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diff --git a/doc/draft-richardson-ipsec-opportunistic.txt b/doc/draft-richardson-ipsec-opportunistic.txt deleted file mode 100644 index 4c87d857a..000000000 --- a/doc/draft-richardson-ipsec-opportunistic.txt +++ /dev/null @@ -1,2688 +0,0 @@ - - -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 - other groups may also distribute working documents as Internet- - Drafts. - - Internet-Drafts are draft documents valid for a maximum of six months - and may be updated, replaced, or obsoleted by other documents at any - time. It is inappropriate to use Internet-Drafts as reference - material or to cite them other than as "work in progress." - - The list of current Internet-Drafts can be accessed at http:// - www.ietf.org/ietf/1id-abstracts.txt. - - The list of Internet-Draft Shadow Directories can be accessed at - http://www.ietf.org/shadow.html. - - 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. - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 1] - -Internet-Draft opportunistic May 2003 - - - 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 - - - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 2] - -Internet-Draft opportunistic May 2003 - - -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. - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 3] - -Internet-Draft opportunistic May 2003 - - -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. - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 4] - -Internet-Draft opportunistic May 2003 - - - 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] - - - - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 5] - -Internet-Draft opportunistic May 2003 - - -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. - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 6] - -Internet-Draft opportunistic May 2003 - - - 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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 7] - -Internet-Draft opportunistic May 2003 - - - 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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 8] - -Internet-Draft opportunistic May 2003 - - - 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. - - - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 9] - -Internet-Draft opportunistic May 2003 - - -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. - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 10] - -Internet-Draft opportunistic May 2003 - - -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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 11] - -Internet-Draft opportunistic May 2003 - - - 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. - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 12] - -Internet-Draft opportunistic May 2003 - - -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. - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 13] - -Internet-Draft opportunistic May 2003 - - - 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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 14] - -Internet-Draft opportunistic May 2003 - - - 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. - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 15] - -Internet-Draft opportunistic May 2003 - - - 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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 16] - -Internet-Draft opportunistic May 2003 - - - 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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 17] - -Internet-Draft opportunistic May 2003 - - - 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. - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 18] - -Internet-Draft opportunistic May 2003 - - - 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.) - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 19] - -Internet-Draft opportunistic May 2003 - - - 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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 20] - -Internet-Draft opportunistic May 2003 - - -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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 21] - -Internet-Draft opportunistic May 2003 - - - 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. - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 22] - -Internet-Draft opportunistic May 2003 - - - 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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 23] - -Internet-Draft opportunistic May 2003 - - -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.) - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 24] - -Internet-Draft opportunistic May 2003 - - - --------------------------------------------------------------------- - - - 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. - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 25] - -Internet-Draft opportunistic May 2003 - - -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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 26] - -Internet-Draft opportunistic May 2003 - - - 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. - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 27] - -Internet-Draft opportunistic May 2003 - - -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. - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 28] - -Internet-Draft opportunistic May 2003 - - -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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 29] - -Internet-Draft opportunistic May 2003 - - -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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 30] - -Internet-Draft opportunistic May 2003 - - - 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. - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 31] - -Internet-Draft opportunistic May 2003 - - -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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 32] - -Internet-Draft opportunistic May 2003 - - - 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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 33] - -Internet-Draft opportunistic May 2003 - - -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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 34] - -Internet-Draft opportunistic May 2003 - - -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.) - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 35] - -Internet-Draft opportunistic May 2003 - - - (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)------ - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 36] - -Internet-Draft opportunistic May 2003 - - - <==========(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. - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 37] - -Internet-Draft opportunistic May 2003 - - - (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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 38] - -Internet-Draft opportunistic May 2003 - - - 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. - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 39] - -Internet-Draft opportunistic May 2003 - - -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 - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 40] - -Internet-Draft opportunistic May 2003 - - - encrypted to SG-A, decrypted by SG-A and passed to Alice at (10). - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 41] - -Internet-Draft opportunistic May 2003 - - -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. - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 42] - -Internet-Draft opportunistic May 2003 - - -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. - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 43] - -Internet-Draft opportunistic May 2003 - - -13. IANA Considerations - - There are no known numbers which IANA will need to manage. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 44] - -Internet-Draft opportunistic May 2003 - - -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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 45] - -Internet-Draft opportunistic May 2003 - - -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. - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 46] - -Internet-Draft opportunistic May 2003 - - - [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 - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 47] - -Internet-Draft opportunistic May 2003 - - -Full Copyright Statement - - Copyright (C) The Internet Society (2003). All Rights Reserved. - - This document and translations of it may be copied and furnished to - others, and derivative works that comment on or otherwise explain it - or assist in its implementation may be prepared, copied, published - and distributed, in whole or in part, without restriction of any - kind, provided that the above copyright notice and this paragraph are - included on all such copies and derivative works. However, this - document itself may not be modified in any way, such as by removing - the copyright notice or references to the Internet Society or other - Internet organizations, except as needed for the purpose of - developing Internet standards in which case the procedures for - copyrights defined in the Internet Standards process must be - followed, or as required to translate it into languages other than - English. - - The limited permissions granted above are perpetual and will not be - revoked by the Internet Society or its successors or assigns. - - This document and the information contained herein is provided on an - "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING - TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING - BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION - HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF - MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. - -Acknowledgement - - Funding for the RFC Editor function is currently provided by the - Internet Society. - - - - - - - - - - - - - - - - - - - -Richardson & Redelmeier Expires November 19, 2003 [Page 48] - |