Operations Area Working Group F. Baker
Internet-Draft Cisco Systems
Intended status: BCP P. Hoffman
Expires: April 21, 2013 VPN Consortium
October 18, 2012
On Firewalls in Internet Security
draft-ietf-opsawg-firewalls-01
Abstract
This document discusses the most important operational and security
implications of using modern firewalls in networks. It makes
recommendations for operators of firewalls, as well as for firewall
vendors.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 21, 2013.
Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Modern Firewall Features That Should Not Be Confused
with Firewalling . . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. High-Level Firewall Concepts . . . . . . . . . . . . . . . . . 4
2.1. The End-to-End Principle . . . . . . . . . . . . . . . . . 4
2.2. Building a Communication . . . . . . . . . . . . . . . . . 5
3. Firewalling Strategies . . . . . . . . . . . . . . . . . . . . 6
3.1. Blocking Traffic Unless It Is Explicitly Allowed . . . . . 7
3.2. Typical Firewall Categories . . . . . . . . . . . . . . . 7
3.3. Newer categories of firewalling . . . . . . . . . . . . . 8
4. Recommendations for Operators . . . . . . . . . . . . . . . . 8
5. Recommendations for Firewall Vendors . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. IPv4 NATs Are Not Security Devices . . . . . . . . . 10
Appendix B. Origin Reputation and Firewalls . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
In this document, a firewall is defined as a device or software that
imposes a policy whose effect is "a stated type of packets may or may
not pass from A to B". All modern firewalls allow an administrator
to change the policies in the firewall, although the ease of
administration for making those changes, and the granularity of the
policies, vary widely between firewalls and vendors.
Given this definition, it is easy to see that there is a perimeter
(the position between A and B) in which the specific security policy
applies. In typical deployed networks, there are usually some easy-
to-define perimeters. If two or more networks that are connected by
a single device, the perimeter is inside the device. If that device
is a firewall, it can impose a security policy at the shared
perimeters of those networks.
Many firewalls also employ some perimeters that are not as easy to
define. Some of these perimeters in modern firewalls include:
o An application-layer gateway (ALG) in front of a server creates a
perimeter between that server and the network it is connected to.
The ALG blocks some of the flows in the application protocol based
on policies such as "do not all traffic from this network" and "do
not allow the client to send a message of this type".
o Routing domains that are controlled with role-based administration
create perimeters in a routed network. Role-based administration
makes rules such as "Domain X cannot see Domain Y in its routing
table"; this prevents any host in Domain X from sending traffic to
any host in Domain Y.
o [[[ MORE HERE with other interesting perimeters ]]]
Modern firewalls apply perimeters at three layers:
Layer 3: Most firewalls can filter based on source and destination
IPv4 addresses. Many (but, frustratingly, not all) firewalls can
filter based on IPv6 addresses.
Layer 4: Most firewalls can filter based on TCP and UDP ports.
Many (but, frustratingly, not all) firewalls can also filter based
on transports other than TCP and UDP.
Layer 7: Modern firewalls can filter based on the application
protocol contents, such as to allow or block certain types of
protocol-defined messages, or based on the contents of those
messages.
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Note that many firewall devices can only create policies at one or
two of the layers.
Hardware-based firewalls by their nature inspect traffic flowing
through them, sometimes using proprietary mechanisms to make traffic
analysis as fast as possible on the given hardware. Some firewalls
use network visibility protocols such as NetFlow and sFlow to help
capture and analyze traffic. [[ References needed ]]
1.1. Modern Firewall Features That Should Not Be Confused with
Firewalling
There are a few features that appear in any firewall devices that
have become associated with firewalls but in fact are not used for
firewalling. Those non-firewalling features include:
Network Address Translation (NAT) [RFC2993], which is not used for
security policy
IPsec [RFC4301], which is used for virtual private networks
(VPNs). Although the core IPsec protocol has firewalling in it,
when IPsec appears in a firewall device, it is normally only
associated with the application of authenticated encryption and
integrity protection of traffic.
"SSL VPN" is a set of technologies that rely on tunneling traffic
through the TLS [RFC5246] protocol running on port 443. Some
firewalls offer SSL VPNs as an alternative to IPsec.
Traffic prioritization is a feature common in firewalls, but does
not meet the definition of firewalling at all.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Some terms which have specific meanings in this document (such as
"firewall") are defined earlier in this section.
2. High-Level Firewall Concepts
2.1. The End-to-End Principle
One common complaint about firewalls in general is that they violate
the End-to-End Principle [EndToEnd]. The End-to-End Principle is
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often incorrectly stated as requiring that "application specific
functions ought to reside in the end hosts of a network rather than
in intermediary nodes, provided they can be implemented 'completely
and correctly' in the end hosts" or that "there should be no state in
the network."
What it actually says is heavily nuanced, and is a line of reasoning
applicable when considering any two communication layers. The
document says that it "presents a design principle that helps guide
placement of functions among the modules of a distributed computer
system. The principle, called the end-to-end argument, suggests that
functions placed at low levels of a system may be redundant or of
little value when compared with the cost of providing them at that
low level."
In other words, the End-to-End Argument is not a prohibition against
lower layer retries of transmissions, which can be important in
certain LAN technologies, nor of the maintenance of state, nor of
consistent policies imposed for security reasons. It is, however, a
plea for simplicity. Any behavior of a lower communication layer,
whether found in the same system as the higher layer (and especially
application) functionality or in a different one, that from the
perspective of a higher layer introduces inconsistency, complexity,
or coupling extracts a cost. That cost may be in user satisfaction,
difficulty of management or fault diagnosis, difficulty of future
innovation, reduced performance, or other forms. Such costs need to
be clearly and honestly weighed against the benefits expected, and
used only if the benefit outweighs the cost.
From that perspective, introduction of a policy that prevents
communication under an understood set of circumstances, whether it is
to prevent access to pornographic sites or prevents traffic that can
be characterized as an attack, does not fail the end to end argument;
there are any number of possible sites on the network that are
inaccessible at any given time, and the presence of such a policy is
easily explained and understood.
What does fail the end-to-end argument is behavior that is
intermittent, difficult to explain, or unpredictable. If I can
sometimes reach a site and not at other times, or reach it using this
host or application but not another, I wonder why that is true, and
may not even know where to look for the issue.
2.2. Building a Communication
Any communication requires at least three components:
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o a sender, someone or some thing that sends a message,
o a receiver, someone or some thing that receives the message, and
o a channel, which is a medium by which the message is communicated.
In the Internet, the IP network is the channel; it may traverse
something as simple as a directly connected cable or as complex as a
sequence of ISPs, but it is the means of communication. In normal
communications, a sender sends a message via the channel to the
receiver, who is willing to receive and operate on it. In contrast,
attacks are a form of harassment. A receiver exists, but is
unwilling to receive the message, has no application to operate on
it, or is by policy unwilling to. Attacks on infrastructure occur
when message volume overwhelms infrastructure or uses infrastructure
but has no obvious receiver.
By that line of reasoning, a firewall primarily protects
infrastructure, by preventing traffic that would attack it from it.
The best prophylactic might use a procedure for the dissemination of
flow specification rules from [RFC5575] to drop traffic sent by an
unauthorized or inappropriate sender or which has no host or
application willing to receive it as close as possible to the sender.
In other words, as discussed in Section 1, a firewall compares to the
human skin, and has as its primary purpose the prophylactic defense
of a network. By extension, the firewall also protects a set of
hosts and applications, and the bandwidth that serves them, as part
of a strategy of defense in depth. A firewall is not itself a
security strategy; the analogy to the skin would say that a body
protected only by the skin has an immune system deficiency and cannot
be expected to long survive. That said, every security solution has
a set of vulnerabilities; the vulnerabilities of a layered defense is
the intersection of the vulnerabilities of the various layers (e.g.,
a successful attack has to thread each layer of defense).
3. Firewalling Strategies
There is a great deal of tension in firewall policies between two
primary goals of networking: the security goal of "block traffic
unless it is explicitly allowed" and the networking goal of "trust
hosts with new protocols". The two inherently cannot coexist easily
in a set of policies for a firewall.
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3.1. Blocking Traffic Unless It Is Explicitly Allowed
The security goal of "block traffic unless it is explicitly allowed"
prevents useful new applications. This problem has been seen
repeatedly over the past decade: a new and useful application
protocol is deployed, but it cannot get wide adoption because it is
blocked by firewalls. The result has been a tendency to try to run
new protocols over established applications, particularly over HTTP
[RFC3205]. The result is protocols that do not work as well they
might if they were designed from scratch.
Worse, the same goal prevents the deployment of useful transports
other than TCP, UDP, and ICMP. A conservative firewall that only
knows those three transports will block new transports such as SCTP
[RFC4960]; this in turn causes the Internet to not be able to grow in
a healthy fashion. Many firewalls will also block TCP and UDP
options they don't understand, and this has the same unfortunate
result.
[[[ MORE HERE about forcing more costly and error-prone layer 7
inspection ]]]
3.2. Typical Firewall Categories
Most IPv4 firewalls have pre-configured security policies that fall
into one of the following categories:
I: Block all outside-initiated traffic, allow all inside-initiated
traffic
II: Same as I, but allow outside-initiated traffic to some
specific inside hosts. The specified hosts are often added by IP
address (or sometimes by DNS host name), and the host may be
limited to particular transport and application protocols. For
example, a rule might allow traffic destined to 203.0.113.226 on
TCP ports 80 and 443.
III: Same as I or II, but allow some outside-initiated traffic
over some protocols to all hosts. For example, a firewall
protecting a farm of web servers might want to allow traffic using
TCP ports 80 and 443 to all addresses protected by the firewall so
that new servers can be deployed without having to update the
firewall rules.
Firewalls that understand IPv6 may have a fourth category:
IV: Allow nearly all outside-initiated traffic. [[[ MORE HERE
about why this is considered a good idea by some and a bad idea by
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others ]]]]
3.3. Newer categories of firewalling
[[[ MORE HERE on blocking traffic based on dynamic origin reputation
such as the long-expired vyncke-advanced-ipv6-security ]]]
4. Recommendations for Operators
[[[ MORE HERE with the following outline ]]]
Firewalling strategies
None. This is really the operator's choice.
Be aware that deep packet inspection causes varying amounts of
delay in firewalls, particularly for long-lived flows
Don't enforce protocol semantics in the firewall
Applications are easier to change than firewalls
Avoid using application-layer gateways for firewalling
Use the security in the applications servers instead
Servers are easier to change than firewalls
However, ALGs are useful for IPv4-IPv6 conversion and proxying
in some protocols
Allow fragments
Except in specific protocols where layer 7 content filtering is
deemed crucial
Document your intended firewall strategy and settings
Be sure that other operators of the firewall are able to see it
Don't rely on a NAT for security (see Appendix A)
If using IPsec or SSL VPN, test whether the filtering rules for the
rest of the firewall apply
5. Recommendations for Firewall Vendors
[[[ MORE HERE with the following outline ]]]
Make a set of NAT-like rules for IPv6 easily choosable
Interface for pinholing of IPv4 NATs needs clearly identify security
issues
Follow the BEHAVE RFC rules for binding timeouts on NATs
Keep a summary log of non-normal events to aid reviewing
Make leaving notes about the firewalling rules easy and useful
Implement draft-ietf-pcp-base and probably the follow-on protocols
from that WG
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6. IANA Considerations
None.
7. Security Considerations
This document is all about security considerations. It introduces no
new ones.
8. Acknowledgements
Warren Kumari commented on this document.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[EndToEnd]
Saltzer, JH., Reed, DP., and DD. Clark, "End-to-end
arguments in system design", ACM Transactions on Computer
Systems (TOCS) v.2 n.4, p277-288, Nov 1984.
[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
November 2000.
[RFC3205] Moore, K., "On the use of HTTP as a Substrate", BCP 56,
RFC 3205, February 2002.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5575] Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
and D. McPherson, "Dissemination of Flow Specification
Rules", RFC 5575, August 2009.
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Appendix A. IPv4 NATs Are Not Security Devices
Their security is a side-effect of their design. [[[ MORE HERE about
the history and why some operators mistake the security policy of
NATs with firewalls. ]]]
[[[ MORE HERE about how pinholes mess badly that security policy. ]]]
[[[ MORE HERE about PCP and how to integrate it with a firewall
security policy. ]]]
Recommendations for deploying NATs in firewalls include:
o NATs should only be used when more IPv4 addresses are needed
o Operators should not pinhole to addresses that are unpredictably
assigned by DHCP
Appendix B. Origin Reputation and Firewalls
[[[ MORE HERE with the following outline ]]]
Letting someone else curate your security policy
Different types of reputation for different layers
draft-ietf-repute-model
draft-vyncke-advanced-ipv6-security
draft-hallambaker-omnibroker
Recommendations
Check logs to be sure updates are happening
Check vendors' policies
Authors' Addresses
Fred Baker
Cisco Systems
Email: fred@cisco.com
Paul Hoffman
VPN Consortium
Email: paul.hoffman@vpnc.org
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