Re: [Ntp] Antw: Re: New I-D: NTP Port Randomization (draft-gont-ntp-port-randomization-00.txt)

Fernando Gont <fgont@si6networks.com> Wed, 24 April 2019 07:07 UTC

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To: Ulrich Windl <Ulrich.Windl@rz.uni-regensburg.de>, mlichvar@redhat.com
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From: Fernando Gont <fgont@si6networks.com>
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Date: Wed, 24 Apr 2019 08:36:58 +0200
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Subject: Re: [Ntp] Antw: Re: New I-D: NTP Port Randomization (draft-gont-ntp-port-randomization-00.txt)
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On 24/4/19 08:12, Ulrich Windl wrote:
>>>> Fernando Gont <fgont@si6networks.com> schrieb am 24.04.2019 um 01:17 in
> Nachricht <890da574-a283-9ccc-28fc-19d500920d36@si6networks.com>om>:
>> On 23/4/19 10:48, Miroslav Lichvar wrote:
>>> On Fri, Apr 19, 2019 at 12:56:04AM +0200, Fernando Gont wrote:
>>>> On 18/4/19 14:36, Miroslav Lichvar wrote:
>>>>> The source port in client mode packets can be random and it can also
>>>>> change with each request, which might be recommended in the document.
>>>>> Most NTP clients do that.
>>>>
>>>> FWIW, I'm running ntpd 4.2.8p11@1.3728‑o on Ubuntu, and all packets use
>>>> src port 123.
>>>
>>> Yes, ntpd and ntpdate (without ‑q or ‑d) seem to be in the minority of
>>> clients using source port 123.
>>
>> Any clues whether in other cases they explicitly randomize the source
>> port, or rely on the kernel doing port randomization?
> 
> Basically I think there are only two cases:
> 1) You care and pick a port number like 123
> 2) You don't care and use 0 as port number, causing the OS to pick one for
> you
> 
> I see no point of explicitly selecting a random post, specifically when you
> can't be sure whether another process has picked that already, too.

Well, if you care about doing port randomization, but the OS does not
implement it, maybe that's your only option?

-- yes, you don't know if the port has been picked already, so you'd
have to consider the case where bind() fails, and you need to pick a
different random port and try again...


-- 
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492