evdns: add DNS_OPTION_NAMESERVERS_NO_DEFAULT/EVDNS_BASE_NAMESERVERS_NO_DEFAULT

- DNS_OPTION_NAMESERVERS_NO_DEFAULT
  Do not "default" nameserver (i.e. "127.0.0.1:53") if there is no nameservers
  in resolv.conf, (iff DNS_OPTION_NAMESERVERS is set)

- EVDNS_BASE_NAMESERVERS_NO_DEFAULT
  If EVDNS_BASE_INITIALIZE_NAMESERVERS isset, do not add default
  nameserver if there are no nameservers in resolv.conf (just set
  DNS_OPTION_NAMESERVERS_NO_DEFAULT internally)

Fixes: #569
(cherry picked from commit e5b8f4c1925867d8e4cd7dc0390e5141d7ef1106)

evdns.c

@@ -3327,10 +3327,16 @@ search_request_finished(struct evdns_request *const handle) {
 
 static void
 evdns_resolv_set_defaults(struct evdns_base *base, int flags) {
+	int add_default = flags & DNS_OPTION_NAMESERVERS;
+	if (flags & DNS_OPTION_NAMESERVERS_NO_DEFAULT)
+		add_default = 0;
+
 	/* if the file isn't found then we assume a local resolver */
 	ASSERT_LOCKED(base);
-	if (flags & DNS_OPTION_SEARCH) search_set_from_hostname(base);
+	if (flags & DNS_OPTION_SEARCH)
-	if (flags & DNS_OPTION_NAMESERVERS) evdns_base_nameserver_ip_add(base,"127.0.0.1");
+		search_set_from_hostname(base);
+	if (add_default)
+		evdns_base_nameserver_ip_add(base, "127.0.0.1");
 }
 
 #ifndef EVENT__HAVE_STRTOK_R
@@ -3626,9 +3632,14 @@ evdns_base_resolv_conf_parse_impl(struct evdns_base *base, int flags, const char
 	char *resolv;
 	char *start;
 	int err = 0;
+	int add_default;
 
 	log(EVDNS_LOG_DEBUG, "Parsing resolv.conf file %s", filename);
 
+	add_default = flags & DNS_OPTION_NAMESERVERS;
+	if (flags & DNS_OPTION_NAMESERVERS_NO_DEFAULT)
+		add_default = 0;
+
 	if (flags & DNS_OPTION_HOSTSFILE) {
 		char *fname = evdns_get_default_hosts_filename();
 		evdns_base_load_hosts(base, fname);
@@ -3664,7 +3675,7 @@ evdns_base_resolv_conf_parse_impl(struct evdns_base *base, int flags, const char
 		}
 	}
 
-	if (!base->server_head && (flags & DNS_OPTION_NAMESERVERS)) {
+	if (!base->server_head && add_default) {
 		/* no nameservers were configured. */
 		evdns_base_nameserver_ip_add(base, "127.0.0.1");
 		err = 6;
@@ -3965,7 +3976,12 @@ evdns_base_new(struct event_base *event_base, int flags)
 
 	TAILQ_INIT(&base->hostsdb);
 
-#define EVDNS_BASE_ALL_FLAGS (0x8001)
+#define EVDNS_BASE_ALL_FLAGS ( \
+	EVDNS_BASE_INITIALIZE_NAMESERVERS | \
+	EVDNS_BASE_DISABLE_WHEN_INACTIVE  | \
+	EVDNS_BASE_NAMESERVERS_NO_DEFAULT | \
+	0)
+
 	if (flags & ~EVDNS_BASE_ALL_FLAGS) {
 		flags = EVDNS_BASE_INITIALIZE_NAMESERVERS;
 		log(EVDNS_LOG_WARN,
@@ -3976,10 +3992,15 @@ evdns_base_new(struct event_base *event_base, int flags)
 
 	if (flags & EVDNS_BASE_INITIALIZE_NAMESERVERS) {
 		int r;
+		int opts = DNS_OPTIONS_ALL;
+		if (flags & EVDNS_BASE_NAMESERVERS_NO_DEFAULT) {
+			opts |= DNS_OPTION_NAMESERVERS_NO_DEFAULT;
+		}
+
 #ifdef _WIN32
 		r = evdns_base_config_windows_nameservers(base);
 #else
-		r = evdns_base_resolv_conf_parse(base, DNS_OPTIONS_ALL, "/etc/resolv.conf");
+		r = evdns_base_resolv_conf_parse(base, opts, "/etc/resolv.conf");
 #endif
 		if (r == -1) {
 			evdns_base_free_and_unlock(base, 0);

include/event2/dns.h

@@ -209,6 +209,9 @@ extern "C" {
     DNS_OPTION_HOSTSFILE   | \
     0                        \
 )
+/* Do not "default" nameserver (i.e. "127.0.0.1:53") if there is no nameservers
+ * in resolv.conf, (iff DNS_OPTION_NAMESERVERS is set) */
+#define DNS_OPTION_NAMESERVERS_NO_DEFAULT 16
 
 /* Obsolete name for DNS_QUERY_NO_SEARCH */
 #define DNS_NO_SEARCH DNS_QUERY_NO_SEARCH
@@ -233,6 +236,10 @@ struct event_base;
 /** Flag for evdns_base_new: Do not prevent the libevent event loop from
  * exiting when we have no active dns requests. */
 #define EVDNS_BASE_DISABLE_WHEN_INACTIVE 0x8000
+/** Flag for evdns_base_new: If EVDNS_BASE_INITIALIZE_NAMESERVERS isset, do not
+ * add default nameserver if there are no nameservers in resolv.conf
+ * @see DNS_OPTION_NAMESERVERS_NO_DEFAULT */
+#define EVDNS_BASE_NAMESERVERS_NO_DEFAULT 0x10000
 
 /**
   Initialize the asynchronous DNS library.
@@ -243,7 +250,7 @@ struct event_base;
 
   @param event_base the event base to associate the dns client with
   @param flags any of EVDNS_BASE_INITIALIZE_NAMESERVERS|
-    EVDNS_BASE_DISABLE_WHEN_INACTIVE
+    EVDNS_BASE_DISABLE_WHEN_INACTIVE|EVDNS_BASE_NAMESERVERS_NO_DEFAULT
   @return evdns_base object if successful, or NULL if an error occurred.
   @see evdns_base_free()
  */
@@ -478,7 +485,7 @@ int evdns_base_set_option(struct evdns_base *base, const char *option, const cha
 
   @param base the evdns_base to which to apply this operation
   @param flags any of DNS_OPTION_NAMESERVERS|DNS_OPTION_SEARCH|DNS_OPTION_MISC|
-    DNS_OPTION_HOSTSFILE|DNS_OPTIONS_ALL
+    DNS_OPTION_HOSTSFILE|DNS_OPTIONS_ALL|DNS_OPTION_NAMESERVERS_NO_DEFAULT
   @param filename the path to the resolv.conf file
   @return 0 if successful, or various positive error codes if an error
     occurred (see above)

test/regress_dns.c

@@ -204,7 +204,7 @@ dns_resolve_reverse(void *ptr)
 {
 	struct in_addr in;
 	struct event_base *base = event_base_new();
-	struct evdns_base *dns = evdns_base_new(base, 1/* init name servers */);
+	struct evdns_base *dns = evdns_base_new(base, EVDNS_BASE_INITIALIZE_NAMESERVERS);
 	struct evdns_request *req = NULL;
 
 	tt_assert(base);
@@ -1004,6 +1004,59 @@ end:
 		event_base_free(inactive_base);
 }
 
+static void
+dns_initialize_nameservers_test(void *arg)
+{
+	struct basic_test_data *data = arg;
+	struct event_base *base = data->base;
+	struct evdns_base *dns = NULL;
+
+	dns = evdns_base_new(base, 0);
+	tt_assert(dns);
+	tt_int_op(evdns_base_get_nameserver_addr(dns, 0, NULL, 0), ==, -1);
+	evdns_base_free(dns, 0);
+
+	dns = evdns_base_new(base, EVDNS_BASE_INITIALIZE_NAMESERVERS);
+	tt_assert(dns);
+	tt_int_op(evdns_base_get_nameserver_addr(dns, 0, NULL, 0), ==, sizeof(struct sockaddr));
+
+end:
+	if (dns)
+		evdns_base_free(dns, 0);
+}
+#ifndef _WIN32
+#define RESOLV_FILE "empty-resolv.conf"
+static void
+dns_nameservers_no_default_test(void *arg)
+{
+	struct basic_test_data *data = arg;
+	struct event_base *base = data->base;
+	struct evdns_base *dns = NULL;
+	int ok = access(RESOLV_FILE, R_OK);
+
+	tt_assert(ok);
+
+	dns = evdns_base_new(base, 0);
+	tt_assert(dns);
+	tt_int_op(evdns_base_get_nameserver_addr(dns, 0, NULL, 0), ==, -1);
+
+	/* We cannot test
+	 * EVDNS_BASE_INITIALIZE_NAMESERVERS|EVDNS_BASE_NAMESERVERS_NO_DEFAULT
+	 * because we cannot mock "/etc/resolv.conf" (yet). */
+
+	evdns_base_resolv_conf_parse(dns,
+		DNS_OPTIONS_ALL|DNS_OPTION_NAMESERVERS_NO_DEFAULT, RESOLV_FILE);
+	tt_int_op(evdns_base_get_nameserver_addr(dns, 0, NULL, 0), ==, -1);
+
+	evdns_base_resolv_conf_parse(dns, DNS_OPTIONS_ALL, RESOLV_FILE);
+	tt_int_op(evdns_base_get_nameserver_addr(dns, 0, NULL, 0), ==, sizeof(struct sockaddr));
+
+end:
+	if (dns)
+		evdns_base_free(dns, 0);
+}
+#endif
+
 /* === Test for bufferevent_socket_connect_hostname */
 
 static int total_connected_or_failed = 0;
@@ -2332,6 +2385,13 @@ struct testcase_t dns_testcases[] = {
 	{ "disable_when_inactive_no_ns", dns_disable_when_inactive_no_ns_test,
 	  TT_FORK|TT_NEED_BASE, &basic_setup, NULL },
 
+	{ "initialize_nameservers", dns_initialize_nameservers_test,
+	  TT_FORK|TT_NEED_BASE, &basic_setup, NULL },
+#ifndef _WIN32
+	{ "nameservers_no_default", dns_nameservers_no_default_test,
+	  TT_FORK|TT_NEED_BASE, &basic_setup, NULL },
+#endif
+
 	{ "getaddrinfo_async", test_getaddrinfo_async,
 	  TT_FORK|TT_NEED_BASE, &basic_setup, (char*)"" },
 	{ "getaddrinfo_cancel_stress", test_getaddrinfo_async_cancel_stress,