dns.c

Go to the documentation of this file.

/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 1999 - 2006 Thorsten Lockert
 *
 * Written by Thorsten Lockert <tholo@trollphone.org>
 *
 * Funding provided by Troll Phone Networks AS
 *
 * 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 DNS Support for Asterisk
 *
 * \author Thorsten Lockert <tholo@trollphone.org>
 *
 * \par Reference
 * - DNR SRV records http://www.ietf.org/rfc/rfc2782.txt
 *
 */
 
/*** MODULEINFO
    <support_level>core</support_level>
 ***/
 
#include "asterisk.h"
 
#include "asterisk/network.h"
#include <arpa/nameser.h>   /* res_* functions */
#include <resolv.h>
 
#include "asterisk/channel.h"
#include "asterisk/dns.h"
#include "asterisk/endian.h"
 
/*! \brief The maximum size permitted for the answer from the DNS server */
#define MAX_SIZE 4096
 
#ifdef __PDP_ENDIAN
#if __BYTE_ORDER == __PDP_ENDIAN
#define DETERMINED_BYTE_ORDER __LITTLE_ENDIAN
#endif
#endif
#if __BYTE_ORDER == __BIG_ENDIAN
#define DETERMINED_BYTE_ORDER __BIG_ENDIAN
#endif
#if __BYTE_ORDER == __LITTLE_ENDIAN
#define DETERMINED_BYTE_ORDER __LITTLE_ENDIAN
#endif
 
#ifndef HAVE_RES_NINIT
AST_MUTEX_DEFINE_STATIC(res_lock);
#endif
 
/* The dns_HEADER structure definition below originated
   in the arpa/nameser.h header file distributed with ISC
   BIND, which contains the following copyright and license
   notices:
 
 * ++Copyright++ 1983, 1989, 1993
 * -
 * Copyright (c) 1983, 1989, 1993
 *    The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *  This product includes software developed by the University of
 *  California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * -
 * Portions Copyright (c) 1993 by Digital Equipment Corporation.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies, and that
 * the name of Digital Equipment Corporation not be used in advertising or
 * publicity pertaining to distribution of the document or software without
 * specific, written prior permission.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND DIGITAL EQUIPMENT CORP. DISCLAIMS ALL
 * WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS.   IN NO EVENT SHALL DIGITAL EQUIPMENT
 * CORPORATION BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
 * DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
 * PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS
 * ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
 * SOFTWARE.
 * -
 * --Copyright--
 */
 
typedef struct {
    unsigned    id:16;          /*!< query identification number */
#if DETERMINED_BYTE_ORDER == __BIG_ENDIAN
            /* fields in third byte */
    unsigned    qr:1;           /*!< response flag */
    unsigned    opcode:4;       /*!< purpose of message */
    unsigned    aa:1;           /*!< authoritative answer */
    unsigned    tc:1;           /*!< truncated message */
    unsigned    rd:1;           /*!< recursion desired */
            /* fields in fourth byte */
    unsigned    ra:1;           /*!< recursion available */
    unsigned    unused:1;       /*!< unused bits (MBZ as of 4.9.3a3) */
    unsigned    ad:1;           /*!< authentic data from named */
    unsigned    cd:1;           /*!< checking disabled by resolver */
    unsigned    rcode:4;        /*!< response code */
#endif
#if DETERMINED_BYTE_ORDER == __LITTLE_ENDIAN
            /* fields in third byte */
    unsigned    rd:1;           /*!< recursion desired */
    unsigned    tc:1;           /*!< truncated message */
    unsigned    aa:1;           /*!< authoritative answer */
    unsigned    opcode:4;       /*!< purpose of message */
    unsigned    qr:1;           /*!< response flag */
            /* fields in fourth byte */
    unsigned    rcode:4;        /*!< response code */
    unsigned    cd:1;           /*!< checking disabled by resolver */
    unsigned    ad:1;           /*!< authentic data from named */
    unsigned    unused:1;       /*!< unused bits (MBZ as of 4.9.3a3) */
    unsigned    ra:1;           /*!< recursion available */
#endif
            /* remaining bytes */
    unsigned    qdcount:16;     /*!< number of question entries */
    unsigned    ancount:16;     /*!< number of answer entries */
    unsigned    nscount:16;     /*!< number of authority entries */
    unsigned    arcount:16;     /*!< number of resource entries */
} dns_HEADER;
 
struct dn_answer {
    unsigned short rtype;       /*!< The resource record type. */
    unsigned short class;       /*!< The resource record class. */
    unsigned int ttl;           /*!< The resource record time to live. */
    unsigned short size;        /*!< The resource record size. */
} __attribute__((__packed__));
 
/*!
 * \brief Tries to find the position of the next field in the DNS response.
 *
 * \internal
 *
 * \param  s    A char pointer to the current frame in the DNS response.
 * \param  len  The remaining available length of the DNS response.
 *
 * \return The position of the next field
 * \retval -1 if there are no remaining fields
 */
static int skip_name(unsigned char *s, int len)
{
    int x = 0;
 
    while (x < len) {
        if (*s == '\0') {
            s++;
            x++;
            break;
        }
 
        if ((*s & 0xc0) == 0xc0) {
            s += 2;
            x += 2;
            break;
        }
 
        x += *s + 1;
        s += *s + 1;
    }
 
    /* If we are out of room to search, return failure. */
    if (x >= len) {
        return AST_DNS_SEARCH_FAILURE;
    }
 
    /* Return the value for the current position in the DNS response. This is the start
    position of the next field. */
    return x;
}
 
/*!
 * \brief Advances the position of the DNS response pointer by the size of the current field.
 *
 * \internal
 *
 * \param  dns_response   A pointer to a char pointer to the current field in the DNS response.
 * \param  remaining_len  The remaining available length in the DNS response to search.
 * \param  field_size     A positive value representing the size of the current field
                          pointed to by the dns_response parameter.
 *
 * \return The remaining length in the DNS response
 * \retval -1 there are no frames remaining in the DNS response
 */
static int dns_advance_field(unsigned char **dns_response, int remaining_len, int field_size)
{
    if (dns_response == NULL || field_size < 0 || remaining_len < field_size) {
        return AST_DNS_SEARCH_FAILURE;
    }
 
    *dns_response += field_size;
    remaining_len -= field_size;
 
    return remaining_len;
}
 
#ifndef HAVE_RES_NINIT
/*!
 * \brief Handles the DNS search if the system has RES_INIT.
 *
 * \internal
 *
 * \param  dname             Domain name to lookup (host, SRV domain, TXT record name).
 * \param  rr_class          Record Class (see "man res_search").
 * \param  rr_type           Record type (see "man res_search").
 * \param  dns_response      The full DNS response.
 * \param  dns_response_len  The length of the full DNS response.
 *
 * \return The length of the DNS response
 * \retval -1 on search failure
 */
static int dns_search_res(const char *dname, int rr_class, int rr_type,
    unsigned char *dns_response, int dns_response_len)
{
 
    int ret = AST_DNS_SEARCH_FAILURE;
 
    ast_mutex_lock(&res_lock);
    res_init();
    ret = res_search(dname,
                     rr_class,
                     rr_type,
                     dns_response,
                     dns_response_len);
 
#ifdef HAVE_RES_CLOSE
    res_close();
#endif
 
    ast_mutex_unlock(&res_lock);
 
    return ret;
}
#else
/*!
 * \brief Handles the DNS search if the system has RES_NINIT.
 *
 * \internal
 *
 * \param  dname             Domain name to lookup (host, SRV domain, TXT record name).
 * \param  rr_class          Record Class (see "man res_search").
 * \param  rr_type           Record type (see "man res_search").
 * \param  dns_response      The full DNS response.
 * \param  dns_response_len  The length of the full DNS response.
 *
 * \return The length of the DNS response
 * \retval -1 on search failure
 */
static int dns_search_res(const char *dname, int rr_class, int rr_type,
    unsigned char *dns_response, int dns_response_len)
{
 
    int ret = AST_DNS_SEARCH_FAILURE;
    struct __res_state dns_state;
 
    memset(&dns_state, 0, sizeof(dns_state));
    res_ninit(&dns_state);
    ret = res_nsearch(&dns_state,
                      dname,
                      rr_class,
                      rr_type,
                      dns_response,
                      dns_response_len);
 
#ifdef HAVE_RES_NDESTROY
    res_ndestroy(&dns_state);
#else
    res_nclose(&dns_state);
#endif
 
    return ret;
}
#endif
 
/*!
 * \brief Parse DNS lookup result, call callback
 *
 * \internal
 *
 * \param  context   Void pointer containing data to use in the callback functions.
 * \param  class     Record Class (see "man res_search").
 * \param  type      Record type (see "man res_search").
 * \param  answer    The full DNS response.
 * \param  len       The length of the full DNS response.
 * \param  callback  Callback function for handling the discovered resource records from the DNS search.
 *
 * \retval -1 on search failure
 * \retval  0 on no records found
 * \retval  1 on success
 */
static int dns_parse_answer(void *context,
    int class, int type, unsigned char *answer, int len,
    int (*callback)(void *context, unsigned char *answer, int len, unsigned char *fullanswer))
{
    unsigned char *fullanswer = answer;
    struct dn_answer *ans;
    dns_HEADER *h;
    int ret = 0;
    int res;
    int x;
 
    h = (dns_HEADER *)answer;
    answer += sizeof(dns_HEADER);
    len -= sizeof(dns_HEADER);
 
    for (x = 0; x < ntohs(h->qdcount); x++) {
        if ((res = skip_name(answer, len)) < 0) {
            ast_log(LOG_WARNING, "Couldn't skip over name\n");
            return -1;
        }
        answer += res + 4;  /* Skip name and QCODE / QCLASS */
        len -= res + 4;
        if (len < 0) {
            ast_log(LOG_WARNING, "Strange query size\n");
            return -1;
        }
    }
 
    for (x = 0; x < ntohs(h->ancount); x++) {
        if ((res = skip_name(answer, len)) < 0) {
            ast_log(LOG_WARNING, "Failed skipping name\n");
            return -1;
        }
        answer += res;
        len -= res;
        ans = (struct dn_answer *)answer;
        answer += sizeof(struct dn_answer);
        len -= sizeof(struct dn_answer);
        if (len < 0) {
            ast_log(LOG_WARNING, "Length of DNS answer exceeds frame\n");
            return -1;
        }
 
        if (ntohs(ans->class) == class && ntohs(ans->rtype) == type) {
            if (callback) {
                if ((res = callback(context, answer, ntohs(ans->size), fullanswer)) < 0) {
                    ast_log(LOG_WARNING, "Failed to parse result\n");
                    return -1;
                }
                ret = 1;
            }
        }
        answer += ntohs(ans->size);
        len -= ntohs(ans->size);
    }
    return ret;
}
 
/*!
 * \brief Extended version of the DNS Parsing function.
 *
 * \details Parses the DNS lookup result and notifies the observer of each discovered
 *          resource record with the provided callback.
 *
 * \internal
 *
 * \param  context           Void pointer containing data to use in the callback functions.
 * \param  rr_class          Record Class (see "man res_search").
 * \param  rr_type           Record type (see "man res_search").
 * \param  answer            The full DNS response.
 * \param  answer_len        The length of the full DNS response.
 * \param  response_handler  Callback function for handling the DNS response.
 * \param  record_handler    Callback function for handling the discovered resource records from the DNS search.
 *
 * \retval -1 on search failure
 * \retval  0 on no records found
 * \retval  1 on success
 */
static int dns_parse_answer_ex(void *context, int rr_class, int rr_type, unsigned char *answer, int answer_len,
    int (*response_handler)(void *context, unsigned char *dns_response, int dns_response_len, int rcode),
    int (*record_handler)(void *context, unsigned char *record, int record_len, int ttl))
{
    unsigned char *dns_response = answer;
    dns_HEADER *dns_header = (dns_HEADER *)answer;
 
    struct dn_answer *ans;
    int res, x, pos, dns_response_len, ret;
 
    dns_response_len = answer_len;
    ret = AST_DNS_SEARCH_NO_RECORDS;
 
    /* Invoke the response_handler callback to notify the observer of the raw DNS response */
    response_handler(context, dns_response, dns_response_len, ntohs(dns_header->rcode));
 
    /* Verify there is something to parse */
    if (answer_len == 0) {
        return ret;
    }
 
    /* Try advancing the cursor for the dns header */
    if ((pos = dns_advance_field(&answer, answer_len, sizeof(dns_HEADER))) < 0) {
        ast_log(LOG_WARNING, "Length of DNS answer exceeds available search frames\n");
        return AST_DNS_SEARCH_FAILURE;
    }
 
    /* Skip domain name and QCODE / QCLASS */
    for (x = 0; x < ntohs(dns_header->qdcount); x++) {
        if ((res = skip_name(answer, pos)) < 0) {
            ast_log(LOG_WARNING, "Failed skipping name\n");
            return AST_DNS_SEARCH_FAILURE;
        }
 
        /* Try advancing the cursor for the name and QCODE / QCLASS fields */
        if ((pos = dns_advance_field(&answer, pos, res + 4)) < 0) {
            return AST_DNS_SEARCH_FAILURE;
        }
    }
 
    /* Extract the individual records */
    for (x = 0; x < ntohs(dns_header->ancount); x++) {
        if ((res = skip_name(answer, pos)) < 0) {
            ast_log(LOG_WARNING, "Failed skipping name\n");
            return AST_DNS_SEARCH_FAILURE;
        }
 
        /* Try advancing the cursor to the current record */
        if ((pos = dns_advance_field(&answer, pos, res)) < 0) {
            ast_log(LOG_WARNING, "Length of DNS answer exceeds available search frames\n");
            return AST_DNS_SEARCH_FAILURE;
        }
 
        /* Cast the current value for the answer pointer as a dn_answer struct */
        ans = (struct dn_answer *) answer;
 
        /* Try advancing the cursor to the end of the current record  */
        if ((pos = dns_advance_field(&answer, pos, sizeof(struct dn_answer)))  < 0) {
            ast_log(LOG_WARNING, "Length of DNS answer exceeds available search frames\n");
            return AST_DNS_SEARCH_FAILURE;
        }
 
        /* Skip over the records that do not have the same resource record class and type we care about */
        if (ntohs(ans->class) == rr_class && ntohs(ans->rtype) == rr_type) {
            /* Invoke the record handler callback to deliver the discovered record */
            record_handler(context, answer, ntohs(ans->size), ntohl(ans->ttl));
            /*At least one record was found */
            ret = AST_DNS_SEARCH_SUCCESS;
        }
 
        /* Try and update the field to the next record, but ignore any errors that come
         * back because this may be the end of the line. */
        pos = dns_advance_field(&answer, pos, ntohs(ans->size));
    }
 
    return ret;
}
 
/*!
 * \brief Lookup record in DNS
 *
 * \note Asterisk DNS is synchronous at this time. This means that if your DNS does not
 *       work properly, Asterisk might not start properly or a channel may lock.
*/
int ast_search_dns(void *context,
       const char *dname, int class, int type,
       int (*callback)(void *context, unsigned char *answer, int len, unsigned char *fullanswer))
{
#ifdef HAVE_RES_NINIT
    struct __res_state dnsstate;
#endif
    unsigned char answer[MAX_SIZE];
    int res, ret = -1;
 
#ifdef HAVE_RES_NINIT
    memset(&dnsstate, 0, sizeof(dnsstate));
    res_ninit(&dnsstate);
    res = res_nsearch(&dnsstate, dname, class, type, answer, sizeof(answer));
#else
    ast_mutex_lock(&res_lock);
    res_init();
    res = res_search(dname, class, type, answer, sizeof(answer));
#endif
    if (res > 0) {
        if ((res = dns_parse_answer(context, class, type, answer, res, callback)) < 0) {
            ast_log(LOG_WARNING, "DNS Parse error for %s\n", dname);
            ret = -1;
        } else if (res == 0) {
            ast_debug(1, "No matches found in DNS for %s\n", dname);
            ret = 0;
        } else
            ret = 1;
    }
#ifdef HAVE_RES_NINIT
#ifdef HAVE_RES_NDESTROY
    res_ndestroy(&dnsstate);
#else
    res_nclose(&dnsstate);
#endif
#else
#ifdef HAVE_RES_CLOSE
    res_close();
#endif
    ast_mutex_unlock(&res_lock);
#endif
 
    return ret;
}
 
enum ast_dns_search_result ast_search_dns_ex(void *context, const char *dname, int rr_class, int rr_type,
       int (*response_handler)(void *context, unsigned char *dns_response, int dns_response_len, int rcode),
       int (*record_handler)(void *context, unsigned char *record, int record_len, int ttl))
{
    int ret, dns_response_len;
    unsigned char dns_response[MAX_SIZE];
 
    /* Assert that the callbacks are not NULL */
    ast_assert(response_handler != NULL);
    ast_assert(record_handler != NULL);
 
    /* Try the DNS search. */
    dns_response_len = dns_search_res(dname,
                                      rr_class,
                                      rr_type,
                                      dns_response,
                                      sizeof(dns_response));
 
    if (dns_response_len < 0) {
        ast_debug(1, "DNS search failed for %s\n", dname);
        response_handler(context, (unsigned char *)"", 0, NXDOMAIN);
        return AST_DNS_SEARCH_FAILURE;
    }
 
    /* Parse records from DNS response */
    ret = dns_parse_answer_ex(context,
                              rr_class,
                              rr_type,
                              dns_response,
                              dns_response_len,
                              response_handler,
                              record_handler);
 
    /* Handle the return code from parsing the DNS response */
    if (ret == AST_DNS_SEARCH_FAILURE) {
        /* Parsing Error */
        ast_log(LOG_WARNING, "DNS Parse error for %s\n", dname);
    } else if (ret == AST_DNS_SEARCH_NO_RECORDS) {
        /* No results found */
        ast_debug(1, "DNS search yielded no results for %s\n", dname);
    }
 
    return ret;
}
 
struct ao2_container *ast_dns_get_nameservers(void)
{
#ifdef HAVE_RES_NINIT
    struct __res_state dnsstate;
#endif
    struct __res_state *state;
    struct ao2_container *nameservers;
    int i;
 
    nameservers = ast_str_container_alloc_options(AO2_ALLOC_OPT_LOCK_NOLOCK, 3);
    if (!nameservers) {
        return NULL;
    }
 
#ifdef HAVE_RES_NINIT
    memset(&dnsstate, 0, sizeof(dnsstate));
    res_ninit(&dnsstate);
    state = &dnsstate;
#else
    ast_mutex_lock(&res_lock);
    res_init();
    state = &_res;
#endif
 
    for (i = 0; i < state->nscount; i++) {
        char addr[INET6_ADDRSTRLEN];
        const char *addrp = NULL;
 
        /* glibc sets sin_family to 0 when the nameserver is an IPv6 address */
        if (state->nsaddr_list[i].sin_family) {
            addrp = inet_ntop(AF_INET, &state->nsaddr_list[i].sin_addr, addr, sizeof(addr));
#if defined(HAVE_RES_NINIT) && defined(HAVE_STRUCT___RES_STATE__U__EXT_NSADDRS)
        } else if (state->_u._ext.nsaddrs[i]) {
            addrp = inet_ntop(AF_INET6, &state->_u._ext.nsaddrs[i]->sin6_addr, addr, sizeof(addr));
#endif
        }
 
        if (addrp) {
            ast_debug(1, "Discovered nameserver: %s\n", addrp);
            ast_str_container_add(nameservers, addrp);
        }
    }
 
#ifdef HAVE_RES_NINIT
#ifdef HAVE_RES_NDESTROY
    res_ndestroy(&dnsstate);
#else
    res_nclose(&dnsstate);
#endif
#else
#ifdef HAVE_RES_CLOSE
    res_close();
#endif
    ast_mutex_unlock(&res_lock);
#endif
 
    return nameservers;
}