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by Graham Williams
Duck Duck Go


IP Forwarding: Home Networks

Debian Packages: iptables

Consider a home network of GNU/Linux and MS/Windows machines, connected via ethernet. A GNU/Linux machine can connect to the Internet via PPP. We want to provide network access from all machines. We use iptables (for kernel version 2.4).

Suppose modern ([*]) is the host which will connect to the Internet using PPP. After installing iptables do the following on this host which will serve as the Internet gateway:

  # iptables --flush
  # iptables --table nat --flush
  # iptables --delete-chain
  # iptables --table nat --delete-chain
  # iptables --table nat --append POSTROUTING --out-interface ppp0 -j MASQUERADE
  # iptables --append FORWARD --in-interface eth0 -j ACCEPT

This clears the rules for filtering and then adds a rule to provide the IP forwarding. Now we need to turn it on for the kernel:

  # echo 1 > /proc/sys/net/ipv4/ip_forward

And that's it! This host, modern ([*]) will now act as a gateway to the Internet for your local machines.

There is some setup needed to have this survive a reboot. One approach is to do this through init.d, as explained in Section 35.2. The first step is to create a script file called /etc/init.d/myfirewall containing:

#! /bin/sh
# Set up a firewall for IP Masquerading

case "$1" in
    echo -n "Starting IP Masquerading: myfirewall"
    iptables --flush
    iptables --table nat --flush
    iptables --delete-chain
    iptables --table nat --delete-chain
    iptables --table nat --append POSTROUTING --out-interface ppp0 -j MASQUERADE
    iptables --append FORWARD --in-interface eth0 -j ACCEPT
    echo 1 > /proc/sys/net/ipv4/ip_forward
    echo "." 
    echo -n "Stopping IP Masquerading: myfirewall"
    echo 0 > /proc/sys/net/ipv4/ip_forward
    echo "." 
    echo "Not implemented." 
    sh $0 stop
    sh $0 start
    echo "Usage: /etc/init.d/myfirewall {start|stop|restart|force-reload|reload}"
    exit 1

exit 0

Then the firewall can be turned on and off with:

  $ wajig start myfirewall
  $ wajig stop myfirewall

To have it started at boot and stopped at shutdown:

  # update-rc.d myfirewall start 40 S . stop 89 0 6 .

This creates the following links:

   /etc/rc0.d/K89myfirewall -> ../init.d/myfirewall
   /etc/rc6.d/K89myfirewall -> ../init.d/myfirewall
   /etc/rcS.d/S40myfirewall -> ../init.d/myfirewall

Another approach is to only turn it on and off as a PPP connection is established. See the scripts in /usr/share/doc/iptables/examples for details.

Now rose ([*]) and inco (36.50), machines on the local home network, can have their network interface set up:

iface eth0 inet static
        gateway             (modern)

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