Re: [Ntp] I-D Action: draft-ietf-ntp-port-randomization-00.txt

Heiko Gerstung <> Wed, 13 November 2019 13:27 UTC

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Date: Wed, 13 Nov 2019 14:27:04 +0100
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Thread-Topic: [Ntp] I-D Action: draft-ietf-ntp-port-randomization-00.txt
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From: Heiko Gerstung <>
To: Miroslav Lichvar <>, "" <>
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Subject: Re: [Ntp] I-D Action: draft-ietf-ntp-port-randomization-00.txt
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Hi Miroslav,

I find the reasoning very convincing and very well explained. 

Best Regards,

Heiko Gerstung 
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Am 13.11.19, 12:22 schrieb "ntp im Auftrag von Miroslav Lichvar" < im Auftrag von>gt;:

    On Fri, Nov 01, 2019 at 08:58:37AM -0700, wrote:
    This document describes a per-association and per-request approach for
    randomizing the client's port number with some of their advantages and
    disadvantages. It currently strongly recommends the per-association
    approach for all clients.
    For a typical client running on an OS it is about opening and closing
    sockets. Should it keep one socket open for the lifetime of the
    association, or should it open a new socket before each request?
    I'd like to propose changing the recommendation to the per-request
    approach for the following reasons:
    - It seems to be the current practice. From the implementations
      that I'm familiar with and that use a random source port, each one
      opens a new socket for each request, at least by default. On my
      public servers I see that most clients do change their port over
    - It doesn't seem like a good idea to require a client to keep its
      port open when it's not waiting for a response. If it received a
      valid response, or didn't receive a valid response in few seconds
      after sending the request, it should close the port.
    - The port number may be discoverable by other means, so it should
      change frequently. For example, the attacker could try sending
      packets to all ports and observe which one changed a value reported
      by the client in a monitoring protocol (e.g. mode 6). If the
      attacker can determine the port number, which cannot be prevented in
      the general case, the time for which it is useful for attacks should
      be limited.
    Does that make sense?
    Miroslav Lichvar
    ntp mailing list