9.2. Obey Data Terminal Ready and Data Carrier Detect
The RS-232 Data Terminal Ready signal is
lowered when the computer wishes the modem to hang up. The
computer wishes to hang up when people have ended their login
session ends or when they fail to respond to the
login: prompt.
Using a modem cable that has DTR wired and a
modem that is configured to obey DTR is essential
to prevent denial of service attacks upon the access to the
console.
Without DTR a caller can simply hold the
modem line open, denying system administrators access to the
console.
The RS-232 Data Carrier Detect signal is
lowered when the user hangs up.
Using a modem cable that has DCD wired and
a modem that is configured to assert DCD is
essential to prevent people dialling in after a user has hang up
and from carrying on their session.
Without DCD the session is not cleared when
an accidental disconnection occurs. This allows any subsequent
caller to resume the previous session. The machine is totally
compromised if the previous user was root.