Installing and Administering PPP

Chapter 5 131
Security Techniques
Dial-Back
Dial-Back
PPP supports the ability to maintain a connection when calling a modem
that has a dial-back security feature. The Systems file chat script option
\M allows this by disabling delivery of SIGHUP to pppd. This signal
usually results from loss of Carrier Detect and tells pppd to abruptly
disconnect from the active session.
Dial-Back Process
Typically, an answering modem with dial-back capability responds to a
call by taking the following steps:
1. Challenges incoming callers with a prompt string.
2. Accepts the input identifying the caller.
3. Hangs up the call.
4. Calls a number associated with the caller’s identification.
5. Re-establishes a carrier.
The calling modem might then demand the same type of identification
before allowing remote data to flow through its serial interface to the
local system.
Blocking SIGHUP with Chat Script \M Option
The calling system’s pppd must be prepared for the temporary lack of a
Carrier Detect signal from its modem during the dial-back from the
remote modem. To avoid receiving a SIGHUP, pppd instructs the UNIX
system’s serial drivers not to deliver the signal. In other words it says,
“Temporarily treat the serial interface as if it were connected to a local
device like a terminal or printer, instead of a modem.” pppd does this by
specifying \M in the ‘send’ phase of a Systems chat script. See
ppp.Systems(4) for details on \M and \m chat script options.