path: root/doc/draft-richardson-ipsec-opportunistic.txt

Independent submission                                     M. Richardson
Internet-Draft                                                       SSW
Expires: November 19, 2003                                 D. Redelmeier
                                                                  Mimosa
                                                            May 21, 2003


     Opportunistic Encryption using The Internet Key Exchange (IKE)
              draft-richardson-ipsec-opportunistic-11.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   This Internet-Draft will expire on November 19, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document describes opportunistic encryption (OE) using the
   Internet Key Exchange (IKE) and IPsec.  Each system administrator
   adds new resource records to his or her Domain Name System (DNS) to
   support opportunistic encryption.  The objective is to allow
   encryption for secure communication without any pre-arrangement
   specific to the pair of systems involved.

   DNS is used to distribute the public keys of each system involved.
   This is resistant to passive attacks.  The use of DNS Security
   (DNSSEC) secures this system against active attackers as well.



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   As a result, the administrative overhead is reduced from the square
   of the number of systems to a linear dependence, and it becomes
   possible to make secure communication the default even when the
   partner is not known in advance.

   This document is offered up as an Informational RFC.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Specification  . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  Impacts on IKE . . . . . . . . . . . . . . . . . . . . . . . . 21
   5.  DNS issues . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   6.  Network address translation interaction  . . . . . . . . . . . 28
   7.  Host implementations . . . . . . . . . . . . . . . . . . . . . 29
   8.  Multi-homing . . . . . . . . . . . . . . . . . . . . . . . . . 30
   9.  Failure modes  . . . . . . . . . . . . . . . . . . . . . . . . 32
   10. Unresolved issues  . . . . . . . . . . . . . . . . . . . . . . 34
   11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   12. Security considerations  . . . . . . . . . . . . . . . . . . . 42
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 44
   14. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 45
       Normative references . . . . . . . . . . . . . . . . . . . . . 46
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 47
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 48

























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1. Introduction

1.1 Motivation

   The objective of opportunistic encryption is to allow encryption
   without any pre-arrangement specific to the pair of systems involved.
   Each system administrator adds public key information to DNS records
   to support opportunistic encryption and then enables this feature in
   the nodes' IPsec stack.  Once this is done, any two such nodes can
   communicate securely.

   This document describes opportunistic encryption as designed and
   mostly implemented by the Linux FreeS/WAN project.  For project
   information, see http://www.freeswan.org.

   The Internet Architecture Board (IAB) and Internet Engineering
   Steering Group (IESG) have taken a strong stand that the Internet
   should use powerful encryption to provide security and privacy [4].
   The Linux FreeS/WAN project attempts to provide a practical means to
   implement this policy.

   The project uses the IPsec, ISAKMP/IKE, DNS and DNSSEC protocols
   because they are standardized, widely available and can often be
   deployed very easily without changing hardware or software or
   retraining users.

   The extensions to support opportunistic encryption are simple.  No
   changes to any on-the-wire formats are needed.  The only changes are
   to the policy decision making system.  This means that opportunistic
   encryption can be implemented with very minimal changes to an
   existing IPsec implementation.

   Opportunistic encryption creates a "fax effect".  The proliferation
   of the fax machine was possible because it did not require that
   everyone buy one overnight.  Instead, as each person installed one,
   the value of having one increased - as there were more people that
   could receive faxes.  Once opportunistic encryption is installed it
   automatically recognizes other boxes using opportunistic encryption,
   without any further configuration by the network administrator.  So,
   as opportunistic encryption software is installed on more boxes, its
   value as a tool increases.

   This document describes the infrastructure to permit deployment of
   Opportunistic Encryption.

   The term S/WAN is a trademark of RSA Data Systems, and is used with
   permission by this project.




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1.2 Types of network traffic

   To aid in understanding the relationship between security processing
   and IPsec we divide network traffic into four categories:

   * Deny: networks to which traffic is always forbidden.

   * Permit: networks to which traffic in the clear is permitted.

   * Opportunistic tunnel: networks to which traffic is encrypted if
      possible, but otherwise is in the clear or fails depending on the
      default policy in place.

   * Configured tunnel: networks to which traffic must be encrypted, and
      traffic in the clear is never permitted.

   Traditional firewall devices handle the first two categories.  No
   authentication is required.  The permit policy is currently the
   default on the Internet.

   This document describes the third category - opportunistic tunnel,
   which is proposed as the new default for the Internet.

   Category four, encrypt traffic or drop it, requires authentication of
   the end points.  As the number of end points is typically bounded and
   is typically under a single authority, arranging for distribution of
   authentication material, while difficult, does not require any new
   technology.  The mechanism described here provides an additional way
   to distribute the authentication materials, that of a public key
   method that does not require deployment of an X.509 based
   infrastructure.

   Current Virtual Private Networks can often be replaced by an "OE
   paranoid" policy as described herein.

1.3 Peer authentication in opportunistic encryption

   Opportunistic encryption creates tunnels between nodes that are
   essentially strangers.  This is done without any prior bilateral
   arrangement.  There is, therefore, the difficult question of how one
   knows to whom one is talking.

   One possible answer is that since no useful authentication can be
   done, none should be tried.  This mode of operation is named
   "anonymous encryption".  An active man-in-the-middle attack can be
   used to thwart the privacy of this type of communication.  Without
   peer authentication, there is no way to prevent this kind of attack.




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   Although a useful mode, anonymous encryption is not the goal of this
   project.  Simpler methods are available that can achieve anonymous
   encryption only, but authentication of the peer is a desireable goal.
   The latter is achieved through key distribution in DNS, leveraging
   upon the authentication of the DNS in DNSSEC.

   Peers are, therefore, authenticated with DNSSEC when available.
   Local policy determines how much trust to extend when DNSSEC is not
   available.

   However, an essential premise of building private connections with
   strangers is that datagrams received through opportunistic tunnels
   are no more special than datagrams that arrive in the clear.  Unlike
   in a VPN, these datagrams should not be given any special exceptions
   when it comes to auditing, further authentication or firewalling.

   When initiating outbound opportunistic encryption, local
   configuration determines what happens if tunnel setup fails.  It may
   be that the packet goes out in the clear, or it may be dropped.

1.4 Use of RFC2119 terms

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [5]


























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2. Overview

2.1 Reference diagram

   ---------------------------------------------------------------------

   The following network diagram is used in the rest of this document as
   the canonical diagram:

                             [Q]  [R]
                              .    .              AS2
     [A]----+----[SG-A].......+....+.......[SG-B]-------[B]
            |                 ......
        AS1 |                 ..PI..
            |                 ......
     [D]----+----[SG-D].......+....+.......[C] AS3



                  Figure 1: Reference Network Diagram

   ---------------------------------------------------------------------

   In this diagram, there are four end-nodes: A, B, C and D.  There are
   three gateways, SG-A, SG-B, SG-D.  A, D, SG-A and SG-D are part of
   the same administrative authority, AS1.  SG-A and SG-D are on two
   different exit paths from organization 1.  SG-B/B is an independent
   organization, AS2.  Nodes Q and R are nodes on the Internet.  PI is
   the Public Internet ("The Wild").

2.2 Terminology

   The following terminology is used in this document:

   Security gateway: a system that performs IPsec tunnel mode
      encapsulation/decapsulation.  [SG-x] in the diagram.

   Alice: node [A] in the diagram.  When an IP address is needed, this
      is 192.1.0.65.

   Bob: node [B] in the diagram.  When an IP address is needed, this is
      192.2.0.66.

   Carol: node [C] in the diagram.  When an IP address is needed, this
      is 192.1.1.67.

   Dave: node [D] in the diagram.  When an IP address is needed, this is
      192.3.0.68.



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   SG-A: Alice's security gateway.  Internally it is 192.1.0.1,
      externally it is 192.1.1.4.

   SG-B: Bob's security gateway.  Internally it is 192.2.0.1, externally
      it is 192.1.1.5.

   SG-D: Dave's security gateway.  Also Alice's backup security gateway.
      Internally it is 192.3.0.1, externally it is 192.1.1.6.

   -  A single dash represents clear-text datagrams.

   =  An equals sign represents phase 2 (IPsec) cipher-text datagrams.

   ~  A single tilde represents clear-text phase 1 datagrams.

   #  A hash sign represents phase 1 (IKE) cipher-text datagrams.

   .  A period represents an untrusted network of unknown type.

   Configured tunnel: a tunnel that is directly and deliberately hand
      configured on participating gateways.  Configured tunnels are
      typically given a higher level of trust than opportunistic
      tunnels.

   Road warrior tunnel: a configured tunnel connecting one node with a
      fixed IP address and one node with a variable IP address.  A road
      warrior (RW) connection must be initiated by the variable node,
      since the fixed node cannot know the current address for the road
      warrior.

   Anonymous encryption: the process of encrypting a session without any
      knowledge of who the other parties are.  No authentication of
      identities is done.

   Opportunistic encryption: the process of encrypting a session with
      authenticated knowledge of who the other parties are.

   Lifetime: the period in seconds (bytes or datagrams) for which a
      security association will remain alive before needing to be re-
      keyed.

   Lifespan: the effective time for which a security association remains
      useful.  A security association with a lifespan shorter than its
      lifetime would be removed when no longer needed.  A security
      association with a lifespan longer than its lifetime would need to
      be re-keyed one or more times.

   Phase 1 SA: an ISAKMP/IKE security association sometimes referred to



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      as a keying channel.

   Phase 2 SA: an IPsec security association.

   Tunnel: another term for a set of phase 2 SA (one in each direction).

   NAT: Network Address Translation (see [20]).

   NAPT: Network Address and Port Translation (see [20]).

   AS: an autonomous system (AS) is a group of systems (a network) that
      are under the administrative control of a single organization.

   Default-free zone: a set of routers that maintain a complete set of
      routes to all currently reachable destinations.  Having such a
      list, these routers never make use of a default route.  A datagram
      with a destination address not matching any route will be dropped
      by such a router.


2.3 Model of operation

   The opportunistic encryption security gateway (OE gateway) is a
   regular gateway node as described in [2] section 2.4 and [3] with the
   additional capabilities described here and in [7].  The algorithm
   described here provides a way to determine, for each datagram,
   whether or not to encrypt and tunnel the datagram.  Two important
   things that must be determined are whether or not to encrypt and
   tunnel and, if so, the destination address or name of the tunnel end
   point which should be used.

2.3.1 Tunnel authorization

   The OE gateway determines whether or not to create a tunnel based on
   the destination address of each packet.  Upon receiving a packet with
   a destination address not recently seen, the OE gateway performs a
   lookup in DNS for an authorization resource record (see Section 5.2).
   The record is located using the IP address to perform a search in the
   in-addr.arpa (IPv4) or ip6.arpa (IPv6) maps.  If an authorization
   record is found, the OE gateway interprets this as a request for a
   tunnel to be formed.

2.3.2 Tunnel end-point discovery

   The authorization resource record also provides the address or name
   of the tunnel end point which should be used.

   The record may also provide the public RSA key of the tunnel end



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   point itself.  This is provided for efficiency only.  If the public
   RSA key is not present, the OE gateway performs a second lookup to
   find a KEY resource record for the end point address or name.

   Origin and integrity protection of the resource records is provided
   by DNSSEC ([16]).  Section 3.2.4.1 documents an optional restriction
   on the tunnel end point if DNSSEC signatures are not available for
   the relevant records.

2.3.3 Caching of authorization results

   The OE gateway maintains a cache, in the forwarding plane, of source/
   destination pairs for which opportunistic encryption has been
   attempted.  This cache maintains a record of whether or not OE was
   successful so that subsequent datagrams can be forwarded properly
   without additional delay.

   Successful negotiation of OE instantiates a new security association.
   Failure to negotiate OE results in creation of a forwarding policy
   entry either to drop or transmit in the clear future datagrams.  This
   negative cache is necessary to avoid the possibly lengthy process of
   repeatedly looking up the same information.

   The cache is timed out periodically, as described in Section 3.4.
   This removes entries that are no longer being used and permits the
   discovery of changes in authorization policy.

























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3. Specification

   The OE gateway is modeled to have a forwarding plane and a control
   plane.  A control channel, such as PF_KEY, connects the two planes.
   (See [6].) The forwarding plane performs per datagram operations.
   The control plane contains a keying daemon, such as ISAKMP/IKE, and
   performs all authorization, peer authentication and key derivation
   functions.

3.1 Datagram state machine

   Let the OE gateway maintain a collection of objects -- a superset of
   the security policy database (SPD) specified in [7].  For each
   combination of source and destination address, an SPD object exists
   in one of five following states.  Prior to forwarding each datagram,
   the responder uses the source and destination addresses to pick an
   entry from the SPD.  The SPD then determines if and how the packet is
   forwarded.

3.1.1 Non-existent policy

   If the responder does not find an entry, then this policy applies.
   The responder creates an entry with an initial state of "hold policy"
   and requests keying material from the keying daemon.  The responder
   does not forward the datagram, rather it attaches the datagram to the
   SPD entry as the  "first" datagram and retains it for eventual
   transmission in a new state.

3.1.2 Hold policy

   The responder requests keying material.  If the interface to the
   keying system is lossy (PF_KEY, for instance, can be), the
   implementation SHOULD include a mechanism to retransmit the keying
   request at a rate limited to less than 1 request per second.  The
   responder does not forward the datagram.  It attaches the datagram to
   the SPD entry as the "last" datagram where it is retained for
   eventual transmission.  If there is a datagram already so stored,
   then that already stored datagram is discarded.

   Because the "first" datagram is probably a TCP SYN packet, the
   responder retains the "first" datagram in an attempt to avoid waiting
   for a TCP retransmit.  The responder retains the "last" datagram in
   deference to streaming protocols that find it useful to know how much
   data has been lost.  These are recommendations to decrease latency.
   There are no operational requirements for this.






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3.1.3 Pass-through policy

   The responder forwards the datagram using the normal forwarding
   table.  The responder enters this state only by command from the
   keying daemon, and upon entering this state, also forwards the
   "first" and "last" datagrams.

3.1.4 Deny policy

   The responder discards the datagram.  The responder enters this state
   only by command from the keying daemon, and upon entering this state,
   discards the "first" and "last" datagrams.  Local administration
   decides if further datagrams cause ICMP messages to be generated
   (i.e.  ICMP Destination Unreachable, Communication Administratively
   Prohibited.  type=3, code=13).

3.1.5 Encrypt policy

   The responder encrypts the datagram using the indicated security
   association database (SAD) entry.  The responder enters this state
   only by command from the keying daemon, and upon entering this state,
   releases and forwards the "first" and "last" datagrams using the new
   encrypt policy.

   If the associated SAD entry expires because of byte, packet or time
   limits, then the entry returns to the Hold policy, and an expire
   message is sent to the keying daemon.

   All states may be created directly by the keying daemon while acting
   as a responder.

3.2 Keying state machine - initiator

   Let the keying daemon maintain a collection of objects.  Let them be
   called "connections" or "conn"s.  There are two categories of
   connection objects: classes and instances.  A class represents an
   abstract policy - what could be.  An instance represents an actual
   connection - what is implemented at the time.

   Let there be two further subtypes of connections: keying channels
   (Phase 1 SAs) and data channels (Phase 2 SAs).  Each data channel
   object may have a corresponding SPD and SAD entry maintained by the
   datagram state machine.

   For the purposes of opportunistic encryption, there MUST, at least,
   be connection classes known as "deny", "always-clear-text", "OE-
   permissive", and "OE-paranoid".  The latter two connection classes
   define a set of source and/or destination addresses for which



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   opportunistic encryption will be attempted.  The administrator MAY
   set policy options in a number of additional places.  An
   implementation MAY create additional connection classes to further
   refine these policies.

   The simplest system may need only the "OE-permissive" connection, and
   would list its own (single) IP address as the source address of this
   policy and the wild-card address 0.0.0.0/0 as the destination IPv4
   address.  That is, the simplest policy is to try opportunistic
   encryption with all destinations.

   The distinction between permissive and paranoid OE use will become
   clear in the state transition differences.  In general a permissive
   OE will, on failure, install a pass-through policy, while a paranoid
   OE will, on failure, install a drop policy.

   In this description of the keying machine's state transitions, the
   states associated with the keying system itself are omitted because
   they are best documented in the keying system ([8], [9] and [10] for
   ISAKMP/IKE), and the details are keying system specific.
   Opportunistic encryption is not dependent upon any specific keying
   protocol, but this document does provide requirements for those using
   ISAKMP/IKE to assure that implementations inter-operate.

   The state transitions that may be involved in communicating with the
   forwarding plane are omitted.  PF_KEY and similar protocols have
   their own set of states required for message sends and completion
   notifications.

   Finally, the retransmits and recursive lookups that are normal for
   DNS are not included in this description of the state machine.

3.2.1 Nonexistent connection

   There is no connection instance for a given source/destination
   address pair.  Upon receipt of a request for keying material for this
   source/destination pair, the initiator searches through the
   connection classes to determine the most appropriate policy.  Upon
   determining an appropriate connection class, an instance object is
   created of that type.  Both of the OE types result in a potential OE
   connection.

   Failure to find an appropriate connection class results in an
   administrator defined default.

   In each case, when the initiator finds an appropriate class for the
   new flow, an instance connection is made of the class which matched.




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3.2.2 Clear-text connection

   The non-existent connection makes a transition to this state when an
   always-clear-text class is instantiated, or when an OE-permissive
   connection fails.  During the transition, the initiator creates a
   pass-through policy object in the forwarding plane for the
   appropriate flow.

   Timing out is the only way to leave this state (see Section 3.2.7).

3.2.3 Deny connection

   The empty connection makes a transition to this state when a deny
   class is instantiated, or when an OE-paranoid connection fails.
   During the transition, the initiator creates a deny policy object in
   the forwarding plane for the appropriate flow.

   Timing out is the only way to leave this state (see Section 3.2.7).

3.2.4 Potential OE connection

   The empty connection makes a transition to this state when one of
   either OE class is instantiated.  During the transition to this
   state, the initiator creates a hold policy object in the forwarding
   plane for the appropriate flow.

   In addition, when making a transition into this state, DNS lookup is
   done in the reverse-map for a TXT delegation resource record (see
   Section 5.2).  The lookup key is the destination address of the flow.

   There are three ways to exit this state:

   1.  DNS lookup finds a TXT delegation resource record.

   2.  DNS lookup does not find a TXT delegation resource record.

   3.  DNS lookup times out.

   Based upon the results of the DNS lookup, the potential OE connection
   makes a transition to the pending OE connection state.  The
   conditions for a successful DNS look are:

   1.  DNS finds an appropriate resource record

   2.  It is properly formatted according to Section 5.2

   3.  if DNSSEC is enabled, then the signature has been vouched for.




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   Note that if the initiator does not find the public key present in
   the TXT delegation record, then the public key must be looked up as a
   sub-state.  Only successful completion of all the DNS lookups is
   considered a success.

   If DNS lookup does not find a resource record or DNS times out, then
   the initiator considers the receiver not OE capable.  If this is an
   OE-paranoid instance, then the potential OE connection makes a
   transition to the deny connection state.  If this is an OE-permissive
   instance, then the potential OE connection makes a transition to the
   clear-text connection state.

   If the initiator finds a resource record but it is not properly
   formatted, or if DNSSEC is enabled and reports a failure to
   authenticate, then the potential OE connection should make a
   transition to the deny connection state.  This action SHOULD be
   logged.  If the administrator wishes to override this transition
   between states, then an always-clear class can be installed for this
   flow.  An implementation MAY make this situation a new class.

3.2.4.1 Restriction on unauthenticated TXT delegation records

   An implementation SHOULD also provide an additional administrative
   control on delegation records and DNSSEC.  This control would apply
   to delegation records (the TXT records in the reverse-map) that are
   not protected by DNSSEC.  Records of this type are only permitted to
   delegate to their own address as a gateway.  When this option is
   enabled, an active attack on DNS will be unable to redirect packets
   to other than the original destination.

3.2.5 Pending OE connection

   The potential OE connection makes a transition to this state when the
   initiator determines that all the information required from the DNS
   lookup is present.  Upon entering this state, the initiator attempts
   to initiate keying to the gateway provided.

   Exit from this state occurs either with a successfully created IPsec
   SA, or with a failure of some kind.  Successful SA creation results
   in a transition to the key connection state.

   Three failures have caused significant problems.  They are clearly
   not the only possible failures from keying.

   Note that if there are multiple gateways available in the TXT
   delegation records, then a failure can only be declared after all
   have been tried.  Further, creation of a phase 1 SA does not
   constitute success.  A set of phase 2 SAs (a tunnel) is considered



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   success.

   The first failure occurs when an ICMP port unreachable is
   consistently received without any other communication, or when there
   is silence from the remote end.  This usually means that either the
   gateway is not alive, or the keying daemon is not functional.  For an
   OE-permissive connection, the initiator makes a transition to the
   clear-text connection but with a low lifespan.  For an OE-pessimistic
   connection, the initiator makes a transition to the deny connection
   again with a low lifespan.  The lifespan in both cases is kept low
   because the remote gateway may be in the process of rebooting or be
   otherwise temporarily unavailable.

   The length of time to wait for the remote keying daemon to wake up is
   a matter of some debate.  If there is a routing failure, 5 minutes is
   usually long enough for the network to re-converge.  Many systems can
   reboot in that amount of time as well.  However, 5 minutes is far too
   long for most users to wait to hear that they can not connect using
   OE.  Implementations SHOULD make this a tunable parameter.

   The second failure occurs after a phase 1 SA has been created, but
   there is either no response to the phase 2 proposal, or the initiator
   receives a negative notify (the notify must be authenticated).  The
   remote gateway is not prepared to do OE at this time.  As before, the
   initiator makes a transition to the clear-text or the deny connection
   based upon connection class, but this time with a normal lifespan.

   The third failure occurs when there is signature failure while
   authenticating the remote gateway.  This can occur when there has
   been a key roll-over, but DNS has not caught up.  In this case again,
   the initiator makes a transition to the clear-text or the deny
   connection based upon the connection class.  However, the lifespan
   depends upon the remaining time to live in the DNS.  (Note that
   DNSSEC signed resource records have a different expiry time than non-
   signed records.)

3.2.6 Keyed connection

   The pending OE connection makes a transition to this state when
   session keying material (the phase 2 SAs) is derived.  The initiator
   creates an encrypt policy in the forwarding plane for this flow.

   There are three ways to exit this state.  The first is by receipt of
   an authenticated delete message (via the keying channel) from the
   peer.  This is normal teardown and results in a transition to the
   expired connection state.

   The second exit is by expiry of the forwarding plane keying material.



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   This starts a re-key operation with a transition back to pending OE
   connection.  In general, the soft expiry occurs with sufficient time
   left to continue to use the keys.  A re-key can fail, which may
   result in the connection failing to clear-text or deny as
   appropriate.  In the event of a failure, the forwarding plane policy
   does not change until the phase 2 SA (IPsec SA) reaches its hard
   expiry.

   The third exit is in response to a negotiation from a remote gateway.
   If the forwarding plane signals the control plane that it has
   received an unknown SPI from the remote gateway, or an ICMP is
   received from the remote gateway indicating an unknown SPI, the
   initiator should consider that the remote gateway has rebooted or
   restarted.  Since these indications are easily forged, the
   implementation must exercise care.  The initiator should make a
   cautious (rate-limited) attempt to re-key the connection.

3.2.7 Expiring connection

   The initiator will periodically place each of the deny, clear-text,
   and keyed connections into this sub-state.  See Section 3.4 for more
   details of how often this occurs.  The initiator queries the
   forwarding plane for last use time of the appropriate policy.  If the
   last use time is relatively recent, then the connection returns to
   the previous deny, clear-text or keyed connection state.  If not,
   then the connection enters the expired connection state.

   The DNS query and answer that lead to the expiring connection state
   are also examined.  The DNS query may become stale.  (A negative,
   i.e.  no such record, answer is valid for the period of time given by
   the MINIMUM field in an attached SOA record.  See [12] section
   4.3.4.) If the DNS query is stale, then a new query is made.  If the
   results change, then the connection makes a transition to a new state
   as described in potential OE connection state.

   Note that when considering how stale a connection is, both outgoing
   SPD and incoming SAD must be queried as some flows may be
   unidirectional for some time.

   Also note that the policy at the forwarding plane is not updated
   unless there is a conclusion that there should be a change.

3.2.8 Expired connection

   Entry to this state occurs when no datagrams have been forwarded
   recently via the appropriate SPD and SAD objects.  The objects in the
   forwarding plane are removed (logging any final byte and packet
   counts if appropriate) and the connection instance in the keying



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   plane is deleted.

   The initiator sends an ISAKMP/IKE delete to clean up the phase 2 SAs
   as described in Section 3.4.

   Whether or not to delete the phase 1 SAs at this time is left as a
   local implementation issue.  Implementations that do delete the phase
   1 SAs MUST send authenticated delete messages to indicate that they
   are doing so.  There is an advantage to keeping the phase 1 SAs until
   they expire - they may prove useful again in the near future.

3.3 Keying state machine - responder

   The responder has a set of objects identical to those of the
   initiator.

   The responder receives an invitation to create a keying channel from
   an initiator.

3.3.1 Unauthenticated OE peer

   Upon entering this state, the responder starts a DNS lookup for a KEY
   record for the initiator.  The responder looks in the reverse-map for
   a KEY record for the initiator if the initiator has offered an
   ID_IPV4_ADDR, and in the forward map if the initiator has offered an
   ID_FQDN type.  (See [8] section 4.6.2.1.)

   The responder exits this state upon successful receipt of a KEY from
   DNS, and use of the key to verify the signature of the initiator.

   Successful authentication of the peer results in a transition to the
   authenticated OE Peer state.

   Note that the unauthenticated OE peer state generally occurs in the
   middle of the key negotiation protocol.  It is really a form of
   pseudo-state.

3.3.2 Authenticated OE Peer

   The peer will eventually propose one or more phase 2 SAs.  The
   responder uses the source and destination address in the proposal to
   finish instantiating the connection state using the connection class
   table.  The responder MUST search for an identical connection object
   at this point.

   If an identical connection is found, then the responder deletes the
   old instance, and the new object makes a transition to the pending OE
   connection state.  This means that new ISAKMP connections with a



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   given peer will always use the latest instance, which is the correct
   one if the peer has rebooted in the interim.

   If an identical connection is not found, then the responder makes the
   transition according to the rules given for the initiator.

   Note that if the initiator is in OE-paranoid mode and the responder
   is in either always-clear-text or deny, then no communication is
   possible according to policy.  An implementation is permitted to
   create new types of policies such as "accept OE but do not initiate
   it".  This is a local matter.

3.4 Renewal and teardown

3.4.1 Aging

   A potentially unlimited number of tunnels may exist.  In practice,
   only a few tunnels are used during a period of time.  Unused tunnels
   MUST, therefore, be torn down.  Detecting when tunnels are no longer
   in use is the subject of this section.

   There are two methods for removing tunnels: explicit deletion or
   expiry.

   Explicit deletion requires an IKE delete message.  As the deletes
   MUST be authenticated, both ends of the tunnel must maintain the key
   channel (phase 1 ISAKMP SA).  An implementation which refuses to
   either maintain or recreate the keying channel SA will be unable to
   use this method.

   The tunnel expiry method, simply allows the IKE daemon to expire
   normally without attempting to re-key it.

   Regardless of which method is used to remove tunnels, the
   implementation requires a method to determine if the tunnel is still
   in use.  The specifics are a local matter, but  the FreeS/WAN project
   uses the following criteria.  These criteria are currently
   implemented in the key management daemon, but could also be
   implemented at the SPD layer using an idle timer.

   Set a short initial (soft) lifespan of 1 minute since many net flows
   last only a few seconds.

   At the end of the lifespan, check to see if the tunnel was used by
   traffic in either direction during the last 30 seconds.  If so,
   assign a longer tentative lifespan of 20 minutes after which, look
   again.  If the tunnel is not in use, then close the tunnel.




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   The expiring state in the key management system (see Section 3.2.7)
   implements these timeouts.  The timer above may be in the forwarding
   plane, but then it must be re-settable.

   The tentative lifespan is independent of re-keying; it is just the
   time when the tunnel's future is next considered.  (The term lifespan
   is used here rather than lifetime for this reason.) Unlike re-keying,
   this tunnel use check is not costly and should happen reasonably
   frequently.

   A multi-step back-off algorithm is not considered worth the effort
   here.

   If the security gateway and the client host are the same and not a
   Bump-in-the-Stack or Bump-in-the-Wire implementation, tunnel teardown
   decisions MAY pay attention to TCP connection status as reported by
   the local TCP layer.  A still-open TCP connection is almost a
   guarantee that more traffic is expected.  Closing of the only TCP
   connection through a tunnel is a strong hint that no more traffic is
   expected.

3.4.2 Teardown and cleanup

   Teardown should always be coordinated between the two ends of the
   tunnel by interpreting and sending delete notifications.  There is a
   detailed sub-state in the expired connection state of the key manager
   that relates to retransmits of the delete notifications, but this is
   considered to be a keying system detail.

   On receiving a delete for the outbound SAs of a tunnel (or some
   subset of them), tear down the inbound ones also and notify the
   remote end with a delete.  If the local system receives a delete for
   a tunnel which is no longer in existence, then two delete messages
   have crossed paths.  Ignore the delete.  The operation has already
   been completed.  Do not generate any messages in this situation.

   Tunnels are to be considered as bidirectional entities, even though
   the low-level protocols don't treat them this way.

   When the deletion is initiated locally, rather than as a response to
   a received delete, send a delete for (all) the inbound SAs of a
   tunnel.  If the local system does not receive a responding delete for
   the outbound SAs, try re-sending the original delete.  Three tries
   spaced 10 seconds apart seems a reasonable level of effort.  A
   failure of the other end to respond after 3 attempts, indicates that
   the possibility of further communication is unlikely.  Remove the
   outgoing SAs.  (The remote system may be a mobile node that is no
   longer present or powered on.)



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   After re-keying, transmission should switch to using the new outgoing
   SAs (ISAKMP or IPsec) immediately, and the old leftover outgoing SAs
   should be cleared out promptly (delete should be sent for the
   outgoing SAs) rather than waiting for them to expire.  This reduces
   clutter and minimizes confusion for the operator doing diagnostics.














































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4. Impacts on IKE

4.1 ISAKMP/IKE protocol

   The IKE wire protocol needs no modifications.  The major changes are
   implementation issues relating to how the proposals are interpreted,
   and from whom they may come.

   As opportunistic encryption is designed to be useful between peers
   without prior operator configuration, an IKE daemon must be prepared
   to negotiate phase 1 SAs with any node.  This may require a large
   amount of resources to maintain cookie state, as well as large
   amounts of entropy for nonces, cookies and so on.

   The major changes to support opportunistic encryption are at the IKE
   daemon level.  These changes relate to handling of key acquisition
   requests, lookup of public keys and TXT records, and interactions
   with firewalls and other security facilities that may be co-resident
   on the same gateway.

4.2 Gateway discovery process

   In a typical configured tunnel, the address of SG-B is provided via
   configuration.  Furthermore, the mapping of an SPD entry to a gateway
   is typically a 1:1 mapping.  When the 0.0.0.0/0 SPD entry technique
   is used, then the mapping to a gateway is determined by the reverse
   DNS records.

   The need to do a DNS lookup and wait for a reply will typically
   introduce a new state and a new event source (DNS replies) to IKE.
   Although a synchronous DNS request can be implemented for proof of
   concept, experience is that it can cause very high latencies when a
   queue of queries must all timeout in series.

   Use of an asynchronous DNS lookup will also permit overlap of DNS
   lookups with some of the protocol steps.

4.3 Self identification

   SG-A will have to establish its identity.  Use an IPv4 ID in phase 1.

   There are many situations where the administrator of SG-A may not be
   able to control the reverse DNS records for SG-A's public IP address.
   Typical situations include dialup connections and most residential-
   type broadband Internet access (ADSL, cable-modem) connections.  In
   these situations, a fully qualified domain name that is under the
   control of SG-A's administrator may be used when acting as an
   initiator only.  The FQDN ID should be used in phase 1.  See Section



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   5.3 for more details and restrictions.

4.4 Public key retrieval process

   Upon receipt of a phase 1 SA proposal with either an IPv4 (IPv6) ID
   or an FQDN ID, an IKE daemon needs to examine local caches and
   configuration files to determine if this is part of a configured
   tunnel.  If no configured tunnels are found, then the implementation
   should attempt to retrieve a KEY record from the reverse DNS in the
   case of an IPv4/IPv6 ID, or from the forward DNS in the case of FQDN
   ID.

   It is reasonable that if other non-local sources of policy are used
   (COPS, LDAP), they be consulted concurrently but some clear ordering
   of policy be provided.  Note that due to variances in latency,
   implementations must wait for positive or negative replies from all
   sources of policy before making any decisions.

4.5 Interactions with DNSSEC

   The implementation described (1.98) neither uses DNSSEC directly to
   explicitly verify the authenticity of zone information, nor uses the
   NXT records to provide authentication of the absence of a TXT or KEY
   record.  Rather, this implementation uses a trusted path to a DNSSEC
   capable caching resolver.

   To distinguish between an authenticated and an unauthenticated DNS
   resource record, a stub resolver capable of returning DNSSEC
   information MUST be used.

4.6 Required proposal types

4.6.1 Phase 1 parameters

   Main mode MUST be used.

   The initiator MUST offer at least one proposal using some combination
   of: 3DES, HMAC-MD5 or HMAC-SHA1, DH group 2 or 5.  Group 5 SHOULD be
   proposed first.  [11]

   The initiator MAY offer additional proposals, but the cipher MUST not
   be weaker than 3DES.  The initiator SHOULD limit the number of
   proposals such that the IKE datagrams do not need to be fragmented.

   The responder MUST accept one of the proposals.  If any configuration
   of the responder is required then the responder is not acting in an
   opportunistic way.




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   SG-A SHOULD use an ID_IPV4_ADDR (ID_IPV6_ADDR for IPv6) of the
   external interface of SG-A for phase 1.  (There is an exception, see
   Section 5.3.) The authentication method MUST be RSA public key
   signatures.  The RSA key for SG-A SHOULD be placed into a DNS KEY
   record in the reverse space of SG-A (i.e.  using in-addr.arpa).

4.6.2 Phase 2 parameters

   SG-A MUST propose a tunnel between Alice and Bob, using 3DES-CBC
   mode, MD5 or SHA1 authentication.  Perfect Forward Secrecy MUST be
   specified.

   Tunnel mode MUST be used.

   Identities MUST be ID_IPV4_ADDR_SUBNET with the mask being /32.

   Authorization for SG-A to act on Alice's behalf is determined by
   looking for a TXT record in the reverse-map at Alice's address.

   Compression SHOULD NOT be mandatory.  It may be offered as an option.































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5. DNS issues

5.1 Use of KEY record

   In order to establish their own identities, SG-A and SG-B SHOULD
   publish their public keys in their reverse DNS via DNSSEC's KEY
   record.  See section 3 of RFC 2535 [16].

   For example:

   KEY 0x4200 4 1 AQNJjkKlIk9...nYyUkKK8

   0x4200: The flag bits, indicating that this key is prohibited for
      confidentiality use (it authenticates the peer only, a separate
      Diffie-Hellman exchange is used for confidentiality), and that
      this key is associated with the non-zone entity whose name is the
      RR owner name.  No other flags are set.

   4: This indicates that this key is for use by IPsec.

   1: An RSA key is present.

   AQNJjkKlIk9...nYyUkKK8: The public key of the host as described in
      [17].

   Use of several KEY records allows for key rollover.  The SIG Payload
   in IKE phase 1 SHOULD be accepted if the public key given by any KEY
   RR validates it.

5.2 Use of TXT delegation record

   Alice publishes a TXT record to provide authorization for SG-A to act
   on Alice's behalf.  Bob publishes a TXT record to provide
   authorization for SG-B to act on Bob's behalf.  These records are
   located in the reverse DNS (in-addr.arpa) for their respective IP
   addresses.  The reverse DNS SHOULD be secured by DNSSEC, when it is
   deployed.  DNSSEC is required to defend against active attacks.

   If Alice's address is P.Q.R.S, then she can authorize another node to
   act on her behalf by publishing records at:

   S.R.Q.P.in-addr.arpa

   The contents of the resource record are expected to be a string that
   uses the following syntax, as suggested in [15].  (Note that the
   reply to query may include other TXT resource records used by other
   applications.)




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   ---------------------------------------------------------------------


   X-IPsec-Server(P)=A.B.C.D KEY

             Figure 2: Format of reverse delegation record

   ---------------------------------------------------------------------

   P: Specifies a precedence for this record.  This is similar to MX
      record preferences.  Lower numbers have stronger preference.

   A.B.C.D: Specifies the IP address of the Security Gateway for this
      client machine.

   KEY: Is the encoded RSA Public key of the Security Gateway.  The key
      is provided here to avoid a second DNS lookup.  If this field is
      absent, then a KEY resource record should be looked up in the
      reverse-map of A.B.C.D.  The key is transmitted in base64 format.

   The pieces of the record are separated by any whitespace (space, tab,
   newline, carriage return).  An ASCII space SHOULD be used.

   In the case where Alice is located at a public address behind a
   security gateway that has no fixed address (or no control over its
   reverse-map), then Alice may delegate to a public key by domain name.

   ---------------------------------------------------------------------


   X-IPsec-Server(P)=@FQDN KEY

      Figure 3: Format of reverse delegation record (FQDN version)

   ---------------------------------------------------------------------

   P: Is as above.

   FQDN: Specifies the FQDN that the Security Gateway will identify
      itself with.

   KEY: Is the encoded RSA Public key of the Security Gateway.

   If there is more than one such TXT record with strongest (lowest
   numbered) precedence, one Security Gateway is picked arbitrarily from
   those specified in the strongest-preference records.





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5.2.1 Long TXT records

   When packed into transport format, TXT records which are longer than
   255 characters are divided into smaller <character-strings>.  (See
   [13] section 3.3 and 3.3.14.) These MUST be reassembled into a single
   string for processing.  Whitespace characters in the base64 encoding
   are to be ignored.

5.2.2 Choice of TXT record

   It has been suggested to use the KEY, OPT, CERT, or KX records
   instead of a TXT record.  None is satisfactory.

   The KEY RR has a protocol field which could be used to indicate a new
   protocol, and an algorithm field which could be used to indicate
   different contents in the key data.  However, the KEY record is
   clearly not intended for storing what are really authorizations, it
   is just for identities.  Other uses have been discouraged.

   OPT resource records, as defined in [14] are not intended to be used
   for storage of information.  They are not to be loaded, cached or
   forwarded.  They are, therefore, inappropriate for use here.

   CERT records [18] can encode almost any set of information.  A custom
   type code could be used permitting any suitable encoding to be
   stored, not just X.509.  According to the RFC, the certificate RRs
   are to be signed internally which may add undesirable and unnecessary
   bulk.  Larger DNS records may require TCP instead of UDP transfers.

   At the time of protocol design, the CERT RR was not widely deployed
   and could not be counted upon.  Use of CERT records will be
   investigated, and may be proposed in a future revision of this
   document.

   KX records are ideally suited for use instead of TXT records, but had
   not been deployed at the time of implementation.

5.3 Use of FQDN IDs

   Unfortunately, not every administrator has control over the contents
   of the reverse-map.  Where the initiator (SG-A) has no suitable
   reverse-map, the authorization record present in the reverse-map of
   Alice may refer to a FQDN instead of an IP address.

   In this case, the client's TXT record gives the fully qualified
   domain name (FQDN) in place of its security gateway's IP address.
   The initiator should use the ID_FQDN ID-payload in phase 1.  A
   forward lookup for a KEY record on the FQDN must yield the



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   initiator's public key.

   This method can also be used when the external address of SG-A is
   dynamic.

   If SG-A is acting on behalf of Alice, then Alice must still delegate
   authority for SG-A to do so in her reverse-map.  When Alice and SG-A
   are one and the same (i.e.  Alice is acting as an end-node) then
   there is no need for this when initiating only.

   However, Alice must still delegate to  herself if she wishes others
   to initiate OE to her.  See Figure 3.

5.4 Key roll-over

   Good cryptographic hygiene says that one should replace public/
   private key pairs periodically.  Some administrators may wish to do
   this as often as daily.  Typical DNS propagation delays are
   determined by the SOA Resource Record MINIMUM parameter, which
   controls how long DNS replies may be cached.  For reasonable
   operation of DNS servers, administrators usually want this value to
   be at least several hours, sometimes as a long as a day.  This
   presents a problem - a new key MUST not be used prior to it
   propagating through DNS.

   This problem is dealt with by having the Security Gateway generate a
   new public/private key pair at least MINIMUM seconds in advance of
   using it.  It then adds this key to the DNS (both as a second KEY
   record and in additional TXT delegation records) at key generation
   time.  Note: only one key is allowed in each TXT record.

   When authenticating, all gateways MUST have available all public keys
   that are found in DNS for this entity.  This permits the
   authenticating end to check both the key for "today" and the key for
   "tomorrow".  Note that it is the end which is creating the signature
   (possesses the private key) that determines which key is to be used.















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6. Network address translation interaction

   There are no fundamentally new issues for implementing opportunistic
   encryption in the presence of network address translation.  Rather
   there are only the regular IPsec issues with NAT traversal.

   There are several situations to consider for NAT.

6.1 Co-located NAT/NAPT

   If SG-A is also performing network address translation on behalf of
   Alice, then the packet should be translated prior to being subjected
   to opportunistic encryption.  This is in contrast to typically
   configured tunnels which often exist to bridge islands of private
   network address space.  SG-A will use the translated source address
   for phase 2, and so SG-B will look up that address to confirm SG-A's
   authorization.

   In the case of NAT (1:1), the address space into which the
   translation is done MUST be globally unique, and control over the
   reverse-map is assumed.  Placing of TXT records is possible.

   In the case of NAPT (m:1), the address will be SG-A.  The ability to
   get KEY and TXT records in place will again depend upon whether or
   not there is administrative control over the reverse-map.  This is
   identical to situations involving a single host acting on behalf of
   itself.  FQDN style can be used to get around a lack of a reverse-map
   for initiators only.

6.2 SG-A behind NAT/NAPT

   If there is a NAT or NAPT between SG-A and SG-B, then normal IPsec
   NAT traversal rules apply.  In addition to the transport problem
   which may be solved by other mechanisms, there is the issue of what
   phase 1 and phase 2 IDs to use.  While FQDN could be used during
   phase 1 for SG-A, there is no appropriate ID for phase 2 that permits
   SG-B to determine that SG-A is in fact authorized to speak for Alice.

6.3 Bob is behind a NAT/NAPT

   If Bob is behind a NAT (perhaps SG-B), then there is, in fact, no way
   for Alice to address a packet to Bob.  Not only is opportunistic
   encryption impossible, but it is also impossible for Alice to
   initiate any communication to Bob.  It may be possible for Bob to
   initiate in such a situation.  This creates an asymmetry, but this is
   common for NAPT.





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7. Host implementations

   When Alice and SG-A are components of the same system, they are
   considered to be a host implementation.  The packet sequence scenario
   remains unchanged.

   Components marked Alice are the upper layers (TCP, UDP, the
   application), and SG-A is the IP layer.

   Note that tunnel mode is still required.

   As Alice and SG-A are acting on behalf of themselves, no TXT based
   delegation record is necessary for Alice to initiate.  She can rely
   on FQDN in a forward map.  This is particularly attractive to mobile
   nodes such as notebook computers at conferences.  To respond, Alice/
   SG-A will still need an entry in Alice's reverse-map.



































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8. Multi-homing

   If there are multiple paths between Alice and Bob (as illustrated in
   the diagram with SG-D), then additional DNS records are required to
   establish authorization.

   In Figure 1, Alice has two ways to exit her network: SG-A and SG-D.
   Previously SG-D has been ignored.  Postulate that there are routers
   between Alice and her set of security gateways (denoted by the +
   signs and the marking of an autonomous system number for Alice's
   network).  Datagrams may, therefore, travel to either SG-A or SG-D en
   route to Bob.

   As long as all network connections are in good order, it does not
   matter how datagrams exit Alice's network.  When they reach either
   security gateway, the security gateway will find the TXT delegation
   record in Bob's reverse-map, and establish an SA with SG-B.

   SG-B has no problem establishing that either of SG-A or SG-D may
   speak for Alice, because Alice has published two equally weighted TXT
   delegation records:

   ---------------------------------------------------------------------


   X-IPsec-Server(10)=192.1.1.5 AQMM...3s1Q==
   X-IPsec-Server(10)=192.1.1.6 AAJN...j8r9==

        Figure 4: Multiple gateway delegation example for Alice

   ---------------------------------------------------------------------

   Alice's routers can now do any kind of load sharing needed.  Both SG-
   A and SG-D send datagrams addressed to Bob through their tunnel to
   SG-B.

   Alice's use of non-equal weight delegation records to show preference
   of one gateway over another, has relevance only when SG-B is
   initiating to Alice.

   If the precedences are the same, then SG-B has a more difficult time.
   It must decide which of the two tunnels to use.  SG-B has no
   information about which link is less loaded, nor which security
   gateway has more cryptographic resources available.  SG-B, in fact,
   has no knowledge of whether both gateways are even reachable.

   The Public Internet's default-free zone may well know a good route to
   Alice, but the datagrams that SG-B creates must be addressed to



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   either SG-A or SG-D; they can not be addressed to Alice directly.

   SG-B may make a number of choices:

   1.  It can ignore the problem and round robin among the tunnels.
       This causes losses during times when one or the other security
       gateway is unreachable.  If this worries Alice, she can change
       the weights in her TXT delegation records.

   2.  It can send to the gateway from which it most recently received
       datagrams.  This assumes that routing and reachability are
       symmetrical.

   3.  It can listen to BGP information from the Internet to decide
       which system is currently up.  This is clearly much more
       complicated, but if SG-B is already participating in the BGP
       peering system to announce Bob, the results data may already be
       available to it.

   4.  It can refuse to negotiate the second tunnel.  (It is unclear
       whether or not this is even an option.)

   5.  It can silently replace the outgoing portion of the first tunnel
       with the second one while still retaining the incoming portions
       of both.  SG-B can, thus, accept datagrams from either SG-A or
       SG-D, but send only to the gateway that most recently re-keyed
       with it.

   Local policy determines which choice SG-B makes.  Note that even if
   SG-B has perfect knowledge about the reachability of SG-A and SG-D,
   Alice may not be reachable from either of these security gateways
   because of internal reachability issues.

   FreeS/WAN implements option 5.  Implementing a different option is
   being considered.  The multi-homing aspects of OE are not well
   developed and may be the subject of a future document.















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9. Failure modes

9.1 DNS failures

   If a DNS server fails to respond, local policy decides whether or not
   to permit communication in the clear as embodied in the connection
   classes in Section 3.2.  It is easy to mount a denial of service
   attack on the DNS server responsible for a particular network's
   reverse-map.  Such an attack may cause all communication with that
   network to go in the clear if the policy is permissive, or fail
   completely if the policy is paranoid.  Please note that this is an
   active attack.

   There are still many networks that do not have properly configured
   reverse-maps.  Further, if the policy is not to communicate, the
   above denial of service attack isolates the target network.
   Therefore, the decision of whether or not to permit communication in
   the clear MUST be a matter of local policy.

9.2 DNS configured, IKE failures

   DNS records claim that opportunistic encryption should occur, but the
   target gateway either does not respond on port 500, or refuses the
   proposal.  This may be because of a crash or reboot, a faulty
   configuration, or a firewall filtering port 500.

   The receipt of ICMP port, host or network unreachable messages
   indicates a potential problem, but MUST NOT cause communication to
   fail immediately.  ICMP messages are easily forged by attackers.  If
   such a forgery caused immediate failure, then an active attacker
   could easily prevent any encryption from ever occurring, possibly
   preventing all communication.

   In these situations a clear log should be produced and local policy
   should dictate if communication is then permitted in the clear.

9.3 System reboots

   Tunnels sometimes go down because the remote end crashes,
   disconnects, or has a network link break.  In general there is no
   notification of this.  Even in the event of a crash and successful
   reboot, other SGs don't hear about it unless the rebooted SG has
   specific reason to talk to them immediately.  Over-quick response to
   temporary network outages is undesirable.  Note that a tunnel can be
   torn down and then re-established without any effect visible to the
   user except a pause in traffic.  On the other hand, if one end
   reboots, the other end can't get datagrams to it at all (except via
   IKE) until the situation is noticed.  So a bias toward quick response



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   is appropriate even at the cost of occasional false alarms.

   A mechanism for recovery after reboot is a topic of current research
   and is not specified in this document.

   A deliberate shutdown should include an attempt, using deletes, to
   notify all other SGs currently connected by phase 1 SAs that
   communication is about to fail.  Again, a remote SG will assume this
   is a teardown.  Attempts by the remote SGs to negotiate new tunnels
   as replacements should be ignored.  When possible, SGs should attempt
   to preserve information about currently-connected SGs in non-volatile
   storage, so that after a crash, an Initial-Contact can be sent to
   previous partners to indicate loss of all previously established
   connections.





































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10. Unresolved issues

10.1 Control of reverse DNS

   The method of obtaining information by reverse DNS lookup causes
   problems for people who cannot control their reverse DNS bindings.
   This is an unresolved problem in this version, and is out of scope.












































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11. Examples

11.1 Clear-text usage (permit policy)

   Two example scenarios follow.  In the first example GW-A (Gateway A)
   and GW-B (Gateway B) have always-clear-text policies, and in the
   second example they have an OE policy.

   ---------------------------------------------------------------------


     Alice         SG-A       DNS       SG-B           Bob
      (1)
       ------(2)-------------->
       <-----(3)---------------
      (4)----(5)----->
                      ----------(6)------>
                                          ------(7)----->
                                         <------(8)------
                      <----------(9)------
       <----(10)-----
      (11)----------->
                      ----------(12)----->
                                          -------------->
                                         <---------------
                      <-------------------
       <-------------

                Figure 5: Timing of regular transaction

   ---------------------------------------------------------------------

   Alice wants to communicate with Bob.  Perhaps she wants to retrieve a
   web page from Bob's web server.  In the absence of opportunistic
   encryptors, the following events occur:

   (1) Human or application 'clicks' with a name.

   (2) Application looks up name in DNS to get IP address.

   (3) Resolver returns A record to application.

   (4) Application starts a TCP session or UDP session and OS sends
      datagram.

   (5) Datagram is seen at first gateway from Alice (SG-A).  (SG-A makes
      a transition through Empty connection to always-clear connection
      and instantiates a pass-through policy at the forwarding plane.)



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   (6) Datagram is seen at last gateway before Bob (SG-B).

   (7) First datagram from Alice is seen by Bob.

   (8) First return datagram is sent by Bob.

   (9) Datagram is seen at Bob's gateway.  (SG-B makes a transition
      through Empty connection to always-clear connection and
      instantiates a pass-through policy at the forwarding plane.)

   (10) Datagram is seen at Alice's gateway.

   (11) OS hands datagram to application.  Alice sends another datagram.

   (12) A second datagram traverses the Internet.


11.2 Opportunistic encryption

   In the presence of properly configured opportunistic encryptors, the
   event list is extended.

   ---------------------------------------------------------------------


     Alice          SG-A      DNS       SG-B           Bob
      (1)
       ------(2)-------------->
       <-----(3)---------------
      (4)----(5)----->+
                     ----(5B)->
                     <---(5C)--
                     ~~~~~~~~~~~~~(5D)~~~>
                     <~~~~~~~~~~~~(5E1)~~~
                     ~~~~~~~~~~~~~(5E2)~~>
                     <~~~~~~~~~~~~(5E3)~~~
                     #############(5E4)##>
                     <############(5E5)###
                              <----(5F1)--
                              -----(5F2)->
                     #############(5G1)##>
                              <----(5H1)--
                              -----(5H2)->
                     <############(5G2)###
                     #############(5G3)##>
                      ============(6)====>
   		                       ------(7)----->
                                         <------(8)------



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                     <==========(9)======
       <-----(10)----
      (11)----------->
                      ==========(12)=====>
                                          -------------->
                                         <---------------
                      <===================
       <-------------

        Figure 6: Timing of opportunistic encryption transaction

   ---------------------------------------------------------------------

   (1) Human or application clicks with a name.

   (2) Application initiates DNS mapping.

   (3) Resolver returns A record to application.

   (4) Application starts a TCP session or UDP.

   (5) SG-A (host or SG) sees datagram to target, and buffers it.

   (5B) SG-A asks DNS for TXT record.

   (5C) DNS returns TXT record(s).

   (5D) Initial IKE Main Mode Packet goes out.

   (5E) IKE ISAKMP phase 1 succeeds.

   (5F) SG-B asks DNS for TXT record to prove SG-A is an agent for
      Alice.

   (5G) IKE phase 2 negotiation.

   (5H) DNS lookup by responder (SG-B).

   (6) Buffered datagram is sent by SG-A.

   (7) Datagram is received by SG-B, decrypted, and sent to Bob.

   (8) Bob replies, and datagram is seen by SG-B.

   (9) SG-B already has tunnel up with SG-A, and uses it.

   (10) SG-A decrypts datagram and gives it to Alice.




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   (11) Alice receives datagram.  Sends new packet to Bob.

   (12) SG-A gets second datagram, sees that tunnel is up, and uses it.

   For the purposes of this section, we will describe only the changes
   that occur between Figure 5 and Figure 6.  This corresponds to time
   points 5, 6, 7, 9 and 10 on the list above.

11.2.1 (5) IPsec datagram interception

   At point (5), SG-A intercepts the datagram because this source/
   destination pair lacks a policy (the non-existent policy state).  SG-
   A creates a hold policy, and buffers the datagram.  SG-A requests
   keys from the keying daemon.

11.2.2 (5B) DNS lookup for TXT record

   SG-A's IKE daemon, having looked up the source/destination pair in
   the connection class list, creates a new Potential OE connection
   instance.  SG-A starts DNS queries.

11.2.3 (5C) DNS returns TXT record(s)

   DNS returns properly formed TXT delegation records, and SG-A's IKE
   daemon causes this instance to make a transition from Potential OE
   connection to Pending OE connection.

   Using the example above, the returned record might contain:

   ---------------------------------------------------------------------


   X-IPsec-Server(10)=192.1.1.5 AQMM...3s1Q==

         Figure 7: Example of reverse delegation record for Bob

   ---------------------------------------------------------------------

    with SG-B's IP address and public key listed.

11.2.4 (5D) Initial IKE main mode packet goes out

   Upon entering Pending OE connection, SG-A sends the initial ISAKMP
   message with proposals.  See Section 4.6.1.

11.2.5 (5E1) Message 2 of phase 1 exchange

   SG-B receives the message.  A new connection instance is created in



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   the unauthenticated OE peer state.

11.2.6 (5E2) Message 3 of phase 1 exchange

   SG-A sends a Diffie-Hellman exponent.  This is an internal state of
   the keying daemon.

11.2.7 (5E3) Message 4 of phase 1 exchange

   SG-B responds with a Diffie-Hellman exponent.  This is an internal
   state of the keying protocol.

11.2.8 (5E4) Message 5 of phase 1 exchange

   SG-A uses the phase 1 SA to send its identity under encryption.  The
   choice of identity is discussed in Section 4.6.1.  This is an
   internal state of the keying protocol.

11.2.9 (5F1) Responder lookup of initiator key

   SG-B asks DNS for the public key of the initiator.  DNS looks for a
   KEY record by IP address in the reverse-map.  That is, a KEY resource
   record is queried for 4.1.1.192.in-addr.arpa (recall that SG-A's
   external address is 192.1.1.4).  SG-B uses the resulting public key
   to authenticate the initiator.  See Section 5.1 for further details.

11.2.10 (5F2) DNS replies with public key of initiator

   Upon successfully authenticating the peer, the connection instance
   makes a transition to authenticated OE peer on SG-B.

   The format of the TXT record returned is described in Section 5.2.

11.2.11 (5E5) Responder replies with ID and authentication

   SG-B sends its ID along with authentication material.  This is an
   internal state for the keying protocol.

11.2.12 (5G) IKE phase 2

11.2.12.1 (5G1) Initiator proposes tunnel

   Having established mutually agreeable authentications (via KEY) and
   authorizations (via TXT), SG-A proposes to create an IPsec tunnel for
   datagrams transiting from Alice to Bob.  This tunnel is established
   only for the Alice/Bob combination, not for any subnets that may be
   behind SG-A and SG-B.




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11.2.12.2 (5H1) Responder determines initiator's authority

   While the identity of SG-A has been established, its authority to
   speak for Alice has not yet been confirmed.  SG-B does a reverse
   lookup on Alice's address for a TXT record.

   Upon receiving this specific proposal, SG-B's connection instance
   makes a transition into the potential OE connection state.  SG-B may
   already have an instance, and the check is made as described above.

11.2.12.3 (5H2) DNS replies with TXT record(s)

   The returned key and IP address should match that of SG-A.

11.2.12.4 (5G2) Responder agrees to proposal

   Should additional communication occur between, for instance, Dave and
   Bob using SG-A and SG-B, a new tunnel (phase 2 SA) would be
   established.  The phase 1 SA may be reusable.

   SG-A, having successfully keyed the tunnel, now makes a transition
   from Pending OE connection to Keyed OE connection.

   The responder MUST setup the inbound IPsec SAs before sending its
   reply.

11.2.12.5 (5G3) Final acknowledgment from initiator

   The initiator agrees with the responder's choice and sets up the
   tunnel.  The initiator sets up the inbound and outbound IPsec SAs.

   The proper authorization returned with keys prompts SG-B to make a
   transition to the keyed OE connection state.

   Upon receipt of this message, the responder may now setup the
   outbound IPsec SAs.

11.2.13 (6) IPsec succeeds, and sets up tunnel for communication between
        Alice and Bob

   SG-A sends the datagram saved at step (5) through the newly created
   tunnel to SG-B, where it gets decrypted and forwarded.  Bob receives
   it at (7) and replies at (8).

11.2.14 (9) SG-B already has tunnel up with G1 and uses it

   At (9), SG-B has already established an SPD entry mapping Bob->Alice
   via a tunnel, so this tunnel is simply applied.  The datagram is



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   encrypted to SG-A, decrypted by SG-A and passed to Alice at (10).


















































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12. Security considerations

12.1 Configured vs opportunistic tunnels

   Configured tunnels are those which are setup using bilateral
   mechanisms: exchanging public keys (raw RSA, DSA, PKIX), pre-shared
   secrets, or by referencing keys that are in known places
   (distinguished name from LDAP, DNS).  These keys are then used to
   configure a specific tunnel.

   A pre-configured tunnel may be on all the time, or may be keyed only
   when needed.  The end points of the tunnel are not necessarily
   static: many mobile applications (road warrior) are considered to be
   configured tunnels.

   The primary characteristic is that configured tunnels are assigned
   specific security properties.  They may be trusted in different ways
   relating to exceptions to firewall rules, exceptions to NAT
   processing, and to bandwidth or other quality of service
   restrictions.

   Opportunistic tunnels are not inherently trusted in any strong way.
   They are created without prior arrangement.  As the two parties are
   strangers, there MUST be no confusion of datagrams that arrive from
   opportunistic peers and those that arrive from configured tunnels.  A
   security gateway MUST take care that an opportunistic peer can not
   impersonate a configured peer.

   Ingress filtering MUST be used to make sure that only datagrams
   authorized by negotiation (and the concomitant authentication and
   authorization) are accepted from a tunnel.  This is to prevent one
   peer from impersonating another.

   An implementation suggestion is to treat opportunistic tunnel
   datagrams as if they arrive on a logical interface distinct from
   other configured tunnels.  As the number of opportunistic tunnels
   that may be created automatically on a system is potentially very
   high, careful attention to scaling should be taken into account.

   As with any IKE negotiation, opportunistic encryption cannot be
   secure without authentication.  Opportunistic encryption relies on
   DNS for its authentication information and, therefore, cannot be
   fully secure without a secure DNS.  Without secure DNS, opportunistic
   encryption can protect against passive eavesdropping but not against
   active man-in-the-middle attacks.






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12.2 Firewalls versus Opportunistic Tunnels

   Typical usage of per datagram access control lists is to implement
   various kinds of security gateways.  These are typically called
   "firewalls".

   Typical usage of a virtual private network (VPN) within a firewall is
   to bypass all or part of the access controls between two networks.
   Additional trust (as outlined in the previous section) is given to
   datagrams that arrive in the VPN.

   Datagrams that arrive via opportunistically configured tunnels MUST
   not be trusted.  Any security policy that would apply to a datagram
   arriving in the clear SHOULD also be applied to datagrams arriving
   opportunistically.

12.3 Denial of service

   There are several different forms of denial of service that an
   implementor should concern themselves with.  Most of these problems
   are shared with security gateways that have large numbers of mobile
   peers (road warriors).

   The design of ISAKMP/IKE, and its use of cookies, defend against many
   kinds of denial of service.  Opportunism changes the assumption that
   if the phase 1 (ISAKMP) SA is authenticated, that it was worthwhile
   creating.  Because the gateway will communicate with any machine, it
   is possible to form phase 1 SAs with any machine on the Internet.























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13. IANA Considerations

   There are no known numbers which IANA will need to manage.
















































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14. Acknowledgments

   Substantive portions of this document are based upon previous work by
   Henry Spencer.

   Thanks to Tero Kivinen, Sandy Harris, Wes Hardarker, Robert
   Moskowitz, Jakob Schlyter, Bill Sommerfeld, John Gilmore and John
   Denker for their comments and constructive criticism.

   Sandra Hoffman and Bill Dickie did the detailed proof reading and
   editing.








































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Normative references

   [1]   Redelmeier, D. and H. Spencer, "Opportunistic Encryption",
         paper http://www.freeswan.org/freeswan_trees/freeswan-1.91/doc/
         opportunism.spec, May 2001.

   [2]   Defense Advanced Research Projects Agency (DARPA), Information
         Processing Techniques Office and University of Southern
         California (USC)/Information Sciences Institute, "Internet
         Protocol", STD 5, RFC 791, September 1981.

   [3]   Braden, R. and J. Postel, "Requirements for Internet gateways",
         RFC 1009, June 1987.

   [4]   IAB, IESG, Carpenter, B. and F. Baker, "IAB and IESG Statement
         on Cryptographic Technology and the Internet", RFC 1984, August
         1996.

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

   [6]   McDonald, D., Metz, C. and B. Phan, "PF_KEY Key Management API,
         Version 2", RFC 2367, July 1998.

   [7]   Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401, November 1998.

   [8]   Piper, D., "The Internet IP Security Domain of Interpretation
         for ISAKMP", RFC 2407, November 1998.

   [9]   Maughan, D., Schneider, M. and M. Schertler, "Internet Security
         Association and Key Management Protocol (ISAKMP)", RFC 2408,
         November 1998.

   [10]  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
         RFC 2409, November 1998.

   [11]  Kivinen, T. and M. Kojo, "More MODP Diffie-Hellman groups for
         IKE", RFC 3526, March 2003.

   [12]  Mockapetris, P., "Domain names - concepts and facilities", STD
         13, RFC 1034, November 1987.

   [13]  Mockapetris, P., "Domain names - implementation and
         specification", STD 13, RFC 1035, November 1987.

   [14]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
         August 1999.



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   [15]  Rosenbaum, R., "Using the Domain Name System To Store Arbitrary
         String Attributes", RFC 1464, May 1993.

   [16]  Eastlake, D., "Domain Name System Security Extensions", RFC
         2535, March 1999.

   [17]  Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name
         System (DNS)", RFC 3110, May 2001.

   [18]  Eastlake, D. and O. Gudmundsson, "Storing Certificates in the
         Domain Name System (DNS)", RFC 2538, March 1999.

   [19]  Durham, D., Boyle, J., Cohen, R., Herzog, S., Rajan, R. and A.
         Sastry, "The COPS (Common Open Policy Service) Protocol", RFC
         2748, January 2000.

   [20]  Srisuresh, P. and M. Holdrege, "IP Network Address Translator
         (NAT) Terminology and Considerations", RFC 2663, August 1999.


Authors' Addresses

   Michael C. Richardson
   Sandelman Software Works
   470 Dawson Avenue
   Ottawa, ON  K1Z 5V7
   CA

   EMail: mcr@sandelman.ottawa.on.ca
   URI:   http://www.sandelman.ottawa.on.ca/


   D. Hugh Redelmeier
   Mimosa
   Toronto, ON
   CA

   EMail: hugh@mimosa.com













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Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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