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Internet Engineering Task Force P. Savola
Internet Draft CSC/FUNET
Expiration Date: March 2003
September 2002
Firewalling Considerations for IPv6
draft-savola-v6ops-firewalling-00.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
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Abstract
There are quite a few potential problems regarding firewalling or
packet filtering in IPv6 environment. These include slight ambiguity
in the IPv6 specification, problems parsing packets beyond unknown
Extension Headers and Destination Options, and introduction of end-
to-end encrypted traffic and peer-to-peer applications. There may
also be need to extend packet matching to include some Extension
Header or Destination Option fields. This draft discusses these
issues to raise awareness and proposes some tentative solutions or
workarounds.
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Table of Contents
1. Introduction ............................................... 2
2. Ambiguous Text in the IPv6 Specification ................... 3
2.1. The Problem ............................................ 3
2.2. Possible Solutions ..................................... 4
3. Parsing Extension Header Chains ............................ 4
3.1. The Problem ............................................ 4
3.2. Possible Solutions ..................................... 4
4. Parsing Unknown Destination Options and Security Policy .... 5
4.1. The Problem ............................................ 5
4.2. Possible Solutions ..................................... 5
5. Firewalls and End-to-End IPSEC-encrypted ESP-traffic ....... 6
5.1. The Problem ............................................ 6
5.2. Possible Solutions ..................................... 6
6. Firewalls and Interactions with Peer-to-Peer Applications .. 6
6.1. The Problem ............................................ 6
6.2. Possible Solutions ..................................... 7
7. Security Considerations .................................... 7
8. Acknowledgements ........................................... 7
9. References ................................................. 7
9.1. Normative References ................................... 7
9.2. Informative References ................................. 7
Author's Address ............................................... 8
A. Possible Desirable Header Field Matching Extensions ........ 8
1. Introduction
There are quite a few potential problems regarding firewalling or
packet filtering in IPv6 environment. These include slight ambiguity
in the IPv6 specification, problems parsing packets beyond unknown
Extension Headers and Destination Options, and introduction of end-
to-end encrypted traffic and peer-to-peer applications. There may
also be need to extend packet matching to include some Extension
Header or Destination Option fields. This draft discusses these
issues to raise awareness and proposes some tentative solutions or
workarounds.
In section 2, slightly ambiguous text in the IPv6 specification is
discussed. In section 3, syntactical problem with parsing unknown
Extension Headers is pointed out. In section 4, a similar problem
with Destination Options is discussed in the context of security
policy. In section 5, implications of end-to-end encrypted traffic
are considerated. In section 6, similar implications of peer-to-peer
applications are mentioned. In appendix A, some possibly useful
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packet matching extensions for IPv6 are brought up.
In this document, the term "firewall" is used to mean any kind of
packet filter; no special features (like statefullness or L7 packet
inspection) is assumed.
2. Ambiguous Text in the IPv6 Specification
2.1. The Problem
The [IPV6] specification forbids skipping over any of the headers
before processing them or processing them at all before reaching the
destination (section 4):
"With one exception, Extension Headers are not examined or processed
by any node along a packet's delivery path, until the packet reaches
the node (or each of the set of nodes, in the case of multicast)
identified in the Destination Address field of the IPv6 header.
There, normal demultiplexing on the Next Header field of the IPv6
header invokes the module to process the first Extension Header, or
the upper-layer header if no Extension Header is present. The
contents and semantics of each Extension Header determine whether or
not to proceed to the next header. Therefore, Extension Headers must
be processed strictly in the order they appear in the packet; a
receiver must not, for example, scan through a packet looking for a
particular kind of Extension Header and process that header prior to
processing all preceding ones."
And:
"If, as a result of processing a header, a node is required to
proceed to the next header but the Next Header value in the current
header is unrecognized by the node, it should discard the packet and
send an ICMP Parameter Problem message to the source of the packet,
with an ICMP Code value of 1 ("unrecognized Next Header type
encountered") and the ICMP Pointer field containing the offset of the
unrecognized value within the original packet. The same action
should be taken if a node encounters a Next Header value of zero in
any header other than an IPv6 header."
Similar applies to the specified Destination Options processing
behaviour: if the Option Type has been specified so that the packet
should not be processed further in the case of unrecognized options
(ie. the highest-order two bits are not "00"), should the firewall
also discard the packet and/or send ICMP Parameter Problem message
back to the source?
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Are these also to be done by intermediate nodes (which, by some
definition, should not be processing Extension Headers or Destination
Options Header with Hop-by-Hop options as an exception); this seems
unlikely.
This wording clearly does not take into the account that there might
be middleboxes, or non-final destinations, that could be processing
the packet.
2.2. Possible Solutions
The correct behaviour must be made clear; the wording should be
clarified. Clarifications might be needed at least on:
1. whether intermediate nodes should be taken into account in the
text describing the header processing
2. intermediate nodes' behaviour when detecting unrecognized
headers
3. Parsing Extension Header Chains
3.1. The Problem
IPv4 [RFC1122] [RFC1812] silently ignores options it does not
recognize; options have a specific, pre-defined format. IPv6
Extension Headers are structured differently: the header format can
change, and generally it is not possible to parse the header, or
proceed to the following Extension Headers unless the processing of
the previous header has been implemented.
The above is problematic as it is often the case that a packet filter
will want to examine terminal headers, e.g. TCP or UDP. That is not
possible if there is a problem processing any one of the preceding
headers.
3.2. Possible Solutions
In the generic case, even ignoring the IPv6 specification, unknown
headers cannot be skipped over except by making some very wild
guesses of the content. Thus the solutions (or work-arounds) are:
1. always keep the packet filter up-to-date, so that it can parse
all types of Extension Headers
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2. never introduce new Extension Headers, except possibly in a
very controlled manner; use Destination Options instead
3. standardize the format (for at least the first N bytes
including at least the length and the next header value) of
possibly later specified new Extension Headers, so that
skipping over headers could be possible
The first is not a workable solution in a generic case at least if
it's expected that new Extension Headers should be introducable: the
lifetime of firewall devices and software seems to be much longer
than one would expect. For example, it is good to consider the case
of buggy firewalls and ECN support [ECN]: even though software fixes
may have been available for a long time, upgrades have not taken
place, hindering the deployment of new technology.
The second seems quite a bleak work-around, but as currently
specified, there is little choice; most (if not all) new features can
probably be implemented using Destination Options. However, it's
still good to document and understand this deployment and
specification deadlock.
The third might be doable but it would require some standardization
effort.
4. Parsing Unknown Destination Options and Security Policy
4.1. The Problem
Similar to the above, Destination Options may also include unknown
options. However, the options are encoded in the TLV-format. So,
skipping over unknown options is technically possible.
However, especially in a very controlled environments, where a
firewall may implement a strict security policy, it may be desirable
to reject any options the firewall does not recognize (which may
cause the end-nodes to do something that has not been anticipated in
the security policy controlled by the firewall).
4.2. Possible Solutions
No protocol action seems to be necessary provided that the
implementation would not, in this case, send ICMP messages or discard
packets upon receiving an unknown header.
However, it may be desirable for firewall implementations to have a
feature controlling the handling behaviour of unrecognized
Destination Options.
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5. Firewalls and End-to-End IPSEC-encrypted ESP-traffic
5.1. The Problem
With the promise of the restoration of end-to-end transparency, and
if at least some of the challenges for implementing PKI's are worked
around, it may be possible that the amount of end-to-end encrypted
traffic will increase enermously. The traffic is likely to be
encrypted using IPSEC.
In this case, on-the-path observers (such as a firewall) do not have
the possibility to examine usually critical headers (such as
TCP/UDP). This may result in an administrative decision to disable
IPSEC-encrypted traffic by filtering it out.
5.2. Possible Solutions
It would be desirable, if the users wish to do so, be able to have
the firewall or some node the firewall is configured to trust as an
intermediary in SPI negotiation/configuration. However, even though
this may mitigate the risks somewhat, but it appears that SPI's could
be reused (without the intermediary) in such a way that entirely
different kind of traffic could be sent. There is no fix for this,
by the definition of End-to-End cryptography.
One could try to encode some interesting values, e.g. protocol
numbers and ports in the Flow Label field; one problem here is the
relatively limited length of 20 bits. But this would have to be done
in the source node, which is not usually (at least completely)
trusted in this context.
There appears to be no solution for this, which is indeed a feature
of end-to-end cryptography.
6. Firewalls and Interactions with Peer-to-Peer Applications
6.1. The Problem
As above, the restoration of end-to-end transparency provides a
possibility for a more wide-spread use of peer-to-peer applications.
Such applications are often a bit problematic from the firewall
perspective: it is often the practise to allow outbound (from the
protected site) traffic while allowing in only the related traffic
(and naturally some other administratively permitted traffic). Being
able to run (some) peer-to-peer applications easily in a controlled
environment would be valuable.
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6.2. Possible Solutions
One workaround would be to try to standardize some default port
ranges (in an application-specific manner) for such applications as
these, for example in the above 32768 range. In this way, a site
could enable/disable (default) port ranges for (some) peer-to-peer
applications at will. A major disadvantage here would be that this
could violate the trust model: some applications could intentionally
try to use some other's port range to gain entry through the firewall
even if the default range was blocked. This implies there's at least
some form of trust here.
A real, but possibly quite a complex solution would be to implement
some form of peer-to-peer "pinholing" [MIDCOM]. This hasn't yet been
standardized even for IPv4 (though the concept is quite protocol-
independent).
7. Security Considerations
This draft discusses security considerations related to IPv6
firewalling. When discussing potential solutions for problems, the
weaknesses are also pointed out.
In general, the firewall does not, and cannot, often trust the
node(s) it protects. These may even belong to different
administrative entit(y/ies). In that case, making compromises will
usually open some holes in the firewall.
8. Acknowledgements
Brian Carpenter suggested an IPv6 firewall could support P2P
pinholing.
9. References
9.1. Normative References
[IPV6] Deering, S., Hinden, R., "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
9.2. Informative References
[RFC1122] Braden, R. (Editor), "Requirements for Internet Hosts
-- Communication Layers", RFC1122, October 1989.
[RFC1812] Baker, F. (Editor), "Requirements for IP Version 4
Routers", RFC1812, June 1995.
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[ECN] Garzik, J., "ECN-under-Linux Unofficial Vendor Support
Page", http://gtf.org/garzik/ecn/, March 2002.
[MIDCOM] Srisuresh, P. et al, "Middlebox communication
architecture and framework", RFC3303, August 2002.
[RHHAOSEC] Savola, P. "Security of IPv6 Routing Header and
Home Address Options", work-in-progress,
draft-savola-ipv6-rh-ha-security-02.txt, March 2002.
[MIPV6] Johnson, D., et al, "Mobility Support in IPv6",
draft-ietf-mobileip-ipv6-18.txt, work-in-progress, June 2002.
[MIPV6CAO] O'Neill, A., "MIPv6 Care of Address Option",
draft-oneill-mipv6-cao-00.txt, work-in-progress, Sep 2002.
Author's Address
Pekka Savola
CSC/FUNET
Espoo, Finland
EMail: psavola@funet.fi
A. Possible Desirable Header Field Matching Extensions
As Destination options and Extension Header types are taken into use,
it may be desirable for a firewall to support some matching against
certain header fields. These include, for example:
- whether or not a specific Extension Header or a Destination
Option is detected
- behaviour when an unknown (or specified) Extension Header or
Destination Option is detected
- (Routing Header -specific) being able to match segments left
(mainly, whether it is zero or not), type and the next-to-be-
swapped destination(s) [RHHAOSEC]
- (Home Address Option [MIPV6] -specific) being able to match
against the home address
- (Care-of Address Option [MIPV6COA] -specific) being able to match
against the care-of address used for ingress filtering
- (ESP/AH -specific) being able to match against SPI
- (Tunneled-traffic specific) being able to match against the
embedded IPv4 address in e.g. 6to4, ISATAP, etc.
Some of these are much more useful than others; the list is only
meant to give ideas about possibly useful (in some scenarios, at
least) functionalities.
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