add mDNS lookup method to libosmo-mslookup (#2)

Add the first actually useful lookup method to the mslookup library: multicast
DNS.

The server side is added in a subsequent commit, when the mslookup server is
implemented for the osmo-hlr program.

Use custom DNS encoding instead of libc-ares (which we use in OsmoSGSN
already), because libc-ares is only a DNS client implementation and we will
need both client and server.

Resubmit of f10463c5fc6d9e786ab7c648d99f7450f9a25906 after being
reverted in 110a49f69f29fed844d8743b76fd748f4a14812a. This new version
skips the mslookup_client_mdns test if multicast is not supported in the
build environment. I have verified that it doesn't break the build
anymore in my own OBS namespace.

Related: OS#4237, OS#4361
Patch-by: osmith, nhofmeyr
Change-Id: I3c340627181b632dd6a0d577aa2ea2a7cd035c0c

1 files changed, 54 insertions, 0 deletions

diff --git a/include/osmocom/mslookup/mdns_msg.h b/include/osmocom/mslookup/mdns_msg.h
new file mode 100644
index 0000000..ae7dd17
--- /dev/null
+++ b/include/osmocom/mslookup/mdns_msg.h
@@ -0,0 +1,54 @@
+/* Copyright 2019 by sysmocom s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * All Rights Reserved
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <stdint.h>
+#include "mdns_rfc.h"
+
+struct osmo_mdns_record {
+	struct llist_head list;
+	enum osmo_mdns_rfc_record_type type;
+	uint16_t length;
+	uint8_t *data;
+};
+
+struct osmo_mdns_msg_request {
+	uint16_t id;
+	char *domain;
+	enum osmo_mdns_rfc_record_type type;
+};
+
+struct osmo_mdns_msg_answer {
+	uint16_t id;
+	char *domain;
+	/*! list of osmo_mdns_record. */
+	struct llist_head records;
+};
+
+int osmo_mdns_msg_request_encode(void *ctx, struct msgb *msg, const struct osmo_mdns_msg_request *req);
+struct osmo_mdns_msg_request *osmo_mdns_msg_request_decode(void *ctx, const uint8_t *data, size_t data_len);
+
+void osmo_mdns_msg_answer_init(struct osmo_mdns_msg_answer *answer);
+int osmo_mdns_msg_answer_encode(void *ctx, struct msgb *msg, const struct osmo_mdns_msg_answer *ans);
+struct osmo_mdns_msg_answer *osmo_mdns_msg_answer_decode(void *ctx, const uint8_t *data, size_t data_len);
+int osmo_mdns_result_from_answer(struct osmo_mslookup_result *result, const struct osmo_mdns_msg_answer *ans);
+
+struct osmo_mdns_record *osmo_mdns_record_txt_keyval_encode(void *ctx, const char *key, const char *value_fmt, ...);
+int osmo_mdns_record_txt_keyval_decode(const struct osmo_mdns_record *rec,
+				       char *key_buf, size_t key_size, char *value_buf, size_t value_size);