Only using masquerading on internet facing server

[john doe <johndoe65534@mail.com>]

Hi,

I have one internet facing server that is doing masquerading (server a).
Behind that server I have an other server (server b).

Server a is the only one server that should do NAT .

Ip range on server a: 172.17.232.0/24
IP range on server b: 192.168.3.0/24

I have configured server a to MASQUERADE both IP ranges.

If I enable MASQUERADING on server b everything works as expected but as
soon as I disabled MASQUERADING on server b the hosts behind it don't
have internet access for example.
What do I need to do on server a to properly MASQUERADE server b?

P.S. Server a is on Debian Stretch.

--
John Doe