





Network Working Group                                        R. Atkinson
Request for Comments: 4822                              Extreme Networks
Obsoletes: 2082                                                 M. Fanto
Updates: 2453                                                       NIST
Category: Standards Track                                  February 2007


                   RIPv2 Cryptographic Authentication

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 IETF Trust (2007).

IESG Note

   In the interests of encouraging rapid migration away from Keyed-MD5
   and its known weakness, the IESG has approved this document even
   though it does not meet the guidelines in BCP 107 (RFC 4107).
   However, the IESG stresses that automated key management should be
   used to establish session keys and urges that the future work on key
   management described in Section 5.6 of this document should be
   performed as soon as possible.

Abstract

   This note describes a revision to the RIPv2 Cryptographic
   Authentication mechanism originally specified in RFC 2082.  This
   document obsoletes RFC 2082 and updates RFC 2453.  This document adds
   details of how the SHA family of hash algorithms can be used with
   RIPv2 Cryptographic Authentication, whereas the original document
   only specified the use of Keyed-MD5.  Also, this document clarifies a
   potential issue with an active attack on this mechanism and adds
   significant text to the Security Considerations section.










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1.  Introduction

   Growth in the Internet has made us aware of the need for improved
   authentication of routing information.  RIPv2 provides for
   unauthenticated service (as in classical RIP), or password
   authentication.  Both are vulnerable to passive attacks currently
   widespread in the Internet.  Well-understood security issues exist in
   routing protocols [Bell89].  Cleartext passwords, originally
   specified for use with RIPv2, are widely understood to be vulnerable
   to easily deployed passive attacks [HA94].

   The original RIPv2 cryptographic authentication specification, RFC
   2082 [AB97], used the Keyed-MD5 cryptographic mechanism.  While there
   are no openly published attacks on that mechanism, some reports
   [Dobb96a, Dobb96b] create concern about the ultimate strength of the
   MD5 cryptographic hash function.  Further, some end users,
   particularly several different governments, require the use of the
   SHA hash function family rather than any other such function for
   policy reasons.  Finally, the original specification uses a hashing
   construction widely believed to be weaker than the HMAC construction
   used with the algorithms added in this revision of the specification.

   This document obsoletes the original specification, RFC 2082 [AB97].
   This specification differs from RFC 2082 by adding support for the
   SHA family of hash algorithms and the HMAC technique, while retaining
   the original Keyed-MD5 algorithm and mode.  As the original RIPv2
   Cryptographic Authentication mechanism was algorithm-independent,
   backwards compatibility is retained.  This requirement for backwards
   compatibility precludes making significant protocol changes.  So,
   this document limits changes to the addition of support for an
   additional family of cryptographic algorithms.  The original
   specification has been very widely implemented, is known to be widely
   interoperable, and is also widely deployed.

   The authors do NOT believe that this specification is the final
   answer to RIPv2 authentication and encourage the reader to consult
   the Security Considerations section of this document for more
   details.

   If RIPv2 authentication is disabled, then only simple
   misconfigurations are detected.  The original RIPv2 authentication
   mechanism relied upon reused cleartext passwords.  Use of cleartext
   password authentication can protect against accidental
   misconfigurations if that were the only concern, but is not helpful
   from a security perspective.  By simply capturing information on the
   wire -- straightforward even in a remote environment -- a hostile





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   entity can read the cleartext RIPv2 password and use that knowledge
   to inject false information into the routing system via the RIPv2
   routing protocol.

   This mechanism is intended to reduce the risk of a successful passive
   attack upon RIPv2 deployments.  That is, deployment of this mechanism
   greatly reduces the vulnerability of the RIPv2-based routing system
   from a passive attack.  When cryptographic authentication is enabled,
   we transmit the output of a keyed cryptographic one-way function in
   the authentication field of the RIPv2 packet, instead of sending a
   cleartext reusable password in the RIPv2 packet.  The RIPv2
   Authentication Key is known only to the authorized parties of the
   RIPv2 session.  The RIPv2 Authentication Key is never sent over the
   network in the clear.

   In this way, protection is afforded against forgery or message
   modification.  While it is possible to replay a message until the
   sequence number changes, a sequence number can be used to reduce
   replay risks.  The mechanism does not provide confidentiality, since
   messages stay in the clear.  Since the objective of a routing
   protocol is to advertise the routing topology, confidentiality is not
   normally required for routing protocols.

   Other relevant rationales for the approach are that MD5 and SHA-1 are
   both being used for other purposes and are therefore generally
   already present in IP routers, as is some form of password
   management.

1.1.  Terminology

   In this document, the words "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in [BCP14] and indicate requirement levels for compliant or
   conformant implementations.

2.  Implementation Approach

   Implementation requires use of a special packet format, special
   authentication procedures, and also management controls.
   Implementers need to remember that the Security Considerations
   section is an integral part of this specification and contains
   important parts of this specification.








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2.1.  RIPv2 PDU Format

   The basic RIPv2 message format provides for an 8-octet header with an
   array of 20-octet records as its data content.  When RIPv2
   Cryptographic Authentication is enabled, the same header and content
   are used as with the original RIPv2 specification, but the 16-octet
   "Authentication" password field of the original RIPv2 specification
   is reused to contain a packet offset to the Authentication Data, a
   Key Identifier, the Authentication Data Length, and a non-decreasing
   sequence number.

      AUTHENTICATION TYPE
         The "Authentication Type" is Cryptographic Hash Function, which
         is indicated by the value 3.

      RIPv2 PACKET LENGTH
         An unsigned 16-bit offset from the start of the RIPv2 header to
         the end of the regular RIPv2 packet (not including the
         authentication trailer).

      KEY IDENTIFIER
         An unsigned 8-bit field that contains the Key Identifier or
         Key-ID.  This, in combination with the network interface,
         identifies the RIPv2 Security Association in use for this
         packet.  The RIPv2 Security Association, which is defined in
         Section 2.2 below, includes the Authentication Key that was
         used to create the Authentication Data for this RIPv2 message
         and other parameters.  In implementations supporting more than
         one authentication algorithm, the RIPv2 Security Association
         also includes information about which authentication algorithm
         is in use for this message.  A RIPv2 Security Association is
         always associated with an interface, rather than with a router.
         The actual cryptographic key is part of the RIPv2 Security
         Association.

      AUTHENTICATION DATA LENGTH
         An unsigned 8-bit field that contains the length in octets of
         the trailing Authentication Data field.  The presence of this
         field helps provide cryptographic algorithm independence.

      AUTHENTICATION DATA
         This field contains the cryptographic Authentication Data used
         to validate this packet.  The length of this field is stored in
         the AUTHENTICATION DATA LENGTH field above.







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      SEQUENCE NUMBER
         An unsigned 32-bit sequence number.  The sequence number MUST
         be non-decreasing for all messages sent from a given source
         router with a given Key ID value.

   The authentication trailer contains the Authentication Data, which is
   the output of the keyed cryptographic hash function.  See later
   subsections of this section for details on computing this field.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------------------------------+
   |  Command (1)  | Version (1)   |        Routing Domain (2)     |
   +---------------+---------------+-------------------------------+
   |             0xFFFF            |  Authentication Type=0x0003   |
   +---------------+---------------+---------------+---------------+
   |     RIPv2 Packet Length       |   Key ID      | Auth Data Len |
   +---------------+---------------+---------------+---------------+
   |               Sequence Number (non-decreasing)                |
   +---------------+---------------+---------------+---------------+
   |                      reserved must be zero                    |
   +---------------+---------------+---------------+---------------+
   |                      reserved must be zero                    |
   +---------------+---------------+---------------+---------------+
   |                                                               |
   ~            (RIPv2 Packet Length - 24) bytes of Data           ~
   |                                                               |
   +---------------+---------------+---------------+---------------+
   |             0xFFFF            |            0x0001             |
   +---------------+---------------+---------------+---------------+
   | Authentication Data (variable length; 20 bytes with HMAC-SHA1)|
   +---------------+---------------+---------------+---------------+

2.2.  RIPv2 Security Association

   Understanding the RIPv2 Security Association concept is central to
   understanding this specification.  A RIPv2 Security Association
   contains the set of shared authentication configuration parameters
   needed by the legitimate sender or any legitimate receiver.

   An implementation MUST be able to support at least 2 concurrent RIPv2
   Security Associations on each RIP interface.  This is a functional
   requirement for supporting key rollover.  Support for key rollover is
   mandatory.

   The RIPv2 Security Association, defined below, is selected by the
   sender based on the outgoing router interface.  Each RIPv2 Security
   Association has a lifetime and other configuration parameters



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   associated with it.  In normal operation, a RIPv2 Security
   Association is never used outside its lifetime.  Certain abnormal
   cases are discussed later in this document.

   The minimum data items in a RIPv2 Security Association are as
   follows:

      KEY-IDENTIFIER (KEY-ID)
         The unsigned 8-bit KEY-ID value is used to identify the RIPv2
         Security Association in use for this packet.

         The receiver uses the combination of the interface the packet
         was received upon and the KEY-ID value to uniquely identify the
         appropriate Security Association.

         The sender selects which RIPv2 Security Association to use
         based on the outbound interface for this RIPv2 packet and then
         places the correct KEY-ID value into that packet.  If multiple
         valid and active RIPv2 Security Associations exist for a given
         outbound interface at the time a RIPv2 packet is sent, the
         sender may use any of those security associations to protect
         the packet.

      AUTHENTICATION ALGORITHM
         This specifies the cryptographic algorithm and algorithm mode
         used with the RIPv2 Security Association.  This information is
         never sent in cleartext over the wire.  Because this
         information is not sent on the wire, the implementer chooses an
         implementation specific representation for this information.
         At present, the following values are possible: KEYED-MD5,
         HMAC-SHA-1, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512.

      AUTHENTICATION KEY
         This is the value of the cryptographic authentication key used
         with the associated Authentication Algorithm.  It MUST NOT ever
         be sent over the network in cleartext via any protocol.  The
         length of this key will depend on the Authentication Algorithm
         in use.  Operators should take care to select unpredictable and
         strong keys, avoiding any keys known to be weak for the
         algorithm in use. [ESC05] contains helpful information on both
         key generation techniques and cryptographic randomness.










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      SEQUENCE NUMBER
         This is an unsigned 32-bit number.  For a given KEY-ID value
         and sender, this number MUST NOT decrease.  In normal
         operation, the operator should rekey the RIPv2 session prior to
         reaching the maximum value.  The initial value used in the
         sequence number is arbitrary.  Receivers SHOULD keep track of
         the most recent sequence number received from a given sender.

      START TIME
         This is a local representation of the day and time that this
         Security Association first becomes valid.

      STOP TIME
         This is a local representation of the day and time that this
         Security Association becomes invalid (i.e., when it expires).
         It is permitted, but not recommended, for an operator to
         configure this to "never expire".  The "never expire" value is
         not recommended operational practice because it reduces
         security as compared with periodic rekeying.  Normally, a RIPv2
         Security Association is deleted at its STOP TIME.  However,
         there are certain pathological cases, which are discussed in
         Section 5.1.

   The authentication trailer consists of the Authentication Data, which
   is the output of the keyed cryptographic hash function.  See later
   subsections of this section for details on computing this field.

2.3.  Basic Authentication Processing

   When the authentication type is "Cryptographic Hash Function",
   message processing is changed in message creation and reception as
   compared with the original RIPv2 specification in [Mal94].

   This section describes the message processing generically.
   Additional algorithm-dependent processing that is required is
   described in separate, subsequent sections of this document.  As of
   this writing, there are 2 kinds of algorithm-dependent processing.
   One covers the "Keyed-MD5" algorithm.  The other covers the
   "HMAC-SHA1" family of algorithms.

2.3.1.  Message Generation

   The RIPv2 Packet is created as usual, with these exceptions:

   (1) The UDP checksum SHOULD be calculated, but MAY be set to zero
       because any of the cryptographic authentication mechanisms in
       this specification will provide stronger integrity protection
       than the standard UDP checksum.



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   (2) The Authentication Type field indicates Cryptographic
       Authentication (3).

   (3) The Authentication "password" field is reused to store a packet
       offset to the Authentication Data, a Key Identifier, the
       Authentication Data Length, and a non-decreasing sequence number.

   See also Section 2.2 above on RIPv2 Security Association for other
   important background information.

   When creating the RIPv2 Packet, the following process is followed:

   (1) The Packet Length field of the RIPv2 header indicates the size of
       the main body of the RIPv2 packet.

   (2) An appropriate RIPv2 Security Association is selected for use
       with this packet, based on the outbound interface for the packet.
       Any valid RIPv2 Security Association for that outbound interface
       may be used.  The Authentication Data Offset, Key Identifier, and
       Authentication Data Length fields are filled in appropriately.

   (3) Algorithm-dependent processing occurs now, either for the
       "Keyed-MD5" algorithm or for the "HMAC-SHA1" algorithm family.
       See the respective sub-sections (below) for details of this
       algorithm-dependent processing.

   (4) The resulting Authentication Data value is written into the
       Authentication Data field.  The trailing pad (if any) is not
       actually transmitted, as it is entirely predictable from the
       message length and Authentication Algorithm in use.

2.3.2.  Message Reception

   When the message is received, the process is reversed:

   (1) The received Authentication Data is set aside and stored for
       later use,

   (2) The appropriate RIPv2 Security Association is determined from the
       value of the Key Identifier field and the interface the packet
       was received on.  If there is no valid RIPv2 Security Association
       for the received Key Identifier on the interface that the packet
       was received on, then:

       (a) all processing of the incoming packet ceases, and

       (b) a security event SHOULD be logged by the RIPv2 subsystem of
           the receiving system.  That security event should indicate at



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           least the day/time that the bad packet was received, the
           Source IP Address of the received RIPv2 packet, the Key-ID
           field value, the interface the bad packet arrived upon, and
           the fact that no valid RIPv2 Security Association was found
           for that interface and Key-ID combination.

   (3) Algorithm-dependent processing is performed, using the algorithm
       specified by the appropriate RIPv2 Security Association for this
       packet.  This results in calculation of the Authentication Data
       based on the information in the received RIPv2 packet and
       information from the appropriate RIPv2 Security Association for
       that packet.

   (4) The calculated Authentication Data result is compared with the
       received Authentication Data.

   (5) If the calculated authentication data result does not match the
       received Authentication Data field, then:

       (a) the message MUST be discarded without being processed, and

       (b) a security event SHOULD be logged by the RIPv2 subsystem of
           the receiving system.  That security event SHOULD indicate at
           least the day/time that the bad packet was received, the
           Source IP Address of the received RIPv2 packet, the Key-ID
           field value, the interface the bad packet arrived upon, and
           the fact that RIPv2 Authentication failed upon receipt of the
           packet.

   (6) If the neighbor has been heard from recently enough to have
       viable routes in the local routing table, and the received
       sequence number is less than the last sequence number received,
       then the message MUST be discarded unprocessed.  If the received
       sequence number is less than the last sequence number received,
       that fact SHOULD be logged as a security event.  This logged
       security event SHOULD indicate at least the day/time that the bad
       packet was received, the Source IP Address of the received RIPv2
       packet, the Key-ID field value, and the fact that an out-of-order
       RIPv2 sequence number was received.

       When connectivity to the neighbor has been lost, the receiver
       SHOULD be ready to accept either:

         - a message with a sequence number of zero.

         - a message with a higher sequence number than the last
           received sequence number.




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   (7) Acceptable messages are now truncated to the RIPv2 message
       itself, minus the authentication trailer, and are processed
       normally (i.e., in accordance with the RIPv2 base specification
       in RFC 2453 [Mal98]).  The last received sequence number for this
       RIPv2 Security Association and sender is also updated.

   NOTA BENE: A router that has forgotten its current sequence number
   but remembers its Security Association MUST send its first packet
   with a sequence number of zero.  This leaves a small opening for a
   replay attack.  To reduce the risk of such attacks by precluding the
   situation where a router has forgotten its current sequence number,
   implementers SHOULD provide non-volatile storage for all components
   of a RIPv2 Security Association, and receiving systems SHOULD provide
   non-volatile storage for the last received sequence number from each
   sender.  See also the Security Considerations section of this
   document.

2.4.  Keyed-MD5 Algorithm-Dependent Processing

   This section describes the algorithm-dependent processing steps
   applicable when the "Keyed-MD5" authentication algorithm is in use.
   The RIPv2 Authentication Key is always 16 octets when "Keyed-MD5" is
   in use.

   (1) The RIPv2 Authentication Key is appended to the RIPv2 packet in
       memory.

   (2) The Trailing Pad for MD5 and message length fields are added in
       memory.  The diagram below shows how these additions appear when
       appended in memory:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Authentication Key                        |
      /                      (16 octets long)                         /
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       zero or more pad octets (as defined by RFC 1321)        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   64-bit message length MSW                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   64-bit message length LSW                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (3) The Authentication Data is then calculated according to the MD5
       algorithm defined by RFC 1321 [Rivest92].






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2.5.  HMAC-SHA1 Algorithm-Dependent Processing

   This section describes the processing steps for HMAC Authentication.
   While HMAC was originally documented in [KMC97], for this
   specification, the terminology used in [FIPS-198] is used.  While the
   current specification only provides full details for HMAC
   Authentication using the National Institute of Standards and
   Technology (NIST) SHA-1 algorithm (and its direct derivatives), this
   same basic process could be used with other cryptographic hash
   functions in the future.  Because the RIPv2 packet is only hashed
   once, the overhead of the double hashing in this process is
   negligible.

   The US NIST Secure Hash Standard (SHS), defined by [FIPS-180-2],
   includes specifications for SHA-1, SHA-256, SHA-384, and SHA-512.
   This specification defines processing for each of these.

   The output of the cryptographic computations (e.g., HMAC-SHA1) is NOT
   truncated for RIPv2 Cryptographic Authentication.

   The Authentication Data Length is equal to the Message Digest Size
   for the hash algorithm in use.

   Any key value known to be weak with an algorithm defined by the NIST
   Secure Hash Standard MUST NOT be used with such an algorithm in an
   implementation of this specification.  US NIST is the authoritative
   source for public information on weak keys for those algorithms.

   In the algorithm description below, the following nomenclature, which
   is consistent with [FIPS-198], is used:

         H    is the specific hashing algorithm,
              for example, SHA-1 or SHA-256.
         Ko   is the cryptographic key used with the hash algorithm.
         B    is the block-size of H, measured in octets, not bits.
              Note that B is the internal block size, not the hash size.
              For SHA-1   and SHA-256:  B == 64.
              For SHA-384 and SHA-512:  B == 128
         L    is the length of the hash, measured in octets, not bits.
              For example, with SHA-1, L == 20.
         XOR  is the exclusive-or operation.
         Opad is the hexadecimal value 0x5c repeated B times.
         Ipad is the hexadecimal value 0x36 repeated B times.
         Apad is the hexadecimal value 0x878FE1F3 repeated (L/4) times.







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   (1) PREPARATION OF KEY
       In this application, Ko is always L octets long.

       If the Authentication Key is L octets long, then Ko is set equal
       to the Authentication Key.  If the Authentication Key is more
       than L octets long, then Ko is set to H(Authentication Key).  If
       the Authentication Key is less than L octets long, then Ko is set
       to the Authentication Key with zeros appended to the end of the
       Authentication Key such that Ko is L octets long.

   (2) FIRST HASH
       First, the RIPv2 packet's Authentication Data field is filled
       with the value Apad.

       Then, a first hash, also known as the inner hash, is computed as
       follows:
               First-Hash = H(Ko XOR Ipad || (RIPv2 Packet))

   (3) SECOND HASH
       Then a second hash, also known as the outer hash, is computed as
       follows:
               Second-Hash = H(Ko XOR Opad || First-Hash)

   (4) RESULT
       The result Second-Hash becomes the authentication data that is
       sent in the Authentication Data field of the RIPv2 packet.  The
       length of the Authentication Data field is always identical to
       the message digest size of the hash function H that is being
       used.

       This also implies that use of hash functions with larger output
       sizes will also increase the size of the packet as transmitted on
       the wire.

3.  Management Procedures

   Key management is an important component of this mechanism and proper
   implementation is central to providing the intended level of risk
   reduction.

3.1.  Key Management Requirements

   It is strongly desirable that a hypothetical security breach in one
   Internet protocol not automatically compromise other Internet
   protocols.  The Authentication Key of this specification SHOULD NOT
   be configured or stored using protocols (e.g., RADIUS) or
   cryptographic algorithms that have known flaws.




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   Implementations MUST support the storage of more than one key at the
   same time, although it is recognized that only one key will normally
   be active on an interface.  Implementations MUST associate a specific
   Security Association lifetime (i.e., date/time first valid and
   date/time no longer valid) and a key identifier with each key.
   Implementations also MUST support manual key distribution.  An
   example of manual key distribution is having the privileged user
   typing in the key, key lifetime, and key identifier on the router
   console.  An operator may configure the Security Association lifetime
   to infinite, which means that the session is never rekeyed.  However,
   instead, it is strongly recommended that operators rekey regularly,
   using a moderately short Security Association lifetime (e.g., 24
   hours).

   This specification requires support for at least two authentication
   algorithms, so the implementation MUST require that the
   authentication algorithm be specified for each key when the other key
   information is entered.  Manual deletion of active Security
   Associations MUST be supported.

   It is likely that the IETF will define a standard key management
   protocol for use with routing protocols.  It is strongly desirable to
   use an IETF standards-track key management protocol to distribute
   RIPv2 Authentication Keys among communicating RIPv2 implementations.
   Such a protocol would provide scalability and significantly reduce
   the human administrative burden.  The Key-ID field can be used as a
   hook between RIPv2 and such a future protocol.

   Key management protocols have a long history of subtle flaws that are
   often discovered long after the protocol was first described in
   public.  To avoid having to change all RIPv2 implementations should
   such a flaw be discovered, integrated key management protocol
   techniques were deliberately omitted from this specification.

3.2.  Key Management Procedures

   As with all security methods using keys, it is necessary to change
   the RIPv2 Authentication Key on a regular basis.  To maintain routing
   stability during such changes, implementations MUST be able to store
   and use more than one RIPv2 Authentication Key on a given interface
   at the same time.

   Each key will have its own Key Identifier (KEY-ID), which is stored
   locally.  The combination of the Key Identifier and the interface
   associated with the message uniquely identifies the Authentication
   Algorithm and RIPv2 Authentication Key in use.





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   As noted above in Section 2.3.1, the party creating the RIPv2 message
   will select a valid RIPv2 Security Association from the set of valid
   RIPv2 Security Associations for that interface.  The receiver MUST
   use the Key Identifier and receiving interface to determine which
   RIPv2 Security Association to use for authentication of the received
   message.  More than one RIPv2 Security Association MAY be associated
   with an interface at the same time.  The receiver MUST NOT simply try
   all RIPv2 Security Associations (i.e., keys) that might be configured
   for RIPv2 on the receiving interface, as that creates an easily
   exploited denial-of-service attack on the RIP subsystem of the
   receiver.  (At least one widely used implementation of the previous
   version of this specification violates these requirements as of the
   publication date of this document and has consequent security
   vulnerabilities.)

   Hence, it is possible to have fairly smooth RIPv2 Security
   Association (i.e., key) rollovers, without losing legitimate RIPv2
   messages due to an invalid shared key and without requiring people to
   change all the keys at once.  To ensure a smooth rollover, each
   communicating RIPv2 system must be updated with the new RIPv2
   Security Association (including the new key) several minutes before
   the current RIPv2 Security Association will expire and several
   minutes before the new RIPv2 Security Association lifetime begins.
   Also, the new RIPv2 Security Association should have a lifetime that
   starts several minutes before the old RIPv2 Security Association
   expires.  This gives time for each system to learn of the new
   security association before that security association will be used.
   It also ensures that the new security association will begin use and
   the current security association will go out of use before the
   current security association's lifetime expires.  For the duration of
   the overlap in security association lifetimes, a system may receive
   messages corresponding to either security association and
   successfully authenticate the message.  The Key-ID in the received
   message is used to select the appropriate security association (i.e.,
   key) to be used for authentication.

4.  Conformance Requirements

   For this specification, the term "conformance" has identical meaning
   to the phrase "full compliance".

   The Keyed MD5 authentication algorithm and the HMAC-SHA1 algorithm
   MUST be implemented by all conforming implementations.  In addition,
   the HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 algorithms SHOULD be
   implemented.  MD5 is defined in [Rivest92].  SHA-1, SHA-256, SHA-384,
   and SHA-512 have been defined by the US NIST in [FIPS-180-2].





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   A conforming implementation MAY also support additional
   authentication algorithms, provided those additional algorithms are
   publicly and openly specified.

   Manual key distribution as described above MUST be supported by all
   conforming implementations.  All implementations MUST support the
   smooth key rollover described under "Key Management Procedures".
   This also means that implementations MUST support at least 2
   concurrent RIPv2 Security Associations.

   The user documentation provided with the implementation ought to
   contain clear instructions on how to configure the implementation
   such that smooth key rollover occurs successfully.

   Implementations SHOULD support a standard key management protocol for
   secure distribution of RIPv2 Authentication Keys once such a key
   management protocol is standardized by the IETF.

   The Security Considerations section of this document is an integral
   part of the specification, not just a discussion of the protocol.

5.  Security Considerations

   This entire memo describes and specifies an authentication mechanism
   for the RIPv2 routing protocol that is believed to be secure against
   passive attacks.  The term "passive attack" is defined in RFC 1704
   [HA94].  The analysis contained in RFC 1704 motivated this work.
   Passive attacks are clearly widespread in the Internet at present
   [HA94].

   Protection against active attacks is incomplete in this current
   specification.  The main issue relative to active attacks lies in the
   need to support the case where another router has recently rebooted
   and that router lacks the non-volatile storage needed to remember the
   RIPv2 Security Association(s) and last received RIPv2 sequence
   number(s) across that reboot.

5.1.  Known Pathological Cases

   Two known pathological cases exist that MUST be handled by
   implementations.  Both of these are failures of the network manager.
   Each of these should be exceedingly rare in normal operation.

   (1) During key rollover, devices might exist that have not yet been
       successfully configured with the new key.  Therefore, routers
       SHOULD implement an algorithm that detects the set of RIPv2
       Security Associations being used by its neighbors, and transmit
       its messages using both the new and old RIPv2 Security



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       Associations (i.e., keys) until all of the neighbors are using
       the new security association or the lifetime of the old security
       association expires.  Under normal circumstances, this elevated
       transmission rate will exist for a single RIP update interval.

   (2) In the event that the last RIPv2 Security Association of an
       interface expires, it is unacceptable to revert to an
       unauthenticated condition, and not advisable to disrupt routing.
       Therefore, the router MUST send a "last RIPv2 Security
       Association expiration" notification to the network manager
       (e.g., via SYSLOG, SNMP, and/or other means) and SHOULD treat
       that last Security Association as having an infinite lifetime
       until the lifetime is extended, the Security Association is
       deleted by network management, or a new security association is
       configured.

   In some circumstances, the practice described in (2) can leave an
   opening to an active attack on the RIPv2 routing subsystem.
   Therefore, any actual occurrence of a RIPv2 Security Association
   expiration MUST cause a security event to be logged by the
   implementation.  This log item MUST include at least a note that the
   RIPv2 Authentication Key expired, the RIP routing protocol
   instance(s) affected, the routing interfaces affected, the Key-ID
   that is affected, and the current date/time.  Operators are
   encouraged to check such logs as an operational security practice to
   help detect active attacks on the RIPv2 routing subsystem.  Further,
   implementations SHOULD provide a configuration knob ("fail secure")
   to let a network operator prefer to have the RIPv2 routing fail when
   the last key expires, rather than continue using RIPv2 in an insecure
   manner.

5.2 Network Management Considerations

   Also, the use of SNMP, even SNMPv3 with cryptographic authentication
   and cryptographic confidentiality enabled, to modify or configure the
   RIPv2 Security Associations, or any component of the security
   association (for example, the cryptographic key), is NOT RECOMMENDED.
   This practice would create a potential for a cascading vulnerability,
   whereby a compromise in the SNMP security implementation would
   necessarily lead to a compromise not only of the local routing table
   (which could be accessed via SNMP) but also of all other routers that
   receive RIPv2 packets (directly or indirectly) from the compromised
   router.








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   Similarly, the use of protocols not designed and evaluated for use in
   key management (e.g., RADIUS, Diameter) to configure the security
   association is also NOT RECOMMENDED.  Reading the Security
   Associations via SNMP is allowed, but the information is to be
   treated as security-sensitive and protected by using the priv mode.

   Also, the use of SNMP to configure which form of RIPv2 authentication
   is in use is also NOT RECOMMENDED because of a similar cascading
   failure issue.  Any future revision of the RIPv2 Management
   Information Base (MIB) [MB94] should consider making the
   rip2IfConfAuthType object read-only.  Further, this object would need
   a new enum value to accommodate the RIPv2 cryptographic
   authentication type.  In addition, the compliance statement for this
   MIB does not have a MIN-ACCESS for this object.  At a minimum, if the
   MIB is updated, a new compliance statement SHOULD be written for this
   object that allows this object to be implemented as read-only.  For
   the rip2ifConfAuthKey object, since this object always returns ''H
   when read, the object's MIN-ACCESS in any revised compliance
   statement SHOULD be not-accessible if the MIB is updated.

   Further, for similar reasons, any future revisions to the RIPv2
   Management Information Base (MIB) SHOULD deprecate or omit any
   objects that would permit the writing of any RIPv2 Security
   Association or RIPv2 Security Association component (e.g., the
   cryptographic key).

   Also, it is RECOMMENDED that any future revisions to the RIPv2
   Management Information Base (MIB) consider adding MIB objects to hold
   information about any RIPv2 security events that might have occurred,
   and MIB objects that could be used to read the set of security events
   that have been logged by the RIPv2 subsystem.  For each security
   event mentioned in this document, it is also RECOMMENDED that
   appropriate notifications be included, with a MAX-ACCESS of
   Accessible-for-notify, in any future versions of the RIPv2 MIB
   module.

5.3.  Key Management Considerations

   For the past several years, manual configuration (e.g., via a
   console) has been commonly used to create and modify RIPv2 Security
   Associations.  There are a number of large-scale RIP deployments
   today that successfully use manual configuration of RIPv2 Security
   Associations.  There are also sites that use scripts (e.g., combining
   Tcl/Expect, PERL, and SSHv2) with a site-specific configuration
   database and secure console connections to dynamically manage all
   aspects of their router configurations, including their RIPv2
   Security Associations.  This last approach is similar to the current
   IETF approach to Network Configuration (NetConf) standards.



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   Recent IETF Multicast Security (MSEC) working group efforts into
   multicast key management appear promising.  Several large RIPv2
   deployments happen to also have deployed the Kerberos authentication
   system.  Recent IETF work into the use of Kerberos for Internet Key
   Negotiation (KINK) also seems relevant; one might use Kerberos to
   support RIPv2 key management functions for use at sites that have
   already deployed Kerberos.  It is hoped that in the future the IETF
   will standardize a key management protocol suitable for managing
   RIPv2 Security Associations.

5.4.  Assurance Considerations

   Users need to understand that the quality of the security provided by
   this mechanism depends completely on the strength of the implemented
   authentication algorithms, the strength of the key being used, and
   the correct implementation of the security mechanism in all
   communicating RIPv2 implementations.  This mechanism also depends on
   the RIPv2 Authentication Key being kept confidential by all parties.
   If any of these are incorrect or insufficiently secure, then no real
   security will be provided to the users of this mechanism.

   Use of high-assurance development methods is RECOMMENDED for
   implementations of this specification, in order to reduce the risk of
   subtle implementation flaws that might adversely impact the
   operational risk reduction that this specification seeks to provide.

5.5.  Confidentiality and Traffic Analysis Considerations

   Confidentiality is not provided by this mechanism.  It is generally
   considered that an IP routing protocol does not require
   confidentiality, as the purpose of any routing protocols is to
   disseminate information about the topology of the network.

   Protection against traffic analysis is also not provided.  Mechanisms
   such as bulk link encryption SHOULD be used when protection against
   traffic analysis is required [CKHD89].

5.6.  Other Security Considerations

   Separately, the receipt of a RIPv2 packet using cryptographic
   authentication but containing an invalid or unknown Key-ID value
   might indicate an active attack on the RIP routing subsystem and is a
   significant security event.  Therefore, any actual receipt of a RIPv2
   packet using cryptographic authentication and containing an unknown,
   expired, or otherwise invalid KEY-ID value SHOULD cause a security
   event to be logged by the implementation.  This log item SHOULD
   include at least the fact that the invalid KEY-ID was received, the
   source IP address of the packet containing the invalid KEY-ID, the



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   interface(s) the packet was received on, the KEY-ID received, and the
   current date/time.

   A subtle user-interface consideration also should be noted.  If a
   user interface only permits the entry of human-readable text (e.g., a
   password in US-ASCII format) for use as a cryptographic key,
   significant numbers of bits of the cryptographic key in use become
   predictable, thereby reducing the strength of the key in this
   context.  For this reason, implementations of this specification
   SHOULD support the entry of RIPv2 cryptographic authentication keys
   in hexadecimal format.

5.7.  Future Security Directions

   Specification and deployment of a standards-track key management
   protocol that supports this RIPv2 cryptographic authentication
   mechanism would be a significant next step in operational risk
   reduction and might actually increase the ease of deployment and
   operation of this mechanism.  Such specification is beyond the scope
   of this document.  Recent IETF work in MSEC and KINK working groups
   appears promising in this regard.  Recent IETF work in the NETCONF
   working group towards standardizing methods for secure configuration
   management of routers is also relevant.

   Finally, we observe that this mechanism is not the final word on
   RIPv2 authentication.  Rather, it is believed that this particular
   mechanism represents a significant risk reduction over previous
   methods (e.g., plaintext passwords), while remaining straightforward
   to implement correctly and also straightforward to deploy.

   User communities that believe this mechanism is not adequate to their
   needs are encouraged to consider using digital signatures with RIPv2.
   [MBW97] specifies the use of OSPF with Digital signatures; that
   document might be a starting point for creating such a specification
   for the RIPv2 protocol.  Digital signatures are significantly more
   expensive computationally and are also significantly more difficult
   to deploy operationally, as compared with the mechanism specified
   here.  However, it appears likely that much of the mechanism in this
   document could be reused with digital signatures.

6.  Acknowledgments

   Fred Baker was co-author of the earlier RIPv2 MD5 Authentication
   document [AB97].  This document is a direct derivative of that
   earlier document, though it has been significantly reworked.  The
   current authors would like to thank Bill Burr, Tim Polk, John Kelsey,
   and Morris Dworkin of (US) NIST for review of versions of this
   document.



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7.  Normative References

   [BCP14]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [Mal98]      Malkin, G., "RIP Version 2", STD 56, RFC 2453, November
                1998.

   [FIPS-180-2] National Institute of Standards and Technology, "Secure
                Hash Standard", FIPS PUB 180-2, August 2002,
                <http://csrc.nist.gov/publications/fips/fips180-2/
                fips180-2.pdf>.

   [FIPS-198]   National Institute of Standards and Technology, "The
                Keyed-Hash Message Authentication Code (HMAC)", FIPS PUB
                198, March 2002, <http://csrc.nist.gov/publications/
                fips/fips198/fips-198a.pdf>.

8.  Informative References

   [AB97]       Baker, F. and R. Atkinson, "RIP-2 MD5 Authentication",
                RFC 2082, January 1997.

   [Bell89]     S. Bellovin, "Security Problems in the TCP/IP Protocol
                Suite", ACM Computer Communications Review, Volume 19,
                Number 2, pp. 32-48, April 1989.

   [CKHD89]     Cole Jr, Raymond, Donald Kallgren, Richard Hale, and
                John R. Davis, "Multilevel Secure Mixed-Media
                Communication Networks", Proceedings of the IEEE
                Military Communications Conference (MILCOM '89), IEEE,
                1989.

   [Dobb96a]    Dobbertin, H., "Cryptanalysis of MD5 Compress",
                Technical Report, 2 May 1996.  (Presented at Rump
                Session of EuroCrypt 1996.)

   [Dobb96b]    Dobbertin, H., "The Status of MD5 After a Recent
                Attack", CryptoBytes, Vol. 2, No. 2, Summer 1996.

   [ESC05]      Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                "Randomness Requirements for Security", BCP 106, RFC
                4086, June 2005.

   [HA94]       Haller, N. and R. Atkinson, "On Internet
                Authentication", RFC 1704, October 1994.





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   [KMC97]      Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
                Keyed-Hashing for Message Authentication", RFC 2104,
                February 1997.

   [Mal94]      Malkin, G., "RIP Version 2 - Carrying Additional
                Information", RFC 1723, November 1994.

   [MB94]       Malkin, G. and F. Baker, "RIP Version 2 MIB Extension",
                RFC 1724, November 1994.

   [MBW97]      Murphy, S., Badger, M., and B. Wellington, "OSPF with
                Digital Signatures", RFC 2154, June 1997.

   [Rivest92]   Rivest, R., "The MD5 Message-Digest Algorithm", RFC
                1321, April 1992.

Authors' Addresses

   R. Atkinson
   Extreme Networks
   3585 Monroe Street
   Santa Clara, CA 95051
   USA

   Phone: +1 (408) 579-2800
   EMail: rja@extremenetworks.com


   M. Fanto
   (US) National Institute of Standards and Technology
   Gaithersburg, MD 20878
   USA

   Phone: +1 (301) 975-2000
   EMail: mattjf@umd.edu
   Web:   http://csrc.nist.gov















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Full Copyright Statement

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