Defining the Conversation on the Dial-up Link

The dial-out machine and its remote peer communicate across the PPP link by negotiating and exchanging various instructions. When configuring a dial-out machine, you need to determine what instructions are required by the local and remote modems. Then you create a file that is called a chat script that contains these instructions. This section discusses information about configuring modems and creating chat scripts.

Contents of the Chat Script

Each remote peer that the dial-out machine needs to connect to probably requires its own chat script.

The contents of the chat script are determined by the requirements of your modem model or ISDN TA, and the remote peer. These contents appear as a set of expect-send strings. The dial-out machine and its remote peers exchange the strings as part of the communications initiation process.

An expect string contains characters that the dial-out host machine expects to receive from the remote peer to initiate conversation. A send string contains characters that the dial-out machine sends to the remote peer after receiving the expect string.

Information in the chat script usually includes the following:

• Modem commands, often referred to as AT commands, which enable the modem to transmit data over the telephone

• Phone number of the target peer

  This phone number might be the number that is required by your ISP, or a dial-in server at a corporate site, or an individual machine.

• Time-out value, if required

• Login sequence that is expected from the remote peer

• Login sequence that is sent by the dial-out machine

Chat Script Examples

This section contains chat scripts that you can use as a reference for creating your own chat scripts. The modem manufacturer's guide and information from your ISP and other target hosts contain chat requirements for the modem and your target peers. In addition, numerous PPP web sites have sample chat scripts.

Basic Modem Chat Script

The following is a basic chat script that you can use as a template for creating your own chat scripts.

ABORT   BUSY
ABORT   'NO CARRIER'
REPORT  CONNECT
TIMEOUT 10
"" AT&F1M0&M5S2=255
SAY     "Calling myserver\n"
TIMEOUT 60
OK      "ATDT1-123-555-1212"
ogin: pppuser
ssword: \q\U
% pppd

The next table describes the contents of the chat script.

/etc/ppp/myisp-chat.tmpl Chat Script Template

This release includes the /etc/ppp/myisp-chat.tmpl, which you can modify for use at your site. /etc/ppp/myisp-chat.tmpl is similar to the basic modem chat script except that the template does not include a login sequence.

ABORT   BUSY
ABORT   'NO CARRIER'
REPORT  CONNECT
TIMEOUT 10
""      "AT&F1"
OK      "AT&C1&D2"
SAY     "Calling myisp\n"
TIMEOUT 60
OK      "ATDT1-123-555-1212"
CONNECT \c

Modem Chat Script for Calling an ISP

Use the next chat script as a template for calling an ISP from a dial-out machine with a U.S. Robotics Courier modem.

ABORT   BUSY
ABORT   'NO CARRIER'
REPORT  CONNECT
TIMEOUT 10
"" AT&F1M0&M5S2=255
SAY     "Calling myisp\n"
TIMEOUT 60
OK      "ATDT1-123-555-1212"
CONNECT \c
\r \d\c
SAY "Connected; running PPP\n"

The following table describes the contents of the chat script.

Basic Chat Script Enhanced for a UNIX-Style Login

The next chat script is a basic script that is enhanced for calling a remote Oracle Solaris peer or other UNIX-type peer. This chat script is used in How to Create the Instructions for Calling a Peer.

        SAY "Calling the peer\n"
        TIMEOUT 10
        ABORT BUSY
        ABORT 'NO CARRIER'
        ABORT ERROR
        REPORT CONNECT
        "" AT&F1&M5S2=255
        TIMEOUT 60
        OK ATDT1-123-555-1234
        CONNECT \c
        SAY "Connected; logging in.\n"
        TIMEOUT 5
        ogin:--ogin: pppuser
        TIMEOUT 20
        ABORT 'ogin incorrect'
        ssword: \qmypassword
        "% " \c
        SAY "Logged in.  Starting PPP on peer system.\n" 
        ABORT 'not found'
        "" "exec pppd"
        ~ \c

The following table explains the parameters of the chat script.

Starting PPP right after the CONNECT \c is often called a PAP login by ISPs, though the PAP login is actually not part of PAP authentication.

The phrase ogin:--ogin: pppuser instructs the modem to send the user name pppuser in response to the login prompt from the dial-in server. pppuser is a special PPP user account name that was created for remote user1 on the dial-in server. For instructions about creating PPP user accounts on a dial-in server, refer to How to Configure Users of the Dial-in Server.

Chat Script for External ISDN TA

The following chat script is for calling from a dial-out machine with a ZyXEL omni.net. ISDN TA.

        SAY "Calling the peer\n"
        TIMEOUT 10
        ABORT BUSY
        ABORT 'NO CARRIER'
        ABORT ERROR
        REPORT CONNECT
        "" AT&FB40S83.7=1&K44&J3X7S61.3=1S0=0S2=255
        OK ATDI18882638234
        CONNECT \c
        \r \d\c
        SAY "Connected; running PPP\n"

The following table explains the parameters of the chat script.

Refer to the chat(1M) man page for descriptions of options and other detailed information about the chat script. For an explanation of expect-send strings, refer to Chat-Script Field in /etc/uucp/Systems File.

For More Chat Script Examples

A number of web sites offer sample chat scripts and assistance in creating the chat scripts. For example, see http://ppp.samba.org/ppp/index.html.

Invoking the Chat Script

You call chat scripts by using the connect option. You can use connect "chat ..." in any PPP configuration file or on the command line.

Chat scripts are not executable, but the program that is invoked by connect must be executable. You might use the chat utility as the program to be invoked by connect. In this instance, if you store the chat script in an external file through the -f option, then your chat script file is not executable.

The chat program that is described in chat(1m) executes the actual chat script. The pppd daemon invokes the chat program whenever pppd encounters the connect "chat ..." option.

How to Invoke a Chat Script (Task)

1. Create the chat script as an ASCII file.
2. Invoke the chat script in any PPP configuration file by using the following syntax:

   connect 'chat  -f /etc/ppp/chatfile'

   The -f flag indicates that a file name is to follow. /etc/ppp/chatfile represents the name of the chat file.

3. Give read permission for the external chat file to the user who runs the pppd command.

   ---

   Caution - The chat program always runs with the user's privileges, even if the connect 'chat ...' option is invoked from a privileged source. Thus, a separate chat file that is read with the -f option must be readable by the invoking user. This privilege can be a security problem if the chat script contains passwords or other sensitive information.

   ---

Example 8-1 Inline Chat Script

You can place the entire chat script conversation on a single line, similar to the following:

connect 'chat "" "AT&F1" OK ATDT5551212 CONNECT "\c"'

The complete chat script follows the chat keyword. The script terminates with "\c"'. You use this form in any PPP configuration file or on the command line as an argument to pppd.

Chat Script in an External File

If the chat script that is needed for a particular peer is long or complicated, consider creating the script as a separate file. External chat files are easy to maintain and to document. You can add comments to the chat file by preceding the comments with the hash (#) sign.

The procedure How to Create the Instructions for Calling a Peer shows the use of a chat script that is contained in an external file.

Creating a Chat File That Is Executable

You can create a chat file that is an executable script to be run automatically when the dial-up link is initiated. Thus, you can run additional commands during link initiation, such as stty for parity settings, besides the commands that are contained in a traditional chat script.

This executable chat script logs in to an old-style UNIX system that requires 7 bits with even parity. The system then changes to 8 bits with no parity when running PPP.

#!/bin/sh
chat "" "AT&F1" OK "ATDT555-1212" CONNECT "\c"
stty evenp
chat ogin: pppuser ssword: "\q\U" % "exec pppd"
stty -evenp

How to Create an Executable Chat Program

1. Use your text editor to create an executable chat program, such as the previous example.
2. Make the chat program executable.

   # chmod +x /etc/ppp/chatprogram

3. Invoke the chat program.

   connect /etc/ppp/chatprogram

   Chat programs do not have to be located within the /etc/ppp file system. You can store chat programs in any location.