Network Working Group                                          D. Thaler
Internet-Draft                                                 Microsoft
Intended status: Informational                             June 04, 2013
Expires: December 06, 2013

                     Reflections On Host Firewalls


   In today's Internet, the need for firewalls is generally accepted in
   the industry and indeed firewalls are widely deployed in practice.
   Often the result is that software may be running and potentially
   consuming resources, but then communication is blocked by a firewall.
   It's taken for granted that this end state is either desirable or the
   best that can be achieved in practice, rather than (for example) an
   end state where the relevant software is not running or is running in
   a way that would not result in unwanted communication.  In this
   document, we explore the issues behind these assumptions and provide
   suggestions on improving the architecture going forward.

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   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Firewall Rules  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Category 1: Attack Surface Reduction  . . . . . . . . . . . .   5
     3.1.  Stealth Mode  . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Discussion of Approaches  . . . . . . . . . . . . . . . .   6
       3.2.1.  Fix the Software  . . . . . . . . . . . . . . . . . .   6
       3.2.2.  Don't Use the Software  . . . . . . . . . . . . . . .   7
       3.2.3.  Run the Software Behind a Firewall  . . . . . . . . .   7
   4.  Category 2: Security Policy . . . . . . . . . . . . . . . . .   8
     4.1.  Discussion of Approaches  . . . . . . . . . . . . . . . .   8
       4.1.1.  Security Policies in Applications . . . . . . . . . .   9
       4.1.2.  Security Policies in Firewalls  . . . . . . . . . . .   9
       4.1.3.  Security Policies in a Service  . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   In Section 2.1 of "Reflections on Internet Transparency" [RFC4924],
   the IAB provided some thoughts on firewalls and their impact on the
   Internet architecture, including issues around disclosure obligations
   and complexity as applications evolve to circumvent firewalls.  This
   document extends that discussion with additional considerations.

   Traditionally, firewalls have been about arming customers to protect
   against badly written applications and services.  This document
   discusses a number of fundamental questions such as who a firewall is
   meant to protect from what.  It does so primarily, though not
   exclusively, from an end system perspective with a focus on host
   firewalls in particular.

   The Internet Security Glossary [RFC4949] defines a firewall as

      1.  (I) An internetwork gateway that restricts data communication
      traffic to and from one of the connected networks (the one said to

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      be "inside" the firewall) and thus protects that network's system
      resources against threats from the other network (the one that is
      said to be "outside" the firewall).  (See: guard, security

      2.  (O) A device or system that controls the flow of traffic
      between networks using differing security postures.  [SP41]

      Tutorial: A firewall typically protects a smaller, secure network
      (such as a corporate LAN, or even just one host) from a larger
      network (such as the Internet).  The firewall is installed at the
      point where the networks connect, and the firewall applies policy
      rules to control traffic that flows in and out of the protected

      A firewall is not always a single computer.  For example, a
      firewall may consist of a pair of filtering routers and one or
      more proxy servers running on one or more bastion hosts, all
      connected to a small, dedicated LAN (see: buffer zone) between the
      two routers.  The external router blocks attacks that use IP to
      break security (IP address spoofing, source routing, packet
      fragments), while proxy servers block attacks that would exploit a
      vulnerability in a higher-layer protocol or service.  The internal
      router blocks traffic from leaving the protected network except
      through the proxy servers.  The difficult part is defining
      criteria by which packets are denied passage through the firewall,
      because a firewall not only needs to keep unauthorized traffic
      (i.e., intruders) out, but usually also needs to let authorized
      traffic pass both in and out.

   Informally, most people would tend to think of a firewall as
   "something that blocks unwanted traffic" (see [RFC4948] for a
   discussion on many types of unwanted traffic).  A fundamental
   question is, however: "unwanted by whom?"

   Possible answers include end users, application developers, network
   administrators, host administrators, firewall vendors, and content
   providers.  We will exclude by definition the sender of the traffic
   in question, since the sender would generally want such traffic to be
   delivered.  Still, the other entities have different, and often
   conflicting, desires which means that a type of traffic might be
   wanted by one entity and unwanted by another entity.  Thus, not
   surprisingly, there exist various types of firewalls, and various
   types of "arms race" as we will discuss in Section 4.1.2.

1.1.  Terminology

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   In this document we distinguish between a "host firewall" which
   simply intends to protect the single computer on which it runs, and a
   "network firewall" which is located in the network and intends to
   protect the network and any hosts behind it.

   The term "application" is used generically to apply to any component
   that can receive traffic.  In this sense, it could refer to a process
   running on a computer (including a system service) or even to a
   portion of a TCP/IP stack itself, such as a component that responds
   to pings.

2.  Firewall Rules

   Desires for wanted or unwanted traffic can be expressed in terms of
   "allow" vs. "block" rules, with some way to resolve conflicting
   rules.  Many firewalls are actually implemented in terms of such
   rules.  Figure 1 shows some typical sources of such rules.

         Source    | Consumer  | Consumer   | Enterprise | Enterprise
                   | Grade     | Grade      | Grade      | Grade
                   | Host      | Network    | Host       | Network
                   | Firewall  | Firewall   | Firewall   | Firewall
         End user  | Sometimes |            |            |
                   | (as host  |            |            |
                   | admin)    |            |            |
         App       |   Yes     | Sometimes  |            |
         developer |           | (via       |            |
                   |           | protocols) |            |
         Network   |           | Sometimes  |            |   Yes
         admin     |           |            |            |
         Host      | Sometimes |            |    Yes     |
         admin     |           |            |            |
         Firewall  |   Yes     |    Yes     |    Yes     |   Yes
         vendor    |           |            |            |

                     Common sources of firewall rules

                                 Figure 1

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   Figure 1 assumes that network firewalls are administered by network
   administrators (if any), and host firewalls are administered by host
   administrators (if any).  A firewall may also have rules provided by
   the firewall vendor itself.

   End users typically cannot directly provide rules to firewalls,
   except when acting as host administrators.  Application developers
   can, however, provide such rules to some firewalls, such as providing
   rules at installation time, for example by invoking an API provided
   by a host firewall included with the operating system, or by
   providing metadata to the operating system for use by firewalls, or
   by using a protocol such as UPnP-IGD or PCP to communicate with a
   network firewall (see Section 4.1.3 for a longer discussion).

   Firewall rules generally fall into two categories:

   1.  Attack surface reduction: Rules intended to prevent an
       application from doing things the developer does not want it to

   2.  Security policy: Rules intended to prevent an application from
       doing things the developer might want it to do, but an
       administrator does not.

   A firewall is unnecessary if both categories are empty.  We will now
   treat each category in turn.

3.  Category 1: Attack Surface Reduction

   As noted above, this category of firewall rule typically attempts to
   prevent applications from doing things the developer did not intend.

   One might ask whether this category of rules is typically empty, and
   the answer is that it is not today.  The reason generally stems from
   mitigating code injection threats by putting a security barrier in a
   separate process isolated from the potentially compromised process.
   Furthermore, there is also some desire for a "stealth mode" (see

   Hence, typically a firewall will have rules to block everything by
   default.  A one-time, privileged, application install step adds one
   or more Allow rules, and then normal (unprivileged) application
   execution is then constrained by the resulting rules.

3.1.  Stealth Mode

   There is often a desire to hide from address and port scans on a
   public network.  However, compliance to many RFCs requires responding

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   to various messages.  For example, TCP [RFC0793] compliance requires
   sending a RST in response to a SYN when there is no listener, and
   ICMPv6 [RFC4443] compliance requires sending an Echo Reply in
   response to an Echo Request.

   Firewall rules can allow such stealth without changing the statement
   of compliance of the basic protocols.  However, stealth mode could
   instead be implemented as a configurable option in the protocols
   themselves.  For example, rather than a firewall rule to drop a
   certain outbound message after a protocol generates it, fewer
   resources would be consumed if the protocol knew not to generate it
   in the first place.

3.2.  Discussion of Approaches

   When running an application would result in unwanted behavior,
   customers have three choices, which we will discuss in turn:

   a.  fix (or get the developer to fix) the software,

   b.  not use the software, or

   c.  let the software run, but then use a firewall to thwart it and
       prevent it from working in unwanted ways.

3.2.1.  Fix the Software

   Firewall vendors point out that one can more quickly and reliably
   update firewall rules than application software.  Indeed some
   applications might have no way to update them, and support for other
   applications might no longer be available (e.g., if the developers
   are no longer around).  Most modern operating systems (and any
   applications that come with them) have automatic updates, as do some
   independent applications.  But until all applications have automatic
   updates, and automatic updates are actually used, it will still be
   the case that firewall rules can be updated more quickly than
   software patches.  Furthermore, in some contexts (e.g., within some
   enterprises), a possibly lengthy retesting and recertification
   process must be employed before applications can be updated.  In
   short, mechanisms to encourage and ease the use of secure automatic
   software updates are important and would greatly reduce overall

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3.2.2.  Don't Use the Software

   A key question to ask is whether the application could still do
   something useful when firewalled.  If the answer is yes, then not
   using the software is probably unrealistic.  For example, a game with
   both single-player and multi-player capabilities could still be
   useful in single-player mode when firewalled.  If instead the answer
   is no, it is better to not allow the application to run in the first
   place (and some host firewalls can indeed block applications from

3.2.3.  Run the Software Behind a Firewall

   As noted earlier, one disadvantage of this approach is that resources
   still get consumed.  For example, the application can still consume
   memory, CPU, bandwidth (up to the point of blockage), ports in the
   transport layer protocol, and possibly other resources depending on
   the application, for operations that provide no benefit while

   A second important disadvantage of this approach is the bad user
   experience.  Typically the application and the end-user won't know
   why the application doesn't work.  In addition, a poorly designed
   application might not cope well and consume even more resources
   (e.g., retrying an operation that continually fails).  Finally, using
   a firewall without changing the application could impact protocol
   operation and hence have adverse effects on other endpoints; for
   example blocking ICMP adversely affects path MTU discovery which can
   cause problems for other entities (see [RFC4890] for further

   Sandboxed environments, such as those provided by browsers, can be
   thought of as a type of firewall in the more general sense.  For
   example, a cross-site check in a browser can be thought of as a rule
   specified by the "firewall" vendor to block unwanted outbound traffic
   per a "same origin policy" where a script can only communicate with
   the "site" from which the script was obtained, for some definition of
   site such as the scheme and authority in a URI.

   In short, it is important to make applications more aware of the
   constraints of their environment and hence better able to behave well
   when contrained.

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4.  Category 2: Security Policy

   As noted in Section 2, this category of firewall rule typically
   attempts to prevent applications from doing things an administrator
   does not want, even if the application developer does intend them to

   One might ask whether this category of rules is typically empty, and
   the answer varies somewhat.  For enterprise-grade firewalls, it is
   almost never empty, and hence the problems discussed in Section 3.2.3
   can be common here too.  Similarly, for consumer-grade firewalls, it
   is generally not empty but there are some notable exceptions.  For
   example, for the host firewall in the Windows operation system, this
   category always starts empty and most users never change this.  [[Any
   other major firewalls that also contain no such rules by default?]]

   Security policy rules generally fall into two categories:

   1.  Simple policies that the developer would want, but that are
       difficult to implement.  One example might be a policy that an
       application should communicate only within the local network
       (e.g., a home or enterprise) but not be reachable from the global
       Internet or while the device is moved to some public network such
       as a hotspot.  A second example might be the reverse, i.e., a
       policy to communicate over the Internet but not with local
       entities.  The need for this category would be reduced by better
       platform support for such policies, making them easier for
       developers to implement and use.

   2.  Complex policies where the developer cannot possibly be aware of
       specifics.  One example might be a policy to communicate only
       during, or only outside of, normal business hours, where the
       exact hours may vary by location and time of year.  Another
       example might be a policy to avoid communication over links that
       cost too much, where the definition of "too much" may vary by
       customer, and indeed the end host and application might not even
       be aware of the costs.  The need for this category would be
       reduced by better platform support for such policies, allowing
       the application to communicate through some simple API with some
       other library or service that can deal with the specifics.

4.1.  Discussion of Approaches

   Security policy can be implemented in any of three places, which we
   will discuss in turn: the application, a firewall, or a library/
   service that that application explicitly uses.

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4.1.1.  Security Policies in Applications

   In this option, each application must implement support for
   potentially complex security policies, along with ways for
   administrators to configure them.  This is impractical for a number
   of reasons.  First, the complexity makes it difficult to implement
   and error-prone, especially for application developers whose primary
   expertise is not networking.  Second, the potentially large number of
   applications (and application developers) results in an inconsistent
   experience that makes it difficult for an administrator to manage
   common policies across applications, thus driving up training and
   operational costs.

4.1.2.  Security Policies in Firewalls

   Putting security policies in firewalls without explicit interaction
   with the applications results in the problems discussed in
   Section 3.2.3.  In addition, this leads to "arms races" where the
   applications are incented to evolve to get around the security
   policies, since the desires of the end user or developer can conflict
   with the desires of the host or network administrator.  As stated in
   Section 2.1 of [RFC4924]:

      In practice, filtering intended to block or restrict application
      usage is difficult to successfully implement without customer
      consent, since over time developers will tend to re-engineer
      filtered protocols so as to avoid the filters.  Thus over time,
      filtering is likely to result in interoperability issues or
      unnecessary complexity.  These costs come without the benefit of
      effective filtering since many application protocols began to use
      HTTP as a transport protocol after application developers observed
      that firewalls allow HTTP traffic while dropping packets for
      unknown protocols.

   Such arms races stem from inherent tussles between the desires of
   different entities.  For example, the tussle between end user desires
   and network administrator desires led to the arms race between
   network firewalls and deep packet inspection on the one hand, vs. the
   use of tunnels and obfuscation on the other.  Similarly, the tussle
   between application developer desires and network administrator
   desires contributed to the use of HTTP as a transport in order to
   work from within the widest possible set of networks (see
   Section 3.3.1 of [RFC6250] and [I-D.blanchet-iab-internetoverport443]
   for more discussion).

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   Such arms races most typically, though not exclusively, occur with
   network (not host) firewalls.  This is because it is more likely for
   network firewalls to have lack of trust between the policy-desiring
   entities, and less likely that there is any trusted arbiter.

4.1.3.  Security Policies in a Service

   In this approach, applications use a library or other external
   service whereby the applications have explicit knowledge of the
   impact of the security policies (and in a sandboxed environment this
   might be the application's only way to interact with the network).

   Thus in this opt-in approach, applications provide a description of
   the network access requested, and the library/service can ensure that
   applications and/or users are informed in a way they can understand,
   and that administrators can craft policy that affects the

   This approach is very difficult to do in a firewall-specific library/
   service when there can be multiple firewall implementations
   (including ones in the middle of the network), since it is usually
   impractical for an application developer to know about and develop
   for many different APIs.  It is, however, possible to employ this
   approach with a generic library/service that can communicate with
   both applications and firewalls.  Thus application developers and
   firewall developers can use a common platform.

   We observe that this approach is very different from the classic
   firewall approach.  It is, however the approach used by the
   aforementioned Windows firewall, and it is also the approach used by
   the Port Control Protocol (PCP) [RFC6887] in the IETF.  As such, we
   encourage the deployment and use of this model.  [[Any other major
   platforms besides Windows that already use this approach?]]

   Furthermore, while this approach lessens the incentive for arms races
   as discussed above, one important issue still remains.  Namely, there
   is no standard mechanism today for a library to learn complex
   policies from the network.  Further work in this area is needed.

5.  Security Considerations

   There is a common misconception that firewalls protect users from
   malware, when in fact firewalls protect users from buggy software.
   There is some concern that firewalls give users a false sense of
   security; firewalls are not invulnerable and will not prevent malware
   from running if the user allows it.

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   This document has focused primarily on host firewalls.  For
   additional discussion (focused more on network firewalls), see

6.  IANA Considerations

   This document requires no actions by the IANA.

7.  Acknowledgements

   Stuart Cheshire provided the motivation for this document by asking
   the thought-provoking question of why one would want to firewall an
   application rather than simply stop running it.  The ensuring
   discussion, and subsequent IAB tech chat, led to this document.

8.  Informative References

              Baker, F., "On Firewalls in Internet Security", draft-
              baker-opsawg-firewalls-00 (work in progress), January

              Blanchet, M., "Implications of Blocking Outgoing Ports
              Except Ports 80 and 443", draft-blanchet-iab-
              internetoverport443-01 (work in progress), October 2012.

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

   [RFC2979]  Freed, N., "Behavior of and Requirements for Internet
              Firewalls", RFC 2979, October 2000.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4890]  Davies, E. and J. Mohacsi, "Recommendations for Filtering
              ICMPv6 Messages in Firewalls", RFC 4890, May 2007.

   [RFC4924]  Aboba, B. and E. Davies, "Reflections on Internet
              Transparency", RFC 4924, July 2007.

   [RFC4948]  Andersson, L., Davies, E., and L. Zhang, "Report from the
              IAB workshop on Unwanted Traffic March 9-10, 2006", RFC
              4948, August 2007.

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   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", RFC
              4949, August 2007.

   [RFC6250]  Thaler, D., "Evolution of the IP Model", RFC 6250, May

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

Author's Address

   Dave Thaler
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052

   Phone: +1 425 703 8835

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