Practical UNIX and Internet Security, 3rd Edition

Additional Security for Modems

With today’s telephone systems, if you connect your computer’s modem to an outside telephone line, then anybody in the world can call it.

Although usernames and passwords provide a degree of security, they are not foolproof. Users often pick bad passwords, and even good passwords can occasionally be guessed or discovered by other means.

For this reason, a variety of special kinds of modems have been developed that further protect computers from unauthorized access. These modems are more expensive than traditional modems, but they do provide an added degree of security and trust.

Password modems

These modems require the caller to enter a password before the modem connects the caller to the computer. As with regular Unix passwords, the security provided by these modems can be defeated by repeated password guessing or if an authorized person releases his password to somebody who is not authorized. Usually, these modems can store only 1 to 10 passwords. The password stored in the modem should not be the same as the password of any user. Some versions of Unix can be set up to require special passwords for access by modem. Password modems are probably unnecessary on systems of this kind; the addition of yet another password may be more than your users are prepared to tolerate.

Callback setups

As we mentioned earlier in this chapter, these schemes require the caller to enter a username, and then immediately hang up the telephone line. The modem then will call ...