diff --git a/libc/dns/include/resolv_stats.h b/libc/dns/include/resolv_stats.h
new file mode 100644
index 0000000..2aab958
--- /dev/null
+++ b/libc/dns/include/resolv_stats.h
@@ -0,0 +1,73 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#ifndef _RES_STATS_H
+#define _RES_STATS_H
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <time.h>
+
+#include "resolv_params.h"
+
+#define RCODE_INTERNAL_ERROR    254
+#define RCODE_TIMEOUT           255
+
+/*
+ * Resolver reachability statistics and run-time parameters.
+ */
+
+struct __res_sample {
+    time_t			at;    // time in s at which the sample was recorded
+    uint16_t			rtt;   // round-trip time in ms
+    uint8_t			rcode; // the DNS rcode or RCODE_XXX defined above
+};
+
+struct __res_stats {
+    // Stats of the last <sample_count> queries.
+    struct __res_sample		samples[MAXNSSAMPLES];
+    // The number of samples stored.
+    uint8_t			sample_count;
+    // The next sample to modify.
+    uint8_t			sample_next;
+};
+
+/* Calculate the round-trip-time from start time t0 and end time t1. */
+int
+_res_stats_calculate_rtt(const struct timespec* t1, const struct timespec* t0);
+
+/* Initialize a sample for calculating server reachability statistics. */
+extern void
+_res_stats_set_sample(struct __res_sample* sample, time_t now, int rcode, int rtt);
+
+/* Aggregates the reachability statistics for the given server based on on the stored samples. */
+extern void
+_res_stats_aggregate(struct __res_stats* stats, int* successes, int* errors, int* timeouts,
+             int* internal_errors, int* rtt_avg, time_t* last_sample_time);
+
+/* Returns true if the server is considered unusable, i.e. if the success rate is not lower than the
+ * threshold for the stored stored samples. If not enough samples are stored, the server is
+ * considered usable.
+ */
+extern bool
+_res_stats_usable_server(const struct __res_params* params, struct __res_stats* stats);
+
+/* Returns an array of bools indicating which servers are considered good */
+extern void
+_res_stats_get_usable_servers(const struct __res_params* params, struct __res_stats stats[MAXNS],
+        int nscount, bool valid_servers[MAXNS]);
+
+#endif  // _RES_STATS_H