Internet DRAFT - draft-vyncke-6man-segment-routing-security
draft-vyncke-6man-segment-routing-security
6man Group E. Vyncke, Ed.
Internet-Draft S. Previdi
Intended status: Standards Track Cisco Systems, Inc.
Expires: August 29, 2015 D. Lebrun
Universite Catholique de Louvain
February 25, 2015
IPv6 Segment Routing Security Considerations
draft-vyncke-6man-segment-routing-security-02
Abstract
Segment Routing (SR) allows a node to steer a packet through a
controlled set of instructions, called segments, by prepending a SR
header to the packet. A segment can represent any instruction,
topological or service-based. SR allows to enforce a flow through
any path (topological, or application/service based) while
maintaining per-flow state only at the ingress node to the SR domain.
Segment Routing can be applied to the IPv6 data plane with the
addition of a new type of Routing Extension Header. This document
analyzes the security aspects of the Segment Routing Extension Header
(SRH) and how it is used by SR capable nodes to deliver a secure
service.
Requirements Language
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].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 29, 2015.
Vyncke, et al. Expires August 29, 2015 [Page 1]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Segment Routing Documents . . . . . . . . . . . . . . . . 3
2. Threat model . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Source routing threats . . . . . . . . . . . . . . . . . 4
2.2. Applicability of RFC 5095 to SRH . . . . . . . . . . . . 4
2.3. Service stealing threat . . . . . . . . . . . . . . . . . 5
2.4. Topology disclosure . . . . . . . . . . . . . . . . . . . 5
2.5. ICMP Generation . . . . . . . . . . . . . . . . . . . . . 5
3. Security fields in SRH . . . . . . . . . . . . . . . . . . . 6
3.1. Selecting a hash algorithm . . . . . . . . . . . . . . . 7
3.2. Performance impact of HMAC . . . . . . . . . . . . . . . 7
3.3. Pre-shared key management . . . . . . . . . . . . . . . . 8
4. Deployment Models . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Nodes within the SR domain . . . . . . . . . . . . . . . 9
4.2. Nodes outside of the SR domain . . . . . . . . . . . . . 9
4.3. SR path exposure . . . . . . . . . . . . . . . . . . . . 10
4.4. Impact of BCP-38 . . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Manageability Considerations . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
This document analyzes the security threat model, the security issues
and proposed solutions related to the new routing header for segment
routing with an IPv6 data plane.
Vyncke, et al. Expires August 29, 2015 [Page 2]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
The Segment Routing Header (SRH) is simply another type of the
routing header as described in RFC 2460 [RFC2460] and is:
o inserted by a SR edge router when entering the segment routing
domain or by the originating host itself. The source host can
even be outside the SR domain;
o inspected and acted upon when reaching the destination address of
the IP header per RFC 2460 [RFC2460].
Per RFC2460 [RFC2460], routers on the path that simply forward an
IPv6 packet (i.e. the IPv6 destination address is none of theirs)
will never inspect and process the content of SRH. Routers whose one
interface IPv6 address equals the destination address field of the
IPv6 packet MUST to parse the SRH and, if supported and if the local
configuration allows it, MUST act accordingly to the SRH content.
According to RFC2460 [RFC2460], the default behavior of a non SR-
capable router upon receipt of an IPv6 packet with SRH destined to an
address of its, is to:
o ignore the SRH completely if the Segment Left field is 0 and
proceed to process the next header in the IPv6 packet;
o discard the IPv6 packet if Segment Left field is greater than 0,
it MAY send a Parameter Problem ICMP message back to the Source
Address.
1.1. Segment Routing Documents
Segment Routing terminology is defined in
[I-D.ietf-spring-segment-routing] and in
[I-D.ietf-spring-problem-statement]. Segment Routing use cases are
described in [I-D.filsfils-spring-segment-routing-use-cases].
Segment Routing protocol extensions are defined in
[I-D.ietf-isis-segment-routing-extensions], and
[I-D.ietf-ospf-ospfv3-segment-routing-extensions].
Segment Routing IPv6 use cases are described in
[I-D.ietf-spring-ipv6-use-cases]. And the IPv6 Segment Routing
header is described in [I-D.previdi-6man-segment-routing-header].
2. Threat model
Vyncke, et al. Expires August 29, 2015 [Page 3]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
2.1. Source routing threats
Using a SRH is similar to source routing, therefore it has some well-
known security issues as described in RFC4942 [RFC4942] section 2.1.1
and RFC5095 [RFC5095]:
o amplification attacks: where a packet could be forged in such a
way to cause looping among a set of SR-enabled routers causing
unnecessary traffic, hence a Denial of Service (DoS) against
bandwidth;
o reflection attack: where a hacker could force an intermediate node
to appear as the immediate attacker, hence hiding the real
attacker from naive forensic;
o bypass attack: where an intermediate node could be used as a
stepping stone (for example in a De-Militarized Zone) to attack
another host (for example in the datacenter or any back-end
server).
2.2. Applicability of RFC 5095 to SRH
First of all, the reader must remember this specific part of section
1 of RFC5095 [RFC5095], "A side effect is that this also eliminates
benign RH0 use-cases; however, such applications may be facilitated
by future Routing Header specifications.". In short, it is not
forbidden to create new secure type of Routing Header; for example,
RFC 6554 (RPL) [RFC6554] also creates a new Routing Header type for a
specific application confined in a single network.
In the segment routing architecture described in
[I-D.ietf-spring-segment-routing] there are basically two kinds of
nodes (routers and hosts):
o nodes within the SR domain, which is within one single
administrative domain, i.e., where all nodes are trusted anyway
else the damage caused by those nodes could be worse than
amplification attacks: traffic interception, man-in-the-middle
attacks, more server DoS by dropping packets, and so on.
o nodes outside of the SR domain, which is outside of the
administrative segment routing domain hence they cannot be trusted
because there is no physical security for those nodes, i.e., they
can be replaced by hostile nodes or can be coerced in wrong
behaviors.
The main use case for SR consists of the single administrative domain
where only trusted nodes with SR enabled and configured participate
Vyncke, et al. Expires August 29, 2015 [Page 4]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
in SR: this is the same model as in RFC6554 [RFC6554]. All non-
trusted nodes do not participate as either SR processing is not
enabled by default or because they only process SRH from nodes within
their domain.
Moreover, all SR nodes ignore SRH created by outsiders based on
topology information (received on a peering or internal interface) or
on presence and validity of the HMAC field. Therefore, if
intermediate nodes ONLY act on valid and authorized SRH (such as
within a single administrative domain), then there is no security
threat similar to RH-0. Hence, the RFC 5095 [RFC5095] attacks are
not applicable.
2.3. Service stealing threat
Segment routing is used for added value services, there is also a
need to prevent non-participating nodes to use those services; this
is called 'service stealing prevention'.
2.4. Topology disclosure
The SRH may also contains IPv6 addresses of some intermediate SR-
nodes in the path towards the destination, this obviously reveals
those addresses to the potentially hostile attackers if those
attackers are able to intercept packets containing SRH. On the other
hand, if the attacker can do a traceroute whose probes will be
forwarded along the SR path, then there is little learned by
intercepting the SRH itself. Also the clean-bit of SRH can help by
removing the SRH before forwarding the packet to potentially a non-
trusted part of the network.
2.5. ICMP Generation
Per section 4.4 of RFC2460 [RFC2460], when destination nodes (i.e.
where the destination address is one of theirs) receive a Routing
Header with unsupported Routing Type, the required behavior is:
o If Segments Left is zero, the node must ignore the Routing header
and proceed to process the next header in the packet.
o If Segments Left is non-zero, the node must discard the packet and
send an ICMP Parameter Problem, Code 0, message to the packet's
Source Address, pointing to the unrecognized Routing Type.
This required behavior could be used by an attacker to force the
generation of ICMP message by any node. The attacker could send
packets with SRH (with Segment Left set to 0) destined to a node not
supporting SRH. Per RFC2460 [RFC2460], the destination node could
Vyncke, et al. Expires August 29, 2015 [Page 5]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
generate an ICMP message, causing a local CPU utilization and if the
source of the offending packet with SRH was spoofed could lead to a
reflection attack without any amplification.
It must be noted that this is a required behavior for any unsupported
Routing Type and not limited to SRH packets. So, it is not specific
to SRH and the usual rate limiting for ICMP generation is required
anyway for any IPv6 implementation and has been implemented and
deployed for many years.
3. Security fields in SRH
This section summarizes the use of specific fields in the SRH; they
are integral part of [I-D.previdi-6man-segment-routing-header] and
they are again described here for reader's sake. They are based on a
key-hashed message authentication code (HMAC).
The security-related fields in SRH are:
o HMAC Key-id, 8 bits wide;
o HMAC, 256 bits wide (optional, exists only if HMAC Key-id is not
0).
The HMAC field is the output of the HMAC computation (per RFC 2104
[RFC2104]) using a pre-shared key identified by HMAC Key-id and of
the text which consists of the concatenation of:
o the source IPv6 address;
o First Segment field;
o an octet whose bit-0 is the clean-up bit flag and others are 0;
o HMAC Key-id;
o all addresses in the Segment List.
The purpose of the HMAC field is to verify the validity, the
integrity and the authorization of the SRH itself. If an outsider of
the SR domain does not have access to a current pre-shared secret,
then it cannot compute the right HMAC field and the first SR router
on the path processing the SRH and configured to check the validity
of the HMAC will simply reject the packet.
The HMAC field is located at the end of the SRH simply because only
the router on the ingress of the SR domain needs to process it, then
all other SR nodes can ignore it (based on local policy) because they
Vyncke, et al. Expires August 29, 2015 [Page 6]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
trust the upstream router. This is to speed up forwarding operations
because SR routers which do not validate the SRH do not need to parse
the SRH until the end.
The HMAC Key-id field allows for the simultaneous existence of
several hash algorithms (SHA-256, SHA3-256 ... or future ones) as
well as pre-shared keys. This allows for pre-shared key roll-over
when two pre-shared keys are supported for a while when all SR nodes
converged to a fresher pre-shared key. The HMAC Key-id field is
opaque, i.e., it has neither syntax not semantic except as an index
to the right combination of pre-shared key and hash algorithm and
except that a value of 0 means that there is no HMAC field. It could
also allow for interoperation among different SR domains if allowed
by local policy and assuming a collision-free Key Id allocation.
When a specific SRH is linked to a time-related service (such as
turbo-QoS for a 1-hour period) where the DA, Segment ID (SID) are
identical, then it is important to refresh the shared-secret
frequently as the HMAC validity period expires only when the HMAC
Key-id and its associated shared-secret expires.
3.1. Selecting a hash algorithm
The HMAC field in the SRH is 256 bit wide. Therefore, the HMAC MUST
be based on a hash function whose output is at least 256 bits. If
the output of the hash function is 256, then this output is simply
inserted in the HMAC field. If the output of the hash function is
larger than 256 bits, then the output value is truncated to 256 by
taking the least-significant 256 bits and inserting them in the HMAC
field.
SRH implementations can support multiple hash functions but MUST
implement SHA-2 [FIPS180-4] in its SHA-256 variant.
NOTE: SHA-1 is currently used by some early implementations used for
quick interoperations testing, the 160-bit hash value must then be
right-hand padded with 96 bits set to 0. The authors understand that
this is not secure but is ok for limited tests.
3.2. Performance impact of HMAC
While adding a HMAC to each and every SR packet increases the
security, it has a performance impact. Nevertheless, it must be
noted that:
o the HMAC field is used only when SRH is inserted by a device (such
as a home set-up box) which is outside of the segment routing
domain. If the SRH is added by a router in the trusted segment
Vyncke, et al. Expires August 29, 2015 [Page 7]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
routing domain, then, there is no need for a HMAC field, hence no
performance impact.
o when present, the HMAC field MUST only be checked and validated by
the first router of the segment routing domain, this router is
named 'validating SR router'. Downstream routers MAY NOT inspect
the HMAC field.
o this validating router can also have a cache of <IPv6 header +
SRH, HMAC field value> to improve the performance. It is not the
same use case as in IPsec where HMAC value was unique per packet,
in SRH, the HMAC value is unique per flow.
o Last point, hash functions such as SHA-2 have been optmized for
security and performance and there are multiple implementations
with good performance.
With the above points in mind, the performance impact of using HMAC
is minimized.
3.3. Pre-shared key management
The field HMAC Key-id allows for:
o key roll-over: when there is a need to change the key (the hash
pre-shared secret), then multiple pre-shared keys can be used
simultaneously. The validating routing can have a table of <HMAC
Key-id, pre-shared secret> for the currently active and future
keys.
o different algorithm: by extending the previous table to <HMAC Key-
id, hash function, pre-shared secret>, the validating router can
also support simultaneously several hash algorithms (see section
Section 3.1)
The pre-shared secret distribution can be done:
o in the configuration of the validating routers, either by static
configuration or any SDN oriented approach;
o dynamically using a trusted key distribution such as [RFC6407]
The intent of this document is NOT to define yet-another-key-
distribution-protocol.
Vyncke, et al. Expires August 29, 2015 [Page 8]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
4. Deployment Models
4.1. Nodes within the SR domain
A SR domain is defined as a set of interconnected routers where all
routers at the perimeter are configured to insert and act on SRH.
Some routers inside the SR domain can also act on SRH or simply
forward IPv6 packets.
The routers inside a SR domain can be trusted to generate SRH and to
process SRH received on interfaces that are part of the SR domain.
These nodes MUST drop all SRH packets received on an interface that
is not part of the SR domain and containing a SRH whose HMAC field
cannot be validated by local policies. This includes obviously
packet with a SRH generated by a non-cooperative SR domain.
If the validation fails, then these packets MUST be dropped, ICMP
error messages (parameter problem) SHOULD be generated (but rate
limited) and SHOULD be logged.
4.2. Nodes outside of the SR domain
Nodes outside of the SR domain cannot be trusted for physical
security; hence, they need to request by some trusted means (outside
of the scope of this document) a complete SRH for each new connection
(i.e. new destination address). The received SRH MUST include a HMAC
Key-id and HMAC field which is computed correctly (see Section 3).
When an outside node sends a packet with an SRH and towards a SR
domain ingress node, the packet MUST contain the HMAC Key-id and HMAC
field and the the destination address MUST be an address of a SR
domain ingress node .
The ingress SR router, i.e., the router with an interface address
equals to the destination address, MUST verify the HMAC field with
respect to the HMAC Key-id.
If the validation is successful, then the packet is simply forwarded
as usual for a SR packet. As long as the packet travels within the
SR domain, no further HMAC check needs to be done. Subsequent
routers in the SR domain MAY verify the HMAC field when they process
the SRH (i.e. when they are the destination).
If the validation fails, then this packet MUST be dropped, an ICMP
error message (parameter problem) SHOULD be generated (but rate
limited) and SHOULD be logged.
Vyncke, et al. Expires August 29, 2015 [Page 9]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
4.3. SR path exposure
As the intermediate SR nodes addresses appears in the SRH, if this
SRH is visible to an outsider then he/she could reuse this knowledge
to launch an attack on the intermediate SR nodes or get some insider
knowledge on the topology. This is especially applicable when the
path between the source node and the first SR domain ingress router
is on the public Internet.
The first remark is to state that 'security by obscurity' is never
enough; in other words, the security policy of the SR domain MUST
assume that the internal topology and addressing is known by the
attacker. A simple traceroute will also give the same information
(with even more information as all intermediate nodes between SID
will also be exposed). IPsec Encapsulating Security Payload
[RFC4303] cannot be use to protect the SRH as per RFC4303 the ESP
header must appear after any routing header (including SRH).
To prevent a user to leverage the gained knowledge by intercepting
SRH, it it recommended to apply an infrastructure Access Control List
(iACL) at the edge of the SR domain. This iACL will drop all packets
from outside the SR-domain whose destination is any address of any
router inside the domain. This security policy should be tuned for
local operations.
4.4. Impact of BCP-38
BCP-38 [RFC2827], also known as "Network Ingress Filtering", checks
whether the source address of packets received on an interface is
valid for this interface. The use of loose source routing such as
SRH forces packets to follow a path which differs from the expected
routing. Therefore, if BCP-38 was implemented in all routers inside
the SR domain, then SR packets could be received by an interface
which is not expected one and the packets could be dropped.
As a SR domain is usually a subset of one administrative domain, and
as BCP-38 is only deployed at the ingress routers of this
administrative domain and as packets arriving at those ingress
routers have been normally forwarded using the normal routing
information, then there is no reason why this ingress router should
drop the SRH packet based on BCP-38. Routers inside the domain
commonly do not apply BCP-38; so, this is not a problem.
5. IANA Considerations
There are no IANA request or impact in this document.
Vyncke, et al. Expires August 29, 2015 [Page 10]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
6. Manageability Considerations
TBD
7. Security Considerations
Security mechanisms applied to Segment Routing over IPv6 networks are
detailed in Section 3.
8. Acknowledgements
The authors would like to thank Dave Barach and Stewart Bryant for
their contributions to this document.
9. References
9.1. Normative References
[FIPS180-4]
National Institute of Standards and Technology, "FIPS
180-4 Secure Hash Standard (SHS)", March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
[RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
of Type 0 Routing Headers in IPv6", RFC 5095, December
2007.
[RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
of Interpretation", RFC 6407, October 2011.
9.2. Informative References
Vyncke, et al. Expires August 29, 2015 [Page 11]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
[I-D.filsfils-spring-segment-routing-use-cases]
Filsfils, C., Francois, P., Previdi, S., Decraene, B.,
Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E.
Crabbe, "Segment Routing Use Cases", draft-filsfils-
spring-segment-routing-use-cases-01 (work in progress),
October 2014.
[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
Extensions for Segment Routing", draft-ietf-isis-segment-
routing-extensions-03 (work in progress), October 2014.
[I-D.ietf-ospf-ospfv3-segment-routing-extensions]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
Extensions for Segment Routing", draft-ietf-ospf-ospfv3-
segment-routing-extensions-02 (work in progress), February
2015.
[I-D.ietf-spring-ipv6-use-cases]
Brzozowski, J., Leddy, J., Leung, I., Previdi, S.,
Townsley, W., Martin, C., Filsfils, C., and R. Maglione,
"IPv6 SPRING Use Cases", draft-ietf-spring-ipv6-use-
cases-03 (work in progress), November 2014.
[I-D.ietf-spring-problem-statement]
Previdi, S., Filsfils, C., Decraene, B., Litkowski, S.,
Horneffer, M., and R. Shakir, "SPRING Problem Statement
and Requirements", draft-ietf-spring-problem-statement-03
(work in progress), October 2014.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J.,
and E. Crabbe, "Segment Routing Architecture", draft-ietf-
spring-segment-routing-01 (work in progress), February
2015.
[I-D.previdi-6man-segment-routing-header]
Previdi, S., Filsfils, C., Field, B., and I. Leung, "IPv6
Segment Routing Header (SRH)", draft-previdi-6man-segment-
routing-header-05 (work in progress), January 2015.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
Vyncke, et al. Expires August 29, 2015 [Page 12]
Internet-Draft Pv6 Segment Routing Security Considerations February 2015
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC4942] Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
Co-existence Security Considerations", RFC 4942, September
2007.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554, March
2012.
Authors' Addresses
Eric Vyncke (editor)
Cisco Systems, Inc.
De Kleetlaann 6A
Diegem 1831
Belgium
Email: evyncke@cisco.com
Stefano Previdi
Cisco Systems, Inc.
Via Del Serafico, 200
Rome 00142
Italy
Email: sprevidi@cisco.com
David Lebrun
Universite Catholique de Louvain
Place Ste Barbe, 2
Louvain-la-Neuve, 1348
Belgium
Email: david.lebrun@uclouvain.be
Vyncke, et al. Expires August 29, 2015 [Page 13]
