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ntp: improve replay protection in symmetric mode

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Always allow update from the first valid response, even if its transmit
timestamp is not newer than the currently saved timestamp. This shoud
provide a temporary protection in the case where the attacker does have
an authenticated packet from future, but the peers are using the same
polling interval and the protocol is already synchronised. This could be
also useful in the case where the attacker cannot observe the traffic
and authentication is disabled.
This commit is contained in:
Miroslav Lichvar 2016-11-14 09:27:07 +01:00
parent 4b0ef09221
commit cfb3c3ba44
1 changed files with 1 additions and 1 deletions
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2
ntp_core.c
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@ -1458,7 +1458,7 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
if ((inst->mode == MODE_CLIENT && valid_packet && !inst->valid_rx) ||
(inst->mode == MODE_ACTIVE && (valid_packet || !inst->valid_rx) &&
test5 && !UTI_IsZeroNtp64(&message->transmit_ts) &&
(!inst->updated_timestamps ||
(!inst->updated_timestamps || (valid_packet && !inst->valid_rx) ||
UTI_CompareNtp64(&inst->remote_ntp_tx, &message->transmit_ts) < 0))) {
inst->remote_ntp_rx = message->receive_ts;
inst->remote_ntp_tx = message->transmit_ts;