You will remember from Chapter 5, that the
Virtual Brewery has an Ethernet-based IP network using unregistered
network numbers that are reserved for internal use only. Packets to or
from one of these networks are not routed on the Internet; if we were
to have vlager dial into
cowslip and act as a router
for the Virtual Brewery network, hosts within the Brewery's network
could not talk to real Internet hosts directly because their packets
would be dropped silently by the first major router.
To work around this dilemma, we will configure vlager to act as a kind of launch pad for
accessing Internet services. To the outside world, it will present
itself as a normal SLIP-connected Internet host with a registered IP
address (probably assigned by the network provider running cowslip). Anyone logged in to vlager can use text-based programs like
ftp, telnet, or even
lynx to make use of the Internet. Anyone on the
Virtual Brewery LAN can therefore telnet and log in to vlager and use the programs there. For
some applications, there may be solutions that avoid logging in
to vlager. For WWW users, for
example, we could run a so-called proxy server on
vlager, which would relay all
requests from your users to their respective servers.
Having to log in
to vlager to make use of the
Internet is a little clumsy. But apart from eliminating the paperwork
(and cost) of registering an IP network, it has the added benefit of
going along well with a firewall setup. Firewalls are dedicated
hosts used to provide limited Internet access to users on your local
network without exposing the internal hosts to network attacks from
the outside world. Simple firewall configuration is covered in more
detail in Chapter 9. In Chapter 11, we'll discuss a Linux feature called
“IP masquerade” that provides a powerful alternative to
proxy servers.
Assume that the Brewery has been assigned the IP address 192.168.5.74 for SLIP access. All you
have to do to realize that the setup discussed above is to enter this
address into your /etc/hosts file, naming it
vlager-slip. The procedure
for bringing up the SLIP link itself remains unchanged.
