Route VPN gateway traffic via tun0 instead of normal gateway

Question

On my Linux box, I have a tun0 interface by IPIP tunnel like:

tun0      Link encap:IPIP Tunnel  HWaddr   
          inet addr:10.10.10.2  P-t-P:10.10.10.1  Mask:255.255.255.255

and I running an IPsec VPN server (ipsec-tools) on this box. How can I route all VPN traffic through the tun0 interface on the server (like Openswan's leftnexthop )?