Debian Setup als Router

1. Open a new Putty Terminal and connect to server
2. Enter following command to edit interfaces file:

   # vim /etc/network/interfaces

3. Edit the file with the following lines: (add your netmask and gateway)

   /etc/network/interfaces

   auto lo 
   iface lo inet loopback
    
   auto eth0
   iface eth0 inet static
   address 182.x.x.x 
   netmask  x.x.x.x 
   gateway x.x.x.x
    
   auto eth1
   iface eth1 inet static 
   address 192.168.0.1
   netmask x.x.x.x

4. Now edit /etc/sysctl.conf and uncomment:

   # net.ipv4.ip_forward=1>

   so that it reads: net.ipv4.ip_forward=1

   and save it by entering

   :wq

5. To enable IP masquerading, enter following set of commands in terminal:

   # iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
   
   # iptables -A FORWARD -i eth1 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
   
   # iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT 

Update: Fix strange “-–state” causing command to fail and fix nat MASQUERADE to eth0 (wan interface)

Bessere Lösung →> https://gridscale.io/community/knowledgebase/tutorial-debian-routergateway-in-10-minuten/