System information

wanted, and also that there needs to be an identically named context in exten-
sions.conf to define the call flow for unauthenticated calls.
The next option is allowguest, which we’ve disabled as we don’t want to accept any
unauthenticated calls at this time. Keep in mind that for some channels you may ac-
tually want to accept unauthenticated calls. A common use for allowing unauthenti-
cated calls is for companies that allow dialing by uniform resource identifiers (URIs),
like email addresses. If we wanted to allow customers to call us from their phones
without having to authenticate, we could enable guest calls and handle them in the
unauthenticated context defined by the previous option.
You may be wondering why you might ever want to allow unauthenti-
cated calls. The reason is that if you publish your SIP URI on your busi-
ness cards (e.g., sip:[e-mail]), calls to that URI will
fail if your unauthenticated context simply hangs up. What you want
instead is for your unauthenticated context to put incoming calls into a
controlled environment. You may wish to allow the calls, but you won’t
necessarily trust them.
*
The srvlookup option is used to enable Asterisk to perform a lookup via a DNS SRV
record, which is typically used for outbound connections to service providers. We’ll
talk more about Asterisk and DNS in Chapter 12.
The udpbindaddr
†
option takes the value of an IP address or 0.0.0.0 to tell Asterisk which
network interface it should listen to for requests carried by the UDP network transport
protocol (which is the protocol that actually carries the voice channels). By defining
0.0.0.0, we’re instructing the channel driver to listen on all available interfaces. Alter-
natively, we could limit VoIP connections for this protocol to a single interface by
defining the IP address of a specific network interface on our system.
Currently in Asterisk the udpbindaddr and tcpbindaddr options are an
all-or-one proposition. In other words, if you have three NICs in your
system, you can’t restrict VoIP traffic to two of them: it’s either one only,
or all of them.
* The whole concept of security and trust on a VoIP network is something that can become quite
complex. Spammers are already hard at work figuring out this technology, and you need to be
aware of the concepts. We’ll cover this in more depth later in the book, such as in Chapter 7
and Chapter 26.
† The complement to this option is tcpbindaddr, used for listening for requests carried via the TCP network
transport protocol.
Configuring Asterisk | 91