[ntp:questions] Re: server's address in ntp payload?

Brian Utterback brian.utterback at sun.removeme.com
Wed Nov 23 19:44:26 UTC 2005

David Schwartz wrote:

>     I think if you can make the argument that the source address doesn't 
> have to match the destination of the query, you can equally well argue that 
> the destination address doesn't have to match the source of the query. ;)
>     Or, to put it another way, as soon as you layer a request/reply protocol 
> on top of UDP, the default presumption would be that the reply MUST have the 
> same source as the query's destination and vice-versa.

Completely false. Neither RFC 1123 "Requirements for Internet Hosts"
nor RFC 768 "User Datagram Protocol" make any such requirement. RFC 1123
does say that a UDP "SHOULD" be returned with the same source address
as the original destination address, but explicitly does not make that
a "MUST". Further, description of the API for use with UDP in RFC 768
lists the ability to determine the source address of a packet, but does
not list any such ability for the destination, which would be necessary
to accomplish this. The fact that this ability was not listed
and indeed not included in any UDP API until relatively recently should
tell you something about the requirement.

As I have said before, most UDP protocols are designed to handle this
problem. Only UDP protocols that deal with network control and topology
generally have cared about the destination address. The system I am
using has 356 listening UDP ports open right now, and exactly
3 of those have bound all the interfaces: bootps, snmp, and NTP.  Every 
one of the others either does not care or has another mechanism for
handling it, like XIDs. The bootps/DHCP and SNMP need to know the 
destination address in order  for them to respond with the proper 
answers, which is to say the answer is different depending on the 
destination address of the request. What is NTP's excuse?

I would further point out that none of these are bound to all addresses
in order to prevent port hijacking.

"Having them stolen may become our distribution model..."
Nicolas Negroponte on the Hundred Dollar Laptop.
Brian Utterback - OP/N1 RPE, Sun Microsystems, Inc.
Ph:877-259-7345, Em:brian.utterback-at-ess-you-enn-dot-kom

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