142 lines
6.4 KiB
HTML
142 lines
6.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
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<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
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<HTML>
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<HEAD>
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<TITLE>Dialing out with chat</TITLE>
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</HEAD>
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<BODY LANG="EN">
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<A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node113.html">Debugging Your PPP Setup</A>
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<B>Up:</B> <A HREF="node107.html">The Point-to-Point Protocol</A>
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<B> Previous:</B> <A HREF="node111.html">Using Options Files</A>
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<BR> <P>
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<H1><A NAME="SECTION0010500000">Dialing out with chat</A></H1>
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<A NAME="4177"></A>
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<A NAME="4178"></A>
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<A NAME="4179"></A>
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One of the things that may have struck you as inconvenient in the above
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example is that you had to establish the connection manually before you
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could fire up pppd. Unlike dip, pppd does not have
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its own scripting language for dialing the remote system and logging in,
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but rather relies on some external program or shell script to do this.
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The command to be executed can be given to pppd with the
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connect command line option. pppd will redirect the
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command's standard input and output to the serial line. One useful
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program for this is expect, written by Don Libes. It has a very
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powerful language based on Tcl, and was designed exactly for this sort
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of application.
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<P>
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<A NAME="4545"></A>
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<A NAME="4188"></A>
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The pppd package comes along with a similar program called
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chat, which lets you specify a UUCP-style chat script.
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Basically, a chat script consists of an alternating sequence of
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strings that we expect to receive from the remote system, and the
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answers we are to send. We will call the expect and send strings,
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respectively. This is a typical excerpt from a chat script;
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<Pre>
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ogin: b1ff ssword: s3kr3t
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</pre>
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This tells chat to wait for the remote system to send the login
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prompt, and return the login name b1ff. We only wait for
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ogin: so that it doesn't matter if the login prompt starts with
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an uppercase or lowercase l, or if it arrives garbled. The following
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string is an expect-string again that makes chat wait for the
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password prompt, and send our password in response.
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<P>
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This is basically all that chat scripts are about. A complete script to
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dial up a PPP server would, of course, also have to include the appropriate
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modem commands. Assume your modem understands the Hayes command set, and
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the server's telephone number was 318714. The complete chat
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invocation to establish a connection with c3po would then be
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<Pre>
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$ chat -v '' ATZ OK ATDT318714 CONNECT '' ogin: ppp word: GaGariN
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</Pre>
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By definition, the first string must be an expect string, but as the
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modem won't say anything before we have kicked it, we make chat
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skip the first expect by specifying an empty string. We go on and send
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ATZ, the reset command for Hayes-compatible modems, and wait for
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its response (OK). The next string sends the dial command along
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with the phone number to chat, and expects the CONNECT
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message in response. This is followed by an empty string again, because
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we don't want to send anything now, but rather wait for the login
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prompt. The remainder of the chat script works exactly as described
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above.
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<P>
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The -v option makes chat log all activities to the
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syslog daemon's local2 facility.<A HREF="footnode.html#4546"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif"></A>
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<P>
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<A NAME="4212"></A>
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Specifying the chat script on the command line bears a certain risk,
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because users can view a process' command line with the ps
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command. You can avoid this by putting the chat script in a file, say
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dial-c3po. You make chat read the script from the file
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instead of the command line by giving it the -f option, followed
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by the file name. The complete pppd incantation would now look like
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this:
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<Pre>
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# pppd connect "chat -f dial-c3po" /dev/cua3 38400 -detach \
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crtscts modem defaultroute
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</Pre>
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Beside the connect option that specifies the dial-up script,
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we have added two more options to the command line: -detach,
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which tells pppd not to detach from the console and become a
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background process. The modem keyword makes it perform some
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modem-specific actions on the serial device, like hanging up the line
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before and after the call. If you don't use this keyword, pppd
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will not monitor the port's DCD line, and will therefore not detect if
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the remote end hangs up unexpectedly.
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<P>
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The examples shown above were rather simple; chat allows for much
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more complex chat scripts. One very useful feature is the ability to
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specify strings on which to abort the chat with an error. Typical abort
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strings are messages like BUSY, or NO CARRIER, that your
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modem usually generates when the called number is busy, or doesn't pick
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up the phone. To make chat recognize these immediately, rather
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than timing out, you can specify them at the beginning of the script
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using the ABORT keyword:
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<Pre>
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$ chat -v ABORT BUSY ABORT 'NO CARRIER' '' ATZ OK ...
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</Pre>
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In a similar fashion, you may change the timeout value for parts of the
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chat scripts by inserting TIMEOUT options. For details, please
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check the chat(8) manual page.
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<P>
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Sometimes, you'd also want to have some sort of conditional execution
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of parts of the chat script. For instance, when you don't receive the
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remote end's login prompt, you might want to send a BREAK, or a carriage
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return. You can achieve this by appending a sub-script to an expect
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string. It consists of a sequence of send- and expect-strings, just
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like the overall script itself, which are separated by hyphens. The
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sub-script is executed whenever the expected string they are appended to
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is not received in time. In the example above, we would modify the chat
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script as follows:
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<Pre>
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ogin:-BREAK-ogin: ppp ssword: GaGariN
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</Pre>
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Now, when chat doesn't see the remote system send the login
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prompt, the sub-script is executed by first sending a BREAK, and then
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waiting for the login prompt again. If the prompt now appears, the
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script continues as usual, otherwise it will terminate with an error.
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<P>
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<A NAME="4237"></A>
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<A NAME="4547"></A>
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<P>
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<HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node113.html">Debugging Your PPP Setup</A>
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<B>Up:</B> <A HREF="node107.html">The Point-to-Point Protocol</A>
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<B> Previous:</B> <A HREF="node111.html">Using Options Files</A>
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<P><ADDRESS>
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<I>Andrew Anderson <BR>
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Thu Mar 7 23:22:06 EST 1996</I>
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</ADDRESS>
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</BODY>
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</HTML>
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