Classic DHCP over bridge problem

Hi All

I'm looking for a quick fix:

I've got several pc's connected wirelessly to a linux server doing DHCP.

login as: donkey
Password:
Last login: Thu Apr 14 21:18:18 2005 from frog.peanuts.org.za
Have a lot of fun...
www:~ # killproc dhcpd
www:~ # dhcpd
Internet Systems Consortium DHCP Server V3.0.2rc3
Copyright 2004 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/
Wrote 0 deleted host decls to leases file.
Wrote 0 new dynamic host decls to leases file.
Wrote 30 leases to leases file.

DHCP works beautifully...

Some of the wireless clients are running Windows XP, software bridging
to switches and other pcs. These other PC's also get their assigned IP
adresses.

Now I have a unique problem. I need some of these PC's that provide the
software bridging to be assigned specific IP addresses via DHCP.

Problem is, soon as I tell my DHCP server to give a specific IP to a
specific MAC, it assigns that IP to the bridging machine, and all the
devices bridged by it. Consequently only the bridging machine's
networking works.

Free trips to the Comores to eveyrone with answers!!!


Coenraad

Assigning static IP's to the bridging machines is not an option.

Tauno Voipio wrote:
> Coenraad Loubser wrote:
>
>> Hi All
>>
>> I'm looking for a quick fix:
>>
>> I've got several pc's connected wirelessly to a linux server doing DHCP.
>>
>> login as: donkey
>> Password:
>> Last login: Thu Apr 14 21:18:18 2005 from frog.peanuts.org.za
>> Have a lot of fun...
>> www:~ # killproc dhcpd
>> www:~ # dhcpd
>> Internet Systems Consortium DHCP Server V3.0.2rc3
>> Copyright 2004 Internet Systems Consortium.
>> All rights reserved.
>> For info, please visit http://www.isc.org/sw/dhcp/
>> Wrote 0 deleted host decls to leases file.
>> Wrote 0 new dynamic host decls to leases file.
>> Wrote 30 leases to leases file.
>>
>> DHCP works beautifully...
>>
>> Some of the wireless clients are running Windows XP, software bridging
>> to switches and other pcs. These other PC's also get their assigned IP
>> adresses.
>>
>> Now I have a unique problem. I need some of these PC's that provide
>> the software bridging to be assigned specific IP addresses via DHCP.
>>
>> Problem is, soon as I tell my DHCP server to give a specific IP to a
>> specific MAC, it assigns that IP to the bridging machine, and all the
>> devices bridged by it. Consequently only the bridging machine's
>> networking works.
>>
>> Free trips to the Comores to eveyrone with answers!!!
>>
>>
>> Coenraad
>
>
> Can you use something else than the MAC as the client
> identifier for the bridging hosts?
>
> Your problem is in the very nature of bridging and using
> the bridge MAC as client ID.
>
> Maybe putting a DHCP relay in the bridging hosts could
> avoid sending the bridge MAC as the ID for the hosts
> behind the bridge.
>
> Just my EUR 0.02...
>

Thanks, as I suspected. Now if only I could find out what else I can use
for this DHCP server to know this pc needs that address...

Another thing.. whats up with this DHCP server, why does it pick random
IP's cant it just stick to one per host... Hmm
weird, how does it anyway assign different IP's to the bridged clients
under normal circumstances?

Computers suck

Coenraad Loubser wrote:
> Hi All
>
> I'm looking for a quick fix:
>
> I've got several pc's connected wirelessly to a linux server doing DHCP.
>
> login as: donkey
> Password:
> Last login: Thu Apr 14 21:18:18 2005 from frog.peanuts.org.za
> Have a lot of fun...
> www:~ # killproc dhcpd
> www:~ # dhcpd
> Internet Systems Consortium DHCP Server V3.0.2rc3
> Copyright 2004 Internet Systems Consortium.
> All rights reserved.
> For info, please visit http://www.isc.org/sw/dhcp/
> Wrote 0 deleted host decls to leases file.
> Wrote 0 new dynamic host decls to leases file.
> Wrote 30 leases to leases file.
>
> DHCP works beautifully...
>
> Some of the wireless clients are running Windows XP, software bridging
> to switches and other pcs. These other PC's also get their assigned IP
> adresses.
>
> Now I have a unique problem. I need some of these PC's that provide the
> software bridging to be assigned specific IP addresses via DHCP.
>
> Problem is, soon as I tell my DHCP server to give a specific IP to a
> specific MAC, it assigns that IP to the bridging machine, and all the
> devices bridged by it. Consequently only the bridging machine's
> networking works.
>
> Free trips to the Comores to eveyrone with answers!!!
>
>
> Coenraad

Can you use something else than the MAC as the client
identifier for the bridging hosts?

Your problem is in the very nature of bridging and using
the bridge MAC as client ID.

Maybe putting a DHCP relay in the bridging hosts could
avoid sending the bridge MAC as the ID for the hosts
behind the bridge.

Just my EUR 0.02...

--

Tauno Voipio
tauno voipio (at) iki fi

Coenraad Loubser wrote:
>
>> Can you use something else than the MAC as the client
>> identifier for the bridging hosts?
>>
>> Your problem is in the very nature of bridging and using
>> the bridge MAC as client ID.
>>
>> Maybe putting a DHCP relay in the bridging hosts could
>> avoid sending the bridge MAC as the ID for the hosts
>> behind the bridge.
>>
>> Just my EUR 0.02...
>>
>
> Thanks, as I suspected. Now if only I could find out what else I can use
> for this DHCP server to know this pc needs that address...
>
> Another thing.. whats up with this DHCP server, why does it pick random
> IP's cant it just stick to one per host... Hmm
> weird, how does it anyway assign different IP's to the bridged clients
> under normal circumstances?
>

I suggest putting Ethereal into a host hearing the DHCP
exchange and looking at the trace. There may already be
some other means of identifying the clients.

Which DHCP server is it?

--

Tauno Voipio
tauno voipio (at) iki fi

Coenraad Loubser wrote:
> Tauno Voipio wrote:
> > Coenraad Loubser wrote:
> >
> >> Hi All
> >>
> >> I'm looking for a quick fix:
> >>
> >> I've got several pc's connected wirelessly to a linux server doing
DHCP.
> >>
> >> login as: donkey
> >> Password:
> >> Last login: Thu Apr 14 21:18:18 2005 from frog.peanuts.org.za
> >> Have a lot of fun...
> >> www:~ # killproc dhcpd
> >> www:~ # dhcpd
> >> Internet Systems Consortium DHCP Server V3.0.2rc3
> >> Copyright 2004 Internet Systems Consortium.
> >> All rights reserved.
> >> For info, please visit http://www.isc.org/sw/dhcp/
> >> Wrote 0 deleted host decls to leases file.
> >> Wrote 0 new dynamic host decls to leases file.
> >> Wrote 30 leases to leases file.
> >>
> >> DHCP works beautifully...
> >>
> >> Some of the wireless clients are running Windows XP, software
bridging
> >> to switches and other pcs. These other PC's also get their
assigned IP
> >> adresses.
> >>
> >> Now I have a unique problem. I need some of these PC's that
provide
> >> the software bridging to be assigned specific IP addresses via
DHCP.
> >>
> >> Problem is, soon as I tell my DHCP server to give a specific IP to
a
> >> specific MAC, it assigns that IP to the bridging machine, and all
the
> >> devices bridged by it. Consequently only the bridging machine's
> >> networking works.
> >>
> >> Free trips to the Comores to eveyrone with answers!!!
> >>
> >>
> >> Coenraad
> >
> >
> > Can you use something else than the MAC as the client
> > identifier for the bridging hosts?
> >
> > Your problem is in the very nature of bridging and using
> > the bridge MAC as client ID.
> >
> > Maybe putting a DHCP relay in the bridging hosts could
> > avoid sending the bridge MAC as the ID for the hosts
> > behind the bridge.
> >
> > Just my EUR 0.02...
> >
>
> Thanks, as I suspected. Now if only I could find out what else I can
use
> for this DHCP server to know this pc needs that address...
>
> Another thing.. whats up with this DHCP server, why does it pick
random
> IP's cant it just stick to one per host... Hmm
> weird, how does it anyway assign different IP's to the bridged
clients
> under normal circumstances?
>
> Computers suck

Not always ... but

As T.V. suggests, you probably ought to sniff the wire and see exactly
what's passing between the client and server.

That said, I'm suggesting you look at dhcp option "client-identifier"
as a way to key on the "special" machines.

In Linux, it's pretty easy to set client-identifier but in XP it's a
pain-in-the-arse (so what's new, right?).

In Linux, just look at the man pages:
http://default.co.yu/~bc/rtfm/index....=dhclient.conf
and search for "send {" [no quotes]
and look here:
http://default.co.yu/~bc/rtfm/index....d=dhcp-options
for
option dhcp-client-identifier

This is really just a string meant to signify a specific client
regardless of it's MAC address. Normally, it's set to the MAC, but can
be set to something else.

And now we come to Windows. I won't try to explain or vouch for it's
effectiveness. Look here to see the "official" POV:
http://support.microsoft.com/kb/q172408/

My real qualm has to do with just _what_ the wireless AP is futzing
with. I _assume_ it's the MAC address field from the BOOTP days. If
it's mucking with the clientID option, then all bets are off. In case
you have not guessed, I'm pretty "wireless illiterate" re: the details
of wireless bridging.

To take a look at the header layout of DHCP, see:
http://www.networksorcery.com/enp/protocol/dhcp.htm
and
http://www.networksorcery.com/enp/pr...tp/options.htm

The RFC for dhcp is here:
http://www.faqs.org/rfcs/rfc2131.html
and maybe something useful here:
http://www.faqs.org/rfcs/rfc2132.html

hth,
prg

Tauno Voipio <(E-Mail Removed)> wrote:

> Coenraad Loubser wrote:

>> DHCP works beautifully...
>>
>> Some of the wireless clients are running Windows XP, software bridging
>> to switches and other pcs. These other PC's also get their assigned IP
>> adresses.
>>
>> Now I have a unique problem. I need some of these PC's that provide the
>> software bridging to be assigned specific IP addresses via DHCP.
>>
>> Problem is, soon as I tell my DHCP server to give a specific IP to a
>> specific MAC, it assigns that IP to the bridging machine, and all the
>> devices bridged by it. Consequently only the bridging machine's
>> networking works.
>>
>
> Can you use something else than the MAC as the client
> identifier for the bridging hosts?
>
> Your problem is in the very nature of bridging and using
> the bridge MAC as client ID.
>
> Maybe putting a DHCP relay in the bridging hosts could
> avoid sending the bridge MAC as the ID for the hosts
> behind the bridge.

Why should a bridge modify DHCP requests from its bridged
devices? Even if it got an IP address (for management
purposes) assigned, it's still a bridge which should not
modify bridged traffic. I'm sure I've missed something
here.

It would nice if the OP could provide a tcpdump packet
trace of the dhcp traffic. This should clarify what's
up.


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

In comp.os.linux.networking <d3pb0j$qck$(E-Mail Removed)> Horst Knobloch <(E-Mail Removed)> wrote:
> Why should a bridge modify DHCP requests from its bridged
> devices? Even if it got an IP address (for management
> purposes) assigned, it's still a bridge which should not
> modify bridged traffic. I'm sure I've missed something
> here.

It isn't modifying the DHCP request. It's modifying the Ethernet
frame that carries the DHCP request. Specifically, when the bridge
sends a frame on behalf of a device on the far side, the MAC
address the DHCP server will see is the MAC address of the bridge,
not of the device that really sent the request. This is neccesary
because the network on the far side of the bridge from the server
may not have MAC addresses which can be made to look like Ethernet
MAC addresses, may not even have MAC addresses at all, and even if
it did and they could the frames have to go back to the bridge at
the physical level so the server needs to see and use the MAC address
of the bridge.

--
death.net: because for some problems there's only one solution.

Todd Knarr wrote:

> It isn't modifying the DHCP request. It's modifying the Ethernet
> frame that carries the DHCP request. Specifically, when the bridge
> sends a frame on behalf of a device on the far side, the MAC
> address the DHCP server will see is the MAC address of the bridge,
> not of the device that really sent the request. This is neccesary
> because the network on the far side of the bridge from the server
> may not have MAC addresses which can be made to look like Ethernet
> MAC addresses, may not even have MAC addresses at all, and even if
> it did and they could the frames have to go back to the bridge at
> the physical level so the server needs to see and use the MAC address
> of the bridge.

I think you've gone a bridge too far! If bridges changed the mac, stuff
like NetBIOS would fail. Take a look with Ehtereal on a computer that's
connected to a bridge or switch. You'll see the original macs. Bridges
will filter on macs, so that it won't pass traffic that aren't intended for
the other side, but it won't change the ethernet frame.

Todd Knarr <(E-Mail Removed)> wrote:

> In comp.os.linux.networking <d3pb0j$qck$(E-Mail Removed)> Horst
> Knobloch <(E-Mail Removed)> wrote:
>> Why should a bridge modify DHCP requests from its bridged
>> devices? Even if it got an IP address (for management
>> purposes) assigned, it's still a bridge which should not
>> modify bridged traffic. I'm sure I've missed something
>> here.
>
> It isn't modifying the DHCP request. It's modifying the Ethernet
> frame that carries the DHCP request.

It shouldn't do this either. The bridge must be
transparently forward the ethernet frames. (Except if
the destination MAC of the frame is on the same ethernet
port the frame came in, then the frame is dropped).


> Specifically, when the bridge
> sends a frame on behalf of a device on the far side, the MAC
> address the DHCP server will see is the MAC address of the bridge,
> not of the device that really sent the request.

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.

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.


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