ISP Assigned Addresses Global Pool

147 The PIX uses a feature called Dynamic Address Translation to map the Internal address range 192.168.2.024 into the Global range 2.241.11.24829 as connections are opened and addresses are needed. When not in use, as when a machine is turned off or the user has left for the night, the addresses in the global pool that may have been assigned to the user are returned to an open status and assigned to another user. The global pool must be a separate network from both the internal and external networks. As we briefly touched on before, using a global pool and dynamic addressing to assign network addresses has two benefits. It results in good protection by hiding the internal machine needing Internet access. It also allows hundreds of sites to share a 32-machine or smaller network, if the site doesnt have too many users trying to connect to the Internet at any one time. For everything to work as expected, your Internet service provider needs to add a routing table entry on their backbone to route your global pool of addresses to you. In this case, the ISP added a route that pointed network 2.241.11.24829 to the 1.251.174.156 interface on the PIX firewall. The ISP could have also pointed this network to the gateway router 3.96.12.8, in which case we would have needed to add a further route from 3.96.12.8 to the PIXs external interface to complete the connection.

9.2.2 Advantages of the PIX Firewall

One of the PIXs strongest points is that it is a single-point hardware solution that is easy to configure and to administer. Dynamic address translation allows a large network to share a small amount of address space. Most importantly for our purposes in this book, two or more PIX boxes provide an encrypted tunnel across the Internet.

9.2.2.1 Hardware solution

One of the biggest advantages of using the PIX firewall is that it is a separate piece of hardware from the servers that host your critical services. Physical hardware solutions, if they are made well and are easy to configure and maintain, are excellent ways for creating firewalls and VPNs. Network coordinators and their staffs can easily install many PIX firewalls in a large, non-homogenous, world-wide WAN. Hardware implementations are generally more reliable; they have been through more rigorous testing than software solutions, and are sometimes faster in operation since much of the core microcode can be burned onto chips.

9.2.2.2 Superior to Unix and other router firewalls

The PIX runs an operating system unlike those on either the normal Cisco routers or traditional Unix machines, which run routing or gateway software. While Cisco is known for robust software, it is quite complex because it is designed for a variety of routing and connectivity needs. For instance, most versions of Ciscos IOS software support a wide variety of routing protocols, such as RIP, IGRP, BGP, AppleTalk, and IPX. This support, which is perfect for the management of any large network, is considerable overkill if all you need is the single function of packet filtration. By removing all of that extra overhead, the PIX makes an excellent choice for a firewall. 148 As the PIX was designed with only one purpose in mind, that of securing a private network by both filtering traffic and establishing secure links across an unprotected Internet, it is an excellent place to establish logging controls on the network. Further, the PIX machine is a fine encryption device, protecting valuable communication with tried and true cryptographic techniques.

9.2.2.3 Single point of controlfailure

When network trouble is at hand, a clear delineation between the outside world and ones local world is an immense asset. By using a single unit to handle the networks entire firewalling needs, the network coordinator has a clear starting point for troubleshooting. The PIX provides such an architecture. By having a single unit handle all network traffic in this manner, equipment malfunction and software misconfiguration could cause a great amount of havoc in one fell swoop. But Cisco has anticipated this problem with a fail-over, hot-standby mode, whereby two units can be configured in parallel. Should some fault occur to one of the units, the other can support the firewall by maintaining operation transparently.

9.2.2.4 Dynamic address translation

The dynamic translation feature, which allows users to pick up a free address from the Internet pool whenever they need one, does more than just allow many people to share a few NIC registered addresses. It also provides adaptive security to the hosts using the internal addresses. The PIX unit has two interfaces, one for the exterior network and one for the interior one. Because of this, the internal address range is hidden by translating the internal address into the dynamic pool of addresses for Internet exchange. This method provides a high level of security because a threat is much less likely when an attacker doesnt know the architecture, network range, or setup of the internal network

9.2.2.5 PIX acts like a proxy server

Unix-based proxy servers are commonplace in todays world, and most of them serve their function well. Proxy servers do essentially what the PIX firewall was engineered to do: they hide a machine or a network of machines from the outside world by masquerading as the client in network conversations with unknown or unverifiable parties. The PIX is more robust than the standard proxy servers because it provides a quick session bypass once the authenticated parties have shaken hands. Traditional proxy servers maintain state information for the duration of the session, and handle their packet control at the top of the protocol stack. Moreover, because software-driven proxy servers perform packet filtering, they prevent the use of almost all the VPN solutions on the market today.

9.2.2.6 Ease of configuration and maintenance

For the most part, the PIX is very easy to handle in everyday use. Once the configuration is applied and tested, it handles its job silently and without complaint. As is popular in todays world, to further assist the network coordinators that will be installing the device, Cisco has made an HTML front end for configuration. There is of course the old standby option of the command-line-driven interface, which could be used by those more familiar with Ciscos prevalent IOS software.