PPP dial-up server w/ multiple eth adapters: "operation not permitted"

["Andreas Abach" <Abach@LKT.Uni-Erlangen.de>]

Hi there,

I have a strange problem getting a PPP dial-up working.

The machine is running Debian 2.1 and kernel 2.2.13.

I use mgetty which properly transfers the call to pppd. The clients
gets its 
correct IP address, but it can't ping the p-t-p address, nor can I ping
the 
client from the server. If I try to do so I get a "operation not
permitted."

It seems that pppd on the server is confused which ip address to assign
to 
the ppp0 interface. The machine has two network interfaces, eth0 which 
connects it to the rest of the world, and eth1 which connects it to a
LAN.

Since the dial-in machine is being assigned an internal (LAN) address I
need 
proxy-arp. However due to my understanding this can't work if the ppp0 
interface on the server is being assigned the external if address
instead of 
the internal one.

Let me try to draw a simple map of the setup:
                   ___
                 _/   \___
                /         \    internet
                |        _/
                 \______/
                    |
                    |
                    O          eth0 with external IP
                  +---+
              M---|   |        router w/ modem attached
                  +---+
                    O          eth1 w/ internal IP
                    |
                    |
        +---+---+---+---+---+
        |   |   |   |   |   |
       +-+ +-+ +-+ +-+ +-+ +-+
       | | | | | | | | | | | |   LAN machines
       +-+ +-+ +-+ +-+ +-+ +-+


The router's eth1 interface as well as all the LAN machines have
official ip 
addresses from out of the same class C network.

The router's eth0 interface is the router's default gateway.

Questions:

1) Do I need to assign the ppp0 interface a unique IP address? Or can I

leave the default address that pppd assigns (namely the address of the 
external(!) if eth0)? How do I force pppd to assign a specified ip
address? 
Entering an address into options.modem (for serial line /dev/modem)
does NOT 
work for me.

2) What about proxy arp. I had the "proxyarp" entry in "options" but it

wouldn't work this way, so I tried to manually set up a proxy arp entry
that 
connects the client's IP to the router's eth1(!) MAC/ethernet address.
But 
this didn't ehlp either. :-(

Any other gotchas I should watch for?

Thanks,

Ralf, Andreas



-- 
Dipl.-Ing. Andreas Abach * Lehrstuhl fuer Kunststofftechnik Uni Erlangen
 Tel.: +49-9131-85297-06 * Fax.: -09 * email: Abach@LKT.Uni-Erlangen.de
Am Weichselgarten 9 * D-91058 Erlangen * www.keramikspritzgiessen.de
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