Multihomed Hosts

To connect to a TCP/IP network, a computer must have at least one network interface, and each network interface must have its own unique IP address. The IP address that you give to a host is assigned to its network interface, sometimes referred to as the primary network interface.

If you add a second network interface to a machine, it must have its own unique IP number. Adding a second network interface changes the function of a machine from a host to a router. If you add a second network interface to a host and disable routing, the host is then considered 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 do not need to maintain routing tables.
Database servers can have multiple network interfaces for the same reason as NFS servers; that is, 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 by default 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

Become superuser on the prospective multihomed host and do the following:

Create an /etc/hostname.interface file for each additional network interface installed in the machine.
Type:
% touch /etc/notrouter
This creates an empty file called /etc/notrouter.
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. The machine is now a multihomed host.

Refer to the Solaris TCP/IP and Data Communications Administration Guide for more information about configuring multihomed hosts.