Re: [ntpwg] [dhcwg] Re: Network Time Protocol (NTP) Options for DHCPv6

Mark Stapp <mjs@cisco.com> Tue, 27 November 2007 19:39 UTC

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Date: Tue, 27 Nov 2007 14:39:07 -0500
From: Mark Stapp <mjs@cisco.com>
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To: Danny Mayer <mayer@ntp.org>
Subject: Re: [ntpwg] [dhcwg] Re: Network Time Protocol (NTP) Options for DHCPv6
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yes, I think I understand the issue. it's certainly ok to have an option 
payload that uses DNS names if that's what the service needs to use. I 
was only trying to clarify whether it was possible to use addresses or 
not, since it's only possible to use addresses in the dhcpv4 form of the 
option.

maybe what's happening is that the dhcp folks are hearing: "well, yes, 
the dhcpv4 option used addresses, but that's not really being used in 
the real world much, so let's not do the v6 option that way." if that's 
what's happening, it's certainly possible to develop an option format 
using DNS names.

are there circumstances where addresses would be preferred or necessary, 
or would DNS names _always_ be useable? my intuition is that there are 
probably some cases where it might be necessary to supply addresses, so 
it might be safest to develop an encoding that permits both. we have 
examples of things like that: the SIP servers option (RFC3361) allows a 
list of _either_ addresses or names, and the use of suboptions would 
allow mixed lists.

-- Mark

Danny Mayer wrote:
> Mark Stapp wrote:
>> making it possible to convey NTP servers in dhcpv6 doesn't seem to me to 
>> be any different than conveying them in dhcpv4 was. that was done 
>> something like ten years ago, and as far as I know that hasn't been a 
>> problem.
>>
>> I do wonder why some folks seem to think that using DNS names would 
>> somehow be "safer" than using v6 addresses. if someone shipped a server 
>> with a canned list of DNS names for NTP servers, there would be a 
>> problem until the owners of the NTP servers named moved them. I don't 
>> see how that'd be any better than the analogous mistake involving IP 
>> addresses.
>>
> 
> Mark,
> 
> Slipping on my DNS hat for a moment, the whole point of DNS is that you
> don't have to hardcode IP addresses in everything. You also benefit by
> being able to put more than one IP address for the same name. It's safer
> because the admin of the server doesn't have to worry when he moves a
> service from on server to another. All he/she has to do is update the
> DNS and not notify 1 million or so people that it's been moved. Can you
> imagine moving a web server without it?
> 
>> shipping a DHCP server with a canned configuration would not be good, so 
>> let's hope it doesn't happen. Mark Andrews's email seems to me to 
>> summarize what happens: 'home' routers have a dhcp client face and a 
>> dhcp server face, and use the client to populate the server.
>>
>> aside from the catastrophe hypothetical, is there any really strong 
>> reason - anything to do with the NTP protocol - that would prevent the 
>> use of ipv6 addresses?
>>
> 
> It does use IPv6 addresses when it's presented with one. Now what
> happens if that address is not running an NTP server? With DNS you can
> get more than one address back and try another address or requery the
> DNS to see if the address changed.
> 
> Danny
> 

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