is half bridge mode good and proper?

i'm not exactly sure how half bridge mode (zipb or dhcp spoofing) works but
here's my question.

say...

the modem/router gets set to half-bridge mode and NAT should be turned off
and encapsulation set to VC-Mux (in the UK). the WAN interface picks up an
IP address via DHCP (whether its a single static IP or via DHCP).

the modem/router then sits there with DHCP listening on the LAN side.

a PC (or another router/device) which is connected to the LAN port of the
first modem/router then asks for network settings via DHCP.

the modem/router gives the external IP address that it itself obtained via
DHCP to the PC.

the PC then acts as if it has the external IP address on its network card.


but...

what is the internal (LAN) IP address of the first router/modem - should it
be set - and to what?

i've heard that it uses the (IP address - 1 ) - but surely this is a
workaround and a bit of a cludge?

similarly, in the PC what should the gateway address be set to?

thanks,

kevin

On Tue, 01 Aug 2006 22:54:55 +0100, kevin bailey
<(E-Mail Removed)> wrote:

>i'm not exactly sure how half bridge mode (zipb or dhcp spoofing) works but
>here's my question.
>
>say...
>
>the modem/router gets set to half-bridge mode and NAT should be turned off
>and encapsulation set to VC-Mux (in the UK). the WAN interface picks up an
>IP address via DHCP (whether its a single static IP or via DHCP).

Actualy - usualy not DHCP but via PPP

>
>the modem/router then sits there with DHCP listening on the LAN side.
>
>a PC (or another router/device) which is connected to the LAN port of the
>first modem/router then asks for network settings via DHCP.
>
>the modem/router gives the external IP address that it itself obtained via
>DHCP to the PC.
>
>the PC then acts as if it has the external IP address on its network card.
>
>
>but...
>
>what is the internal (LAN) IP address of the first router/modem - should it
>be set - and to what?

It doesn't really have one. This is what bridge-mode is all about.
It acts as a transparent bridge between the PC and the 'net.

Usualy the utils for talking to the modem from the PC will be using
raw Ethernet to achieve this.

>
>i've heard that it uses the (IP address - 1 ) - but surely this is a
>workaround and a bit of a cludge?
>
>similarly, in the PC what should the gateway address be set to?

The gateway address will be passed to the modem via PPP, and set in
the PC via DHCP - just like the IP address.

This is how it is *supposed* to work. I can well beleive that some
units don't behave like this.

Regards,
Harry.


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>>similarly, in the PC what should the gateway address be set to?
>
> The gateway address will be passed to the modem via PPP, and set in
> the PC via DHCP - just like the IP address.

this sounds like half bridge mode whereby the PC (or other device) gets its
network settings via DHCP.

according to Netgear their DG632 acts in full bridge mode - in this case the
PC runs a DHCP request - this goes straight through the modem/router and is
answered by the ISP with the external IP address.

although the gateway gets set to the same address as the external IP address
which is different from when the modem gets network details itself /

there's more stuff going on underneath which its difficult to find out
about!

cheers,

kevin

>
> This is how it is *supposed* to work. I can well beleive that some
> units don't behave like this.
>

kevin bailey wrote:
>>>similarly, in the PC what should the gateway address be set to?
>>
>> The gateway address will be passed to the modem via PPP, and set in
>>the PC via DHCP - just like the IP address.
>
>
> this sounds like half bridge mode whereby the PC (or other device) gets its
> network settings via DHCP.
>
> according to Netgear their DG632 acts in full bridge mode - in this case the
> PC runs a DHCP request - this goes straight through the modem/router and is
> answered by the ISP with the external IP address.
>
> although the gateway gets set to the same address as the external IP address
> which is different from when the modem gets network details itself /

Hmm that confuses me - Assuming the LAN is eth how can it really have
the same address as a PC on the same eth - ie would they both answer an
arp request for that ip with different mac addresses.

>
> there's more stuff going on underneath which its difficult to find out
> about!

Bridging I think is a term that gets used loosely - it is defined
formally for ethernet, but then that's LAN only due to use of broadcast,
so doesn't apply to wan/lan.

I suppose the router means it's trying to hide/auto setup the routing
details for you. I use a PC as a router and have only one ip address -
so can't say from experience what happens.

You mention DHCP - there is no requirement at all to use it to setup an
IP/ethernet network or dsl - it's just convinient and I don't think the
router will pass on to the ISP any DHCP requests.

As Harry said in practice your can get your IP address during the PPP
setup - essential if it's dynamic - irrelevant if it's static (as long
as you know what it is/they are). All 3 methods of connecting dsl
through BT use ppp - but ppp frames don't care what they carry hence no
real need for ip address/gateway on a ppplink (in fact my isp used to
not give a remote ppp address sometimes - ppp just used one picked from
10.0.0.0/8 so you could use it in your ip routing setup).

In practice dsl modems are ATM devices, in the UK with BT you need to
use PPP to connect so the ip packet from your PC arrives at the LAN
interface possibly as payload of an eth frame, gets put in a ppp frame
which gets put in an aal5 frame is padded out to fit into atm cells and
then given over to the modem as a train of cells (not all at once as
there may be, but usually aren't, other cells (OAM) that get priority).

Andy.

On Tue, 01 Aug 2006 23:54:55 +0100, kevin bailey wrote:

> what is the internal (LAN) IP address of the first router/modem - should
> it be set - and to what?
>
> i've heard that it uses the (IP address - 1 ) - but surely this is a
> workaround and a bit of a cludge?


I had an x-modem and that uses a fixed private ip address. The computer
doesn't know about that subnet; so I guess the way it works is that the
address is treated by the PC like any other routable IP address, and modem
just takes it's own traffic.

That device has only one ethernet port. I'm not sure how other such
devices work when they have more than 1 port, but my guess would be that
they either forbid the use of the other ports in half bridge-mode, or
treat the other ports as a separate subnet.

I don't recall setting a default gateway or netmask, or seeing one
in the DHCP lease file (under FreeBSD). I guess if you don't provide a
default gateway, the DHCP interface is treated as the default route, just
like a modem interface would be.

>
> As Harry said in practice your can get your IP address during the PPP
> setup - essential if it's dynamic - irrelevant if it's static (as long
> as you know what it is/they are). All 3 methods of connecting dsl
> through BT use ppp - but ppp frames don't care what they carry hence no
> real need for ip address/gateway on a ppplink (in fact my isp used to
> not give a remote ppp address sometimes - ppp just used one picked from
> 10.0.0.0/8 so you could use it in your ip routing setup).

thanks for that - i forgot that PPP is its own thing and the ethernet type
stuff does not apply. its been a while since college where it was LAN's,
token rings etc and no internet connection.


>
> In practice dsl modems are ATM devices, in the UK with BT you need to
> use PPP to connect so the ip packet from your PC arrives at the LAN
> interface possibly as payload of an eth frame, gets put in a ppp frame
> which gets put in an aal5 frame is padded out to fit into atm cells and
> then given over to the modem as a train of cells (not all at once as
> there may be, but usually aren't, other cells (OAM) that get priority).


more headers than you can shake a stick at!

thanks again

>
> Andy.

On Fri, 04 Aug 2006 11:14:13 +0100, kevin bailey wrote:


>> As Harry said in practice your can get your IP address during the PPP
>> setup - essential if it's dynamic - irrelevant if it's static (as long
>> as you know what it is/they are). All 3 methods of connecting dsl
>> through BT use ppp - but ppp frames don't care what they carry hence no
>> real need for ip address/gateway on a ppplink (in fact my isp used to
>> not give a remote ppp address sometimes - ppp just used one picked from
>> 10.0.0.0/8 so you could use it in your ip routing setup).
>
> thanks for that - i forgot that PPP is its own thing and the ethernet
> type stuff does not apply. its been a while since college where it was
> LAN's, token rings etc and no internet connection.


I think you're confusing half-bridging and full-bridging here; your
original question was about the former.

In the UK full-bridging only works because BT also allows PPPoE to run
over ATM. The PPP is terminated on the PC, and the routable ip address
provided by PPP is associated with the internal PPP logical interface
rather than the the physical ethernet interface. This is very similar to a
dial-up serial modem. The chief problem with this is that it isn't
provided at all by some LLU operators, and most other ISPs provide no
official support for it.

In half-bridging, ordinary PPPoA is terminated on the modem itself, and ip
is bridged to the PC over ethernet. The PC just treats it as a DHCP
interface, and associates the routable IP address with its ethernet
interface. Half-bridging is therefore trivial to setup, and will work with
any ISP.