If I understand it, to use a Linux OS as a router I just have to declare routes and to send it a packet which will just be routed?
sounds way too easy 
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@duponin I think you also need to enable the sysctl net.ipv4.ip_forward. Equivalent for ipv6 is way more complicated (like everything) — https://unix.stackexchange.com/questions/348533/
don't bad speak of IPv6 D: