Practical UNIX and Internet Security, 3rd Edition

Modems and Unix

Unix can use modems both for placing calls (dialing out) and for receiving them (letting other people dial in).

Broadly speaking, there are four ways to initiate a modem call on a Unix system; several of these are considered archaic and obsolete, but are still widely available:

Calls can made with the user-level tip or cu commands

If you call a computer that’s running the Unix operating system, you may be able to use a simple file-transfer system built into tip and cu. Unfortunately, such a system performs no error checking or correction and works only for transferring text files.

Calls can be initiated with a terminal emulator

Terminal-emulation programs are designed to dial up remote systems and behave like a terminal; they also often support reliable file transfers with protocols designed for serial communication. kermit is a once popular terminal-emulation and file-transfer program developed by Columbia University. Versions of kermit are available for an astonishing variety of computer systems. Several other free software terminal emulators, such as minicom, are also commonly used.

Calls can be initiated with UUCP

UUCP (Unix-to-Unix Copy System) is a primitive system for transferring files and performing remote job execution. Although once popular, UUCP has been largely replaced by SLIP and PPP (see the next item).

There are many security issues that arise when using UUCP. Although we detailed those issues in the first and second editions of this book, we’ve removed ...