===== IP Forwarding =====

To enable a machine to act as a gateway for a network:

  echo "1" > /proc/sys/net/ipv4/ip_forward
  iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -j MASQUERADE

Where 192.168.1.0/24 is the network which will use this machine as a gateway.

To make this permanent, on Debian/Ubuntu:

  touch /etc/network/if-up.d/iptables
  chmod 755 /etc/network/if-up.d/iptables

And add the following to the new file:

  #!/bin/bash
  
  echo "1" > /proc/sys/net/ipv4/ip_forward
  iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -j MASQUERADE
  
For Red Hat/Fedora:

  iptables-save

and edit /etc/sysctl.conf, changing the line ''net.ipv4.ip_forward=0'' to:

  net.ipv4.ip_forward=1

Then running sysctl -p to make sysctl restart with the new values.

Good IP Tables Tutorial:
  * [[http://iptables-tutorial.frozentux.net/iptables-tutorial.html]]
  * [[http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch14_:_Linux_Firewalls_Using_iptables]]