How to update routes when packets arrive?

I have three tunnels, tun0, tun1 and tun2, on which traffic can arrive from 10.10.0.0/16. I would like to update routes, prompted by the arrival of packets. If a packet arrives from 10.10.X.Y on tunZ, I would like to update the route-to-10.10.X.0/24 to use tunZ's far-address as the next hop, until further notice. How can I do this with existing Linux tools and configuration? (I could make a libpcap-based tool which spied on traffic from the tunnels and updated rules as needed.)