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

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;
+}