You will want to do a lot of testing with the firewall before you call it good. This is an area that often will cause you problems. If you are using the ufw firewall then you will want to enable it first.
Linux Terminal Server Firewall (DHCP,SSH, Samba)
You will need to enable several ports for a LTSP server as it will provide DHCP on ports 67,68 UDP and you may have Samba enabled on ports 139,445 and of course you will want SSH on port 22.
ufw allow 67/udp
ufw allow 68/udp
ufw allow 445/tcp
ufw allow 13/tcp