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

David Schwartz davids at webmaster.com
Sat Nov 26 03:51:17 UTC 2005

"Danny Mayer" <mayer at ntp.isc.org> wrote in message 
news:43876FE3.1010309 at ntp.isc.org...

> David Schwartz wrote:

>>>I would therefore contend
>>>that a protocol
>>>that has this requirement gratuitiously is flawed.

>>     Right, so NTP is flawed because it does not put the IP address in the
>> payload, thus requiring you to get it from the kernel if you need it. (I
>> don't agree with your premises, but they lead inexorably to this
>> conclusion.)

> As long as you don't receive packets on the wildcard addresses your
> response will go out using the same address as the packet was received
> on. It's the wildcard addresses that cause the problem and you cannot
> know which address the kernel may choose.

    Right. RFC1123 says:

         *    "SHOULD"

              This word or the adjective "RECOMMENDED" means that there
              may exist valid reasons in particular circumstances to
              ignore this item, but the full implications should be
              understood and the case carefully weighed before choosing
              a different course.


     When the local host is multihomed, a UDP-based request/response
      application SHOULD send the response with an IP source address
      that is the same as the specific destination address of the UDP
      request datagram.

    What this means is that a UDP-based request/response application must 
send responses with the source address that was the destination in the reply 
unless the full implications of not doing so are understood and the case is 
carefully weighed. Failure to do this is failure to comply with RFC1123. 
RFC1123 states the host *requirements* for being part of the Intenet.


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