diff --git a/libc/dns/resolv/res_stats.c b/libc/dns/resolv/res_stats.c
new file mode 100644
index 0000000..a3d6f36
--- /dev/null
+++ b/libc/dns/resolv/res_stats.c
@@ -0,0 +1,184 @@
+/*
+ * 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.
+ */
+
+#include <stdbool.h>
+#include <arpa/nameser.h>
+#include <string.h>
+
+#include "resolv_stats.h"
+#include "private/libc_logging.h"
+#include "isc/eventlib.h"
+
+#define DBG 1
+
+/* 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) {
+    // Divide ns by one million to get ms, multiply s by thousand to get ms (obvious)
+    long ms0 = t0->tv_sec * 1000 + t0->tv_nsec / 1000000;
+    long ms1 = t1->tv_sec * 1000 + t1->tv_nsec / 1000000;
+    return (int) (ms1 - ms0);
+}
+
+/* Create a sample for calculating server reachability statistics. */
+void
+_res_stats_set_sample(struct __res_sample* sample, time_t now, int rcode, int rtt)
+{
+    if (DBG) {
+        __libc_format_log(ANDROID_LOG_INFO, "libc", "rcode = %d, sec = %d", rcode, rtt);
+    }
+    sample->at = now;
+    sample->rcode = rcode;
+    sample->rtt = rtt;
+}
+
+/* Clears all stored samples for the given server. */
+void
+_res_stats_clear_samples(struct __res_stats* stats)
+{
+    stats->sample_count = stats->sample_next = 0;
+}
+
+/* Aggregates the reachability statistics for the given server based on on the stored samples. */
+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)
+{
+    int s = 0;   // successes
+    int e = 0;   // errors
+    int t = 0;   // timouts
+    int ie = 0;  // internal errors
+    long rtt_sum = 0;
+    time_t last = 0;
+    int rtt_count = 0;
+    for (int i = 0 ; i < stats->sample_count ; ++i) {
+        // Treat everything as an error that the code in send_dg() already considers a
+        // rejection by the server, i.e. SERVFAIL, NOTIMP and REFUSED. Assume that NXDOMAIN
+        // and NOTAUTH can actually occur for user queries. NOERROR with empty answer section
+        // is not treated as an error here either. FORMERR seems to sometimes be returned by
+        // some versions of BIND in response to DNSSEC or EDNS0. Whether to treat such responses
+        // as an indication of a broken server is unclear, though. For now treat such responses,
+        // as well as unknown codes as errors.
+        switch (stats->samples[i].rcode) {
+        case NOERROR:
+        case NOTAUTH:
+        case NXDOMAIN:
+            ++s;
+            rtt_sum += stats->samples[i].rtt;
+            ++rtt_count;
+            break;
+        case RCODE_TIMEOUT:
+            ++t;
+            break;
+        case RCODE_INTERNAL_ERROR:
+            ++ie;
+            break;
+        case SERVFAIL:
+        case NOTIMP:
+        case REFUSED:
+        default:
+            ++e;
+            break;
+        }
+    }
+    *successes = s;
+    *errors = e;
+    *timeouts = t;
+    *internal_errors = ie;
+    /* If there was at least one successful sample, calculate average RTT. */
+    if (rtt_count) {
+        *rtt_avg = rtt_sum / rtt_count;
+    } else {
+        *rtt_avg = -1;
+    }
+    /* If we had at least one sample, populate last sample time. */
+    if (stats->sample_count > 0) {
+        if (stats->sample_next > 0) {
+            last = stats->samples[stats->sample_next - 1].at;
+        } else {
+            last = stats->samples[stats->sample_count - 1].at;
+        }
+    }
+    *last_sample_time = last;
+}
+
+bool
+_res_stats_usable_server(const struct __res_params* params, struct __res_stats* stats) {
+    int successes = -1;
+    int errors = -1;
+    int timeouts = -1;
+    int internal_errors = -1;
+    int rtt_avg = -1;
+    time_t last_sample_time = 0;
+    _res_stats_aggregate(stats, &successes, &errors, &timeouts, &internal_errors, &rtt_avg,
+            &last_sample_time);
+    if (successes >= 0 && errors >= 0 && timeouts >= 0) {
+        int total = successes + errors + timeouts;
+        if (DBG) {
+            __libc_format_log(ANDROID_LOG_DEBUG, "libc", "NS stats: S %d + E %d + T %d + I %d "
+                 "= %d, rtt = %d, min_samples = %d\n", successes, errors, timeouts, internal_errors,
+                 total, rtt_avg, params->min_samples);
+        }
+        if (total >= params->min_samples && (errors > 0 || timeouts > 0)) {
+            int success_rate = successes * 100 / total;
+            if (DBG) {
+                __libc_format_log(ANDROID_LOG_DEBUG, "libc", "success rate %d%%\n", success_rate);
+            }
+            if (success_rate < params->success_threshold) {
+                // evNowTime() is used here instead of time() to stay consistent with the rest of
+                // the code base
+                time_t now = evNowTime().tv_sec;
+                if (now - last_sample_time > params->sample_validity) {
+                    // Note: It might be worth considering to expire old servers after their expiry
+                    // date has been reached, however the code for returning the ring buffer to its
+                    // previous non-circular state would induce additional complexity.
+                    if (DBG) {
+                        __libc_format_log(ANDROID_LOG_INFO, "libc",
+                            "samples stale, retrying server\n");
+                    }
+                    _res_stats_clear_samples(stats);
+                } else {
+                    if (DBG) {
+                        __libc_format_log(ANDROID_LOG_INFO, "libc",
+                            "too many resolution errors, ignoring server\n");
+                    }
+                    return 0;
+                }
+            }
+        }
+    }
+    return 1;
+}
+
+void
+_res_stats_get_usable_servers(const struct __res_params* params, struct __res_stats stats[MAXNS],
+        int nscount, bool usable_servers[MAXNS]) {
+    unsigned usable_servers_found = 0;
+    for (int ns = 0; ns < nscount; ns++) {
+        bool usable = _res_stats_usable_server(params, &stats[ns]);
+        if (usable) {
+            ++usable_servers_found;
+        }
+        usable_servers[ns] = usable;
+    }
+    // If there are no usable servers, consider all of them usable.
+    // TODO: Explore other possibilities, such as enabling only the best N servers, etc.
+    if (usable_servers_found == 0) {
+        for (int ns = 0; ns < nscount; ns++) {
+            usable_servers[ns] = true;
+        }
+    }
+}