I am trying to create a router from a Debian machine on virtual box, but I can't get it to work properly. The router has got two network adapters, one that's bridged, and the other is running an internal network.

After setting up the router, I'm unable to retrieve an IP-address from the internal/private interface (eth1), and I can't understand why.

After booting up the machine I configured the networks interfaces as such:

nano -w /etc/network/interfaces

# The external WAN interface (eth0)
allow-hotplug eth0
iface eth0 inet dhcp

# The internal LAN interface (eth1)
allow-hotplug eth 1
iface eth1 inet static
address 192.168.42.1
netmask 255.255.255.0
network 192.168.42.0
broadcast 192.168.42.255

pre-up iptables-restore < /etc/iptables.rules

Then I installed dnsmasq and configured it like this:

nano -w /etc/dnsmasq.conf

interface=eth1
listen-address=127.0.0.1
dhcp-range=192.168.42.100,192.168.42.110,12h

And finally i configured iptables:

nano -w /etc/iptables.rules
*nat
-A PREROUTING -i eth0 -p tcp -m tcp --dport 50000 -j DNAT --to-destination
192.168.0.3:50000
-A POSTROUTING -o eth0 -j MASQUERADE
COMMIT

*filter
-A INPUT -i lo -j ACCEPT
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -i eth0 -p tcp -m tcp --dport 222 -j ACCEPT
-A INPUT -i eth0 -j DROP
-A FORWARD -i eth0 -p tcp -m tcp --dport 50000 -m state --state NEW -j
ACCEPT
COMMIT

After this I activated the iptables rules and rebooted, only to get no IP-adress from eth1.

It looks like you have a space between eth and 1 in the hotplug line of /etc/network/interfaces.

You have:

where it should be:

You also need to turn on the forwarding kernel parameter. Add net.ipv4.ip_forward=1 to /etc/sysctl.conf and then run this command as root:

What distribution/version are you running for the client virtual machine?

Is your client vm configured with an internal network?

In your dnsmasq.conf file the listen address should also include your IP address.
listen-address=127.0.0.1,192.168.42.1

You also need to configure gateway:
dhcp-option=3,192.168.42.1
and DNS
dhcp-option=6,192.168.42.1

What iptables rules if any do you have for eth1?
What are the default policies?

I always use NAT on virtual box, particularly when I am changing locations.

I believe some networks might be restrictive to have the physical network interfaces on the underlying hosts to be available for bridge networks.

Regards.