If I understand what you're trying to do, you need to install a couple of things:
sudo apt-get install dhcp3-server resolvconf #also possibly 'radvd'
Enabling traffic through you machine (IPv4):
Ok, now let's change a setting:
sudo nano /etc/sysctl.conf
Delete the # from the start of the line #net.ipv4.ip_forward=1 and press Ctrl x, then y to save.
Run sudo sysctl -p.
Set address for LAN interface (IPv4):
Unless you are 'Computer Sciences Corporation', you should not be using 20.x.x.x addresses. You could use any of the following private addresses ranges instead: 10.x.x.x, 192.168.x.x, or the range 172.16.0.0 – 172.31.255.255 (aka 172.16.0.0/12 or 172.16.0.0 with netmask 255.240.0.0). Run
sudo nano /etc/network/interfaces
Delete the gateway line. This computer is the gateway for the connection, so you do not set a gateway for this interface. Other computers will have this computer's address as their gateway. Change the address line to address 10.0.0.1. Change the broadcast line to broadcast 10.0.0.255. Press Ctrl x, then y to save.
Set DNS server (IPv4 and/or IPv6):
Run
sudo nano /etc/resolvconf/resolv.conf.d/head
(ignore the scary warning. /etc/resolv.conf is autogenerated, so the warning is in the /etc/resolvconf/resolv.conf.d/head file so it will get put at the top of /etc/resolv.conf when /etc/resolv.conf is generated.) To the end of the file, add
nameserver <ip_of_nameserver>
Press Ctrl x and answer yes to saving the file. To finish up, regenerate /etc/resolv.conf so the changes are applied right now:
sudo resolvconf -u
Set up NAT (IPv4)(shudder):
Enable UFW:
sudo ufw enable
Make a persistent NAT rule that is restored at boot by UFW:
sudo nano /etc/ufw/before.rules
Add the line -A POSTROUTING -o ppp0 -j MASQUERADE after the line *nat. Press Ctrl x, then y to save.
Give out addresses (IPv4):
Let's edit some files:
sudo nano /etc/dhcp3/dhcpd.conf /etc/default/dhcp3-server
To the first file, add the following:
subnet 10.0.0.0 netmask 255.255.255.0 {
range 10.0.0.2 10.0.0.254;
option routers 10.0.0.1;
option domain-name-servers <dns_server>;
}
Press Ctrl x, then y to save. In the next file, edit the line INTERFACES="" to say INTERFACES="eth1". Press Ctrl x, then y to save. To start the address handing out daemon, run
sudo /etc/init.d/dhcp3-server start
