IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Intended status: Best Current Practice W. Liu
Expires: November 28, 2016 Huawei Technologies
May 27, 2016
IPv6 Address Usage Recommendations
draft-gont-6man-address-usage-recommendations-00
Abstract
IPv6 hosts typically configure and use a number of addresses of
different scope and stability properties. Recent work has analyzed
the security and privacy implications of IPv6 addressing, and
improved the security and privacy properties of some of the
aforementioned address types. However, advice is still missing
guidance regarding which address properties are desirable in
different scenarios, and how such addresses should be used when they
are configured. This document complements the aforementioned work by
providing advice regarding which address types to configure and how
to employ them in a number of popular scenarios.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 28, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Address Scope Considerations . . . . . . . . . . . . . . . . 3
4. Address Stability Considerations . . . . . . . . . . . . . . 3
5. Usage Type Considerations . . . . . . . . . . . . . . . . . . 5
6. Advice on IPv6 Address Configuration . . . . . . . . . . . . 6
7. Advice on IPv6 Address Usage . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
9. Security Considerations . . . . . . . . . . . . . . . . . . . 6
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
11.1. Normative References . . . . . . . . . . . . . . . . . . 6
11.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
A typical IPv6 host may have multiple IPv6 addresses available, which
may differ in multiple aspects, such as address scope and address
persistence (e.g. stable addresses vs. temporary addresses).
Given previous work in this area [RFC7721], we expect (and assume in
the rest of this document) that implementations have replaced any
schemes that produce predictable addresses with alternative schemes
that avoid such patterns (e.g., RFC7217 in replacement of the
traditional SLAAC addresses that embed link-layer addresses).
There are three parameters that affect the security and privacy
properties of an address:
o Scope
o Stability
o Usage type (client-like "outgoing connections" vs. server-like
"incoming connections")
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Section 3, Section 4, and Section 5 discuss the security and privacy
implications (and associated tradeoffs) of the scope, stability and
usage type properties of IPv6 addresses, respectively.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Address Scope Considerations
The IPv6 address scope can, in some scenarios, limit the attack
exposure of a node as a result of the implicit isolation that may be
implied by a non-global address scope. For example, a node that only
employs link-local addresses may, in principle, only be reached
exposed to attack to other nodes in the local link. Hosts employing
only Unique Local Addresses (ULAs) may be more isolated from attack
than those employing Global Unicast Addresses (GUAs), assuming that
proper packet filtering is enforced on the network edge.
The potential protection provided by a non-global addresses should
not be regarded as a complete security strategy, but rather as a form
of "prophylactic" security (see
[I-D.gont-opsawg-firewalls-analysis]).
We note that the use of non-global addresses is usually limited to a
reduced type of applications/protocol that e.g. are only meant to
operate on a reduced scope, and hence their applicability may be
limited.
A discussion of ULA usage considerations can be found in
[I-D.ietf-v6ops-ula-usage-considerations].
4. Address Stability Considerations
The stability of an address has two associated security/privacy
implications:
o Ability of an attacker to correlate network activity
o Exposure to attack
For obvious reasons, an address that is employed for multiple
communication instances allows the aforementioned network activities
to be correlated. The longer an address is employed (i.e., the more
stable), the longer such correlation will be possible. In the worst-
case scenario, a stable address that is employed for multiple
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communication instances over time will allow all such activities to
be correlated. On the other hand, if a host were to generate (and
eventually "throw away") one new address for each communication
instance (e.g., TCP connection), network activity correlation would
be mitigated.
Typically, when it comes to attack exposure, the longer an address is
employed the longer an attacker is exposed to attacks (e.g. an
attacker has more time to find the address in the first place
[RFC7707]). While such exposure is traditionally associated with the
stability of the address, the usage type of the address (see
Section 5) may also have an impact on attack exposure.
A popular approach to mitigate network activity correlation is that
known as "temporary addresses". Temporary addresses are typically
configured and employed along with stable addresses, with the
temporary addresses being employed for outgoing communications. We
note that the extent to which temporary addresses provide improved
mitigation of network activity correlation and/or reduced attack
exposure may be questionable in a number of scenarios. For example,
a temporary address that is reachable for, say, a few hours has a
questionable "reduced exposure" (particularly when automated attack
tools do not typically require such a long period of time to complete
their task). Similarly, if network activity can be correlated for
the life of such address (e.g., in the order of several hours), there
are scenarios in which such period of time would be long enough for
an attacker to correlate all the network activity he is meaning to
correlate.
NOTE: Ongoing work [I-D.gont-6man-non-stable-iids] aims at
updating [RFC4941] such that temporary addresses can be employed
without the need to configure stable addresses.
In order to better mitigate network activity correlation and/or
possibly reduce host exposure, an implementation might want to either
reduce the preferred lifetime of a temporary address, or even better,
generate one new temporary address for each new transport protocol
instance. The associated lifetime/stability of an address typically
may have a negative impact on the network. For example, if a node
were to employ "throw away" connections, or employ temporary
addresses [RFC4941] with a short preferred lifetime, and the node
were to use lots of outgoing connections, nodes might need to
maintain too many entries in their Neighbor Cache, and a number of
devices (possibly enforcing security policies) might also need to
keep such additional state.
Enforcing a maximum lifetime on IPv6 addresses may cause long-lived
TCP connections to fail. For example, an address becoming "Invalid"
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(after transiting through the "Preferred" and "Deprecated" status)
would cause the TCP connections employing them to break. This, in
turn, would cause e.g. long-lived SSH sessions to break/fail.
In some scenarios, attack exposure may be reduced by limiting the
usage of temporary addresses to outbound connections, and prevent
such addresses from being used for inbound connections (please see
Section 5).
5. Usage Type Considerations
A node that employs one of its addresses to communicate with an
external server (i.e., to perform an "outgoing connection") may cause
such address to become exposed to attack. For example, once the
external server receives an incoming connection, the corresponding
server may scan the client's address for network services. A real-
world instance of this attack scenario has been documented in [Hein].
However, employing an IPv6 address for an outgoing session/connection
need not increase the exposure of local services to the parties to
which the client connects. For example, nodes could employ temporary
addresses only for outgoing connections, but not for incoming
connections. Thus, external nodes that learn about client's
addresses could not really leverage such addresses for actively
contacting the clients.
There are multiple ways in which this could possibly be achieved,
with different implications. Namely:
Run a host-based firewall
Bind services to specific (explicit) addresses
Bind services only to stable addresses
A client could simply run a host-based firewall that only allows
incoming connections on the stable addresses. This is clearly more
of an operational way of achieving the desired functionality, and may
require good firewall/host integration (e.g., the firewall should be
able to tell stable vs. temporary addresses), may require the client
to run additional firewall software for this specific purpose, etc.
Services could be bound to specific (explicit) addresses. However,
there are a number of short-comings associated with this approach.
Firstly, an application would need to be able to learn all of its
addresses and associated stability properties, something that tends
to be non-trivial, non-portable, and that makes the application
unnecessarily protocol-dependent. Secondly, the Sockets API does not
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really allow a socket to be bound to a subset of the node's
addresses. That is, sockets can be bound to a single address or to
all available addresses (wildcard), but not to a subset of all the
available addresses.
Binding services only to stable addresses provides a clean separation
between addresses employed for client-like outgoing connections and
server-like incoming connections. However, we currently lack an
appropriate API for nodes to be able to specify that a socket should
only be bound t stable addresses. This could be considered for
future work.
6. Advice on IPv6 Address Configuration
[TBD]
7. Advice on IPv6 Address Usage
[TBD]
8. IANA Considerations
There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.
9. Security Considerations
This document discusses address usage considerations, and also
describes possible future standards-track work to allow for greater
flexibility in IPv6 address usage.
10. Acknowledgements
[TBD]
11. References
11.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<http://www.rfc-editor.org/info/rfc4941>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<http://www.rfc-editor.org/info/rfc7217>.
11.2. Informative References
[RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
<http://www.rfc-editor.org/info/rfc7707>.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<http://www.rfc-editor.org/info/rfc7721>.
[I-D.ietf-v6ops-ula-usage-considerations]
Liu, B. and S. Jiang, "Considerations For Using Unique
Local Addresses", draft-ietf-v6ops-ula-usage-
considerations-00 (work in progress), February 2016.
[I-D.gont-6man-non-stable-iids]
Gont, F. and S. LIU, "Recommendation on Non-Stable IPv6
Interface Identifiers", draft-gont-6man-non-stable-iids-00
(work in progress), May 2016.
[I-D.gont-opsawg-firewalls-analysis]
Gont, F. and F. Baker, "On Firewalls in Network Security",
draft-gont-opsawg-firewalls-analysis-02 (work in
progress), February 2016.
[Hein] Hein, B., "The Rising Sophistication of Network Scanning",
January 2016, <http://netpatterns.blogspot.be/2016/01/
the-rising-sophistication-of-network.html>.
Authors' Addresses
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Fernando Gont
SI6 Networks / UTN-FRH
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: http://www.si6networks.com
Will(Shucheng) Liu
Huawei Technologies
Bantian, Longgang District
Shenzhen 518129
P.R. China
Email: liushucheng@huawei.com
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