Operations Area Working Group                                   F. Baker
Internet-Draft                                             Cisco Systems
Intended status: BCP                                    January 21, 2012
Expires: July 24, 2012

                   On Firewalls in Internet Security


   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",
   document are to be interpreted as described in [RFC2119].

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   This Internet-Draft is submitted in full conformance with the
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 24, 2012.

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   to this document.  Code Components extracted from this document must
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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

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

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

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

   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

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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

      [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

   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

   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 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

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

6.  Security Considerations

   This note reasons about security considerations.  It introduces no
   new ones.

7.  Acknowledgements

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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

              Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
              Selkirk, "Port Control Protocol (PCP)",
              draft-ietf-pcp-base-22 (work in progress), January 2012.

              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

   Email: fred@cisco.com

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