Network Working Group                      Ned Freed, Innosoft
                                       Kevin Carosso, Innosoft
Internet Draft               <draft-freed-firewall-req-02.txt>

                     An Internet Firewall
                   Transparency Requirement

                        December 1997

                     Status of this Memo

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Copyright (C) The Internet Society (1997).  All Rights

1.  Abstract

This memo defines a basic transparency requirement for
Internet firewalls.  While such a requirement may seem
obvious, the fact of the matter is that firewall behavior is
currently either unspecified or underspecified, and this lack
of specificity often causes problems in practice. This
requirement is intended to be a necessary first step in making
the behavior of firewalls more consistent and correct.

Internet Draft      Firewall Transparency        December 1997

2.  Introduction

The Internet is now being used for an increasing number of
mission critical applications. Because of this many sites find
isolated secure intranets insufficient for their needs, even
when those intranets are based on and use Internet protocols.
Instead they find it necessary to provide direct
communications paths between the unsecured and sometimes
hostile Internet and systems or networks which either deal
with valuable data, provide vital services, or both.

The security concerns that inevitably arise from such setups
are often dealt with by inserting one or more "firewalls" on
the path between the Internet and the internal network. A
"firewall" is an agent which screens network traffic in some
way, blocking traffic it believes to inappropriate, dangerous,
or both.

More specifically, firewalls either act as a protocol end
point (e.g. a SMTP client/server or a Web proxy agent), as a
packet filter, or some combination of both.

When a firewall acts a protocol end point it may

 (1)   implement a "safe" subset of the protocol,

 (2)   perform extensive protocol validity checks,

 (3)   use an implementation methodology designed to minimize
       the liklihood of bugs,

 (4)   run in an insolated, "safe" environment, or

 (5)   use some combination of these techniques in tandem.

In the case of a packet filter the firewall isn't visible as a
protocol end point. Each packet is examined and the firewall
may then

 (1)   pass the packet through to the other side unchanged,

 (2)   drop the packet entirely, or

 (3)   handle the packet itself in some way.

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Firewalls typically base some of their decisions on IP source
and destination addresses and port numbers. For example,
firewalls may

 (1)   block packets from the Internet side that claim a
       source address of a system on the internal network,

 (2)   block TELNET or RLOGIN connections from the Internet to
       the internal network,

 (3)   block SMTP and FTP connections to the Internet from
       internal systems not authorized to send email or move

 (4)   act as an intermediate server in handling SMTP and HTTP
       connections in either direction, or

 (5)   require the use of an access negotiation and
       encapsulation protocol like SOCKS [1] to gain access to
       the Internet, to the internal network, or both.

(This list is only intended to illustrate the sorts of things
firewalls often do; it is by no means exhaustive, nor are all
firewall products able to perform all the operations on this

Unfortunately, the development and deployment of firewalls has
for the most part been ignored by the Internet standards
community. As a result of this inattention the use of
firewalls has solved some old problems, but not without
generating lots of new ones in the process.

This memo is intended to address the new problems firewalls
can cause by establishing a basic transparency requirement for

2.1.  Requirements notation

This document occasionally uses terms that appear in capital
letters. When the terms "MUST", "SHOULD", "MUST NOT", "SHOULD
NOT", and "MAY" appear capitalized, they are being used to
indicate particular requirements of this specification. A
discussion of the meanings of these terms appears in RFC 2119

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3.  The Transparency Requirement

The basic transparency requirement for firewalls is quite

    The introduction of a firewall and any associated
    tunneling or access negotiation facilities MUST NOT cause
    the gratuitous failure of legitimate and standards-
    compliant usage that would work were the firewall not

A necessary corollary to this requirement is that when such
failures do occur it is incumbent on the firewall and
associated software to address the problem: Changes to either
implementations of existing standard protocols or the
protocols themselves MUST NOT be necessary.

Note that this requirement only applies to legitimate protocol
usage and gratuitous faiures -- a firewall is entitled to
block any sort of access that is seen as illegitimate,
regardless of whether or not it is standards-compliant. This
is, after all, the primary reason to have a firewall in the
first place.

4.  Security Considerations

The transparency rule impacts security to the extent that it
precludes certain simpleminded firewall implementation
techniques. Firewall implementors must therefore work a little
harder to achieve a given level of security. However, the
transparency rule in no way prevents an implementor from
achieving whatever level of security is necessary. Moreover, a
little more work up front results in better security in the
long run because techniques that do not interfere with
existing services will almost certainly be more widely
deployed than ones that do interfere and prevent people from
performing useful work.

Now, some firewall implementors will inevitably claim that the
burden of total transparency is overly onerous and that
adequate security cannot be achieved in the face of such a
requirement. And there's no question that meeting the
transparency requirement is more difficult than not doing so.

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Nevertheless, it is important to remember that the only
perfectly secure network is one that doesn't allow any data
through at all, and that the only problem with such a network
is that it is unusable. Anything less is necessarily a
tradeoff between useability and security. And the simple fact
of the matter is that at present firewalls are being
circumvented in ad hoc ways, necessarily weakening security
dramatically, simply because they don't meet this transparency
requirement. In other words, the only reason that some
firewalls remain in use is because they have essentially been
disabled. As such, one reason to have a transparency
requirement is to IMPROVE security.

Good security may occasionallly result in interoperability
failures between components. This is understood. However, this
doesn't mean that gratiutous interoperability failures caused
by security components are acceptable.  They aren't.

5.  Example Violations of the Transparency Rule

This document will conclude with a (long) list of existing
firewall behavior that violates the transparency requirement.

 (1)   In an effort to enhance security by hiding internal
       system names that might otherwise be revealed in email
       headers, some firewalls either strip information from
       or completely delete certain header fields. There are
       even known cases of certain text strings (e.g. names of
       internal hosts systems) being deleted from message

       Blindly deleting information from message body text is
       simply not acceptable.  Consider what happens when a
       string is deleted from a binary part encoded in base64
       simply because it matches some string pattern
       somewhere, or what happens when someone has given their
       own name to their personal system.

       Deleting "Received:" fields from message headers is
       also problematic, as it interferes with message loop
       detection. In addition, some firewalls delete
       "Received:" fields in messages passing from the
       Internet to the local network in addition to messages
       going the other way, and this actually compromises

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       security as it eliminates trace information vital in
       determining a hostile message's possible origin.

       The solution is simple for messages passing from the
       external Internet to internal hosts: Don't delete
       Received: headers because this compromises, rather than
       enhances, security. As for Received: headers going in
       the other direction, one approach is to obscure host
       name information using name replacement, hash
       functions, or encryption, rather than removing the
       field entirely. Simple encryption schemes lead to host
       names are meaningless to outsiders but if need be can
       be analyzed by a security manager to determine the
       actual underlying name.

 (2)   Firewalls implementing Web proxies often trash URLs
       which are very long, contain odd (but legal)
       characters, or contain many separator characters.  The
       result is that Web applications which employ such URLs,
       such as directory applications, tend not to work
       properly through many firewall products, even though
       the URLs being used are completely legal, safe, and

 (3)   Many firewalls which act as SMTP proxy agents implement
       only the most rudimentary form of SMTP service. The
       result is that the ability to use many useful SMTP
       facilities (DSNs, size negotiation, authentication,
       pipelining, etc.) is eliminated. In the case of DSNs
       and authentication once again this action lessens
       security rather than strengthening it.

 (4)   DNS service behind many firewalls works very poorly.
       Firewalls often implement the concept of a split
       between the part of a domain the outside world can see
       and the part the inside world can see. And firewalls
       are often called upon to create DNS setups of this
       sort. This is often done poorly -- perhaps it is just
       too difficult to configure such things properly. The
       net result often is that such restrictions end up
       getting summarily dumped, which again may compromise
       security more than it strengthens it.

       Note that this also makes it hard to deploy a good
       mailer on the inside even if the firewall lets the SMTP

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       traffic through to/from the mail hub.  A mail hub
       inside such a setup cannot get a true picture of the
       outside world, and this once again may end up
       compromising security rather than strengthening it.

 (5)   Many firewalls handle TCP connections in a way which
       lies somewhere between acting as a full-fledged
       application protocol proxy and a transpart connection
       path. Specifically, they intercept all attempts to open
       TCP connections across the firewall and respond to them
       immediately, making the connection initiator think the
       open has succeeded. They then make a connection of
       their own to the actual destination host. If that
       connection succeeds subsequent data is then forwarded
       from one side to the other, possibly with some
       additional examination nd/or modification occurring at
       the firewall, or possibly not.

       The first problem with this technique arises when the
       connection from the firewall to the actual destination
       cannot be opened. There is no way to properly convey
       the semantics of such a failure to the connection
       originator since the firewall has already completed the
       connection to the originator. The firewall must then
       either content itself with simply closing the
       connection or else send some protocol-specific
       response, which applications may interpret quite
       differently than a transport level connection failure.
       (In fact such differing interpretation is required in
       some protocols.)

       In the case of an SMTP transfer to a destination with
       multiple MX and A records, for example, existing
       clients may interpret a successful connection open as
       constituting a delivery attempt requiring no subsequent
       connection attempts to other MX or A records. This in
       turn can lead to delivery failures when one or more
       hosts in an MX or A record list aren't available for a
       prolonged period of time.

       The second problem with this technique arises when
       modifications are made to packets transferred from one
       side to the other without full knowledge of the
       underlying protocol. For example, situations have
       arisen where firewalls attempt to "censor" an SMTP data

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       stream and end up removing data from that stream,
       operating under the assumption that since SMTP uses
       dot-stuffing rather than counted data such editing is
       acceptable. Unfortunately, the SMTP protocol has
       changed in recent years and now includes the BDAT
       command [2] for transferring counted data, among other
       things. Removing octets from a BDAT data stream
       inevitably leads to protocol desynchronization,
       timeouts,transfer failures, and message delivery

       A similar problem can occur with SMTP's VRFY command,
       which is a mandatory part of the SMTP protocol.
       Firewalls like to intercept VRFY because of the
       perception that VRFY exposes internal information
       unnecessarily. (This perception is false as comparable
       exposure occurs with SMTP's RCPT TO, and disabling or
       interfering with RCPT TO breaks the protocol
       completely.) Unfortunately, while it is possible to
       intercept and effectively disable VRFY properly [4],
       several firewall don't do it correctly. Ways of
       disabling VRFY incorrectly include returning a 5XX SMTP
       error unconditionally (which can lead to delivery
       failures when working with clients that do VRFYs prior
       to each RCPT TO) and failing to return a properly
       formatted 2XX SMTP success code (RFC 821 [5] requires
       that the response to a VRFY include an RFC 822 [6]
       address enclosed in angle brackets).

6.  References

[1]  M. Leech, M. Ganis, Y. Lee, R. Kuris, D. Koblas, L.
     Jones, "SOCKS Protocol Version 5", RFC 1928, April, 1996.

[2]  Vaudreuil, G., "SMTP Service Extensions for Transmission
     of Large and Binary MIME Messages", RFC1830, August,

[3]  Bradner, S., "Key Words for Use in RFCs to Indicate
     Requirement Levels", RFC 2119, March 1997.

[4]  Braden, R., "Requirements for Internet Hosts -
     Application and Support", STD 3, RFC 1123,
     USC/Information Sciences Institute, October 1989.

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[5]  Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
     821, USC/Information Sciences Institute, August 1982.

[6]  Crocker, D., "Standard for the Format of ARPA Internet
     Text Messages", STD 11, RFC 822, UDEL, August 1982.

7.  Authors' Addresses

Ned Freed
Innosoft International, Inc.
1050 Lakes Drive
West Covina, CA 91790
 tel: +1 626 919 3600           fax: +1 626 919 3614

Kevin Carosso
Innosoft International, Inc.
1050 Lakes Drive
West Covina, CA 91790
 tel: +1 626 919 3600           fax: +1 626 919 3614

8.  Full Copyright Statement

Copyright (C) The Internet Society (1997). All Rights

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be followed, or as required to translate it into languages
other than English.

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The limited permissions granted above are perpetual and will
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This document and the information contained herein is provided

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