2020-09-15, rev -06: The Network Time Protocol can operate in several modes. Some of these modes are based on the receipt of unsolicited packets, and therefore require the use of a well-known port as the local port number. However, in the case of NTP modes where the use of a well- known port is not required, employing such well-known port unnecessarily increases the ability of attackers to perform blind/ off-path attacks. This document formally updates RFC5905, recommending the use of transport-protocol ephemeral port randomization for those modes where use of the NTP well-known port is not required.
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