Problem with a conditional evaluation from Microsoft to ISC

Bernardo Pita bpita at
Tue Jan 20 14:54:33 UTC 2009

Hi Bruce, thanks for your answer.

I reach the same conclusion about the usage of some part of MAC (last
digit for example) to send one or another file, but like you says,
always the same MAC obtains the same file.

Your approach about using XID it can be a solution, so I testing in few
days and comment if I can reply the Microsoft configuration with your

Thanks for your help, Bernardo.

-------- Mensaje original --------
De: Bruce Hudson <Bruce.Hudson at Dal.Ca>
Para: dhcp-users at
Asunto: Re:Problem with a conditional evaluation from Microsoft to ISC
Fecha: 19/01/2009 18:50
>> Hi, I need to migrate a Microsoft DHCP configuration to ISC DCHP from a
>> client. In the original configuration (Microsoft) they have this
>> conditional evaluation:
>> [($TIME ("integer")&1)==1 ?"file.a.bin":"file.b.bin"]
>> As you can see the idea of the line it's like a random condition $TIME,
>> the server offers a diferente file to the client request. They use this
>> to obtain a basic balancing for two CMTS.
>> I read dhcp-eval and I can't find anything like this... any ideas that
>> allow me to obtain the same result with ISC ?
>     Off the top of my head, the following code should work. It uses the 
> last byte of the client supplied "XID" value (the 8th byte of the the
> packet) as a source of randomness; converts it to is binary value as an
> ASCII string; and extracts the final bit. 
>     if (suffix (binary-to-ascii (2, 8, ".", packet(7, 1)), 1) = 0) {
>         myfile = "file.a.bin";
>     } else {
>         myfile = "file.b.bin";
>     }
>     This is not inline as your original expression seems to be. It stores
> the file name into a variable. You can then use that in what-ever 
> statement the expression was taken from.
>     The client's hardware address might also serve to balance requests.
> It would have the (dis)advantage that a client will always get the same
> --
> Bruce A. Hudson				| Bruce.Hudson at Dal.CA
> UCIS, Networks and Systems		|
> Dalhousie University			|
> Halifax, Nova Scotia, Canada		| (902) 494-3405
> _______________________________________________
> dhcp-users mailing list
> dhcp-users at

Bernardo Pita
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Phone/Fax: (54) 2964-425870
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