Linux box as network gateway

[vmlinux]

> Where 192.168.99.x and 172.16.253.x are the local non-tinc subnets and
> 10.10.10.x are tinc addresses.
> 

Hmm.. I don't think you can bridge the 192.x and 172.x networks. They
need to have the same IP address range/netmask. You could probably
work some hocus-pocus with Netfilter in the nat table between but
that could be a substantial performance hit depending on traffic.