Network configuration

Interface setup (static IP)

Adjust the following according to your network details:

auto ens160
iface ens160 inet static
  address 192.168.170.100
  netmask 255.255.255.0
  network 192.168.170.0
  broadcast 192.168.170.255
  gateway 192.168.170.200
  dns-nameservers <ip of your domain dns>
  dns-search your.domain.com

Setup an interface as a gateway

In the following setup interface eth0 is connected to the internet while eth1 is the gateway of the internal lan that forwards traffic to eth0

auto eth0
iface eth0 inet static
  address 192.168.4.20
  netmask 255.255.255.0
  broadcast 192.168.4.255
  network 192.168.4.0
  gateway 192.168.4.1
  dns-nameservers 147.102.222.210 147.102.222.220 147.102.222.230

auto eth1
iface eth1 inet static
  address 192.168.253.1
  netmask 255.255.255.0
  broadcast 192.168.253.255
  network 192.168.253.0
  up iptables-restore < /etc/ltsp/nat
  up route add -net 192.168.248.0 netmask 255.255.252.0 gw 192.168.253.254

The /etc/ltspnat file should be something similar to:

# Generated by iptables-save v1.4.4 on Fri Nov  4 15:53:53 2011
*nat
:PREROUTING ACCEPT [4:1808]
:POSTROUTING ACCEPT [1:116]
:OUTPUT ACCEPT [1:116]
-A POSTROUTING -s 192.168.253.0/24 -j MASQUERADE
-A POSTROUTING -s 192.168.248.0/22 -j MASQUERADE
COMMIT
# Completed on Fri Nov  4 15:53:53 2011

IP forwarding should be enabled in /etc/sysctl.conf:

...
# Uncomment the next line to enable packet forwarding for IPv4
net.ipv4.ip_forward=1
...

Screenshots

Debian

Ubuntu