Asterisk cmd ExternalIVR

Asterisk External IVR Interface

If you load app_externalivr.so in your Asterisk instance, you will have an ExternalIVR() application available in your dialplan. This application implements a simple protocol for bidirectional communication with an external process, while simultaneous playing audio files to the connected channel (without interruption or blocking).

The arguments to ExternalIVR() consist of the command to execute and any arguments to pass to it, the same as the Asterisk cmd System application accepts. The external command will be executed in a child process, with its standard file handles connected to the Asterisk process as follows:

stdin (0)
DTMF and hangup events will be received on this handle
stdout (1)
Playback and hangup commands can be sent on this handle
stderr (2)
Error messages can be sent on this handle

The application will also create an audio generator to play audio to the channel, and will start playing silence. When your application wants to send audio to the channel, it can send a command (see below) to add file(s) to the generator's playlist. The generator will then work its way through the list, playing each file in turn until it either runs out of files to play, the channel is hung up, or a command is received to clear the list and start with a new file. At any time, more files can be added to the list and the generator will play them in sequence.

While the generator is playing audio (or silence), any DTMF events received on the channel will be sent to the child process (see below). Note that this can happen at any time, since the generator, the child process and the channel thread are all executing independently. It is very important that your external application be ready to receive events from Asterisk at all times (without blocking), or you could cause the channel to become non-responsive.

If the child process dies, ExternalIVR() will notice this and hang up the channel immediately (and also send a message to the log).

DTMF (and other) events

All events will be newline-terminated strings.

Events send to the child's stdin will be in the following format:


The tag can be one of the following characters:

DTMF event for keys 0 through 9
DTMF event for keys A through D
DTMF event for key *
DTMF event for key #
the channel was hung up by the connected party
the previous command was unable to be executed (file does not exist, etc.)
the play list was interrupted (see below)
a file was dropped from the play list due to interruption (the data element will be the dropped file name)
a file has finished playing (the data element will be the file name)

The timestamp will be 10 digits long, and will be a decimal representation of a standard Unix epoch-based timestamp.


All commands must be newline-terminated strings.

The child process can send commands on stdout in the following formats:


The 'S' command checks to see if there is a playable audio file with the specified name, and if so, clear's the generator's playlist and places the file onto the list. Note that the playability check does not take into account transcoding requirements, so it is possible for the file to not be played even though it was found. If the file cannot be found, a 'Z' event (see above) will be sent to the child. If the generator is not currently playing silence, then T and D events will be sent to the child to signal the playlist interruption and notify it of the files that will not be played.

The 'A' command checks to see if there is a playable audio file with the specified name, and if so, adds it to the generator's playlist. The same playability and exception rules apply as for the 'S' command.

The 'H' command stops the generator and hangs up the channel, and logs the supplied message to the Asterisk log.

The 'O' command allows the child to set/clear options in the ExternalIVR() application. The supported options are:

Automatically interrupt and clear the playlist upon reception of DTMF input.
Do not automatically interrupt and clear the playlist upon reception of DTMF input.


Any newline-terminated output generated by the child process on its stderr handle will be copied into the Asterisk log.


Asterisk | Applications | Functions | Variables | Expressions | Asterisk FAQ

Created by: digitalcon, Last modification: Tue 30 of Mar, 2010 (21:55 UTC) by JustRumours
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