Link two Linux system via Serial Port

Why would you want to link two Linux systems via a serial port when a network connection is so fast and easy to configure? The primary purpose is to provide a secure way to maintain your firewall. This project shows you how to hook up two Linux systems via the serial port using point to point protocol (PTPP). The advantage is the ability to establish multiple sessions via the one serial line.

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Hardware

The only special thing here is the cable.

2 Linux systems that need to be connected.
A Null Modem cable. This swaps the transmit and receive lines and appropriate control lines so the two boxes can communicate. You may find that a standard modem cable and a cross-over switchbox is a better option.

Software

You will need the following software:

Linux installed on both boxes.
RPMs can be downloaded from rpmfind.net.
You will need the UUCP package installed so that the 'cu' command is available (at least on the client machine for testing).
See the code section for sample scripts.

Ideas

The following ideas may inspire creative use of this project:

If you must host a web server for a customer, you may only have access via dialup or serial port connection. Connecting your LAN to their network may be unacceptable.
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Tell me about your implementation.

Notes

You can test connection to the other system using 'cu -l ttyS0', enter Linux commands and use '~.[ENTER]' to get back your local command prompt.
You should remove any network connections otherwise things may not work correctly.
Serial port A is /dev/ttyS0.
Serial port B is /dev/ttyS1.

Code

The following configuration should be applied to each PC.

Plug in the null modem cable.

On the client PC (ie PC1), type:
slattach -p slip -s 115200 /dev/ttyS1 &
ifconfig sl0 192.168.0.1 pointopoint 192.168.0.2 up
route add -host 192.168.0.2 dev sl0

On the server PC (ie PC2), type:
slattach -p slip -s 115200 /dev/ttyS0 &
ifconfig sl0 192.168.0.2 pointopoint 192.168.0.1 up
route add -host 192.168.0.1 dev sl0

Now test using the following commands on the client:
ping 192.168.0.2
telnet 192.168.0.2

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