Internet DRAFT - draft-ietf-iab-firewall-req
draft-ietf-iab-firewall-req
Network Working Group Ned Freed, Innosoft
Internet Draft <draft-ietf-iab-firewall-req-00.txt>
Behavior of and Requirements for
Internet Firewalls
June 1999
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (1999). All Rights
Reserved.
1. Abstract
This memo defines behavioral characteristics of and
interoperability requirements for Internet firewalls. While
most of these things may seem obvious, current firewall
behavior is often 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
Internet Draft Firewal Requirements June 1999
making the behavior of firewalls more consistent and correct.
2. Introduction
The Internet is 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 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.
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
[2].
3. Characteristics
Firewalls either act as a protocol end point and relay (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,
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(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.
Firewalls acting as packet filters aren't visible as protocol
end points. The firewall examines each packet and then
(1) passes the packet through to the other side unchanged,
(2) drops the packet entirely, or
(3) handles the packet itself in some way.
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
files,
(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 such as SOCKS [1] to gain access
to the Internet, to the internal network, or both.
(This list of decision criteria is only intended to illustrate
the sorts of factors firewalls often consider; it is by no
means exhaustive, nor are all firewall products able to
perform all the operations on this list.)
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4. Firewall Requirements
Applications have to continue to work properly in the presence
of firewalls. This translates into the following transparency
requirement:
The introduction of a firewall and any associated
tunneling or access negotiation facilities MUST NOT cause
gratuitous failures of legitimate and standards-compliant
usage that would work were the firewall not present.
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 failures -- a firewall is entitled to
block any sort of access that a site deems illegitimate,
regardless of whether or not the attemped access is
standards-compliant. This is, after all, the primary reason to
have a firewall in the first place.
Also note that it is perfectly permissible for a firewall to
provide additional facilities applications can use to
authenticate or authorize various sorts of connections. For
example, the SOCKS protocol [1] provides such a facility.
However, firewalls MUST NOT require application implementation
of such facilities in order to operate.
5. Application Requirements
Firewalls are a fact of life that application protocols must
face. As such, application protocols SHOULD be designed to
facilititate operation across firewalls, as long as such
design choices don't adversely impact the application in other
ways. In addition, application protocol specifications MAY
include material defining requirements firewalls must meet to
properly handle a given application protocol.
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6. 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. 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.
Some firewall implementors may 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 is no question that meeting the transparency requirement
is more difficult than not doing so.
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. At present firewalls
are being circumvented in ad hoc ways because they don't meet
this transparency requirement and this necessarily weakens
security dramatically. 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.
7. References
[1] M. Leech, M. Ganis, Y. Lee, R. Kuris, D. Koblas, L.
Jones, "SOCKS Protocol Version 5", RFC 1928, April, 1996.
[2] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
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8. Authors Address
Ned Freed
Innosoft International, Inc.
1050 Lakes Drive
West Covina, CA 91790
USA
tel: +1 626 919 3600 fax: +1 626 919 3614
email: ned.freed@innosoft.com
9. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights
Reserved.
This document and translations of it may be copied and
furnished to others, and derivative works that comment on or
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above copyright notice and this paragraph are included on all
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the copyright notice or references to the Internet Society or
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The limited permissions granted above are perpetual and will
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ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
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USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
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