DNS: Create a system-level DNS resolver

Prior to this patch, the DNS core present in master had no default system-level
resolver implementation. Therefore, it was not possible for the DNS core to
perform resolutions unless the libunbound library was installed and the
res_resolver_unbound module was loaded.

This patch introduces a system-level DNS resolver implementation that will
register itself with the lowest consideration priority available (to ensure
that it is to be used only as a last resort). The resolver relies on low-level
DNS search functions to perform a rudimentary DNS search based on a provided
query and then supplies the search results to the DNS core.

ASTERISK-25146 #close
Reported By: Joshua Colp

Change-Id: I3b36ea17b889a98df4f8d80d50bb7ee175afa077

2 files changed, 57 insertions, 10 deletions

diff --git a/include/asterisk/_private.h b/include/asterisk/_private.h
index d49de1789..2966f8787 100644
--- a/include/asterisk/_private.h
+++ b/include/asterisk/_private.h
@@ -29,6 +29,7 @@ int ast_cli_perms_init(int reload);	/*!< Provided by cli.c */
 int dnsmgr_init(void);			/*!< Provided by dnsmgr.c */
 void dnsmgr_start_refresh(void);	/*!< Provided by dnsmgr.c */
 int dnsmgr_reload(void);		/*!< Provided by dnsmgr.c */
+int ast_dns_system_resolver_init(void); /*!< Provided by dns_system_resolver.c */
 void threadstorage_init(void);		/*!< Provided by threadstorage.c */
 int ast_device_state_engine_init(void);	/*!< Provided by devicestate.c */
 int astobj2_init(void);			/*!< Provided by astobj2.c */
diff --git a/include/asterisk/dns.h b/include/asterisk/dns.h
index 4899fa8b4..c193a22aa 100644
--- a/include/asterisk/dns.h
+++ b/include/asterisk/dns.h
@@ -24,17 +24,63 @@
 #ifndef _ASTERISK_DNS_H
 #define _ASTERISK_DNS_H
 
-/*!	\brief	Perform DNS lookup (used by DNS, enum and SRV lookups)
-	\param	context
-	\param	dname	Domain name to lookup (host, SRV domain, TXT record name)
-	\param	class	Record Class (see "man res_search")
-	\param	type	Record type (see "man res_search")
-	\param	callback Callback function for handling DNS result
-	\note   Asterisk DNS is synchronus at this time. This means that if your DNS
-		services does not work, Asterisk may lock while waiting for response.
-*/
+/*! \brief DNS search return values */
+enum ast_dns_search_result {
+	AST_DNS_SEARCH_FAILURE = -1,     /*!< DNS search resulted in failure */
+	AST_DNS_SEARCH_NO_RECORDS = 0,   /*!< DNS search yielded no results */
+	AST_DNS_SEARCH_SUCCESS = 1       /*!< DNS search yielded at least one discovered record */
+};
+
+/*!
+ * \brief Perform DNS lookup (used by DNS, enum and SRV lookups)
+ *
+ * \param  context   Void pointer containing data to use in the callback function.
+ * \param  dname     Domain name to lookup (host, SRV domain, TXT record name).
+ * \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
+ *
+ * \note Asterisk DNS is synchronus at this time. This means that if your DNS
+ *       service does not work, Asterisk may lock while waiting for a response.
+ */
 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));
+	int (*callback)(void *context, unsigned char *answer, int len, unsigned char *fullanswer));
+
+/*!
+ * \brief Extended version of the DNS search function.
+ *
+ * \details Performs a DNS lookup, (used by DNS, enum and SRV lookups), parses the
+ *          results and notifies the observer with the response and discovered records
+ *          via invoking the provided callbacks (used by ast_dns_system_resolver).
+ *
+ * \param  context           Void pointer containing data to use in the handler functions.
+ * \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  response_handler  Callback function for handling the DNS response. Invoked upon
+ *                           completion of the DNS search.
+ * \param  record_handler    Callback function for handling the discovered resource
+ *                           records from the DNS search. Invoked n times, where n is the
+ *                           number of records discovered while parsing the DNS
+ *                           response.
+ *
+ * \retval AST_DNS_SEARCH_FAILURE    on search failure
+ * \retval AST_DNS_SEARCH_NO_RECORDS on no records found
+ * \retval AST_DNS_SEARCH_SUCCESS    on success
+ *
+ * \note Asterisk DNS is synchronus at this time. This means that if your DNS
+ *       service does not work, Asterisk may lock while waiting for a response.
+ */
+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));
 
 /*! \brief Retrieve the configured nameservers of the system */
 struct ao2_container *ast_dns_get_nameservers(void);