Unusual routing problem

["Benjamin Tompkins" <netfilter@dahanga.org>]

I am attempting to route myself an ip block from my office to my home via a
tunnel. Simple enough. The catch is, I only want the tunnel to be used for
lan destined traffic and incoming connections to my IP block. The lan
destined traffic is easy, the trick apparently is getting the block to be
accessible via the internet, without forcing all traffic to use the tunnel.
A diagram.

eth0  (DHCP) cable modem            eth1  (x.x.x.1/28) LAN
                                              \   /
                                      Linux Machine
                                                |
                                 ppp0 (x.x.x.2/30)

Ok, so what I have so far is as follows.

This takes care of access to the office network.
route add -net x.x.x.1 netmask 255.255.255.240 dev eth1
route add -net x.x.x.0 netmask 255.255.254.0 dev ppp0
route add -host x.x.x.1 dev eth0

Now to use the cable for everything else.
iptables -A POSTROUTING -s x.x.x.1/28 -o eth0 -j MASQUERADE

So now I can access my office lan and vice versa, and everything else gets
masqed out the cable. But I'm having a heck of a time letting the box to
know that stuff requested via ppp0, needs to go out ppp0. I have looked at
using the mangle table making rules for input and forward, but am just
missing something along the way. Any help anyone can offer would be greatly
appreciated. Thanks.