Re: Classic DHCP over bridge problem

From: Horst Knobloch (horschti2_at_gmx.de)
Date: 04/16/05 

Date: Sat, 16 Apr 2005 12:07:48 +0200

Todd Knarr <tknarr@silverglass.org> wrote:

> In comp.os.linux.networking <d3pm2t$sc4$1@ulysses.news.tiscali.de> Horst
> Knobloch <horschti2@gmx.de> wrote:
>> Even if a bridge would do this (which it doesn't), it
>> shouldn't confuse the DHCP server. The DHCP server
>> looks into the DHCP request for processing and doesn't
>> bother with the ethernet frame overhead.
>
> How then would the DHCP server match on hardware address when the
> DHCP request doesn't contain a filled-in hardware address field?
> I know it can, so it's got to be looking at the Ethernet MAC
> address on the incoming datagram.

*Inside* the DHCP request there is a field which
contains the client's hardware address. It's called
chaddr in RFC2131.

I don't know how the ISC DHCP server processes it,
however if it looks to the source MAC address of the
ethernet frame instead of the client's hardware address
field (chaddr) inside the DHCP request, it would be
*very* dumb.

>> This should be the normal behavior but may be the
>> XP bridging software behaves otherwise and processes
>> the DHCP traffic in a more "creative" way.
>
> A real hardware Ethernet bridge would retain the MAC address,
> yes. This, however, is Windows doing the bridging. I'm not even
> completely sure it's capable of transmitting with an arbitrary
> MAC address as would be needed to be a real bridge.

Then this piece of software should not be called bridge
because it's none.

Ciao, Horst 

-- 
»When pings go wrong (It hurts me too)« E.Clapton/E.James/P.Tscharn

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