ntp: prevent synchronization to itself

Improve the client's test D to compare the stratum, reference ID,
reference timestamp, and root delay from the received packet with its
own reference data in order to prevent it from synchronizing to itself,
e.g. due to a misconfiguration.

diff --git a/ntp_core.c b/ntp_core.c
index 8ac7e0dc4f27ea363c175dcaaf5a9807e4c25344..7266e2ec1c303066219787d7d5e5a371c477e091 100644 (file)
--- a/ntp_core.c
+++ b/ntp_core.c
@@ -1464,6 +1464,52 @@ check_delay_dev_ratio(NCR_Instance inst, SST_Stats stats,
 
 /* ================================================== */
 
+static int
+check_sync_loop(NCR_Instance inst, NTP_Packet *message, NTP_Local_Address *local_addr,
+                struct timespec *local_ts)
+{
+  double our_root_delay, our_root_dispersion;
+  int are_we_synchronised, our_stratum;
+  struct timespec our_ref_time;
+  NTP_Leap leap_status;
+  uint32_t our_ref_id;
+
+  /* Check if our server may be enabled, i.e. a client or peer can actually
+     be synchronised to us */
+  if (REF_GetMode() != REF_ModeNormal)
+    return 1;
+
+  /* Check if the source indicates that it is synchronised to our address
+     (assuming it uses the same address as the one from which we send requests
+     to the source) */
+  if (message->stratum > 1 &&
+      message->reference_id == htonl(UTI_IPToRefid(&local_addr->ip_addr)))
+    return 0;
+
+  /* Compare our reference data with the source to make sure it is not us
+     (e.g. due to a misconfiguration) */
+
+  REF_GetReferenceParams(local_ts, &are_we_synchronised, &leap_status, &our_stratum,
+                         &our_ref_id, &our_ref_time, &our_root_delay, &our_root_dispersion);
+
+  if (message->stratum == our_stratum &&
+      message->reference_id == htonl(our_ref_id) &&
+      message->root_delay == UTI_DoubleToNtp32(our_root_delay) &&
+      !UTI_IsZeroNtp64(&message->reference_ts)) {
+    NTP_int64 ntp_ref_time;
+
+    UTI_TimespecToNtp64(&our_ref_time, &ntp_ref_time, NULL);
+    if (UTI_CompareNtp64(&message->reference_ts, &ntp_ref_time) == 0) {
+      DEBUG_LOG("Source %s is me", UTI_IPToString(&inst->remote_addr.ip_addr));
+      return 0;
+    }
+  }
+
+  return 1;
+}
+
+/* ================================================== */
+
 static void
 process_sample(NCR_Instance inst, NTP_Sample *sample)
 {
@@ -1716,10 +1762,9 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
        the increase in delay */
     testC = check_delay_dev_ratio(inst, stats, &sample.time, sample.offset, sample.peer_delay);
 
-    /* Test D requires that the remote peer is not synchronised to us to
-       prevent a synchronisation loop */
-    testD = message->stratum <= 1 || REF_GetMode() != REF_ModeNormal ||
-            pkt_refid != UTI_IPToRefid(&local_addr->ip_addr);
+    /* Test D requires that the source is not synchronised to us and is not us
+       to prevent a synchronisation loop */
+    testD = check_sync_loop(inst, message, local_addr, &rx_ts->ts);
   } else {
     remote_interval = local_interval = response_time = 0.0;
     sample.offset = sample.peer_delay = sample.peer_dispersion = 0.0;