VoIP systems take a wide variety of forms. Just about any personal computer is capable of providing VoIP. The Linux platform, in particular, offers a large number of VoIP applications to choose from. In general, the term VoIP is associated with equipment that provides the ability to dial telephone numbers and communicate with parties who have either a VoIP terminal or a traditional analog telephone on the other end of the connection.
Increasing demand for VoIP services has resulted in a broad array of end-user products, including:
• Traditional telephone handsets Usually these units have extra features beyond a simple handset with dial pad. For instance, many have a small LCD screen that may provide browsing, instant messaging, or a telephone directory. They can also be used when configuring the handset to gain access to enhanced features such as conference calls.
• Mobile units Although wireless VoIP products may present additional security challenges if not carefully configured, they are becoming more and more popular—especially since many organizations already have an installed base of 802.11 WiFi networking equipment.
• Softphones With a headset, software, and inexpensive connection service, any workstation can be used as a VoIP unit, often referred to as a softphone. If practical, softphone systems should be avoided where security and privacy are a serious concern. Common software vulnerabilities in personal computers result in unacceptably high risks in the use of softphones. Moreover, using a softphone system conflicts with the need to separate voice and data networks to the greatest extent practical (see the previous "Preventing Converging Networks Attacks" section).
In addition to end-user units, other network elements commonly used in VoIP infrastructures include:
• Media gateways (MGs) These represent the interface between circuit-switched networks and IP networks. MGs focus on the audio signal translation function, performing analog/digital conversion, call origination and reception, and quality improvement functions such as compression or echo cancellation.
• Media gateway controllers (MGCs) These handle the signaling data between the MGs and other network components such as H.323 gatekeepers or SIP servers, or toward SS7 signaling gateways. A single MGC can control multiple MGs, which leads to cost reductions when deploying larger systems.
• Firewalls and session border controllers (SBCs) Whether securing a LAN, encapsulating a DMZ, or just providing protection to a single computer, a firewall is usually the first line of defense against external attackers in today's IP networks. As previously explained, the introduction of firewalls to VoIP networks complicates several aspects of VoIP, most notably communications on dynamic ports and call setup procedures. To overcome some of the problems that firewalls and NAT cause for VoIP, SBCs can be used to exert control over the signal and media streams involved in setting up, conducting, and tearing down calls. Additionally, they can also perform the function of application-level gateways and control the types of calls that can be placed through the networks where they reside.
• Conventional network services and equipment VoIP deployments also need some traditional network services, such as DNS, DHCP, TFTP, SNMP, LDAP, and more. Furthermore, regardless of the type of traffic they carry, all IP networks rely on conventional network equipment—namely switches, routers, and possibly wireless access points.
Finally, depending on the signaling standard of choice, other specialized equipment may be deployed, such as call processors, call managers, gateways, backend servers, etc. These special devices, along with their role in the call setup process, will be detailed in the section "VoIP Network Elements Attacks," later in this chapter.
Of course, the vulnerabilities in VoIP encompass the flaws inherent not only within the VoIP equipment itself, but also in the underlying operating systems, applications, and protocols on which VoIP depends. Therefore, the following two broad classes of attacks targeting devices and network services in VoIP environments have been identified.
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