RFC4552: Authentication/Confidentiality for OSPFv3
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Related keywords:
(ah/esp)
(authentication header)
(encapsulating security payload)
(open shortest path first)
Network Working Group M. Gupta
Request for Comments: 4552 Tropos Networks
Category: Standards Track N. Melam
Juniper Networks
June 2006
Authentication/Confidentiality for OSPFv3
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes means and mechanisms to provide
authentication/confidentiality to OSPFv3 using an IPv6 Authentication
Header/Encapsulating Security Payload (AH/ESP) extension header.
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RFC 4552 Authentication/Confidentiality for OSPFv3 June 2006
Table of Contents
1. Introduction ....................................................2
1.1. Conventions Used in This Document ..........................2
2. Transport Mode vs. Tunnel Mode ..................................3
3. Authentication ..................................................3
4. Confidentiality .................................................3
5. Distinguishing OSPFv3 from OSPFv2 ...............................4
6. IPsec Requirements ..............................................4
7. Key Management ..................................................5
8. SA Granularity and Selectors ....................................7
9. Virtual Links ...................................................8
10. Rekeying .......................................................9
10.1. Rekeying Procedure ........................................9
10.2. KeyRolloverInterval .......................................9
10.3. Rekeying Interval ........................................10
11. IPsec Protection Barrier and SPD ..............................10
12. Entropy of Manual Keys ........................................12
13. Replay Protection .............................................12
14. Security Considerations .......................................12
15. References ....................................................13
15.1. Normative References .....................................13
15.2. Informative References ...................................13
1. Introduction
OSPF (Open Shortest Path First) Version 2 [N1] defines the fields
AuType and Authentication in its protocol header to provide security.
In OSPF for IPv6 (OSPFv3) [N2], both of the authentication fields
were removed from OSPF headers. OSPFv3 relies on the IPv6
Authentication Header (AH) and IPv6 Encapsulating Security Payload
(ESP) to provide integrity, authentication, and/or confidentiality.
This document describes how IPv6 AH/ESP extension headers can be used
to provide authentication/confidentiality to OSPFv3.
It is assumed that the reader is familiar with OSPFv3 [N2], AH [N5],
ESP [N4], the concept of security associations, tunnel and transport
mode of IPsec, and the key management options available for AH and
ESP (manual keying [N3] and Internet Key Exchange (IKE)[I1]).
1.1. Conventions Used in This Document
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 [N7].
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2. Transport Mode vs. Tunnel Mode
The transport mode Security Association (SA) is generally used
between two hosts or routers/gateways when they are acting as hosts.
The SA must be a tunnel mode SA if either end of the security
association is a router/gateway. Two hosts MAY establish a tunnel
mode SA between themselves. OSPFv3 packets are exchanged between
routers. However, since the packets are locally delivered, the
routers assume the role of hosts in the context of tunnel mode SA.
All implementations conforming to this specification MUST support
transport mode SA to provide required IPsec security to OSPFv3
packets. They MAY also support tunnel mode SA to provide required
IPsec security to OSPFv3 packets.
3. Authentication
Implementations conforming to this specification MUST support
authentication for OSPFv3.
In order to provide authentication to OSPFv3, implementations MUST
support ESP and MAY support AH.
If ESP in transport mode is used, it will only provide authentication
to OSPFv3 protocol packets excluding the IPv6 header, extension
headers, and options.
If AH in transport mode is used, it will provide authentication to
OSPFv3 protocol packets, selected portions of IPv6 header, selected
portions of extension headers, and selected options.
When OSPFv3 authentication is enabled,
o OSPFv3 packets that are not protected with AH or ESP MUST be
silently discarded.
o OSPFv3 packets that fail the authentication checks MUST be
silently discarded.
4. Confidentiality
Implementations conforming to this specification SHOULD support
confidentiality for OSPFv3.
If confidentiality is provided, ESP MUST be used.
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When OSPFv3 confidentiality is enabled,
o OSPFv3 packets that are not protected with ESP MUST be silently
discarded.
o OSPFv3 packets that fail the confidentiality checks MUST be
silently discarded.
5. Distinguishing OSPFv3 from OSPFv2
The IP/IPv6 Protocol Type for OSPFv2 and OSPFv3 is the same (89), and
OSPF distinguishes them based on the OSPF header version number.
However, current IPsec standards do not allow using arbitrary
protocol-specific header fields as the selectors. Therefore, the
OSPF version field in the OSPF header cannot be used to distinguish
OSPFv3 packets from OSPFv2 packets. As OSPFv2 is only for IPv4 and
OSPFv3 is only for IPv6, the version field in the IP header can be
used to distinguish OSPFv3 packets from OSPFv2 packets.
6. IPsec Requirements
In order to implement this specification, the following IPsec
capabilities are required.
Transport mode
IPsec in transport mode MUST be supported. [N3]
Multiple Security Policy Databases (SPDs)
The implementation MUST support multiple SPDs with an SPD
selection function that provides an ability to choose a specific
SPD based on interface. [N3]
Selectors
The implementation MUST be able to use source address, destination
address, protocol, and direction as selectors in the SPD.
Interface ID tagging
The implementation MUST be able to tag the inbound packets with
the ID of the interface (physical or virtual) via which it
arrived. [N3]
Manual key support
Manually configured keys MUST be able to secure the specified
traffic. [N3]
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Encryption and authentication algorithms
The implementation MUST NOT allow the user to choose stream
ciphers as the encryption algorithm for securing OSPFv3 packets
since the stream ciphers are not suitable for manual keys.
Except when in conflict with the above statement, the key words
"MUST", "MUST NOT", "REQUIRED", "SHOULD", and "SHOULD NOT" that
appear in the [N6] document for algorithms to be supported are to
be interpreted as described in [N7] for OSPFv3 support as well.
Dynamic IPsec rule configuration
The routing module SHOULD be able to configure, modify, and delete
IPsec rules on the fly. This is needed mainly for securing
virtual links.
Encapsulation of ESP packet
IP encapsulation of ESP packets MUST be supported. For
simplicity, UDP encapsulation of ESP packets SHOULD NOT be used.
Different SAs for different Differentiated Services Code Points
(DSCPs)
As per [N3], the IPsec implementation MUST support the
establishment and maintenance of multiple SAs with the same
selectors between a given sender and receiver. This allows the
implementation to associate different classes of traffic with the
same selector values in support of Quality of Service (QoS).
7. Key Management
OSPFv3 exchanges both multicast and unicast packets. While running
OSPFv3 over a broadcast interface, the authentication/confidentiality
required is "one to many". Since IKE is based on the Diffie-Hellman
key agreement protocol and works only for two communicating parties,
it is not possible to use IKE for providing the required "one to
many" authentication/confidentiality. This specification mandates
the usage of Manual Keying with current IPsec implementations.
Future specifications can explore the usage of protocols like
Kerberized Internet Negotiation of Keys/Group Secure Association Key
Management Protocol (KINK/GSAKMP) when they are widely available. In
manual keying, SAs are statically installed on the routers and these
static SAs are used to authenticate/encrypt packets.
The following discussion explains that it is not scalable and is
practically infeasible to use different security associations for
inbound and outbound traffic to provide the required "one to many"
security. Therefore, the implementations MUST use manually
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configured keys with the same SA parameters (Security Parameter Index
(SPI), keys, etc.) for both inbound and outbound SAs (as shown in
Figure 3).
A |
SAa ------------>|
SAb <------------|
|
B |
SAb ------------>|
SAa <------------| Figure 1
|
C |
SAa/SAb ------------>|
SAa/SAb <------------|
|
Broadcast
Network
If we consider communication between A and B in Figure 1, everything
seems to be fine. A uses security association SAa for outbound
packets and B uses the same for inbound packets and vice versa. Now
if we include C in the group and C sends a packet using SAa, then
only A will be able to understand it. Similarly, if C sends a packet
using SAb, then only B will be able to understand it. Since the
packets are multicast and they are going to be processed by both A
and B, there is no SA for C to use so that both A and B can
understand them.
A |
SAa ------------>|
SAb <------------|
SAc <------------|
|
B |
SAb ------------>|
SAa <------------| Figure 2
SAc <------------|
|
C |
SAc ------------>|
SAa <------------|
SAb <------------|
|
Broadcast
Network
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The problem can be solved by configuring SAs for all the nodes on
every other node as shown in Figure 2. So A, B, and C will use SAa,
SAb, and SAc, respectively, for outbound traffic. Each node will
lookup the SA to be used based on the source (A will use SAb and SAc
for packets received from B and C, respectively). This solution is
not scalable and practically infeasible because a large number of SAs
will need to be configured on each node. Also, the addition of a
node in the broadcast network will require the addition of another SA
on every other node.
A |
SAo ------------>|
SAi <------------|
|
B |
SAo ------------>|
SAi <------------| Figure 3
|
C |
SAo ------------>|
SAi <------------|
|
Broadcast
Network
The problem can be solved by using the same SA parameters (SPI, keys,
etc.) for both inbound (SAi) and outbound (SAo) SAs as shown in
Figure 3.
8. SA Granularity and Selectors
The user SHOULD be given the choice of sharing the same SA among
multiple interfaces or using a unique SA per interface.
OSPFv3 supports running multiple instances over one interface using
the "Instance Id" field contained in the OSPFv3 header. As IPsec
does not support arbitrary fields in the protocol header to be used
as the selectors, it is not possible to use different SAs for
different OSPFv3 instances running over the same interface.
Therefore, all OSPFv3 instances running over the same interface will
have to use the same SA. In OSPFv3 RFC terminology, SAs are per-link
and not per-interface.
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9. Virtual Links
A different SA than the SA of the underlying interface MUST be
provided for virtual links. Packets sent on virtual links use
unicast non-link local IPv6 addresses as the IPv6 source address,
while packets sent on other interfaces use multicast and unicast link
local addresses. This difference in the IPv6 source address
differentiates the packets sent on virtual links from other OSPFv3
interface types.
As the virtual link end point IPv6 addresses are not known, it is not
possible to install SPD/Security Association Database (SAD) entries
at the time of configuration. The virtual link end point IPv6
addresses are learned during the routing table computation process.
The packet exchange over the virtual links starts only after the
discovery of the end point IPv6 addresses. In order to protect these
exchanges, the routing module must install the corresponding SPD/SAD
entries before starting these exchanges. Note that manual SA
parameters are preconfigured but not installed in the SAD until the
end point addresses are learned.
According to the OSPFv3 RFC [N2], the virtual neighbor's IP address
is set to the first prefix with the "LA-bit" set from the list of
prefixes in intra-area-prefix-Link State Advertisements (LSAs)
originated by the virtual neighbor. But when it comes to choosing
the source address for the packets that are sent over the virtual
link, the RFC [N2] simply suggests using one of the router's own
global IPv6 addresses. In order to install the required security
rules for virtual links, the source address also needs to be
predictable. Hence, routers that implement this specification MUST
change the way the source and destination addresses are chosen for
packets exchanged over virtual links when IPsec is enabled.
The first IPv6 address with the "LA-bit" set in the list of prefixes
advertised in intra-area-prefix-LSAs in the transit area MUST be used
as the source address for packets exchanged over the virtual link.
When multiple intra-area-prefix-LSAs are originated, they are
considered concatenated and are ordered by ascending Link State ID.
The first IPv6 address with the "LA-bit" set in the list of prefixes
received in intra-area-prefix-LSAs from the virtual neighbor in the
transit area MUST be used as the destination address for packets
exchanged over the virtual link. When multiple intra-area-prefix-
LSAs are received, they are considered concatenated and are ordered
by ascending Link State ID.
This makes both the source and destination addresses of packets
exchanged over the virtual link predictable when IPsec is enabled.
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10. Rekeying
To maintain the security of a link, the authentication and encryption
key values SHOULD be changed periodically.
10.1. Rekeying Procedure
The following three-step procedure SHOULD be provided to rekey the
routers on a link without dropping OSPFv3 protocol packets or
disrupting the adjacency.
(1) For every router on the link, create an additional inbound SA for
the interface being rekeyed using a new SPI and the new key.
(2) For every router on the link, replace the original outbound SA
with one using the new SPI and key values. The SA replacement
operation should be atomic with respect to sending OSPFv3 packets
on the link so that no OSPFv3 packets are sent without
authentication/encryption.
(3) For every router on the link, remove the original inbound SA.
Note that all routers on the link must complete step 1 before any
begin step 2. Likewise, all the routers on the link must complete
step 2 before any begin step 3.
One way to control the progression from one step to the next is for
each router to have a configurable time constant KeyRolloverInterval.
After the router begins step 1 on a given link, it waits for this
interval and then moves to step 2. Likewise, after moving to step 2,
it waits for this interval and then moves to step 3.
In order to achieve smooth key transition, all routers on a link
should use the same value for KeyRolloverInterval and should initiate
the key rollover process within this time period.
At the end of this procedure, all the routers on the link will have a
single inbound and outbound SA for OSPFv3 with the new SPI and key
values.
10.2. KeyRolloverInterval
The configured value of KeyRolloverInterval should be long enough to
allow the administrator to change keys on all the OSPFv3 routers. As
this value can vary significantly depending upon the implementation
and the deployment, it is left to the administrator to choose an
appropriate value.
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10.3. Rekeying Interval
This section analyzes the security provided by manual keying and
recommends that the encryption and authentication keys SHOULD be
changed at least every 90 days.
The weakest security provided by the security mechanisms discussed in
this specification is when NULL encryption (for ESP) or no encryption
(for AH) is used with the HMAC-MD5 authentication. Any other
algorithm combinations will at least be as hard to break as the ones
mentioned above. This is shown by the following reasonable
assumptions:
o NULL Encryption and HMAC-SHA-1 Authentication will be more
secure as HMAC-SHA-1 is considered to be more secure than
HMAC-MD5.
o NON-NULL Encryption and NULL Authentication combination is not
applicable as this specification mandates authentication when
OSPFv3 security is enabled.
o Data Encryption Security (DES) Encryption and HMAC-MD5
Authentication will be more secure because of the additional
security provided by DES.
o Other encryption algorithms like 3DES and the Advanced
Encryption Standard (AES) will be more secure than DES.
RFC 3562 [I4] analyzes the rekeying requirements for the TCP MD5
signature option. The analysis provided in RFC 3562 is also
applicable to this specification as the analysis is independent of
data patterns.
11. IPsec Protection Barrier and SPD
The IPsec protection barrier MUST be around the OSPF protocol.
Therefore, all the inbound and outbound OSPF traffic goes through
IPsec processing.
The SPD selection function MUST return an SPD with the following rule
for all the interfaces that have OSPFv3
authentication/confidentiality disabled.
No. source destination protocol action
1 any any OSPF bypass
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The SPD selection function MUST return an SPD with the following
rules for all the interfaces that have OSPFv3
authentication/confidentiality enabled.
No. source destination protocol action
2 fe80::/10 any OSPF protect
3 fe80::/10 any ESP/OSPF or AH/OSPF protect
4 src/128 dst/128 OSPF protect
5 src/128 dst/128 ESP/OSPF or AH/OSPF protect
For rules 2 and 4, action "protect" means encrypting/calculating
Integrity Check Value (ICV) and adding an ESP or AH header. For
rules 3 and 5, action "protect" means decrypting/authenticating the
packets and stripping the ESP or AH header.
Rule 1 will bypass the OSPFv3 packets without any IPsec processing on
the interfaces that have OSPFv3 authentication/confidentiality
disabled.
Rules 2 and 4 will drop the inbound OSPFv3 packets that have not been
secured with ESP/AH headers.
ESP/OSPF or AH/OSPF in rules 3 and 5 mean that it is an OSPF packet
secured with ESP or AH.
Rules 2 and 3 are meant to secure the unicast and multicast OSPF
packets that are not being exchanged over the virtual links.
Rules 4 and 5 are meant to secure the packets being exchanged over
virtual links. These rules are installed after learning the virtual
link end point IPv6 addresses. These rules MUST be installed in the
SPD for the interfaces that are connected to the transit area for the
virtual link. These rules MAY alternatively be installed on all the
interfaces. If these rules are not installed on all the interfaces,
clear text or malicious OSPFv3 packets with the same source and
destination addresses as the virtual link end point IPv6 addresses
will be delivered to OSPFv3. Though OSPFv3 drops these packets as
they were not received on the right interface, OSPFv3 receives some
clear text or malicious packets even when the security is enabled.
Installing these rules on all the interfaces ensures that OSPFv3 does
not receive these clear text or malicious packets when security is
enabled. On the other hand, installing these rules on all the
interfaces increases the processing overhead on the interfaces where
there is no other IPsec processing. The decision of whether to
install these rules on all the interfaces or on just the interfaces
that are connected to the transit area is a private decision and
doesn't affect the interoperability in any way. Hence it is an
implementation choice.
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12. Entropy of Manual Keys
The implementations MUST allow the administrator to configure the
cryptographic and authentication keys in hexadecimal format rather
than restricting it to a subset of ASCII characters (letters,
numbers, etc.). A restricted character set will reduce key entropy
significantly as discussed in [I2].
13. Replay Protection
Since it is not possible using the current standards to provide
complete replay protection while using manual keying, the proposed
solution will not provide protection against replay attacks.
Detailed analysis of various vulnerabilities of the routing protocols
and OSPF in particular is discussed in [I3] and [I2]. The conclusion
is that replay of OSPF packets can cause adjacencies to be disrupted,
which can lead to a DoS attack on the network. It can also cause
database exchange process to occur continuously thus causing CPU
overload as well as micro loops in the network.
14. Security Considerations
This memo discusses the use of IPsec AH and ESP headers to provide
security to OSPFv3 for IPv6. Hence, security permeates throughout
this document.
OSPF Security Vulnerabilities Analysis [I2] identifies OSPF
vulnerabilities in two scenarios -- one with no authentication or
simple password authentication and the other with cryptographic
authentication. The solution described in this specification
provides protection against all the vulnerabilities identified for
scenarios with cryptographic authentication with the following
exceptions:
Limitations of manual key:
This specification mandates the usage of manual keys. The following
are the known limitations of the usage of manual keys.
o As the sequence numbers cannot be negotiated, replay protection
cannot be provided. This leaves OSPF insecure against all the
attacks that can be performed by replaying OSPF packets.
o Manual keys are usually long lived (changing them often is a
tedious task). This gives an attacker enough time to discover
the keys.
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o As the administrator is manually configuring the keys, there is
a chance that the configured keys are weak (there are known
weak keys for DES/3DES at least).
Impersonating attacks:
The usage of the same key on all the OSPF routers connected to a link
leaves them all insecure against impersonating attacks if any one of
the OSPF routers is compromised, malfunctioning, or misconfigured.
Detailed analysis of various vulnerabilities of routing protocols is
discussed in [I3].
15. References
15.1. Normative References
[N1] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[N2] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6", RFC 2740,
December 1999.
[N3] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
[N4] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
December 2005.
[N5] Kent, S., "IP Authentication Header", RFC 4302, December 2005.
[N6] Eastlake 3rd, D., "Cryptographic Algorithm Implementation
Requirements for Encapsulating Security Payload (ESP) and
Authentication Header (AH)", RFC 4305, December 2005.
[N7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
15.2. Informative References
[I1] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306,
December 2005.
[I2] Jones, E. and O. Moigne, "OSPF Security Vulnerabilities
Analysis", Work in Progress.
[I3] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to Routing
Protocols", Work in Progress.
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[I4] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
Acknowledgements
The authors would like to extend sincere thanks to Marc Solsona,
Janne Peltonen, John Cruz, Dhaval Shah, Abhay Roy, Paul Wells,
Vishwas Manral, and Sam Hartman for providing useful information and
critiques on this memo. The authors would like to extend special
thanks to Acee Lindem for many editorial changes.
We would also like to thank members of the IPsec and OSPF WG for
providing valuable review comments.
Authors' Addresses
Mukesh Gupta
Tropos Networks
555 Del Rey Ave
Sunnyvale, CA 94085
Phone: 408-331-6889
EMail: mukesh.gupta@tropos.com
Nagavenkata Suresh Melam
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
Phone: 408-505-4392
EMail: nmelam@juniper.net
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