sysctl

This howto guide outlines the steps to enable IP forwarding on Ubuntu 13.04. This is required if you want your system to act as a router.
Check the current setting:

$ sysctl net.ipv4.ip_forward

This should output a 0 or a 1 depending on if it is already enabled or not.
eg: the following shows where ip forwarding is disabled

$ sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 0

Enable the kernel option for IP forwarding:

$ sudo sysctl net.ipv4.ip_forward=1

eg:

$ sudo sysctl net.ipv4.ip_forward=1
net.ipv4.ip_forward = 1

To make these changes perminant across reboots:
Edit the /etc/sysctl.conf file:

$ sudo nano /etc/sysctl.conf

Find the following text:

# Uncomment the next line to enable packet forwarding for IPv4
#net.ipv4.ip_forward=1

Uncomment the net.ipv4.ip_forward=1 line:

# Uncomment the next line to enable packet forwarding for IPv4
net.ipv4.ip_forward=1

Save and exit the file.
Run sysctl -p to make the changes take effect immediatly:

$ sudo sysctl -p

eg:

$ sudo sysctl -p
net.ipv4.ip_forward = 1

Your Ubuntui 13.04 system will now have IP forwarding enabled. Make sure your iptables rules are set up correctly to prevent possible abuse.