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ITL Bulletin for October 2009




ITL Bulletin for October 2009
ITL Bulletin for October 2009



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ITL BULLETIN FOR OCTOBER 2009

PROTECTING INFORMATION SYSTEMS WITH FIREWALLS: REVISED GUIDELINES ON
FIREWALL TECHNOLOGIES AND POLICIES

Shirley Radack, Editor
Computer Security Division
Information Technology Laboratory
National Institute of Standards and Technology
U.S. Department of Commerce

Firewalls are essential devices or programs that help organizations 
protect their networks and systems, and help home users protect their 
computers, from hostile attacks, break-ins, and malicious software. 
Firewalls control the flow of network traffic between networks and 
between hosts that employ different security policies.

Firewalls were originally installed at the perimeter of networks, where 
hostile threats from external intruders could be detected and stopped. 
While these early firewalls provided some protection for an 
organization=E2=80=99s internal systems, they could not recognize all instances 
and all forms of attack. For example, attacks sent from one internal 
host to another often did not pass through the network firewalls.

Networks are now often designed to provide protection at the network 
perimeter as well as at other network locations and to detect both 
external and internal attacks. Firewalls can now be used to restrict 
connectivity to and from internal networks that process personal 
information and carry out sensitive functions, such as accounting and 
personnel tasks. Firewalls can provide an additional layer of security 
by preventing unauthorized access to systems and information, and they 
can protect mobile devices that are placed directly onto external 
networks. To help organizations use today=E2=80=99s firewall technology 
effectively, the Information Technology Laboratory of the National 
Institute of Standards and Technology (NIST) recently revised its guide 
to firewall technology and the development of firewall policies.=C2=A0


NIST Special Publication 800-41, Revision 1, Guidelines on Firewalls and 
Firewall Policy: Recommendations of the National Institute of Standards 
and Technology

Written by Karen Scarfone of NIST and Paul Hoffman of the Virtual 
Private Network Consortium, NIST Special Publication (SP) 800-41, 
Revision 1, replaces an earlier guide to firewalls that had been issued 
in 2002. The updated report provides an overview of firewall technology, 
and helps organizations plan for and implement effective firewalls.


The revised publication explains the technical features of firewalls, 
the types of firewalls that are available for implementation by 
organizations, and their security capabilities.=C2=A0 Organizations are 
advised on the placement of firewalls within the network architecture, 
and on the selection, implementation, testing, and management of 
firewalls. Other issues covered in detail are the development of 
firewall policies, and recommendations on the types of network traffic 
that should be prohibited.=C2=A0


The appendices to the report contain helpful supporting material, 
including a glossary and lists of acronyms and abbreviations used in the 
text of the report. Also included in the appendices are listings of 
in-print and online resources that provide information on the effective 
use of firewalls as a component of a comprehensive approach to 
protecting information and information systems.


The revised guide to firewalls and firewall policies is available from 
the NIST Web page

http://csrc.nist.gov/publications/PubsSPs.html. 


Role of Firewalls in Network Communications

There are several types of firewall technologies, which can be most 
readily distinguished by which parts of network communications they can 
interpret. One way of accomplishing this is by referencing the four 
layers of network communications that are described by the Transmission 
Control Protocol/Internet Protocol (TCP/IP) interconnection standard:



* The application layer, the highest layer, sends and receives data for 
  applications such as Domain Name System (DNS), Hypertext Transfer 
  Protocol (HTTP), and Simple Main Transfer Protocol (SMTP). The 
  application layer itself is composed of layers of protocols, such as 
  for message formats and message handling.



* The transport layer provides connection-oriented or connectionless 
  services for transporting application layer services between networks, 
  and can provide communications reliability services. Transmission 
  Control Protocol (TCP) and User Datagram Protocol (UDP) are commonly 
  used at this layer.

* The internet protocol (or network) layer routes packets across 
  networks using protocols including Internet Protocol version 4 (IPv4), 
  IP version 6 (IPv6), Internet Control Message Protocol (ICMP), and 
  Internet Group Management Protocol (IGMP).

* The hardware (or data link) layer is the lowest layer, controlling 
  communications on the components of the physical network. The Ethernet 
  protocol is a data link layer protocol.



The TCP/IP communications layers work together to transfer data between 
hosts. When a user wants to transfer data across networks, the data is 
passed from the highest layer through intermediate layers to the lowest 
layer, with each layer adding more information. The lowest layer sends 
the accumulated data through the physical network, and the data is then 
passed upwards by the receiving host through the communications layers 
to its destination.


Basic firewalls operate at one or a few layers, usually the lower 
layers, while the more advanced firewalls operate at all of the layers. 
Threats were previously most prevalent in the lower layers of network 
traffic, but now threats are common at the application layer.=C2=A0 Firewalls 
are still needed to stop the significant threats at the lower layers of 
network communications, but the firewalls that examine more layers can 
perform more comprehensive examinations. Firewalls that are effective in 
the application layer can potentially protect advanced applications and 
protocols, and provide services that are user-oriented. For example, a 
firewall that operates only in the lower layers usually cannot identify 
specific users, but a firewall with application layer capabilities can 
provide security services such as enforcing user authentication and 
logging events by specific users.


Firewall Technologies

Firewall technology is often combined with other technologies, such as 
routing and network address translation (NAT) capabilities. Firewalls 
may also include content filtering features and intrusion prevention 
technology. See Section 2 of the report for a discussion of the 
advantages and disadvantages of these firewall technologies.


The older firewalls were primarily packet filter firewalls. These are 
routing devices that provide access control capabilities for host 
addresses and communication sessions. Also known as stateless inspection 
firewalls, these devices do not keep track of the state of each flow of 
traffic that passes though the firewall; as a result, they cannot 
associate multiple requests within a single session to each other. 
Packet filter technology is still employed in most modern firewalls, 
along with other firewall methods. Unlike more advanced filters, packet 
filters do not analyze the content of packets. Their access control 
functionality is governed by a set of directives referred to as a 
ruleset. Packet filtering capabilities are built into most operating 
systems and into devices capable of routing, such as a network router 
that employs access control lists. Packet filters operate at the network 
layer, and can filter both inbound and outbound traffic.


Firewalls with stateful inspection functions improve on the capabilities 
of packet filters by tracking the state of connections and by blocking 
packets that deviate from the expected state. These firewalls have 
greater awareness of the transport layer; packets are intercepted at the 
network layer and inspected for adherence to an existing firewall rule, 
as packet filters do, but stateful inspection firewalls also keep track 
of each connection in a state table that contains information such as 
source IP address, destination IP address, port numbers, and connection 
state information.


Application firewalls add a stateful protocol analysis capability. Some 
vendors refer to this feature as deep packet inspection. Stateful 
protocol analysis improves upon the standard stateful inspection by 
providing basic intrusion detection technology to analyze protocols at 
the application layer and identify suspicious events. These firewalls 
can allow or deny access based on how an application is running over the 
network. An application firewall can determine if an email message 
contains a type of attachment that the organization does not permit, 
determine if protocols are being used incorrectly, block connections 
that are not allowed, and allow or deny access to Web pages. Firewalls 
with both stateful inspection and stateful protocol analysis features 
provide extensive capabilities to detect and prevent attacks, but they 
are not complete intrusion detection and prevention systems (IDPSs).


A firewall that has an application-proxy gateway capability combines 
lower-layer access control with upper-layer functionality. These 
firewalls contain a proxy agent that acts as an intermediary between two 
hosts communicating with each other. The proxy agent never allows a 
direct connection between the hosts. Each successful connection attempt 
results in the creation of two separate connections; one connection is 
between the client and the proxy server, and another connection is 
between the proxy server and the destination address. From the 
perspective of the two hosts, the connection appears to be direct. 
Because external hosts only communicate with the proxy agent, internal 
IP addresses are not visible to the outside world. The proxy agent 
interfaces directly with the firewall ruleset to determine whether 
specific network traffic should be allowed to pass through the firewall.


Some proxy agents can require authentication of each individual network 
user. This authentication process can include user identification (ID) 
and password, hardware or software token, source address, and 
biometrics. Like application firewalls, the proxy gateway operates at 
the application layer and can inspect the actual content of the network 
traffic.


Dedicated proxy servers retain proxy control of traffic, but they 
usually have much more limited firewalling capabilities than 
application-proxy gateway firewalls. Many dedicated proxy servers are 
application-specific, and some actually perform analysis and validation 
of common application protocols. These servers are usually deployed 
behind traditional firewall platforms. The proxy server can filter or 
log incoming traffic forwarded by the main firewall, and then forward 
the traffic to internal systems. A proxy server can also accept outbound 
traffic directly from internal systems, filter or log the traffic, and 
pass it to the firewall for outbound delivery. Lately, the use of 
inbound proxy servers has decreased considerably because they must mimic 
the capabilities of the main server that they are protecting. Most proxy 
servers now in use are outbound proxy servers.


Firewall devices may have to encrypt and decrypt specific network 
traffic flows between the protected network and external networks. This 
function is accomplished through virtual private networks (VPNs), which 
use additional protocols to encrypt traffic and provide user 
authentication and integrity checking. Common VPN architectures are 
gateway-to-gateway and host-to-gateway. Gateway-to-gateway architectures 
connect multiple fixed sites over public lines through the use of VPN 
gateways. An example of this task is the connection of branch offices to 
an organization=E2=80=99s headquarters. The host-to-gateway architecture 
provides a secure connection to the network for individual users, 
usually called remote users, who are physically located outside of the 
organization, such as at home or in a hotel.


Gateway-to-gateway and host-to-gateway VPN functionality is often part 
of the firewall itself. Placing it behind the firewall would require the 
VPN traffic to be passed through the firewall while encrypted, thus 
preventing the firewall from inspecting the traffic. The organization=E2=80=99s 
VPN policy can specify the resources that users and groups are 
authorized to access.


Firewalls at the edge of a network may have to perform client checks for 
incoming connections from remote users and allow or disallow access 
based on those checks. This checking process, commonly called network 
access control (NAC) or network access protection (NAP), allows access 
based on the user=E2=80=99s credentials and the results of the checking process, 
assuring that the user=E2=80=99s computer complies with organizational policy 
concerning the use of patches, security software, and configuration 
settings.


Unified threat management (UTM) systems combine multiple security 
features into a single system, including a firewall, malware detection 
and eradication, and sensing and blocking of suspicious network probes. 
This approach may reduce the complexity of setting and maintaining 
policies on all of the systems that are deployed at the same location on 
a network, but it requires that the UTM have all the desired features to 
meet all security objectives.


Web application firewalls are installed in front of the Web server to 
detect the placement of malicious software on the computer of users who 
are accessing information on the Web, or to deter attempts to solicit 
private information from users. These firewalls, which are considered to 
be different from traditional firewalls, are a relatively new 
technology, with changing capabilities.


Firewalls for virtual infrastructures are another new area of firewall 
technology. These firewalls monitor virtualized networking, which allows 
more than one operating system running on a single computer 
simultaneously to communicate with each other as if they were on a 
standard Ethernet. Network activity that passes directly between 
virtualized operating systems within a host cannot be monitored by an 
external firewall. Some virtualization systems offer built-in firewalls 
or allow third-party software firewalls to be added as plug-ins.


Host-based firewalls for servers and personal firewalls for desktop and 
laptop personal computers (PCs) provide an additional layer of security 
against network-based attacks. These firewalls are software-based, 
residing on the hosts that they are protecting and monitoring and 
controlling the incoming and outgoing network traffic for a single host. 
They can provide more granular protection than network firewalls to meet 
the needs of specific hosts. Host-based firewalls are available as part 
of server operating systems.=C2=A0 Many host-based firewalls can also act as 
intrusion prevention systems (IPSs) that, after detecting an attack in 
progress, take actions to thwart the attacker and prevent damage to the 
targeted host.


NIST Recommendations to Organizations on the Use of Firewalls and Firewall
Policies

NIST recommends that organizations improve the effectiveness and 
security of their information systems by taking the following actions:=C2=A0


* Create a firewall policy that specifies how firewalls should handle 
  inbound and outbound network traffic.


Firewall policies, which are described in Section 4 of the report, 
define how organizational firewalls should handle inbound and outbound 
network traffic for specific IP addresses and address ranges, protocols, 
applications, and content types based on the organization=E2=80=99s information 
security policies. Organizations should conduct risk analyses to 
identify the acceptable types of network traffic and how they must be 
protected, specifying the types of traffic that can pass through the 
firewall and under what circumstances. For example, an organization 
might permit only necessary Internet Protocol (IP) protocols to pass, 
appropriate source and destination IP addresses to be used, particular 
Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) 
ports to be accessed, and certain Internet Control Message Protocol 
(ICMP) types and codes to be used. Generally, all inbound and outbound 
traffic not expressly permitted by the firewall policy should be blocked 
because such traffic is not needed by the organization. This practice 
will reduce the risk of attack and can also decrease the volume of 
traffic carried on the organization=E2=80=99s networks.


* Identify all requirements that should be considered when determining 
  which firewall to implement.


In planning for and selecting firewalls, organizations should determine 
the network areas to be protected, and consider the types of firewall 
technologies that will be most effective for protecting the required 
types of traffic. Organizations should also take into consideration 
firewall performance issues and the integration of the firewall into the 
existing network and security infrastructures. The design of the 
firewall should take into account any requirements relating to physical 
environment and to personnel, and to future needs, such as plans to 
adopt new IPv6 technologies or virtual private networks (VPNs).


* Create rulesets that implement the organization=E2=80=99s firewall policy 
  while supporting firewall performance.


Firewall rulesets should be as specific as possible for the network 
traffic that will be controlled. Organizations should determine the 
types of network traffic that are required, including the protocols that 
the firewall itself may need to use for management. The details of 
creating rulesets will vary according to the type of firewall and the 
specific firewall product, but the performance of many firewalls can be 
improved by optimizing firewall rulesets. For example, some firewalls 
check traffic against rules in a sequential manner until a match is 
found; for these firewalls, rules that have the highest chance of 
matching traffic patterns should be placed at the top of the list 
wherever possible.


* Manage firewall architectures, policies, software, and other 
  components throughout the life of the firewall solutions.


The types of firewalls to deploy and their positions in the 
organization=E2=80=99s networks can affect the security policies that the 
firewalls can enforce. Policy rules should be monitored and changed as 
the organization=E2=80=99s requirements change, such as when new applications or 
hosts are implemented within the network. The performance of firewall 
components should be monitored to enable potential resource issues to be 
identified and addressed before performance is adversely affected. Logs 
and alerts should also be continuously monitored to identify threats, 
both successful and unsuccessful. Firewall rulesets and policies should 
be managed through formal change management control processes to avoid 
any impact on security and on business operations. Ruleset reviews or 
tests should be performed periodically to ensure continued compliance 
with the organization=E2=80=99s policies. Firewall software should be patched as 
vendors provide updates to address any discovered vulnerabilities.


Related Publications

For information about NIST standards and guidelines related to the use 
of firewalls, as well as other security-related publications, see 
NIST'ss Web page.

http://csrc.nist.gov/publications/index.html 


Disclaimer

Any mention of commercial products or reference to commercial 
organizations is for information only; it does not imply recommendation 
or endorsement by NIST nor does it imply that the products mentioned are 
necessarily the best available for the purpose.


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