Maintenance and Security

Caching

Since clients cache the IPs they get from DNS, when they find a working IP, the slow down incurred will be negligible. However, it is highly recommended to remove problematic machines or addresses from the pool.

The example we look at here uses A records; however, it is increasingly common to see round-robin implementations for SRV records. SRV records are used to locate a service within a domain. For example in Microsoft's Active Directory implementation, SRV records are used to locate domain controllers and in our Asterisk setup we would use them to locate our routing service providers. The functionality of round robin doesn't differ for the record you request, however—you still create a pool of IPs in your DNS implementation. The two most commonly used implementations of DNS, Microsoft DNS and BIND (versions above 4.9.7), support SRV records and support round-robin SRV records.

For example to set up multiple SRV records for our SIP implementation, we would add the following to our DNS zone:

sipa	IN	A	10.1.1.1
sipb	IN	A	10.1.1.2
sipc	IN	A	10.1.1.3
sip	IN CNAME		sipa.example.com
	IN CNAME		sipb.example.com
	IN CNAME		sipc.example.com
_sip._udp.example.com.				IN	SRV	20	0	5060
	sip.example.com.

This sets up three SIP servers for us (sipa, sipb, and sipc) and one _sip._udp SRV record. Whenever the SRV record is requested, one of these three SIP servers will be returned.

Support Channels for Asterisk

As an open-source project, we would expect Asterisk to have at least some basic community support that we could rely on. Asterisk does have this and it has quite a bit more as well. It has mailing lists, forums, and IRC as well as official support from Digium. We don't always require commercial support but if running Asterisk is not our core responsibility or if we have other constraints, having paid technical support on hand can be a resource we would welcome.

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Chapter 9

Mailing Lists

There are a few mailing lists available for unofficial Asterisk support, by far the most active being provided by Digium itself. They are frequented by Digium staff as well as Asterisk users and are probably the best source of information when it comes to quick opinions or support from the community. They are found at the following URL:

http://lists.digium.com/mailman/listinfo

The USERS mailing list is the best choice for support issues.

There is also the VOIPSEC mailing list provided by www.voipsa.org, which isn't Asterisk-centric as its main focus is VoIP security on a wider scale. However as Asterisk is one of the most common VoIP solutions, it is a topic of frequent discussion on the list and topics such as firewalling protocols or encrypted communication are directly relevant to anyone responsible for the security of an Asterisk installation.

We may not decide to use these mailing lists as a support mechanism; however, it is worth having a "lurk" and reading through them at least, to give an insight into how other people are using Asterisk and the problems and issues they come across. Such experiences are invaluable in ensuring we do not repeat others' mistakes and will help in increasing our knowledge of Asterisk and associated technologies. The VOIPSEC list for instance has become the focal point of VoIP security and is often the first outlet for information that has an impact on the security of a VoIP implementation.

Forums

You can also obtain some support from the Digium forums, which can be found at http://forums.digium.com/. However, they aren't as busy as the other support available, the mailing lists and IRC being most popular.

IRC (Internet Relay Chat)

Asterisk has a lively community support mechanism provided by its IRC channel. This can be found on the Freenode network, which is a network that comprises almost entirely support channels for free and open source software.

To access this, download a suitable IRC client. mIRC, X-Chat, irssi, and chatzilla are commonly used clients, and most have the address for the Freenode (irc.freenode.net) servers in their default configuration. Once connected to Freenode, join #asterisk. This channel is much like the Digium mailing list, in that it focuses on discussion of the use and administration of Asterisk. It is also frequented by the same people that use the mailing lists: developers, administrators, and users.

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