|CHAT(8)||System Manager's Manual||CHAT(8)|
chat — automated
conversational script with a modem
chat program defines a conversational
exchange between the computer and the modem. Its primary purpose is to
establish a connection between the Point-to-Point Protocol Daemon
(pppd(8)) and the remote's pppd
ECHOkeyword. When echoing is enabled, all output from the modem is echoed to stderr.
REPORT, the resulting strings are written to this file. If this option is not used and you still use
REPORTkeywords, the stderr file is used for the report strings.
-Swill prevent both log messages from
-vand error messages from being logged via syslog(3).
-vand all error messages will be sent to stderr.
chatprogram to terminate with a non-zero error code.
chatprogram will then log all text received from the modem and the output strings sent to the modem to the stderr device. This device is usually the local console at the station running the
chator pppd(8) program.
chatprogram will then log the execution state of the chat script as well as all text received from the modem and the output strings sent to the modem. The default is to log through syslog(3) with level “info”, though this may be altered with the
-foption, then the script is included as parameters to the
The chat script defines the communications.
A script consists of one or more “expect-send” pairs of strings, separated by spaces, with an optional “subexpect-subsend” string pair, separated by a dash as in the following example:
ogin:-BREAK-ogin: ppp ssword: hello2u2
This line indicates that the
should expect the string “ogin:”. If it fails to receive a
login prompt within the time interval allotted, it is to send a break
sequence to the remote and then expect the string “ogin:”. If
the first “ogin:” is received then the break sequence is not
Once it receives the login prompt, the
chat program will send the string ppp and then
expect the prompt “ssword:”. When it receives the prompt for
the password, it will send the password hello2u2.
A carriage return is normally sent following the reply string. It is not expected in the “expect” string unless it is specifically requested by using the \r character sequence.
The expect sequence should contain only what is needed to identify the string. Since it is normally stored on a disk file, it should not contain variable information. It is generally not acceptable to look for time strings, network identification strings, or other variable pieces of data as an expect string.
To help correct for characters which may be corrupted during the initial sequence, look for the string “ogin:” rather than “login:”. It is possible that the leading “l” character may be received in error and you may never find the string even though it was sent by the system. For this reason, scripts look for “ogin:” rather than “login:” and “ssword:” rather than “password:”.
A very simple script might look like this:
ogin: ppp ssword: hello2u2
In other words, expect ....ogin:, send ppp, expect ...ssword:, send hello2u2.
In actual practice, simple scripts are rare. At the very least, you should include sub-expect sequences should the original string not be received. For example, consider the following script:
ogin:--ogin: ppp ssword: hello2u2
This would be a better script than the simple one used earlier. This would look for the same login: prompt. If one is not received, a single return sequence is sent and then it will look for login: again. Should line noise obscure the first login prompt then sending the empty line will usually generate a login prompt again.
Comments can be embedded in the chat script. A comment is a line
which starts with the ‘#’ (hash) character in column 1. Such
comment lines are just ignored by the
If a ‘#’ character is to be expected as the first character of
the expect sequence, you should quote the expect string. If you want to wait
for a prompt that starts with a ‘#’ (hash) character, you
would have to write something like this:
# Now wait for the prompt and send logout string ´# ' logout
Many modems will report the status of the call as a string. These strings may be CONNECT or NO CARRIER or BUSY. It is often desirable to terminate the script should the modem fail to connect to the remote. The difficulty is that a script would not know exactly which modem string it may receive. On one attempt it may receive BUSY, while the next time it may receive NO CARRIER.
These “abort” strings may be specified in the script
ABORT sequence. It is written in the
script as in the following example:
ABORT BUSY ABORT 'NO CARRIER' '' ATZ OK ATDT5551212 CONNECT
This sequence will expect nothing; and then send the string ATZ. The expected response to this is the string OK. When it receives OK, it sends the string ATDT5551212 to dial the telephone. The expected string is CONNECT. If the string CONNECT is received the remainder of the script is executed. However, should the modem find a busy telephone, it will send the string BUSY. This will cause the string to match the abort character sequence. The script will then fail because it found a match to the abort string. If it received the string NO CARRIER, it will abort for the same reason. Either string may be received. Either string will terminate the chat script.
This sequence allows for clearing previously set
are kept in an array of a pre-determined size (at compilation time);
CLR_ABORT will reclaim the space for cleared entries
so that new strings can use that space.
SAY directive allows the script to
send strings to the user at the terminal via standard error. If
chat is being run by
pppd(8), and pppd is running as a daemon
(detached from its controlling terminal), standard error will normally be
redirected to the file /etc/ppp/connect-errors.
SAY strings must be enclosed in single or
double quotes. If carriage return and line feed are needed in the string to
be output, you must explicitly add them to your string.
SAY strings could be used to give
progress messages in sections of the script where you want to have 'ECHO
OFF' but still let the user know what is happening. An example is:
ABORT BUSY ECHO OFF SAY "Dialling your ISP...\n" ´' ATDT5551212 TIMEOUT 120 SAY "Waiting up to 2 minutes for connection ... " CONNECT '' SAY "\nConnected, now logging in ...\n" ogin: account ssword: pass $ SAY "Logged in OK ...\n" etc ...
This sequence will only present the
strings to the user and all the details of the script will remain hidden.
For example, if the above script works, the user will see:
Dialling your ISP... Waiting up to 2 minutes for connection ... Connected, now logging in ... Logged in OK ...
A report string is similar to the
string. The difference is that the strings, and all characters to the next
control character such as a carriage return, are written to the report
The report strings may be used to isolate the transmission rate of
the modem's connect string and return the value to the
chat user. The analysis of the report string logic
occurs in conjunction with the other string processing such as looking for
the expect string. The use of the same string for a report and abort
sequence is probably not very useful; however, it is possible.
The report strings do not change the completion code of the program.
These “report” strings may be specified in the
script using the
REPORT sequence. It is written in
the script as in the following example:
REPORT CONNECT ABORT BUSY '' ATDT5551212 CONNECT '' ogin: account
This sequence will expect nothing; and then send the string ATDT5551212 to dial the telephone. The expected string is CONNECT. If the string CONNECT is received the remainder of the script is executed. In addition the program will write to the expect-file the string “CONNECT” plus any characters which follow it such as the connection rate.
This sequence allows for clearing previously set
strings are kept in an array of a pre-determined size (at compilation time);
CLR_REPORT will reclaim the space for cleared
entries so that new strings can use that space.
The echo options controls whether the output from the modem is
echoed to stderr. This option may be set with the
-e option, but it can also be controlled by the
ECHO keyword. The “expect-send” pair
ECHO ON enables echoing, and
OFF disables it. With this keyword you can select which parts of the
conversation should be visible. For instance, with the following script:
ABORT 'BUSY' ABORT 'NO CARRIER' '' ATZ OK\r\n ATD1234567 \r\n \c ECHO ON CONNECT \c ogin: account
all output resulting from modem configuration and dialing is not visible, but starting with the CONNECT (or BUSY) message, everything will be echoed.
HANGUP options control whether a modem
hangup should be considered as an error or not. This option is useful in
scripts for dialing systems which will hang up and call your system back.
HANGUP options can be
HANGUP is set
OFF and the modem hangs up (e.g., after the first
stage of logging in to a callback system),
continue running the script (e.g., waiting for the incoming call and
second-stage login prompt). As soon as the incoming call is connected, you
should use the
HANGUP ON directive to reinstall
normal hangup signal behavior. Here is an example script:
ABORT 'BUSY' '' ATZ OK\r\n ATD1234567 \r\n \c CONNECT \c ´Callback login:' call_back_ID HANGUP OFF ABORT "Bad Login" ´Callback Password:' Call_back_password TIMEOUT 120 CONNECT \c HANGUP ON ABORT "NO CARRIER" ogin:--BREAK--ogin: real_account etc ...
The initial timeout value is 45 seconds. This may be changed using
The following example illustrates how to change the timeout value for the next expect string:
ATZ OK ATDT5551212 CONNECT TIMEOUT 10 ogin:--ogin: TIMEOUT 5 assword: hello2u2
This will change the timeout to 10 seconds when it expects the login: prompt. The timeout is then changed to 5 seconds when it looks for the password prompt.
The timeout, once changed, remains in effect until it is changed again.
The special reply string of
chat program should send an EOT character
to the remote. This is normally the End-of-file character sequence. A return
character is not sent following the EOT. The EOT sequence may be embedded
into the send string using the sequence ^D.
The special reply string of
cause a break condition to be sent. The break is a special signal on the
transmitter. The normal processing on the receiver is to change the
transmission rate. It may be used to cycle through the available
transmission rates on the remote until you are able to receive a valid login
prompt. The break sequence may be embedded into the send string using the
The expect and reply strings may contain escape sequences. All of the sequences are legal in the reply string. Many are legal in the expect string. Those which are not valid in the expect sequence are so indicated.
BREAK. (Not valid in expect.)
chat program will terminate with the
following completion codes:
chatreceiving a signal such as
Using the termination code, it is possible to determine which event terminated the script. It is possible to decide if the string “BUSY” was received from the modem as opposed to “NO DIAL TONE”. While the first event may be retried, the second will probably have little chance of succeeding during a retry.
chat program is in the public domain.
This is not the GNU public license. If it breaks then you get to keep both
|July 25, 2011||HamBSD-current|