Implementation
Our needs were extremely simple: a routing system that would connect one remote site to a shared Internet router that spoke RIP on our end. The site had one LAN to start with, but would probably be adding a few more.
We decided to use Linux because it was inexpensive enough for a pilot project: it would cost us two PCs plus labor. Also, we would be able to have one machine handle routing services, mail services, limited FTP services, and name services. This saved us the startup costs of a dedicated router plus a UNIX or dedicated DOS box to manage DNS and POP. Our goal, and current implementation, is shown in Figure 1.
We built the first machine out of a surplus (and highly generic) 80486 motherboard with 8Mb of RAM and a 120Mb IDE hard drive. We purchased and installed the Slackware distribution of Linux off a borrowed NEC CD-ROM, making sure to install kernel source code, the PPP daemon, and the various user-level services we wanted to run.
The first thing to do was to modify the kernel to act as a packet forwarder. Slackware came with many excellent precompiled kernels, but IP forwarding was not an option. Apparently, some random RFC specifies that IP forwarding is not to be turned on by default. This actually makes sense: why spend the processor cycles dealing with it if you don’t need to? So, compiling a kernel ourselves was our only option. However, it’s not the only option you have: if you want a precompiled Linux 1.2.0 kernel with IP forwarding, ethernet, and token-ring support, get it from
ftp://moria.co.chatham.ga.us/pub/ vmlinuz.tr.eth.ipfwd
At first, the notion of compiling a kernel seemed daunting. Before this, we’d only re-linked OEM UNIX kernels. It turned out to be much easier than expected; the processor and hard drive were the only things that had to work hard.
The directory /usr/src/linux is, by convention, a symbolic link to the current production version of Linux running on your machine. Finding it is easy: The README file contained therein is fairly short and to the point, and describes how to configure the kernel and install it.
To make a kernel, we typed:
# cd /usr/src/linux
# make config
...
*
* Networking options
*
TCP/IP networking (CONFIG_INET) [y]
IP forwarding/gatewaying (CONFIG_IP_FORWARD) [n] y
# make dep ; make clean
# make zImage
Then, before installing the kernel, in addition to backing up the current kernel, we also made a boot disk, just in case. After putting a floppy in the A: drive (for you DOS types), we typed:
# dd if=/vmlinuz of=/dev/fd0
# cp /vmlinuz /vmlinuz.safe
# mv /usr/src/linux/arch/i386/boot/zImage /vmlinuz
# lilo
Added linux
# sync ; reboot