To allow the icct-net, jupiter-net and saturn-net sub-nets to reach each other they need to have a gateway router defined at each join point. For hosts you have already provided for this by having set the default gateway to the cluster node.

Before you can actually reach the other sub-nets, and the Internet, you still have to configure the cluster servers. These have two interfaces, one in each network that they talk to. These are set up independently to talk to their networks. Before they can perform routing between the two networks, they have to be set up to forward IP packets to one another. By default, this feature is turned off. To turn IP forwarding on, you have to enable IP forwarding in the Linux kernel. Most firewall and routing HOWTOs instruct you to do this by running the following command as root [Rankin, 2005¹⁾, Hubert et al., 2003²⁾]. <cli> root@myhost:~# echo 1 > /proc/sys/net/ipv4/ip_forward </cli>

However, under Ubuntu, you must change that command so that it works under the sudo environment by typing: <cli> icct@myhost:~$ sudo sh -c “echo 1 > /proc/sys/net/ipv4/ip_forward” </cli>

However, this only works until the next boot. To make this change permanent edit the file /etc/network/options and change the line which states ip_forward=no to ip_forward=yes. When this is done it will look like this:

# /etc/network/options - for ICCT network
# set ip_forward=yes if you need to set
# up a machine with two network interface cards
# as a router
ip_forward=yes
spoofprotect=yes
syncookies=no

You may need to reboot to turn this setting on. To reboot, use: <cli> icct@myhost:~$ sudo /sbin/shutdown -r now </cli>

Once IP forwarding is turned on, the interfaces are set up using route add to match the aliases defined in /etc/hosts and Table 1 in our configuration.

You should assign the addresses jupiter-if1 and saturn-if1 to eth0 on both cluster servers. The if1 interface will be serve as the gateway from icct-net to the subnet. Similarly the addresses of jupiter-if2 and saturn-if2 will be assigned to eth1. You will note from the settings in the /etc/hosts file that jupiter-if2 and saturn-if2 are defined as aliases for jupiter and saturn respectively and that jupiter and saturn have already been set as the gateways for the hosts on their respective sub-nets.

To set up the interfaces, we only need to tell jupiter-if1 and saturn-if1 that they can route to icct-net, and jupiter-if2, and saturn-if2 that they are part of jupiter-net and saturn-net respectively. We also need to tell saturn-if1 that it can route to jupiter-net via gateway gw-jupiter, and jupiter-if1 that it can route to saturn-net via gateway gw-saturn. Finally, both interfaces are told that solaris is the default gateway so that any IP packets not intended for the 192.168.0.0 network will be routed via the Linksys gateway router.

To illustrate this in more concrete terms, this is what the settings should look like in /etc/networks/interfaces on jupiter:

  # Interface in icct-net does the routing,
  # so needs to know about the rest of the network.
  
  auto eth0
  iface eth0 inet static
    address 192.168.1.3
    netmask 255.255.255.0
    gateway 192.168.1.1 # default for jupiter is solaris
    # route to saturn-net
    up   route add -net 192.168.3.0 netmask 255.255.255.0 gw 192.168.1.4
    down route del -net 192.168.3.0 netmask 255.255.255.0 gw 192.168.1.4
  
  # Interface in jupiter-net is much simpler
  auto eth0
  iface eth1 inet static
    address 192.168.2.1
    netmask 255.255.255.0

The settings are similar on saturn, just adjust the IP addresses according to Table 2.

Once the cluster gateway routers are configured, and networking is restarted, you should be able to ping any host (including the Internet gateway router and the Internet) from any host on the ICCT network.

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