DNS: Add basic support for asynchronous DNS resolving

This patch is a simple implementation for the proposal discussed in
https://mail.openvswitch.org/pipermail/ovs-dev/2017-August/337038.html and
https://mail.openvswitch.org/pipermail/ovs-dev/2017-October/340013.html.

It enables ovs-vswitchd and other utilities to use DNS names when specifying
OpenFlow and OVSDB remotes.

Below are some of the features and limitations of this patch:
    - Resolving is asynchornous in daemon context, avoiding blocking main loop;
    - Resolving is synchronous in general utility context;
    - Both IPv4 and IPv6 are supported;
    - The resolving API is thread-safe;
    - Depends on the unbound library;
    - When multiple ip addresses are returned, only the first one is used;
    - /etc/nsswitch.conf isn't respected as unbound library doesn't look at it;
    - For async-resolving, caller need to retry later; there is no callback.

Signed-off-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

1 files changed, 36 insertions, 0 deletions

diff --git a/lib/dns-resolve-stub.c b/lib/dns-resolve-stub.c
new file mode 100644
index 000000000..edf8337be
--- /dev/null
+++ b/lib/dns-resolve-stub.c
@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2017, 2018 Nicira, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <config.h>
+#include "dns-resolve.h"
+#include "compiler.h"
+
+void
+dns_resolve_init(void)
+{
+}
+
+bool
+dns_resolve(const char *name OVS_UNUSED, char **addr)
+{
+    *addr = NULL;
+    return false;
+}
+
+void
+dns_resolve_destroy(void)
+{
+}