Chat Scripts

Some legacy servers can require company-specific chat scripts for logging onto the dial-up servers.

A chat script, like other types of scripts, automates the act of typing commands using a keyboard. It consists of commands and responses, made up of groups of expect-response pairs as well as additional control commands, used by the chat script interpreter on the TELE3 SP. The TELE3 SP uses a default chat script that works with most ISPs, but your ISP may require a chat script with specific commands to “chat” with their server. If an ISP requires a specific chat script, it is typically provided to you with your dial-up access information. The default chat script for the TELE3 SP has the following commands:

The first three commands direct the chat script interpreter to abort if any of the strings NO CARRIER, NO DIALTONE, or BUSY are received from the modem.

The next five commands are AT commands that tell the chat interpreter to wait for nothing as
" defines an empty string, and configure the following on the modem: return command responses, don't echo characters, report the connecting baud rate when connected, and return verbose responses.

The next line has OK as the expected string, and the interpreters waits for OK to be returned in response to the previous command, ATV1, before continuing the script. If OK is not returned within the default time period of 50 seconds, the chat interpreter aborts the script and the connection fails. If OK is received, the prefix and phone number of the selected dial-up account is dialled. The \T command is replaced by chat script interpreter with the prefix and phone number of the dial-up account.

In the last line of the script, CONNECT is the expected response from the remote modem. If the modems successfully connect, CONNECT is returned from the TELE3 SP modem.The \D adds a pause of one second to allow the server to start the PPP authentication. The \C command ends the chat script end without sending a carriage return to the modem. The TELE3 SP then attempts to establish a PPP (Point-to-Point Protocol) connection over the serial link. The PPP connection usually includes authentication of the user by using PAP (Password Authentication Protocol) or CHAP (Challenge Handshake Authentication Protocol) from the PPP suite. Once a PPP connection is established, it looks like any other network interface.

Custom Chat Scripts

Custom chat scripts can be used when the ISP dial-up server does not use PAP or CHAP as an authentication protocol to control access. Instead, the ISP requires a user to log onto the dial-up server by prompting for a user name and password before establishing the PPP connection. For the most part, this type of server is part of the legacy systems rooted in the dumb terminal login architecture. Because these types of servers can prompt for a user name and password in a variety of ways or require subsequent commands to initiate the PPP connection, a Chat Script field is provided for you to enter a custom script.

If a custom chat script is required by an ISP for establishing a connection, it is commonly found on their web site or provided with their dial-up access information. Sometimes the scripts can be found by using a search engine on the Internet and using the keywords, “chat script ppp Linux <ISP name>”.

A custom chat script can look like the following script:

TIP: The first character of username and password are ignored during PPP authentication.

The script looks a lot like the previous script with the exception of the commands at the end. There is an empty string (") after CONNECT which sends a carriage return command to the server. The chat interpreter then waits for sername: substring. When a response is returned, the current PPP account user name, substituting the \L command control string, is sent. Then, the chat interpreter waits for the substring assword:, and sends the password, substituting \P with the PPP account password. If either the sername or assword substring are not received within the timeout period, the chat interpreter aborts the dial-up process resulting in a dial-up failure.