Hacking the Cable Modem: Building A Console Cable

The device shown in Figure 17-1 is an RS-232–to–TTL converter board, designed to allow a PC with a serial (RS-232) port to communicate with a device that has a console (TTL) port. External converters such as this are common, and you can purchase one from many online electronics stores. Or, with the right parts, you can build your own inexpensive RS-232–to–TTL converter, known as a console cable.

The Console Port

Many embedded devices (such as switches, routers, cable modems, and so on) have an internal communication port known as a console port. This type of port is typically used for configuring the device and issuing commands with root-level access. If the device is offline, this port can also be used to reconfigure the device locally. However, if it is online, other administration protocols can also be used, such as telnet or rlogin.

Figure 17-1: A professionally developed RS-232 console port

Many cable modems have a clandestine console port left over from debugging during the manufacturing process. This port can sometimes be utilized to access the device's bootloader program or operating system, allowing the user to change many of its internal settings (MAC address, serial number, and so on) or its firmware, and/or execute system commands. Because having the ability to communicate using this port may by itself be enough to hack a cable modem, it is important to know how to communicate using this type of port.

What Is TTL?

Transistor-Transistor Logic (TTL) is an interface often used to communicate between integrated circuits. If a cable modem has an unused console port, that port will most likely be accessible using a TTL-compatible interface. While your computer probably does not have ports that support TTL signals, you can build a port converter from scratch or purchase one from many electronics stores.

The easiest way to connect your computer to a TTL console port is with a serial (RS-232 or DB9) port on your computer. If your computer does not have such a serial port, you can purchase a USB-to-serial adapter for around $20. The cable modem's TTL port will not usually have a connector, so you will most likely have to build one and solder it in. Then, once your computer's serial port is connected to the modem through the RS-232 converter, you can communicate with it through the port using any terminal emulation software, such as HyperTerminal or EtherBoot.

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Hacking the Cable Modem: What Cable Companies Don't Want You to Know by DerEngel September 2006, 320 pp ISBN 9781593271015, $23.95 US order@oreilly.com 1-800-998-9938 1-707-827-7000

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