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One of the things that may have struck you as inconvenient in the above
example is that you had to establish the connection manually before you
could fire up pppd. Unlike dip, pppd does not have
its own scripting language for dialing the remote system and logging in,
but rather relies on some external program or shell script to do this.
The command to be executed can be given to pppd with the
connect command line option. pppd will redirect the
command's standard input and output to the serial line. One useful
program for this is expect, written by Don Libes. It has a very
powerful language based on Tcl, and was designed exactly for this sort
of application.
The pppd package comes along with a similar program called
chat, which lets you specify a UUCP-style chat script.
Basically, a chat script consists of an alternating sequence of
strings that we expect to receive from the remote system, and the
answers we are to send. We will call the expect and send strings,
respectively. This is a typical excerpt from a chat script;
ogin: b1ff ssword: s3kr3t
This tells chat to wait for the remote system to send the login
prompt, and return the login name b1ff. We only wait for
ogin: so that it doesn't matter if the login prompt starts with
an uppercase or lowercase l, or if it arrives garbled. The following
string is an expect-string again that makes chat wait for the
password prompt, and send our password in response.
This is basically all that chat scripts are about. A complete script to
dial up a PPP server would, of course, also have to include the appropriate
modem commands. Assume your modem understands the Hayes command set, and
the server's telephone number was 318714. The complete chat
invocation to establish a connection with c3po would then be
$ chat -v '' ATZ OK ATDT318714 CONNECT '' ogin: ppp word: GaGariN
By definition, the first string must be an expect string, but as the
modem won't say anything before we have kicked it, we make chat
skip the first expect by specifying an empty string. We go on and send
ATZ, the reset command for Hayes-compatible modems, and wait for
its response (OK). The next string sends the dial command along
with the phone number to chat, and expects the CONNECT
message in response. This is followed by an empty string again, because
we don't want to send anything now, but rather wait for the login
prompt. The remainder of the chat script works exactly as described
above.
The -v option makes chat log all activities to the
syslog daemon's local2 facility.
Specifying the chat script on the command line bears a certain risk,
because users can view a process' command line with the ps
command. You can avoid this by putting the chat script in a file, say
dial-c3po. You make chat read the script from the file
instead of the command line by giving it the -f option, followed
by the file name. The complete pppd incantation would now look like
this:
# pppd connect "chat -f dial-c3po" /dev/cua3 38400 -detach \
crtscts modem defaultroute
Beside the connect option that specifies the dial-up script,
we have added two more options to the command line: -detach,
which tells pppd not to detach from the console and become a
background process. The modem keyword makes it perform some
modem-specific actions on the serial device, like hanging up the line
before and after the call. If you don't use this keyword, pppd
will not monitor the port's DCD line, and will therefore not detect if
the remote end hangs up unexpectedly.
The examples shown above were rather simple; chat allows for much
more complex chat scripts. One very useful feature is the ability to
specify strings on which to abort the chat with an error. Typical abort
strings are messages like BUSY, or NO CARRIER, that your
modem usually generates when the called number is busy, or doesn't pick
up the phone. To make chat recognize these immediately, rather
than timing out, you can specify them at the beginning of the script
using the ABORT keyword:
$ chat -v ABORT BUSY ABORT 'NO CARRIER' '' ATZ OK ...
In a similar fashion, you may change the timeout value for parts of the
chat scripts by inserting TIMEOUT options. For details, please
check the chat(8) manual page.
Sometimes, you'd also want to have some sort of conditional execution
of parts of the chat script. For instance, when you don't receive the
remote end's login prompt, you might want to send a BREAK, or a carriage
return. You can achieve this by appending a sub-script to an expect
string. It consists of a sequence of send- and expect-strings, just
like the overall script itself, which are separated by hyphens. The
sub-script is executed whenever the expected string they are appended to
is not received in time. In the example above, we would modify the chat
script as follows:
ogin:-BREAK-ogin: ppp ssword: GaGariN
Now, when chat doesn't see the remote system send the login
prompt, the sub-script is executed by first sending a BREAK, and then
waiting for the login prompt again. If the prompt now appears, the
script continues as usual, otherwise it will terminate with an error.
Next: Debugging Your PPP Setup
Up: The Point-to-Point Protocol
Previous: Using Options Files
Andrew Anderson
Thu Mar 7 23:22:06 EST 1996
