[ntp:hackers] NTP protocol version interoperability

Dowd, Greg Greg.Dowd at microsemi.com
Wed Sep 3 05:44:41 UTC 2014


Actually I think there is pseudocode server functionality in the spec that says that server copies version field from request to reply.  Ergo reply with same version.

On Sep 2, 2014 10:40 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

Jon_Hart at rapid7.com said:
> I've consulted the 5 NTP RFCs for versions 0-4 and while they all (as
> applicable) do talk about compatibility with previous versions, I can't find
> anything that clarifies exactly how/when the various versions should be
> used.

The general idea when you want to update the protocol verson in traditional
client-server network protocols is that you update the server first so it
supports both the old and new versions.  Then you can update the clients one
by one to switch from the old to the new version.

The NTP case may be slightly more complicated since a client can also be a
server.

> When two hosts A and B are communicating with each other using NTP and host
> A sends a message to B using version N, is it required that B interpret the
> message as version N?

It doesn't make sense to do anything else.  If you got a reasonable result
when interpreting it as some other version, why would they have bumped the
version number?

> If B uses a newer version in the response, how should A react?  If B uses an
> older version in the response, how should A react?

In general, A should drop the response if the version doesn't match what they
used in the request.

In the specific case of NTP, the only versions I would pay any attention to
are 4 and 3.  3 is ancient, but lots of systems that that sit in a dark
corner and "just work" don't get updated.  There are probably many of them
out there on the net.  If you don't have to support any systems like that,
just use version 4 everywhere and ignore anything else.



--
These are my opinions.  I hate spam.



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