Linux Network Administrators Guide
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Chapter 8. The Point-to-Point Protocol
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8.4. Using chat to Automate Dialing
One of the things that may have struck you as inconvenient in the previous 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 relies on an 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.
The pppd software package is supplied with a very simple program called chat , which is capable of being used in this way to automate simple login sequences. We'll talk about this command in some detail.
If your login sequence is complex, you will need something more powerful than chat . One useful alternative you might consider is expect , written by Don Libes. It has a very powerful language based on Tcl, and was designed exactly for this sort of application. Those of you whose login sequence requires, for example, challenge/response authentication involving calculator-like key generators will find expect powerful enough to handle the task. Since there are so many possible variations on this theme, we won't describe how to develop an appropriate expect script in this book. Suffice it to say, you'd call your expect script by specifying its name using the pppd connect option. It's also important to note that when the script is running, the standard input and output will be attached to the modem, not to the terminal that invoked pppd . If you require user interaction, you should manage it by opening a spare virtual terminal, or arrange some other means.
The chat command 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 them expect and send strings, respectively. This is a typical excerpt from a chat script:
ogin: b1ff ssword: s3|<r1t
This script tells chat to wait for the remote system to send the login prompt and return the login name b1ff . We wait only 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 another expect string that makes chat wait for the password prompt and send our response password.
This is basically what chat scripts are all about. A complete script to dial up a PPP server would, of course, also have to include the appropriate modem commands. Assume that your modem understands the Hayes command set, and the server's telephone number is 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 then 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 previously. This description probably looks a bit confusing, but we'll see in a moment that there is a way to make chat scripts a lot easier to understand.
The –v option makes chat log all activities to the syslog daemon local2
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