Internet DRAFT - draft-gupta-ospf-ospfv2-sec
draft-gupta-ospf-ospfv2-sec
Network Working Group M. Gupta
Internet Draft Juniper Networks
Document: draft-gupta-ospf-ospfv2-sec-01.txt N. Melam
Intended Status: Proposed Standard Juniper Networks
Expires: Feb 2010 Aug 2009
Authentication/Confidentiality for OSPFv2
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Abstract
This document describes means and mechanisms to provide
authentication/confidentiality to OSPFv2 using IPsec (IP Security).
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].
Table of Contents
1. Introduction...................................................2
2. Transport Mode vs Tunnel Mode..................................3
3. Authentication.................................................3
4. Confidentiality................................................4
5. Distinguishing OSPFv2 from OSPFv3 [N2].........................4
6. IPsec Requirements.............................................4
7. Key Management.................................................5
8. SA Granularity and Selectors...................................7
9. Virtual Links..................................................8
10. Rekeying......................................................8
10.1 . Rekeying Procedure......................................8
10.2 . KeyRolloverInterval.....................................9
10.3 . Rekeying Interval.......................................9
11. IPsec rules..................................................10
12. Entropy of Manual Keys.......................................11
13. Replay Protection............................................11
Security Considerations..........................................11
IANA Considerations..............................................12
Normative References.............................................12
Informative References...........................................13
Acknowledgments..................................................13
Authors' Addresses...............................................13
1.
Introduction
OSPF (Open Shortest Path First) Version 2 [N1] defines the fields
AuType and Authentication in its protocol header to provide security.
These fields do not provide any confidentiality and also the
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authentication provided by these fields is weak [specific problems
here].
The demands for securing the routing protocols have increased since
the OSPFv2 protocol was designed. This document describes how IP
Security (Encapsulating Security Payload and Authentication Header
protocols) can be used to provide integrity, authentication, and/or
confidentiality to OSPFv2.
It is assumed that the reader is familiar with OSPFv2 [N1],
Authentication Header (AH) [N5], Encapsulating Security Payload (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]).
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. OSPFv2 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 confirming to this specification MUST support
transport mode SA to provide required IPsec security to OSPFv2
packets. They MAY also support tunnel mode SA to provide required
IPsec security to OSPFv2 packets.
3.
Authentication
Implementations conforming to this specification MUST support
authentication for OSPFv2.
In order to provide authentication to OSPFv2, implementations MUST
support ESP and MAY support AH.
If ESP in transport mode is used, it will only provide authentication
to OSPFv2 protocol packets excluding the IP header and IP options.
If AH in transport mode is used, it will provide authentication to
OSPFv2 protocol packet, selected portions of IP header and selected
IP options.
When OSPFv2 authentication is enabled,
o OSPFv2 packets that are not protected with AH or ESP MUST be
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silently discarded.
o OSPFv2 packets that fail the authentication checks MUST be
silently discarded.
4.
Confidentiality
Implementations conforming to this specification SHOULD support
confidentiality for OSPFv2.
If confidentiality is provided, ESP MUST be used.
When OSPFv2 confidentiality is enabled,
o OSPFv2 packets that are not protected with ESP MUST be silently
discarded.
o OSPFv2 packets that fail the confidentiality checks MUST be
silently discarded.
5.
Distinguishing OSPFv2 from OSPFv3 [N2]
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 in order to
distinguish OSPFv2 packets from OSPFv3 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 OSPFv2 packets from OSPFv3 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 a specific SPD
selection function. [N3]
Selectors
The implementation MUST be able to use source address, destination
address, protocol, and direction as selectors in the SPD.
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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]
Encryption and authentication algorithms
The implementation MUST NOT allow the user to choose stream
ciphers as the encryption algorithm for securing OSPFv2 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 OSPFv2 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
OSPFv2 exchanges both multicast and unicast packets. While running
OSPFv2 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
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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
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 <------------|
|
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B |
SAb ------------>|
SAa <------------| Figure: 2
SAc <------------|
|
C |
SAc ------------>|
SAa <------------|
SAb <------------|
|
Broadcast
Network
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.
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9.
Virtual Links
A different SA than the SA of the underlying interface MUST be
provided for virtual links. The source IP address of the OSPF
packets sent over the virtual links does not belong to the same
subnet as the interface running OSPFv2. The source IP address of all
the other OSPF packets, however, lies in the same subnet. This
difference in the IP source address differentiates the packets sent
on virtual links from other OSPFv2 interface types.
As the virtual link end point IP 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 IP
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 IP 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.
10.
Rekeying
To maintain the security of a link, the authentication and encryption
key values SHOULD be changed from periodically.
10.1
. Rekeying Procedure
The following three-step procedure SHOULD be provided to rekey the
routers on a link without dropping OSPFv2 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 OSPFv2 packets
on the link so that no OSPFv2 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.
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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 OSPFv2 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 OSPFv2 routers. As
this value can vary significantly depending upon the implementation
and the deployment, it is left to the administrator to choose the
appropriate value.
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 OSPFv2
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.
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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 rules
The following set of transport mode rules can be installed in the SPD
to provide the authentication/confidentiality to OSPFv2 packets.
Outbound Rules for interfaces running OSPFv2 security:
No. source destination protocol action
1 intfPrefix any OSPF apply
Outbound Rules for virtual links running OSPFv2 security:
No. source destination protocol action
2 src/32 dst/32 OSPF apply
Inbound Rules for interfaces running OSPFv2 security:
No. source destination protocol action
3 intfPrefix any ESP/OSPF or AH/OSPF apply
4 intfPrefix any OSPF drop
Inbound Rules for virtual links running OSPFv2 security:
No. source destination protocol action
5 src/32 dst/32 ESP/OSPF or AH/OSPF apply
6 src/32 dst/32 OSPF drop
"intfPrefix" means the prefix of the interface that OSPFv2 is running
on. For example, if the IP address of the interface where OSPFv2 is
configured is 192.0.2.1/24, the value of "intfPrefix" would be
"192.0.2.0/24".
For outbound rules, action "apply" means encrypting/calculating ICV
and adding an ESP or AH header. For inbound rules, action "apply"
means decrypting/authenticating the packets and stripping the ESP or
AH header.
Rules 4 and 6 are to drop the insecure OSPFv2 packets without 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.
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Rules 1, 3 and 4 are meant to secure the unicast and multicast OSPF
packets that are not being exchanged over the virtual links. These
rules MUST only be installed in the security policy database (SPD) of
the interface running OSPFv2 security.
Rules 2, 5 and 6 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 on at
least 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 OSPFv2 packets with the same source and
destination addresses as the virtual link end point IPv6 addresses
will be delivered to OSPFv2. Though OSPFv2 drops these packets
because they were not received on the right interface, OSPFv2
receives some clear text or malicious packets even when the security
is enabled. Installing these rules on all the interfaces insures
that OSPFv2 does not receive these clear text or malicious packets
when security is turned 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
installing 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.
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.
Security Considerations
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This memo discusses the use of IPsec AH and ESP headers in order to
provide security to OSPFv2 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
can not 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.
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].
IANA Considerations
This document has no IANA considerations.
This section should be removed by the RFC Editor to final
publication.
Normative References
[N1] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
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[N2] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for
IPv6", RFC 5340, July 2008.
[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] Manral, V., "Cryptographic Algorithm Implementation for
Encapsulating Security Payload (ESP) and Authentication Header
(AH)", RFC 4835, April 2007.
[N7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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.
[I4] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
[I5] Gupta, M. and N. Melam, "Authentication/Confidentiality for
OSPFv3", RFC 4552, June 2006.
Acknowledgments
This document is widely derived from Authentication/Confidentiality
to OSPFv3 [I5].
Authors' Addresses
Mukesh Gupta
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
Phone: 408-936-4197
EMail: mukesh@juniper.net
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Nagavenkata Suresh Melam
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
Phone: 408-505-4392
EMail: nmelam@juniper.net
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