The configuration that works best for a particular organization depends on three factors:
The objectives of the network, the organization‘s ability to develop and implement the
architectures, and the budget available for the function.
implementations are packet filtering routers, screened host firewalls, dual-homed
firewalls,a nd screened subnet firewalls.
I. Packet Filtering Routers
Most organizations with a n Internet connections have some form of a router
as the interface to the Internet at the perimeter between the organization‘s
internal networks and the external service provider. Many of these routers can
be configured to reject packets that the organization does not allow into the
network. This is a simple but effective way to lower the organization‘s risk
from external attack. The drawbacks to this type of system include a lack of
auditing and strong authentication. Also, the complexity of the access control
lists used to filter the packets can grow and degrade network performance. Fig
6-4 is an example of this type of architecture.
II. Screened Host Firewalls
This architecture combines the packet filtering router with a separate, dedicated
firewall, such as an application proxy server. This approach allows the router to
pre-screen packets to minimize the network traffic and loads on the internal
proxy.The application proxy examines an application layer protocol, such as
HTTP, and perform the proxy services. This separate host is often referred to as a
bastion host; it can be a rich target for external attacks, and should be very
thoroughly secured.Evn though the bastion host/application proxy actually
contains only cached copies of the internal Web documents, it can still present a
promising target, because compromise of the bastion host can disclose the
configuration of internal networks and possibly provide external sources with
internal information. Since the bastion host stands as a sloe defender on the
network perimeter, it is also commonly referred to as the Sacrificial Host.
To its advantage, this configuration requires the external attack to compromise
two separate systems, before the attack can access internal data. Inthis way, the
bastion host protects the data more fully than the router alone. Fig 6-11 shows a
typical configuration of a screened host architectural approach.
Dual-Homed Host Firewalls
The next step up in firewall architectural complexity is the dual-homed host. When this
architectural approach is used, the bastion host contains two NICs (Network Interface
Cards) rather than one, as in the bastion host configuration. One NIC is connected to the
external network, and one is connected to the internal network, providing an additional
layer of protection. With TWO NICs , all traffic must physically go through the firewall
to move between the internal and external networks.
Implementation of this architecture often makes use of NATs. NAT is a method of
mapping real, valid, external IP addresses to special ranges of non-routable internal IP
addresses, thereby creating yet another barrier to intrusion from external attackers.
The internal addresses used by NAT consist of three different ranges. Organizations that
need Class A addresses can use the 10.x.x.x range, which has over 16.5 million usable
addresses. Organization‘s that need Class B addresses can use the 192.168.x.x range,
which has over 65,500 addresses. Finally , organiazations with smaller needs , such as
those needing onlya few Class C addresses, can use the c172.16.0.0 to range,
which hs over 16 Class C addresses or about 4000 usable addresses.
See table 6-4 for a recap of the IP address ranges reseved fro non-public networks.
Messages sent with internal addresses within these three internal use addresses is directly
connected to the external network, and avoids the NAT server, its traffic cannot be routed
on the public network. Taking advantage of this , NAT prevents external attacks from
reaching internal machines with addresses in specified ranges.If the NAT server is a
multi-homed bastion host, it translates between the true, external IP addresses assigned to
the organization by public network naming authorities ansd the internally assigned, nonroutable IP addresses. NAT translates by dynamically assigning addresses to internal
communications and tracking the conversions with sessions to determine which incoming
message is a response to which outgoing traffic. Fig 6-12 shows a typical configuration
of a dual homed host firewall that uses NAT and proxy access to protect the internal
Another benefit of a dual-homed host is its ability to translate between
many different protocols at their respective data link layers, including Ethernet , Token
Ring, Fiber Distributed Data interface (FDDI) , and Asynchronous Transfer Method
(ATM). On the downside, if this dual-homed host is compromised, it can disable the
connection to the external network, and as traffic volume increases, it can become overloaded. Compared to more complex solutions, however, this architecture provides strong
overall protection with minimal expense.
Screened Subnet Firewalls (with DMZ)
The dominant architecture used today is the screened subnet firewall. The architecture of
a screened subnet firewall provides a DMZ. The DMZ can be a dedicated port on the
firewall device linking a single bastion host, or it can be connected to a screened subnet,
as shown in Fig 6-13. Until recently , servers providing services through an untrusted
network were commonly placed in the DMZ. Examples of these include Web servers, file
transfer protocol (FTP) servers, and certain database servers. More recent strategies using
proxy servers have provided much more secure solutions.
A common arrangement finds the subnet firewall consisting of two or more internal
bastion hosts behind a packet filtering router, with each host protecting the trusted
network. There are many variants of the screened subnet architecture. The first general
model consists of two filtering routers, with one or more dual-homed bastion hosts
between them. In the second general model, as illustrated in Fig 6-13 , the connections
are routed as follows:
Connections from the outside or un trusted network are routed through an
external filtering router.
Connections from the outside or un trusted network are routed into-and then
out of – a routing firewall to the separate network segment known as the DMZ.
Connections into the trusted internal network are allowed only from the
DMZ bastion host servers.
The screened subnet is an entire network segment that performs two functions: it protects
the DMZs systems and information from outside threats by providing a network of
intermediate security; and it protects the internal networks by limiting how external
connections can gain access to internal systems. Although extremely secure, the screened
subnet can be expensive to implement and complex to configure and manage. The value
of the information it protects must justify the cost.
Another facet of the DMZ is the creation of an area of known as an extranet. AN extranet
is a segment of the DMZ where additional authentication and authorization controls are
put into place to provide services that are not available to the general public. An example
would be an online retailer that allows anyone to browse the product catalog and place
items into a shopping cart, but will require extra authentication and authorization when
the customer is ready to check out and place an order.
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