I am a big fan of Ubuntu’s Uncomplicated firewall because it is very simple to use.
I cannot remember if ufw is installed by default so to be sure
$ sudo apt-get install ufw
As part of my project to use Sonofon Mobile Broadband as backup for my ADSL I needed ufw to NAT the connections through the
ppp-connection.
The solution is pretty straight forward:
In the file /etc/default/ufw
change the parameter DEFAULT_FORWARD_POLICY
DEFAULT_FORWARD_POLICY="ACCEPT"
also configure /etc/ufw/sysctl.conf
to allow forwarding (the parameter is commented out by default)
net.ipv4.ip_forward=1
the final step is to add NAT to ufw’s configuration. Add the following to /etc/ufw/before.rules
just after the header
# nat Table rules
*nat
:POSTROUTING ACCEPT [0:0]
# Forward traffic through ppp0 - Change to match you out-interface
-A POSTROUTING -s 192.168.1.0/24 -o ppp0 -j MASQUERADE
# don't delete the 'COMMIT' line or these nat table rules won't
# be processed
COMMIT
Enable the changes
$ sudo ufw disable && sudo ufw enable