Linux Network Administrators Guide
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Chapter 7. Serial Line IP
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7.3. Dealing with Private IP Networks
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.
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