[DNS] forward zone requests

[azharoth71]

Hello,
I've 3 lans connected trough vpn
the dns zones are : a.local,  b.local, c.local, routing works, i can ping every lan

So, i want reach the other networks, via dns requests
suppose i'm in a.local and want to connect to server.b.local

right, in ipfire this works, by defining zone e relative dns servers

In zentyal i don't know how to do it in simple way

Thanks
Alessio