Option 82 or subnet: who wins?

Simon Hobson dhcp1 at thehobsons.co.uk
Tue Mar 17 08:14:21 UTC 2009


David McGaughey wrote:

>  > Moving the class definition outside the subnet definition means that
>>  the class won't have a router or netmask (or other values from that
>>  subnet) as inherited values. These values can then be picked up from
>>  the lower precedent subnet. Ususally this is what you want and is why
>>  the usual thing is to put class definitions in the global scope.
>
>Of all the help and tutoring that you all have provided, this is the most
>unexpected statement of them all.  I realize that scope may be placed into a
>class so that it might be inherited within a match.  But to have the class
>acquire scope from where it is declared is - unexpected to me.  I can't read
>it in the man page.  I see some hint there "Each of these declarations
>itself appear within a lexical scope..." But that's a pretty big jump (in my
>mind) to picking up scope from lexical context.  Look at this class
>definition:
>
>class "ms_rm116_sw2650_hp1.0.17"
>{
>  match if (
>          option agent.remote-id = 0:1d:b3:1f:c2:80
>      and
>          option agent.circuit-id = 0:11
>       );
>}
>
>It's a global class definition having nothing to do with subnet or even
>client MAC.  Why would lexical context attach scope baggage to it?
>Evidently - it just does.  My only reason for placing the class definitions
>into the subnet context was to have it close to the pool that would consume
>it - for debugging: HA!

It comes up regularly in relation to host statements - to the extend 
that code was added to spit out a warning if a host statement is 
declared anywhere by the global scope. The assumption by many is that 
by putting a host statement inside a subnet, it's only valid for a 
client in that subnet - instead they end up here with a weird problem 
of a client getting the wrong router !


I guess there are things that with hindsight and a clean sheet would 
probably have been done differently - I know I've often looked back 
at stuff I've done and thought that way. But it's what we've got. A 
regular call is for more rules checking, but I think the feeling 
there is that it would either be complicated to check, or restrict 
flexibility to do so (some people have been "quite creative").

-- 
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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