Operations Area Working Group                                    F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Best Current Practice                          F. Baker
Expires: March 18, 2016                                    Cisco Systems
                                                              P. Hoffman
                                                          VPN Consortium
                                                      September 15, 2015

                   On Firewalls in Internet Security


   This document analyzes the role of firewalls in Internet security,
   and suggests a line of reasoning about their usage.  It analyzes
   common kinds of firewalls and the claims made for them.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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 March 18, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Reasoning about Firewalls . . . . . . . . . . . . . . . . . .   3
     3.1.  The Role of Firewalls in Internet Security  . . . . . . .   3
     3.2.  The End-to-End Principle  . . . . . . . . . . . . . . . .   4
     3.3.  Building a communication  . . . . . . . . . . . . . . . .   5
     3.4.  The middle way  . . . . . . . . . . . . . . . . . . . . .   5
   4.  Common kinds of firewalls . . . . . . . . . . . . . . . . . .   6
     4.1.  Perimeter security: Protection from aliens and intruders    6
     4.2.  Pervasive access control  . . . . . . . . . . . . . . . .   8
     4.3.  Intrusion Management: Contract and Reputation filters . .   8
   5.  Firewalling Strategies  . . . . . . . . . . . . . . . . . . .  10
     5.1.  Blocking Traffic Unless It Is Explicitly Allowed (default
           deny) . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Assumptions on IP addresses and Transport Protocol Port
       Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  State Associated with Packet Filtering  . . . . . . . . . . .  11
   8.  Enforcing Protocol Syntax at the Firewall . . . . . . . . . .  12
   9.  Performing Deep Packet Inspection . . . . . . . . . . . . . .  12
   10. Recommendations . . . . . . . . . . . . . . . . . . . . . . .  13
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     14.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   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.

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   Prophylactic perimeter security in the form of firewalls, and the
   proper use of them, have been a fractious sub-topic in this area.
   This document suggests a line of reasoning about the use of
   firewalls, and attempts to end the bickering on the topic, which is,
   for the most part, of little value in illuminating the discussion.
   It also analyzes common kinds of firewalls and the claims made for

2.  Terminology

   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.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Reasoning about Firewalls

3.1.  The Role of Firewalls in Internet Security

   One could compare the role of firewalls in prophylactic perimeter
   security to that of 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.

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3.2.  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 nodes of a network rather than
   in intermediary nodes, provided they can be implemented 'completely
   and correctly' in the end nodes" 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
   e.g. 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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3.3.  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 3.1, 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).

3.4.  The middle way

   There is therefore no one way to prevent attacks; there are different
   kinds of firewalls, and they address different views of the network
   (please see Section 4 for further discussion).  A zone-based firewall
   (Section 4.1) views the network as containing zones of trust, and
   deems applications inside its zone of protection to be trustworthy.
   A role-based firewall (Section 4.2) identifies parties on the basis

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   of membership in groups, and prevents unauthorized communication
   between groups.  A reputation, anomaly, or signature-based intrusion
   management system (Section 4.3) 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 4.1, 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 [RFC6887], or
   PCP, is an example of a protocol designed for that purpose.

4.  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,

   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.

4.1.  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 enforce simple permission
   rules, such as that external web clients are permitted to access a
   specific web server or that external SMTP MTAs are permitted to
   access internal SMTP MTAs.  Apart from those rules, a session may be

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   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.

   In many scenarios the existence and definition of zone-based
   perimeter defenses is arguably a side-effect of the deployment of
   Network Address Translation [RFC2993].  Since e.g. a single address
   is shared among multiple systems, the NAT device needs to translate
   both the IP addresses and the transport protocol ports in order to
   multiplex multiple communication instances from different nodes in
   the same external address.  Thus, the NAT device must keep a state
   table to know how to translate the IP addresses and transport
   protocol ports of incoming packets.  Packets originating from the
   internal network will either match an existing entry in the state
   table, or create a new one.  On the other hand, packets originating
   in the external network will either match an existing entry in the
   state table, or be dropped.  Thus, as a side effect, NATs implicitly
   require that communication be initiated from the internal network,
   and only allow return traffic from the external network.

   Some applications make the mistake of coupling application identities
   to network layer addresses, and hence employ such addresses in the
   application protocol.  Thus, Network Address Translation forces the
   translator to interpret packet payloads and change addresses where
   used by applications.

   As a result, 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.  This has slowed the deployment of SCTP [RFC4960], since a
   firewall will often not 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.

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4.2.  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 as 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

4.3.  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:

   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

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      attacks within one or several flows.  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, since e.g.
      temporary addresses [RFC4941] 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 zero-day 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 can 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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5.  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.

5.1.  Blocking Traffic Unless It Is Explicitly Allowed (default deny)

   Many networks enforce the so-called "default deny" policy, in which
   traffic is blocked unless it is explicitly allowed.  The rationale
   for such policy is that it is easier to open "holes" in a firewall
   for allowing specific protocols, than trying to block all protocols
   that might be employed as an attack vectors; and that a network
   should only support the protocols it has been explicitly designed to

   The drawback of this approach is that 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 specified, 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

6.  Assumptions on IP addresses and Transport Protocol Port Numbers

   In a number of scenarios, firewalls rules have traditionally been
   specified in terms of the associated IP addresses and transport
   protocol port numbers.  In general, this assumes that the associated
   IP addresses are stable, and that there is a "well known" transport
   protocol port number associated with each application.

   In the IPv4 world, IP addresses may be considered rather stable.
   However, IPv6 introduces the concept of "temporary addresses"
   [RFC4941] which, by definition, change over time.  This may prevent
   the enforcement of filtering policies based on specific IPv6

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   addresses, or may lead to filtering based on a specific address
   prefixes (as opposed to specific IPv6 addresses).  In some scenarios,
   from the point of view of enforcing filtering policies, it might be
   desirable to disable temporary addresses altogether.

      For example, an administrator might prefer that the caching DNS
      server or SMTP MTA always employ the same source IPv6 address, as
      opposed to the temporary addresses that change over time.

   The server-side transport protocol port is generally the so-called
   "well-known port" corresponding to the associated application.  While
   widespread, this practice should probably be considered a kludge/
   short-cut rather than a "design principle" that can be relied upon
   for the general case.  For example, use of DNS SRV records [RFC2782],
   or applications such as "portmapper" [Portmap] [RFC1833] might mean
   that the associated transport protocol port number cannot be assumed
   to be well-known, but rather needs to be dynamically learned.

7.  State Associated with Packet Filtering

   There are two main paradigms for packet filtering:

   o  Stateless filtering

   o  Stateful filtering

   Stateless filtering implies that the decision on whether to allow or
   block a specific packet is solely based on the contents of such
   packet.  One common example of such paradigm is the enforcement of
   network ingress filtering [RFC2827], in which packets may be blocked
   based on their IP addresses.  Stateless filtering scales well, since
   there are no state requirements on the filtering device other than
   that associated with maintaining the filtering rules to be applied to
   incoming packets.

   On the other hand, stateful filtering implies that the decision on
   whether to allow or block a packet is not only based on the contents
   of the packet, but also on the existence (or lack of) previous
   traffic/state associated with such packet.  A common example of such
   paradigm is a firewall that "allows outbound connection requests and
   only allows return traffic from the external network" (such as the
   policy implicitly enforced my most NAT devices).  For obvious
   reasons, the firewall needs to maintain state in order to be able to
   enforce such policies.  For example, a firewall may need to keep
   track of all on-going communication instances, possibly applying
   timeouts and garbage collection on the associated state table.

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   Stateful filtering tends to allow more powerful packet filtering, at
   the expense of increased state.  Thus, stateful filtering may be
   desirable when trying to perform deep packet inspection, but may be
   undesirable when the firewall is meant to block some Denial of
   Service attacks, since the firewall itself may become "the weakest
   link in the chain".

8.  Enforcing Protocol Syntax at the Firewall

   Some firewalls try to enforce the protocol syntax by checking that
   only packets complying with existing protocol definitions are
   allowed.  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] has been that common
   firewalls often test reserved/unused bits and require them to be set
   to zero to close covert channels.  When a new protocol or feature is
   defined, the firewall needs to stop applying that rule, and that can
   be difficult to make happen.

   A somewhat related concept is that of traffic normalization (or
   "scrubbing"), in which the filtering device can "normalize" traffic
   by e.g. clearing bits that are expected to be cleared, changing some
   protocol fields such that they are within "normal" ranges, etc. (see
   e.g. the discussion of "traffic normalization" in [OpenBSD-PF]).
   While this can have the useful effect of blocking DoS attacks to
   sloppy implementations that do not enforce sanity checks on the
   received packets, it also has the nasty side-effect of making
   innovation difficult, or even breaking deployed protocols.

      For example, some firewalls are known enforce a default packet
      normalization policy that clears the TCP URG bit, as a result of
      the TCP urgent mechanism being associated with some popular DoS
      attacks.  Widespread deployment of such firewalls has essentially
      rendered the TCP urgent mechanism unusable, leading to its
      eventual formal deprecation in [RFC6093].

9.  Performing Deep Packet Inspection

   While filtering packets based on the layer-3 protocol header fields
   is rather simple and straight-forward, performing packet filtering
   based on the upper layer protocols can be a challenging task.

   For example, IP fragmentation may make this task quite challenging,
   since even the very layer-4 protocol header could be present in a
   non-first fragment.  In a similar vein, IPv6 extension headers may
   represent a challenge for a filtering device, since they can result

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   in long IPv6 extension header chains [RFC7112]

   This problem is exacerbated as one tries to filter packets based on
   upper layer protocol contents, since many of such protocols implement
   some form of fragmentation/segmentation and reassembly.  In many
   cases, the reassembly process could possibly lead to different
   results, and this may be exploited by attackers for circumventing
   security controls [Ptacek1998] [RFC6274].

   In general, the upper in the protocol stack that packet filtering is
   performed, the more state that is required at the filtering device.
   And when stateful packet filtering is warranted, its associated
   security implications should be considered.

10.  Recommendations

   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'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 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 4.1, 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

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   traffic, but the side-effects of preventing protocol improvement and
   application innovation are destructive and unnecessary.

11.  IANA Considerations

   This memo asks the IANA for no new parameters.  It can before
   publication as an RFC by the RFC Editor.

12.  Security Considerations

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

13.  Acknowledgements

   This document is based on [I-D.ietf-opsawg-firewalls-00] authored by
   Fred Baker, and [I-D.ietf-opsawg-firewalls-01] authored by Paul

14.  References

14.1.  Normative References

   [RFC1833]  Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
              RFC 1833, DOI 10.17487/RFC1833, August 1995,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3205]  Moore, K., "On the use of HTTP as a Substrate", BCP 56,
              RFC 3205, DOI 10.17487/RFC3205, February 2002,

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,

   [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
              P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
              DOI 10.17487/RFC6887, April 2013,

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   [RFC7112]  Gont, F., Manral, V., and R. Bonica, "Implications of
              Oversized IPv6 Header Chains", RFC 7112,
              DOI 10.17487/RFC7112, January 2014,

14.2.  Informative References

              Gont, F., Hilliard, N., Doering, G., LIU, S., and W.
              Kumari, "Operational Implications of IPv6 Packets with
              Extension Headers", draft-gont-v6ops-ipv6-ehs-packet-
              drops-00 (work in progress), July 2015.

              Baker, F., "On Firewalls in Internet Security", draft-
              ietf-opsawg-firewalls-00 (work in progress), June 2012.

              Baker, F. and P. Hoffman, "On Firewalls in Internet
              Security", draft-ietf-opsawg-firewalls-01 (work in
              progress), October 2012.

              Vyncke, E., Yourtchenko, A., and M. Townsley, "Advanced
              Security for IPv6 CPE", draft-vyncke-advanced-
              ipv6-security-03 (work in progress), October 2011.

              OpenBSD, , "pf(4) manual page: pf -- packet filter", 2015,

   [Portmap]  Wikipedia, , "Portmap", 2014,

              Ptacek, T. and T. Newsham, "Insertion, Evasion and Denial
              of Service: Eluding Network Intrusion Detection", 1998,

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,

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   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <http://www.rfc-editor.org/info/rfc2827>.

   [RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
              DOI 10.17487/RFC2993, November 2000,

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,

   [RFC5575]  Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
              and D. McPherson, "Dissemination of Flow Specification
              Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,

   [RFC6092]  Woodyatt, J., Ed., "Recommended Simple Security
              Capabilities in Customer Premises Equipment (CPE) for
              Providing Residential IPv6 Internet Service", RFC 6092,
              DOI 10.17487/RFC6092, January 2011,

   [RFC6093]  Gont, F. and A. Yourtchenko, "On the Implementation of the
              TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093,
              January 2011, <http://www.rfc-editor.org/info/rfc6093>.

   [RFC6274]  Gont, F., "Security Assessment of the Internet Protocol
              Version 4", RFC 6274, DOI 10.17487/RFC6274, July 2011,

   [Saltzer]  Saltzer, J., Reed, D., and D. Clark, "End-to-end arguments
              in system design", ACM Transactions on Computer Systems
              (TOCS) v.2 n.4, p277-288, Nov 1984.

Authors' Addresses

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   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com

   Fred Baker
   Cisco Systems
   Santa Barbara, California  93117

   Email: fred@cisco.com

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org

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