res_pjsip: Enable PJSIP DNS client support.

This change enables DNS client support within PJSIP. System
nameservers are automatically discovered using res_init or
res_ninit. If this fails then PJSIP will resort to using
gethostbyname for resolution.

By enabling this support we gain SRV support, failover, and
weight support.

(closes issue ASTERISK-23435)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/3343/
........

Merged revisions 410795 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410796 65c4cc65-6c06-0410-ace0-fbb531ad65f3

CHANGES

@@ -241,6 +241,13 @@ res_mwi_external
    and other modules that depend on it cannot be built or loaded with
    app_voicemail present.
 
+res_pjsip
+------------------
+ * DNS functionality will now automatically be enabled if the system configured
+   nameservers can be retrieved. If the system configured nameservers can not be
+   retrieved the functionality will resort to using system resolution. Functionalty
+   such as SRV records and failover will not be available if system resolution
+   is in use.
 
 ------------------------------------------------------------------------------
 --- Functionality changes from Asterisk 11 to Asterisk 12 --------------------

include/asterisk/dns.h

@@ -36,4 +36,7 @@
 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));
 
+/*! \brief Retrieve the configured nameservers of the system */
+struct ao2_container *ast_dns_get_nameservers(void);
+
 #endif /* _ASTERISK_DNS_H */

main/dns.c

@@ -296,3 +296,47 @@ int ast_search_dns(void *context,
 
 	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++) {
+		ast_str_container_add(nameservers, ast_inet_ntoa(state->nsaddr_list[i].sin_addr));
+	}
+
+#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;
+}

res/res_pjsip.c

@@ -2305,6 +2305,8 @@ static int load_module(void)
 		return AST_MODULE_LOAD_DECLINE;
 	}
 
+	ast_sip_initialize_dns();
+
 	pjsip_tsx_layer_init_module(ast_pjsip_endpoint);
 	pjsip_ua_init_module(ast_pjsip_endpoint, NULL);
 
@@ -2395,6 +2397,7 @@ static int reload_module(void)
 		return AST_MODULE_LOAD_DECLINE;
 	}
 	ast_res_pjsip_init_options_handling(1);
+	ast_sip_initialize_dns();
 	return 0;
 }
 

res/res_pjsip/config_system.c

@@ -25,6 +25,7 @@
 #include "asterisk/sorcery.h"
 #include "include/res_pjsip_private.h"
 #include "asterisk/threadpool.h"
+#include "asterisk/dns.h"
 
 #define TIMER_T1_MIN 100
 #define DEFAULT_TIMER_T1 500
@@ -174,3 +175,70 @@ void ast_sip_destroy_system(void)
 	ast_sorcery_unref(system_sorcery);
 }
 
+static int system_create_resolver_and_set_nameservers(void *data)
+{
+	struct ao2_container *discovered_nameservers;
+	struct ao2_iterator it_nameservers;
+	char *nameserver;
+	pj_status_t status;
+	pj_dns_resolver *resolver;
+	pj_str_t nameservers[PJ_DNS_RESOLVER_MAX_NS];
+	unsigned int count = 0;
+
+	discovered_nameservers = ast_dns_get_nameservers();
+	if (!discovered_nameservers) {
+		ast_log(LOG_ERROR, "Could not retrieve local system nameservers, resorting to system resolution\n");
+		return 0;
+	}
+
+	if (!ao2_container_count(discovered_nameservers)) {
+		ast_log(LOG_ERROR, "There are no local system nameservers configured, resorting to system resolution\n");
+		ao2_ref(discovered_nameservers, -1);
+		return -1;
+	}
+
+	if (!(resolver = pjsip_endpt_get_resolver(ast_sip_get_pjsip_endpoint()))) {
+		status = pjsip_endpt_create_resolver(ast_sip_get_pjsip_endpoint(), &resolver);
+		if (status != PJ_SUCCESS) {
+			ast_log(LOG_ERROR, "Could not create DNS resolver(%d), resorting to system resolution\n", status);
+			return 0;
+		}
+	}
+
+	it_nameservers = ao2_iterator_init(discovered_nameservers, 0);
+	while ((nameserver = ao2_iterator_next(&it_nameservers))) {
+		pj_strset2(&nameservers[count++], nameserver);
+		ao2_ref(nameserver, -1);
+
+		if (count == (PJ_DNS_RESOLVER_MAX_NS - 1)) {
+			break;
+		}
+	}
+	ao2_iterator_destroy(&it_nameservers);
+
+	status = pj_dns_resolver_set_ns(resolver, count, nameservers, NULL);
+
+	/* Since we no longer need the nameservers we can drop the list of them */
+	ao2_ref(discovered_nameservers, -1);
+
+	if (status != PJ_SUCCESS) {
+		ast_log(LOG_ERROR, "Could not set nameservers on DNS resolver in PJSIP(%d), resorting to system resolution\n",
+			status);
+		return 0;
+	}
+
+	if (!pjsip_endpt_get_resolver(ast_sip_get_pjsip_endpoint())) {
+		status = pjsip_endpt_set_resolver(ast_sip_get_pjsip_endpoint(), resolver);
+		if (status != PJ_SUCCESS) {
+			ast_log(LOG_ERROR, "Could not set DNS resolver in PJSIP(%d), resorting to system resolution\n", status);
+			return 0;
+		}
+	}
+
+	return 0;
+}
+
+void ast_sip_initialize_dns(void)
+{
+	ast_sip_push_task_synchronous(NULL, system_create_resolver_and_set_nameservers, NULL);
+}

res/res_pjsip/include/res_pjsip_private.h

@@ -72,6 +72,11 @@ int ast_sip_initialize_system(void);
  */
 void ast_sip_destroy_system(void);
 
+/*!
+ * \brief Initialize nameserver configuration
+ */
+void ast_sip_initialize_dns(void);
+
 /*!
  * \brief Initialize global configuration
  *