configuring ppp and lan

[richard@the-place.net]

I'm finally beginning to have some internet here in the Appennine alps.
No land line so I have acquired a usb gsm modem. (To be truthful, I
still don't know if it will work at the house which also has no mobile
signal, but I have a directional antenna with a 10dB gain that may fix
that...)

To the point.  Trying it out where there is a gsm signal, I have got ppp
running, but cannot configure the box with the modem so that both the
LAN and the PPP work at the same time.  If I 'poff', the lan sort of
works.  If I 'ifdown eth1' (the wifi built into t he box), the ppp
connection resolves correctly.  dhclient rewrites resolv.conf giving it
one of the nameservers of the provider (Wind), and the router/switch
address as the second nameserver.  I'm not sure why.  The router (D-Link
DI524) doesn't actualy seem to do any DNS (although it knows the
hostnames and IPs on the internal subnet), so that may be a part of the
problem, but the correct nameserver ought to be accessed at least some
of the time. dhclient also writes the routing table and maybe I have got
something wrong here.  netstat -r gives

Destination     Gateway  Genmask         Flags   MSS Window  irtt  Iface
joiner.mulinoca *        255.255.255.255 UH        0 0          0  ppp0
default         *        0.0.0.0         U         0 0          0  ppp0

when pppd is up.  And when eth1 is up, it says

Destination    Gateway       Genmask        Flags  MSS Window irtt Iface
192.168.133.0  *             255.255.255.0  U        0 0         0 eth1
default        192.168.133.1 0.0.0.0        UG       0 0         0 eth1

with both up I get

Destination    Gateway        Genmask         Flags MSS Windo irtt
Iface
joiner.mulinoc *              255.255.255.255 UH      0 0        0 ppp0
192.168.133.0  *              255.255.255.0   U       0 0        0 eth1
default         192.168.133.1 0.0.0.0         UG      0 0        0 eth1

joiner.mulinocarletti.net is the box with both the modem and the lan
connection.  the lan as you can see is 192.168.133.x and the modem local
address insists on being 192.168.100.101.  joiner has both addresses in
its /etc/hosts, which is I suppose the reason the name is shown in the
routing table rather than the IP allocated by wind.it.

I feel even more confused now, so I hope this makes sense to somebody.
I wondered if I should reconfigure the whole lan to be on the same
subnet as the Wind modem, though I cannot see why that should be
necessary.

Any ideas?

-- 
richard