Debian Router - Network Interfaces Configuration

In preparation of using debian as a router, We'll configure network interfaces, after performing initial setup.  

  In this debian router articles series also:

1. Introduction.
2. Hardware Requirements.
3. Software installation.
4. Basic Setup.
5. /etc/network/Interfaces Configuration. (We are Here!)
6. Unbound dns server setup.
7. DHCP server setup.
8. Squid setup.
9. Final step iptables and sysctl.conf configuration.

We'll modify /etc/network/interfaces for setting up the network, as said previously eth0, eth1 are used for KISS sake only.   Lets start our network interfaces automatically after boot, add the folowing:

# automatic start the following interfaces:
auto eth0 eth1

  If you use usb ethernet cards, then its helpful to start/stop interfaces  whenever you plug/unplug usb cards, then add the following:

# usb interfaces support for plug/unplug:
allow-hotplug eth0 eth1

  eth0 is the primary ethernet get its internet/wan ip address from WAN-AP so add the following:

# primary ethernet (connected to the WAN-AP)
iface eth0 inet dhcp

  eth1 is the secondary ethernet connected to LAN-AP, we need to assign it a static ip address by our DHCP server, add the following:

# secondery ethernet (connected to LAN-AP)
iface eth1 inet static  
 address 10.5.5.1
 netmask 255.255.255.0  
 network 10.5.5.0  
 broadcast 10.5.5.255  
 gateway 10.5.5.1

    After applying these changes your /etc/network/interfaces may will look similar to the following:  

# The loopback network interface:  
iface lo inet loopback
  
# automatic start the following interfaces:  
auto lo eth0 eth1  

  
# usb interfaces support for plug/unplug  
allow-hotplug eth0 eth1  

  
# primary ethernet (connected to the WAN-AP)  
iface eth0 inet dhcp  

  
# secondery ethernet (connected to LAN-AP)  
iface eth1 inet static  
 address 10.5.5.1  
 netmask 255.255.255.0  
 network 10.5.5.0  
 broadcast 10.5.5.255  
 gateway 10.5.5.1  

  Next Unbound dns server setup