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[services] ; Special services such as conferencing could be defined here
When you define a channel (which is not done in the extensions.conf file), one of the
required parameters in each channel definition is context. The context is the point in
the dialplan where connections coming from that channel will arrive. So the way you
plug a channel into the dialplan is through the context (see Figure 6-2).
Figure 6-2. Relation between channel configuration (on the left) and contexts in the
dialplan (on the right)
This is one of the most important concepts to understand when
dealing with channels and dialplans. Troubleshooting call flow is
much easier once you understand the relationship between the
channel context (which tells the channel where to plug into the
dialplan) and the dialplan context (which is where we create the
call flow that happens when the call arrives).
An important (perhaps the most important) use of contexts is to provide privacy and
security. By using contexts correctly, you can give some channels access to features
2 The default context used to be a popular way to whip up simple configurations, but this proved to be some‐
what problematic for security. Best practice these days is to avoid all use of it.
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Chapter 6: Dialplan Basics
(such as long-distance calling) that aren’t made available to others. If you do not
design your dialplan carefully, you may inadvertently allow others to fraudulently use
your system. Please keep this in mind as you build your Asterisk system; there are
many bots on the internet that were specifically written to identify and exploit poorly
secured Asterisk systems.
The Asterisk wiki outlines several steps you should take to keep
your Asterisk system secure. It is vitally important that you read
and understand this page. If you ignore the security precautions
outlined there, you may end up allowing anyone and everyone to
make long-distance or toll calls at your expense!
If you don’t take the security of your Asterisk system seriously, you
may end up paying—literally. Please take the time and effort to
secure your system from toll fraud.
Extensions
In the telecommunications industry the word extension typically has referred to a
numeric identifier that, when dialed, will ring a phone (or system resource such as
voicemail or a queue). In Asterisk, an extension is far more powerful, as it defines the
unique series of steps (each step containing an application) through which Asterisk
will take that call.
Within each context, we can define as many (or few) extensions as required. When a
particular extension is triggered (by an incoming channel), Asterisk will follow the
steps defined for that extension. It is the extensions, therefore, that specify what hap‐
pens to calls as they make their way through the dialplan. Although extensions can, of
course, be used to specify phone extensions in the traditional sense (i.e., extension
153 will cause the SIP telephone set on John’s desk to ring), in an Asterisk dialplan,
they can be used for much more.
The syntax for an extension is the word exten, followed by an arrow formed by the
equals sign and the greater-than sign, like this:
exten =>
This is followed by the name (or number) of the extension.
When dealing with traditional telephone systems, we tend to think of extensions as
the numbers you would dial to make another phone ring. In Asterisk, extension
names can be any combination of numbers and letters. Over the course of this chap‐
ter and the next, we’ll use both numeric and alphanumeric extensions.
Dialplan Syntax
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Assigning names to extensions may seem like an unusual concept,
but when you realize that SIP supports dialing by all sorts of char‐
acter combinations (anything that is a valid URI, strictly speaking),
it makes perfect sense. This is one of the features that makes Aster‐
isk so flexible and powerful.
Each step in an extension has three components:
• The name (or number) of the extension
• The priority (each extension can include multiple steps; the step number is called
the “priority”)
• The application (or command) that will take place at that step
These three components are separated by commas, like this:
exten => name,priority,application()