Creating a Multihomed Host

By default, TCP/IP considers any machine with multiple network interfaces to be a router. However, you can change a router into a multihomed host--a machine with more than one network interface that does not run routing protocols or forward IP packets. You typically configure the following types of machines as multihomed hosts:

• NFS servers, particularly large data centers, can be attached to more than one network in order to share files among a large pool of users. These servers don't need to maintain routing tables.

• Database servers can have multiple network interfaces for the same reason as NFS servers--to provide resources to a large pool of users.

• Firewall gateways are machines that provide the connection between a company's network and public networks such as the Internet. Administrators set up firewalls as a security measure. When configured as a firewall, the host will not pass packets between the networks attached to it. On the other hand, it can still provide standard TCP/IP services, such as ftp or rlogin, to authorized users.

Since TCP/IP considers any machine with multiple network interfaces to be a router, you need to perform a few operations to turn it into a multihomed host.

How to Create a Multihomed Host

1. Become superuser on the prospective multihomed host.

2. Create an /etc/hostname.interface file for each additional network interface installed in the machine.

3. Type:

   % touch /etc/notrouter

   This creates an empty file called /etc/notrouter.

4. Reboot the machine.

When the machine reboots, the startup script looks for the presence of the /etc/notrouter file. If the file exists, the startup script does not run in.routed -s or in.rdisc -r, and does not turn on IP forwarding on all interfaces configured "up" by ifconfig. This happens regardless of whether an /etc/gateways file exists. Thus the machine is now a multihomed host.