
 }

 append_attr_address(&attr, STUN_MAPPED_ADDRESS, src, &resplen, &respleft);
 resp->msglen = htons(resplen);
 resp->msgtype = htons(STUN_BINDRESP);
 stun_send(s, src, resp);
 ast_stun_request(s, src, combined, NULL);
 ret = AST_STUN_ACCEPT;
 break;
 default:
 if (ast_debug_stun_packet_is_allowed)
 ast_verbose("Dunno what to do with STUN message %04x (%s)\n", (unsigned)ntohs(hdr->msgtype), stun_msg2str(ntohs(hdr->msgtype)));
 }
 }
 return ret;
}

/*! \brief Extract the STUN_MAPPED_ADDRESS from the stun response.
 * This is used as a callback for stun_handle_response
 * when called from ast_stun_request.
 */
static int stun_get_mapped(struct stun_attr *attr, void *arg)
{
 struct stun_addr *addr = (struct stun_addr *)(attr + 1);
 struct sockaddr_in *sa = (struct sockaddr_in *)arg;

 if (ntohs(attr->attr) != STUN_MAPPED_ADDRESS || ntohs(attr->len) != 8)
 return 1; /* not us. */
 sa->sin_port = addr->port;
 sa->sin_addr.s_addr = addr->addr;
 return 0;
}

int ast_stun_request(int s, struct sockaddr_in *dst,
 const char *username, struct sockaddr_in *answer)
{
 struct stun_header *req;
 struct stun_header *rsp;
 unsigned char req_buf[1024];
 unsigned char rsp_buf[1024];
 int reqlen, reqleft;
 struct stun_attr *attr;
 int res = -1;
 int retry;

 if (answer) {
 /* Always clear answer in case the request fails. */
 memset(answer, 0, sizeof(struct sockaddr_in));
 }

 /* Create STUN bind request */
 req = (struct stun_header *) req_buf;
 stun_req_id(req);
 reqlen = 0;
 reqleft = sizeof(req_buf) - sizeof(struct stun_header);
 attr = (struct stun_attr *) req->ies;
 if (username) {
 append_attr_string(&attr, STUN_USERNAME, username, &reqlen, &reqleft);
 }
 req->msglen = htons(reqlen);
 req->msgtype = htons(STUN_BINDREQ);

 for (retry = 0; retry++ < 3;) { /* XXX make retries configurable */
 /* send request, possibly wait for reply */
 struct sockaddr_in src;
 socklen_t srclen;
 struct timeval start;

 /* Send STUN message. */
 res = stun_send(s, dst, req);
 if (res < 0) {
 ast_debug_stun(1, "stun_send try %d failed: %s\n", retry, strerror(errno));
 break;
 }
 if (!answer) {
 /* Successful send since we don't care about any response. */
 res = 0;
 break;
 }

 start = ast_tvnow();
try_again:
 /* Wait for response. */
 {
 struct pollfd pfds = { .fd = s, .events = POLLIN };
 int ms;

 ms = ast_remaining_ms(start, 3000);
 if (ms <= 0) {
 /* No response, timeout */
 res = 1;
 continue;
 }
 res = ast_poll(&pfds, 1, ms);
 if (res < 0) {
 /* Error */
 continue;
 }
 if (!res) {
 /* No response, timeout */
 res = 1;
 continue;
 }
 }

 /* Read STUN response. */
 memset(&src, 0, sizeof(src));
 srclen = sizeof(src);
 /* XXX pass sizeof(rsp_buf) - 1 in the size, because stun_handle_packet might
 * write past the end of the buffer.
 */
 res = recvfrom(s, rsp_buf, sizeof(rsp_buf) - 1,
 0, (struct sockaddr *) &src, &srclen);
 if (res < 0) {
 ast_debug_stun(1, "recvfrom try %d failed: %s\n", retry, strerror(errno));
 break;
 }

 /* Process the STUN response. */
 rsp = (struct stun_header *) rsp_buf;
 if (ast_stun_handle_packet(s, &src, rsp_buf, res, stun_get_mapped, answer)
 || (rsp->msgtype != htons(STUN_BINDRESP)
 && rsp->msgtype != htons(STUN_BINDERR))
 || stun_id_cmp(&req->id, &rsp->id)) {
 /* Bad STUN packet, not right type, or transaction ID did not match. */
 memset(answer, 0, sizeof(struct sockaddr_in));

 /* Was not a resonse to our request. */
 goto try_again;
 }
 /* Success. answer contains the external address if available. */
 res = 0;
 break;
 }
 return res;
}

static char *handle_cli_stun_set_debug(struct ast_cli_entry *e, int cmd, struct ast_cli_args *a)
{
 switch (cmd) {
 case CLI_INIT:
 e->command = "stun set debug {on|off}";
 e->usage =
 "Usage: stun set debug {on|off}\n"
 " Enable/Disable STUN (Simple Traversal of UDP through NATs)\n"
 " debugging\n";
 return NULL;
 case CLI_GENERATE:
 return NULL;
 }

 if (a->argc != e->args)
 return CLI_SHOWUSAGE;

 if (!strncasecmp(a->argv[e->args-1], "on", 2))
 ast_debug_category_set_sublevel(AST_LOG_CATEGORY_STUN_PACKET, AST_LOG_CATEGORY_ENABLED);
 else if (!strncasecmp(a->argv[e->args-1], "off", 3))
 ast_debug_category_set_sublevel(AST_LOG_CATEGORY_STUN_PACKET, AST_LOG_CATEGORY_DISABLED);
 else
 return CLI_SHOWUSAGE;

 ast_cli(a->fd, "STUN Debugging %s\n", ast_debug_stun_packet_is_allowed ? "Enabled" : "Disabled");
 return CLI_SUCCESS;
}

static struct ast_cli_entry cli_stun[] = {
 AST_CLI_DEFINE(handle_cli_stun_set_debug, "Enable/Disable STUN debugging"),
};

static uintmax_t debug_category_stun_id;

uintmax_t ast_debug_category_stun_id(void)
{
 return debug_category_stun_id;
}

static uintmax_t debug_category_stun_packet_id;

uintmax_t ast_debug_category_stun_packet_id(void)
{
 return debug_category_stun_packet_id;
}

static void stun_shutdown(void)
{
 ast_cli_unregister_multiple(cli_stun, sizeof(cli_stun) / sizeof(struct ast_cli_entry));

 ast_debug_category_unregister(AST_LOG_CATEGORY_STUN_PACKET);
 ast_debug_category_unregister(AST_LOG_CATEGORY_STUN);
}

/*! \brief Initialize the STUN system in Asterisk */
void ast_stun_init(void)
{
 ast_cli_register_multiple(cli_stun, sizeof(cli_stun) / sizeof(struct ast_cli_entry));
 ast_register_cleanup(stun_shutdown);

 debug_category_stun_id = ast_debug_category_register(AST_LOG_CATEGORY_STUN);
 debug_category_stun_packet_id = ast_debug_category_register(AST_LOG_CATEGORY_STUN_PACKET);
}