I've set up an L2TP/IPsec VPN using OpenSwan and xl2tpd using the "nodefaultroute" ppp option. The server's virtual IP address is set to 10.10.31.1 and it allocates addresses in the range 10.10.31.2-254. This sets up PPP links between 10.10.31.1 <-> 10.10.31.2 etc. and this works fine - the VPN-connected hosts can communicate with the server.
How can I set up the ppp link so the server end looks like the entire range 10.10.0.0 instead of just that IP address? I am hoping that connecting clients will then route any requests for that subnet via the VPN interface, where the server can catch them and act as a gateway. Going in the other direction, the "proxyarp" option will allow the VPN server to catch traffic back to the VPN clients.
One obvious solution to this is to use the VPN server as the default route. I would prefer not to do this, so that only the traffic specific to that subnet goes through the VPN. Is this even possible?