Operations Area Working Group F. Baker
Internet-Draft Cisco Systems
Intended status: BCP June 12, 2012
Expires: December 14, 2012
On Firewalls in Internet Security
draft-ietf-opsawg-firewalls-00
Abstract
There is an ongoing discussion regarding the place of firewalls in
security. This note is intended to capture and try to make sense out
of it.
Requirements Language
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].
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 14, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Common kinds of firewalls . . . . . . . . . . . . . . . . . . 3
2.1. Perimeter security: Protection from aliens and
intruders . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Pervasive access control . . . . . . . . . . . . . . . . . 5
2.3. Intrusion Management: Contract and Reputation filters . . 5
3. Reasoning about Firewalls . . . . . . . . . . . . . . . . . . 7
3.1. The End-to-End Principle . . . . . . . . . . . . . . . . . 7
3.2. Building a communication . . . . . . . . . . . . . . . . . 8
3.3. The middle way . . . . . . . . . . . . . . . . . . . . . . 8
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
There is an ongoing discussion regarding the place of firewalls in
security. This note is intended to capture and try to make sense out
of it.
The IETF has a long and fractured discussion on security. Many early
RFCs simply didn't address the topic - and said as much. When the
IESG started complaining about that, it was told that there was no
market interest in the topic that was measurable in money spent.
Those who *were* interested in the topic set forth frameworks, rules,
and procedures without necessarily explaining how they would be
useful in deployment, and dismissed questions as "from those who
don't understand." In many cases, as a result, deployments have been
underwhelming in both quantity and quality, and the Internet is noted
for its problems with security. What is clear is that people need to
think clearly about security, their own and that of others. What is
not clear is how to do so in a coherent and scalable manner.
Prophylactic perimeter security in the form of firewalls, and the
proper use of them, have been a fractious sub-topic in this area.
One could compare them to the human skin. The service that the skin
performs for the rest of the body is to keep common crud out, and as
a result prevent much damage and infection that could otherwise
occur. The body supplies prophylactic perimeter security for itself
and then presumes that the security perimeter has been breached; real
defenses against attacks on the body include powerful systems that
detect changes (anomalies) counterproductive to human health, and
recognizable attack syndromes such as common or recently-seen
diseases. One might well ask, in view of those superior defenses,
whether there is any value in the skin at all; the value is easily
stated, however. It is not in preventing the need for the stronger
solutions, but in making their expensive invocation less needful and
more focused.
This note will address common kinds of firewalls and the claims made
for them. It will suggest a line of reasoning about the use of
firewalls. It will attempt to end the bickering on the topic, which
is, for the most part, of little value in illuminating the
discussion.
2. Common kinds of firewalls
There are at least three common kinds of firewalls:
o Context or Zone-based firewalls, that protect systems within a
perimeter from systems outside it,
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o Pervasive routing-based measures, which protect intermingled
systems from each other by enforcing role-based policies, and
o Systems that analyze application behavior and trigger on events
that are unusual, match a signature, or involve an untrusted peer.
2.1. Perimeter security: Protection from aliens and intruders
As discussed in [RFC6092], the most common kind of firewall is used
at the perimeter of a network. Perimeter security assumes two
things: that applications and equipment inside the perimeter are
under the control of the local administration and are therefore
probably doing reasonable things, and that applications and equipment
outside the perimeter are unknown. It may make simple permission
rules, such as that external web clients are permitted to access a
specific web server or that SMTP peers are permitted to access
internal SMTP MTAs. Apart from those rules, a session may be
initiated from inside the perimeter, and responses from outside will
be allowed through the firewall, but sessions may never be initiated
from outside.
In addition, perimeter firewalls often perform some level of testing,
either as application proxies or through deep packet inspection, to
verify that the protocol claimed to be being passed is in fact the
protocol being passed.
The existence and definition of zone-based perimeter defenses is
arguably a side-effect of the deployment of Network Address
Translation [RFC2993]; applications frequently make the mistake of
coupling application identities to network layer addresses, and in so
doing make two other coupling assumptions: that an address useful to
and understood by one application is useful to and understood by
another, and that addresses are unlikely to change within a time
frame useful to the application. Network Address Translation forces
the translator to interpret packet payloads and change addresses
where used by applications. If the transport or application headers
are not understood by the translator, this has the effect of damaging
or preventing communication. Detection of such issues can be sold as
a security feature, although it is really a side-effect of a failure.
While this can have useful side-effects, such as preventing the
passage of attack traffic that masquerades as some well-known
protocol, it also has the nasty side-effect of making innovation
difficult. For example, One of the issues in the deployment of
Explicit Congestion Notification [RFC3168], for example, has been
that common firewalls often test unused bits and require them to be
set to zero to close covert channels. A similar problem has slowed
the deployment of SCTP [RFC4960], in that a firewall will often not
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permit a protocol it doesn't know even if a user behind it opens the
session. When a new protocol or feature is defined, the firewall
needs to stop applying that rule, and that can be difficult to make
happen.
2.2. Pervasive access control
Another access control model, often called "Role-based", tries to
control traffic in flight regardless of the perimeter. Given a rule
that equipment located in a given routing domain or with a specific
characteristic (such as "student dorms") should not be able to access
equipment in another domain or with a specific characteristic (such
as "academic records"), it might prevent routing from announcing the
second route in the domain of the first, or it might tag individual
packets ("I'm from the student dorm") and filter on those tags at
enforcement points throughout network. Such rules can be applied to
individuals are well as equipment; in that case, the host needs to
tag the traffic, or there must be a reliable correlation between
equipment and its user.
One common use of this model is in data centers, in which physical or
virtual machines from one tenant (which is not necessarily an "owner"
as much as it is a context in which the system is used) might be co-
resident with physical or virtual machines from another. Inter-
tenant attacks, espionage, and fraud are prevented by enforcing a
rule that traffic from systems used by any given tenant is only
delivered to other systems used by the same tenant. This might, of
course have nuances; under stated circumstances, identified systems
or identified users might be able to cross such a boundary.
The major impediment in deployment is complexity. The administration
has the option to assign policies for individuals on the basis of
their current location (e.g. as the cross-product of people,
equipment, and topology), meaning that policies can multiply wildly.
The administrator that applies a complex role-based access policy is
probably most justly condemned to live in the world he or she has
created.
2.3. Intrusion Management: Contract and Reputation filters
The model proposed in Advanced Security for IPv6 CPE
[I-D.vyncke-advanced-ipv6-security] could be compared to purchasing
an anti-virus software package for one's computer. The proposal is
to install a set of filters, perhaps automatically updated, that
identify "bad stuff" and make it inaccessible, while not impeding
anything else.
It depends on four basic features:
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o A frequently-updated signature-based Intrusion Prevention System
which inspects a pre-defined set of protocols at all layers (from
layer-3 to layer-7) and uses a vast set of heuristics to detect
attacks within one or several flow. Upon detection, the flow is
terminated and an event is logged for further optional auditing.
o A centralized reputation database that scores prefixes for degree
of trust. This is unlikely to be on addresses per se, as Privacy
Addresses change regularly and frequently.
o Local correlation of attack-related information, and
o Global correlation of attacks seen, in a reputation database
The proposal doesn't mention anomaly-based intrusion detection, which
could be used to detect day-zero attacks and new applications or
attacks. This would be an obvious extension.
The comparison to anti-virus software is real; anti-virus software
uses similar algorithms, but on API calls or on data exchanged rather
than on network traffic, and for identified threats is often
effective.
The proposal also has weaknesses:
o People don't generally maintain anti-virus packages very well,
letting contracts expire,
o Reputation databases have a bad reputation for distributing
information which is incorrect or out of date,
o Anomaly-based analysis identifies changes but is often ineffective
in determining whether new application or application behaviors
are pernicious (false positives). Someone therefore has to
actively decide - a workload the average homeowner might have
little patience for, and
o Signature-based analysis applies to attacks that have been
previously identified, and must be updated as new attacks develop.
As a result, in a world in which new attacks literally arise
daily, the administrative workload and be intense, and reflexive
responses like accepting https certificates that are out of date
or the download and installation of unsigned software on the
assumption that the site admin is behind are themselves vectors
for attack.
Security has to be maintained to be useful, because attacks are
maintained.
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3. Reasoning about Firewalls
3.1. The End-to-End Principle
One common complaint about firewalls in general is that they violate
the End-to-End Principle [Saltzer]. The End-to-End Principle is
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.
[Saltzer] "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.
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3.2. Building a communication
Any communication requires at least three components:
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 [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).
3.3. The middle way
There is therefore no one way to prevent attacks; as noted in
Section 2, there are different kinds of firewalls, and they address
different views of the network. A zone-based firewall (Section 2.1)
views the network as containing zones of trust, and deems
applications inside its zone of protection to be trustworthy. A
role-based firewall (Section 2.2) identifies parties on the basis of
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membership in groups, and prevents unauthorized communication between
groups. A reputation, anomaly, or signature-based intrusion
management system depends on active administration, and permits known
applications to communicate while excluding unknown or known-evil
applications. In each case, the host or application is its own final
bastion of defense, but preventing a host from accepting incoming
traffic (so-called "host firewalls") does not defend infrastructure.
Each type of prophylactic has a purpose, and none of them is a
complete prophylactic defense.
Each type of defense, however, can be assisted by enabling an
application running in a host to inform the network of what it is
willing to receive. As noted in Section 2.1, a zone-based firewall,
generally denies all incoming sessions and permits responses to
sessions initiated outbound from the zone, but can in some cases be
configured to also permit specific classes of incoming session
requests, such as WWW or SMTP to an appropriate server. A simple way
to enable a zone-based firewall to prevent attacks on infrastructure
(traffic to an un instantiated address or to an application that is
off) while not impeding traffic that has a willing host and
application would be for the application to inform the firewall of
that willingness to receive. The Port Control Protocol
[I-D.ietf-pcp-base], or PCP, is an example of a protocol designed for
that purpose.
4. Recommendations
A general recommendation for the IETF: the IETF should not seek to
standardize something that is not being requested by consumers or
industry.
Zone-based firewalls, when used, SHOULD exclude all session
initiation from outside the zone regardless of attributes such as the
use of IPsec. They SHOULD also facilitate the use of a protocol such
as PCP by hosts to identify traffic (IPsec AH, IPsec ESP, transports
in general, or transports using specified destination port ranges)
that they are willing to receive, and interpret that into rules
permitting specified traffic to those specific systems. Being fully
automated and easily understood, such firewalls are appropriate for
networks with passive administration.
Role-based firewalls can be implemented using routing technology.
For example, if Alice should not be able to send a message to Bob,
Alice might not be able to obtain Bob's address from DNS, Alice's
routing system might not have a route to Bob, or Bob's routing system
might not have a route to Alice. Role-based firewalls can also be
implemented using filtering technology; Alice, Alice's router, Bob's
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router, or Bob may have a filter that prevents communication between
them. While there can be issues in specific cases, a routing
implementation is generally more scalable and more easily managed.
Reputation, anomaly, or signature-based intrusion management is
generally proprietary; a service maintains the list of exclusions,
which must be updated as new kinds of attacks are developed.
Implementations SHOULD be designed for frequent and scalable
updating.
As further discussed in Section 2.1, firewalls of any type SHOULD NOT
attempt to perform the kind of deep packet inspection and surgery
that is common with Network Address Translators [RFC2993]. There is
marginal value in detecting the spoofing of applications by attack
traffic, but the side-effects of preventing protocol improvement and
application innovation are destructive and unnecessary.
Apart from ICMP, tunnel encapsulations, routing protocols, and
infrastructure protocols intended to manage network configuration and
use of addresses such as DNS or DHCP, applications MUST NOT expect a
peer to be able to interpret network layer addresses carried in their
payload. Network layer addresses carried for documentation purposes,
such as in an SMTP envelope or a syslog message, have other value and
don't violate this recommendation.
5. IANA Considerations
This memo asks the IANA for no new parameters.
Note to RFC Editor: This section will have served its purpose if it
correctly tells IANA that no new assignments or registries are
required, or if those assignments or registries are created during
the RFC publication process. From the author's perspective, it may
therefore be removed upon publication as an RFC at the RFC Editor's
discretion.
6. Security Considerations
This note reasons about security considerations. It introduces no
new ones.
7. Acknowledgements
Warren Kumari commented on this note.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[I-D.ietf-pcp-base]
Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)",
draft-ietf-pcp-base-26 (work in progress), June 2012.
[I-D.vyncke-advanced-ipv6-security]
Vyncke, E., Yourtchenko, A., and M. Townsley, "Advanced
Security for IPv6 CPE",
draft-vyncke-advanced-ipv6-security-03 (work in progress),
October 2011.
[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
November 2000.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5575] Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
and D. McPherson, "Dissemination of Flow Specification
Rules", RFC 5575, August 2009.
[RFC6092] Woodyatt, J., "Recommended Simple Security Capabilities in
Customer Premises Equipment (CPE) for Providing
Residential IPv6 Internet Service", RFC 6092,
January 2011.
[Saltzer] 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.
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Author's Address
Fred Baker
Cisco Systems
Santa Barbara, California 93117
USA
Email: fred@cisco.com
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