Network Working Group N. Williams Request for Comments: 5660 Sun Category: Standards Track October 2009
IPsec Channels: Connection Latching
Abstract
This document specifies, abstractly, how to interface applications and transport protocols with IPsec so as to create "channels" by latching "connections" (packet flows) to certain IPsec Security Association (SA) parameters for the lifetime of the connections. Connection latching is layered on top of IPsec and does not modify the underlying IPsec architecture.
Connection latching can be used to protect applications against accidentally exposing live packet flows to unintended peers, whether as the result of a reconfiguration of IPsec or as the result of using weak peer identity to peer address associations. Weak association of peer ID and peer addresses is at the core of Better Than Nothing Security (BTNS); thus, connection latching can add a significant measure of protection to BTNS IPsec nodes.
Finally, the availability of IPsec channels will make it possible to use channel binding to IPsec channels.
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License.
Table of Contents
1. Introduction ....................................................3 1.1. Conventions Used in This Document ..........................4 2. Connection Latching .............................................4 2.1. Latching of Quality-of-Protection Parameters ...............8 2.2. Connection Latch State Machine .............................9 2.3. Normative Model: ULP Interfaces to the Key Manager ........12 2.3.1. Race Conditions and Corner Cases ...................17 2.3.2. Example ............................................18 2.4. Informative Model: Local Packet Tagging ...................19 2.5. Non-Native Mode IPsec .....................................21 2.6. Implementation Note Regarding Peer IDs ....................22 3. Optional Features ..............................................22 3.1. Optional Protection .......................................22 4. Simultaneous Latch Establishment ...............................23 5. Connection Latching to IPsec for Various ULPs ..................23 5.1. Connection Latching to IPsec for TCP ......................24 5.2. Connection Latching to IPsec for UDP with Simulated Connections .....................................24 5.3. Connection Latching to IPsec for UDP with Datagram-Tagging APIs .....................................25 5.4. Connection Latching to IPsec for SCTP .....................25 5.5. Handling of BROKEN State for TCP and SCTP .................26 6. Security Considerations ........................................27 6.1. Impact on IPsec ...........................................27 6.2. Impact on IPsec of Optional Features ......................28 6.3. Security Considerations for Applications ..................28 6.4. Channel Binding and IPsec APIs ............................29 6.5. Denial-of-Service Attacks .................................29 7. Acknowledgements ...............................................30 8. References .....................................................30 8.1. Normative References ......................................30 8.2. Informative References ....................................30
IPsec protects packets with little or no regard for stateful packet flows associated with upper-layer protocols (ULPs). This exposes applications that rely on IPsec for session protection to risks associated with changing IPsec configurations, configurations that allow multiple peers access to the same addresses, and/or weak association of peer IDs and their addresses. The latter can occur as a result of "wildcard" matching in the IPsec Peer Authorization Database (PAD), particularly when Better Than Nothing Security (BTNS) [RFC5387] is used.
Applications that wish to use IPsec may have to ensure that local policy on the various end-points is configured appropriately [RFC5406] [USING-IPSEC]. There are no standard Application Programming Interfaces (APIs) to do this (though there are non- standard APIs, such as [IP_SEC_OPT.man]) -- a major consequence of which, for example, is that applications must still use hostnames (and, e.g., the Domain Name System [RFC1034]) and IP addresses in existing APIs and must depend on an IPsec configuration that they may not be able to verify. In addition to specifying aspects of required Security Policy Database (SPD) configuration, application specifications must also address PAD/SPD configuration to strongly bind individual addresses to individual IPsec identities and credentials (certificates, public keys, etc.).
IPsec is, then, quite cumbersome for use by applications. To address this, we need APIs to IPsec. Not merely APIs for configuring IPsec, but also APIs that are similar to the existing IP APIs (e.g., "BSD Sockets"), so that typical applications making use of UDP [RFC0768], TCP [RFC0793], and Stream Control Transmission Protocol (SCTP) [RFC4960] can make use of IPsec with minimal changes.
This document describes the foundation for IPsec APIs that UDP and TCP applications can use: a way to bind the traffic flows for, e.g., TCP connections to security properties desired by the application. We call these "connection latches" (and, in some contexts, "IPsec channels"). The methods outlined below achieve this by interfacing ULPs and applications to IPsec.
If widely adopted, connection latching could make application use of IPsec much simpler, at least for certain classes of applications.
Connection latching, as specified herein, is primarily about watching updates to the SPD and Security Association Database (SAD) to detect changes that are adverse to an application's requirements for any given packet flow, and to react accordingly (such as by synchronously alerting the ULP and application before packets can be sent or
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received under the new policy). Under no circumstance are IPsec policy databases to be modified by connection latching in any way that can persist beyond the lifetime of the related packet flows, nor reboots. Under no circumstance is the PAD to be modified at all by connection latching. If all optional features of connection latching are excluded, then connection latching can be implemented as a monitor of SPD and SAD changes that intrudes in their workings no more than is needed to provide synchronous alerts to ULPs and applications.
We assume the reader is familiar with the IPsec architecture [RFC4301] and Internet Key Exchange Protocol version 2 (IKEv2) [RFC4306].
Note: the terms "connection latch" and "IPsec channel" are used interchangeably below. The latter term relates to "channel binding" [RFC5056]. Connection latching is suitable for use in channel binding applications, or will be, at any rate, when the channel bindings for IPsec channels are defined (the specification of IPsec channel bindings is out of scope for this document).
Note: where this document mentions IPsec peer "ID" it refers to the Internet Key Exchange (IKE) peer ID (e.g., the ID derived from a peer's cert, as well as the cert), not the peer's IP address.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
Abstract function names are all capitalized and denoted by a pair of parentheses. In their descriptions, the arguments appear within the parentheses, with optional arguments surrounded by square brackets. Return values, if any, are indicated by following the function argument list with "->" and a description of the return value. For example, "FOO(3-tuple, [message])" would be a function named "FOO" with two arguments, one of them optional, and returning nothing, whereas "FOOBAR(handle) -> state" would be a function with a single, required argument that returns a value. The values' types are described in the surrounding text.
An "IPsec channel" is a packet flow associated with a ULP control block, such as a TCP connection, where all the packets are protected by IPsec SAs such that:
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o the peer's identity is the same for the lifetime of the packet flow;
o the quality of IPsec protection used for the packet flow's individual packets is the same for all of them for the lifetime of the packet flow.
An IPsec channel is created when the associated packet flow is created. This can be the result of a local operation (e.g., a connect()) that causes the initial outgoing packet for that flow to be sent, or it can be the result of receiving the first/initiating packet for that flow (e.g., a TCP SYN packet).
An IPsec channel is destroyed when the associated packet flow ends. An IPsec channel can also be "broken" when the connection latch cannot be maintained for some reason (see below), in which case the ULP and application are informed.
IPsec channels are created by "latching" various parameters listed below to a ULP connection when the connections are created. The REQUIRED set of parameters bound in IPsec channels is:
o Type of protection: confidentiality and/or integrity protection;
o Transport mode versus tunnel mode;
o Quality of protection (QoP): cryptographic algorithm suites, key lengths, and replay protection (see Section 2.1);
o Local identity: the local ID asserted to the peer, as per the IPsec processing model [RFC4301] and BTNS [RFC5386];
o Peer identity: the peer's asserted and authorized IDs, as per the IPsec processing model [RFC4301] and BTNS [RFC5386].
The SAs that protect a given IPsec channel's packets may change over time in that they may expire and be replaced with equivalent SAs, or they may be re-keyed. The set of SAs that protect an IPsec channel's packets need not be related by anything other than the fact that they must be congruent to the channel (i.e., the SAs' parameters must match those that are latched into the channel). In particular, it is desirable that IPsec channels survive the expiration of IKE_SAs and child SAs because operational considerations of the various key exchange protocols then cannot affect the design and features of connection latching.
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When a situation arises in which the SPD is modified, or an SA is added to the SAD, such that the new policy and/or SA are not congruent to an established channel (see previous paragraph), then we consider this a conflict. Conflict resolution is addressed below.
Requirements and recommendations:
o If an IPsec channel is desired, then packets for a given connection MUST NOT be sent until the channel is established.
o If an IPsec channel is desired, then inbound packets for a given connection MUST NOT be accepted until the channel is established. That is, inbound packets for a given connection arriving prior to the establishment of the corresponding IPsec channel must be dropped or the channel establishment must fail.
o Once an IPsec channel is established, packets for the latched connection MUST NOT be sent unprotected nor protected by an SA that does not match the latched parameters.
o Once an IPsec channel is established, packets for the latched connection MUST NOT be accepted unprotected nor protected by an SA that does not match the latched parameters. That is, such packets must either be dropped or cause the channel to be terminated and the application to be informed before data from such a packet can be delivered to the application.
o Implementations SHOULD provide programming interfaces for inquiring the values of the parameters latched in a connection.
o Implementations that provide such programming interfaces MUST make available to applications all relevant and available information about a peer's ID, including authentication information. This includes the peer certificate, when one is used, and the trust anchor to which it was validated (but not necessarily the whole certificate validation chain).
o Implementations that provide such programming interfaces SHOULD make available to applications any information about local and/or remote public and private IP addresses, in the case of NAT- traversal.
o Implementations that provide such programming interfaces SHOULD make available to applications the inner and outer local and peer addresses whenever the latched connection uses tunnel-mode SAs.
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o Implementations SHOULD provide programming interfaces for setting the values of the parameters to be latched in a connection that will be initiated or accepted, but these interfaces MUST limit what values applications may request according to system policy (i.e., the IPsec PAD and SPD) and the application's local privileges.
(Typical system policy may not allow applications any choices here. Policy extensions allowing for optional protection are described in Section 3.1.)
o Implementations SHOULD create IPsec channels automatically by default when the application does not explicitly request an IPsec channel. Implementations MAY provide a way to disable automatic creation of connection latches.
o The parameters latched in an IPsec channel MUST remain unchanged once the channel is established.
o Timeouts while establishing child SAs with parameters that match those latched into an IPsec channel MUST be treated as packet loss (as happens, for example, when a network partitions); normal ULP and/or application timeout handling and retransmission considerations apply.
o Implementations that have a restartable key management process (or "daemon") MUST arrange for existing latched connections to either be broken and disconnected, or for them to survive the restart of key exchange processes. (This is implied by the above requirements.) For example, if such an implementation relies on keeping some aspects of connection latch state in the restartable key management process (e.g., values that potentially have large representations, such as BTNS peer IDs), then either such state must be restored on restart of such a process, or outstanding connection latches must be transitioned to the CLOSED state.
o Dynamic IPsec policy (see Section 3.1) related to connection latches, if any, MUST be torn down when latched connections are torn down, and MUST NOT survive reboots.
o When IKE dead-peer detection (DPD) concludes that the remote peer is dead or has rebooted, then the system SHOULD consider all connection latches with that peer to be irremediably broken.
We describe two models, one of them normative, of IPsec channels for native IPsec implementations. The normative model is based on abstract programming interfaces in the form of function calls between ULPs and the key management component of IPsec (basically, the SAD,
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augmented with a Latch Database (LD)). The second model is based on abstract programming interfaces between ULPs and the IPsec (Encapsulating Security Payload / Authentication Header (ESP/AH)) layer in the form of meta-data tagging of packets within the IP stack.
The two models given below are not, however, entirely equivalent. One model cannot be implemented with Network Interface cards (NICs) that offload ESP/AH but that do not tag incoming packets passed to the host processor with SA information, nor allow the host processor to so tag outgoing packets. That same model can be easily extended to support connection latching with unconnected datagram "sockets", while the other model is rigidly tied to a notion of "connections" and cannot be so extended. There may be other minor differences between the two models. Rather than seek to establish equivalency for some set of security guarantees, we instead choose one model to be the normative one.
We also provide a model for non-native implementations, such as bump- in-the-stack (BITS) and Security Gateway (SG) implementations. The connection latching model for non-native implementations is not full- featured as it depends on estimating packet flow state, which may not always be possible. Nor can non-native IPsec implementations be expected to provide APIs related to connection latching (implementations that do could be said to be native). As such, this third model is not suitable for channel binding applications [RFC5056].
In IPsec, the assumption of IKE initiator/responder roles is non- deterministic. That is, sometimes an IKE SA and child SAs will be initiated by the "client" (e.g., the caller of the connect() BSD sockets function) and sometimes by the "server" (e.g., the caller of the accept() BSD Sockets function). This means that the negotiation of quality of protection is also non-deterministic unless one of the peers offers a single cryptographic suite in the IKE negotiation.
When creating narrow child SAs with traffic selectors matching the connection latch's 5-tuple, it is possible to constrain the IKE Quality-of-Protection negotiation to a single cryptographic suite. Therefore, implementations SHOULD provide an API for requesting the use of such child SAs. Implementors SHOULD consider an application request for a specific QoP to imply a request for narrow child SAs.
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When using SAs with traffic selectors encompassing more than just a single flow, then the system may only be able to latch a set of cryptographic suites, rather than a single cryptographic suite. In such a case, an implementation MUST report the QoP being used as indeterminate.
Connection latches can exist in any of the following five states:
o LISTENER
o ESTABLISHED
o BROKEN (there exist SAs that conflict with the given connection latch, conflicting SPD changes have been made, or DPD has been triggered and the peer is considered dead or restarted)
o CLOSED (by the ULP, the application or administratively)
and always have an associated owner, or holder, such as a ULP transmission control block (TCB).
A connection latch can be born in the LISTENER state, which can transition only to the CLOSED state. The LISTENER state corresponds to LISTEN state of TCP (and other ULPs) and is associated with IP 3-tuples, and can give rise to new connection latches in the ESTABLISHED state.
A connection latch can also be born in the ESTABLISHED and BROKEN states, either through the direct initiative of a ULP or when an event occurs that causes a LISTENER latch to create a new latch (in either ESTABLISHED or BROKEN states). These states represent an active connection latch for a traffic flow's 5-tuple. Connection latches in these two states can transition to the other of the two states, as well as to the CLOSED state.
Connection latches remain in the CLOSED state until their owners are informed except where the owner caused the transition, in which case this state is fleeting. Transitions from ESTABLISHED or BROKEN states to the CLOSED state should typically be initiated by latch owners, but implementations SHOULD provide administrative interfaces through which to close active latches.
Connection latches transition to the BROKEN state when there exist SAs in the SAD whose traffic selectors encompass the 5-tuple bound by the latch, and whose peer and/or parameters conflict with those bound by the latch. Transitions to the BROKEN state also take place when
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SPD changes occur that would cause the latched connection's packets to be sent or received with different protection parameters than those that were latched. Transitions to the BROKEN state are also allowed when IKEv2 DPD concludes that the remote peer is dead or has rebooted. Transitions to the BROKEN state always cause the associated owner to be informed. Connection latches in the BROKEN state transition back to ESTABLISHED when all SA and/or SPD conflicts are cleared.
Most state transitions are the result of local actions of the latch owners (ULPs). The only exceptions are: birth into the ESTABLISHED state from latches in the LISTENER state, transitions to the BROKEN state, transitions from the BROKEN state to ESTABLISHED, and administrative transitions to the CLOSED state. (Additionally, see the implementation note about restartable key management processes in Section 2.)
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The state diagram below makes use of conventions described in Section 1.1 and state transition events described in Section 2.3.
The details of the transitions depend on the model of connection latching followed by any given implementation. See the following sections.
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2.3. Normative Model: ULP Interfaces to the Key Manager
This section describes the NORMATIVE model of connection latching.
In this section, we describe connection latching in terms of a function-call interface between ULPs and the "key manager" component of a native IPsec implementation. Abstract interfaces for creating, inquiring about, and releasing IPsec channels are described.
This model adds a service to the IPsec key manager (i.e., the component that manages the SAD and interfaces with separate implementations of, or directly implements, key exchange protocols): management of connection latches. There is also a new IPsec database, the Latch Database (LD), that contains all connection latch objects. The LD does not persist across system reboots.
The traditional IPsec processing model allows the concurrent existence of SAs with different peers but overlapping traffic selectors. Such behavior, in this model, directly violates the requirements for connection latching (see Section 2). We address this problem by requiring that connection latches be broken (and holders informed) when such conflicts arise.
The following INFORMATIVE figure illustrates this model and API in terms that are familiar to many implementors, though not applicable to all:
The ULP interfaces to the IPsec LD are as follows:
o CREATE_LISTENER_LATCH(3-tuple, [type and quality-of-protection parameters]) -> latch handle | error
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If there is no conflicting connection latch object in the LISTENER state for the given 3-tuple (local address, protocol, and local port number), and local policy permits it, then this operation atomically creates a connection latch object in the LISTENER state for the given 3-tuple.
When a child SA is created that matches a listener latch's 3-tuple, but not any ESTABLISHED connection latch's 5-tuple (local address, remote address, protocol, local port number, and remote port number), then the key manager creates a new connection latch object in the ESTABLISHED state. The key manager MUST inform the holder of the listener latch of connection latches created as a result of the listener latch; see the "ALERT()" interface below.
o CREATE_CONNECTION_LATCH(5-tuple, [type and quality-of-protection parameters], [peer ID], [local ID]) -> latch handle | error
If a) the requested latch does not exist (or exists, but is in the CLOSED state), b) all the latch parameters are provided, or if suitable SAs exist in the SAD from which to derive them, and c) if there are no conflicts with the SPD and SAD, then this creates a connection latch in the ESTABLISHED state. If the latch parameters are not provided and no suitable SAs exist in the SAD from which to derive those parameters, then the key manager MUST initiate child SAs, and if need be, IKE_SA, from which to derive those parameters.
The key manager MAY delay the child SA setup and return immediately after the policy check, knowing that the ULP that requested the latch will subsequently output a packet that will trigger the SA establishment. Such an implementation may require an additional, fleeting state in the connection latch state machine, a "LARVAL" state, so to speak, that is not described herein.
If the connection latch ultimately cannot be established, either because of conflicts or because no SAs can be established with the peer at the destination address, then an error is returned to the ULP. (If the key manager delayed SA establishment, and SA establishment ultimately fails, then the key manager has to inform the ULP, possibly asynchronously. This is one of several details that implementors who use a LARVAL state must take care of.)
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o RELEASE_LATCH(latch object handle)
Changes the state of the given connection latch to CLOSED; the connection latch is then deleted.
The key manager MAY delete any existing child SAs that match the given latch if it had been in the ESTABLISHED states. If the key manager does delete such SAs, then it SHOULD inform the peer with an informational Delete payload (see IKEv2 [RFC4306]).
o FIND_LATCH(5-tuple) -> latch handle | error
Given a 5-tuple returns a latch handle (or an error).
Returns all available information about the given latch, including its current state (or an error).
The IPsec LD interface to the ULP is as follows:
o ALERT(latch object handle, 5-tuple, new state, [reason])
Alerts a ULP as to an asynchronous state change for the given connection latch and, optionally, provides a reason for the change.
This interface is to be provided by each ULP to the key manager. The specific details of how this interface is provided are implementation details, thus not specified here (for example, this could be a "callback" function or "closure" registered as part of the CREATE_LISTENER_LATCH() interface, or it could be provided when the ULP is loaded onto the running system via a registration interface provided by the key manager).
Needless to say, the LD is updated whenever a connection latch object is created, deleted, or broken.
The API described above is a new service of the IPsec key manager. In particular, the IPsec key manager MUST prevent conflicts amongst latches, and it MUST prevent conflicts between any latch and existing or proposed child SAs as follows:
o Non-listener connection latches MUST NOT be created if there exist conflicting SAs in the SAD at the time the connection latch is requested or would be created (from a listener latch). A child SA
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conflicts with another, in view of a latch, if and only if: a) its traffic selectors and the conflicting SA's match the given latch's, and b) its peer, type-of-protection, or quality-of- protection parameters differ from the conflicting SA.
o Child SA proposals that would conflict with an extant connection latch and whose traffic selectors can be narrowed to avoid the conflict SHOULD be narrowed (see Section 2.9 of [RFC4306]); otherwise, the latch MUST be transitioned to the BROKEN state.
o Where child SA proposals that would conflict with an extant connection latch cannot be narrowed to avoid the conflict, the key manager MUST break the connection latch and inform the holder (i.e., the ULP) prior to accepting the conflicting SAs.
Finally, the key manager MUST protect latched connections against SPD changes that would change the quality of protection afforded to a latched connection's traffic, or which would bypass it. When such a configuration change takes place, the key manager MUST respond in either of the following ways. The REQUIRED to implement behavior is to transition into the BROKEN state all connection latches that conflict with the given SPD change. An OPTIONAL behavior is to logically update the SPD as if a PROTECT entry had been added at the head of the SPD-S with traffic selectors matching only the latched connection's 5-tuple, and with processing information taken from the connection latch. Such updates of the SPD MUST NOT survive system crashes or reboots.
ULPs create latched connections by interfacing with IPsec as follows:
o For listening end-points, the ULP will request a connection latch listener object for the ULP listener's 3-tuple. Any latching parameters requested by the application MUST be passed along.
o When the ULP receives a packet initiating a connection for a 5-tuple matching a 3-tuple listener latch, then the ULP will ask the key manager whether a 5-tuple connection latch was created. If not, then the ULP will either reject the new connection or accept it and inform the application that the new connection is not latched.
o When initiating a connection, the ULP will request a connection latch object for the connection's 5-tuple. Any latching parameters requested by the application MUST be passed along. If no latch can be created, then the ULP MUST either return an error to the application or continue with the new connection and inform the application that the new connection is not latched.
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o When a connection is torn down and no further packets are expected for it, then the ULP MUST request that the connection latch object be destroyed.
o When tearing down a listener, the ULP MUST request that the connection latch listener object be destroyed.
o When a ULP listener rejects connections, the ULP will request the destruction of any connection latch objects that may have been created as a result of the peer's attempt to open the connection.
o When the key manager informs a ULP that a connection latch has transitioned to the BROKEN state, then the ULP MUST stop sending packets and MUST drop all subsequent incoming packets for the affected connection until it transitions back to ESTABLISHED. Connection-oriented ULPs SHOULD act as though the connection is experiencing packet loss.
o When the key manager informs a ULP that a connection latch has been administratively transitioned to the CLOSED state, then connection-oriented ULPs MUST act as though the connection has been reset by the peer. Implementations of ULPs that are not connection-oriented, and which have no API by which to simulate a reset, MUST drop all inbound packets for that connection and MUST NOT send any further packets -- the application is expected to detect timeouts and act accordingly.
The main benefit of this model of connection latching is that it accommodates IPsec implementations where ESP/AH handling is implemented in hardware (for all or a subset of the host's SAD), even where the hardware does not support tagging inbound packets with the indexes of SAD entries corresponding to the SAs that protected them.
ULPs MUST drop inbound packets and stop sending packets immediately upon receipt of a connection latch break message. Otherwise, the ULP will not be able to distinguish inbound packets that were protected consistently with the connection's latch from inbound packets that were not. This may include dropping inbound packets that were protected by a suitable SA; dropping such packets is no different, from the ULP's point of view, than packet loss elsewhere on the network at the IP layer or below -- harmless, from a security point of view as the connection fails safe, but it can result in retransmits.
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Another race condition is as follows. A PROTECTed TCP SYN packet may be received and decapsulated, but the SA that protected it could have expired before the key manager creates the connection latch that would be created by that packet. In this case, the key manager will have to initiate new child SAs so as to determine what the sender's peer ID is so it can be included in the connection latch. Here, there is no guarantee that the peer ID for the new SAs will be the same as those of the peer that sent the TCP SYN packet. This race condition is harmless: TCP will send a SYN+ACK to the wrong peer, which will then respond with a RST -- the connection latch will reflect the new peer however, so if the new peer is malicious it will not be able to appear to be the old peer. Therefore, this race condition is harmless.
Consider several systems with a very simple PAD containing a single entry like so:
Child SA Rule Remote ID IDs allowed SPD Search by ---- --------- ----------- ------------- 1 <any valid to trust anchor X> 192.0.2/24 by-IP
Figure 3: Example PAD
And a simple SPD like so:
Rule Local Remote Next Action TS TS Proto ---- ----- ------ ----- ---------------- 1 192.0.2/24:ANY 192.0.2/24:1-5000 TCP PROTECT(ESP,...) 1 192.0.2/24:1-5000 192.0.2/24:ANY TCP PROTECT(ESP,...) 1 ANY ANY ANY BYPASS
Figure 4: [SG-A] SPD Table
Effectively this says: for TCP ports 1-5000 in our network, allow only peers that have credentials issued by CA X and PROTECT that traffic with ESP, otherwise, bypass all other traffic.
Now let's consider two hosts, A and B, in this network that wish to communicate using port 4000, and a third host, C, that is also in the same network and wishes to attack A and/or B. All three hosts have credentials and certificates issued by CA X. Let's also imagine that A is connected to its network via a wireless link and is dynamically addressed.
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B is listening on port 4000. A initiates a connection from port 32800 to B on port 4000.
We'll assume no IPsec APIs, but that TCP creates latches where possible.
We'll consider three cases: a) A and B both support connection latching, b) only A does, c) only B does. For the purposes of this example, the SAD is empty on all three hosts when A initiates its TCP connection to B on port 4000.
When an application running on A initiates a TCP connection to B on port 4000, A will begin by creating a connection latch. Since the SAD is empty, A will initiate an IKEv2 exchange to create an IKE_SA with B and a pair of child SAs for the 5-tuple {TCP, A, 32800, B, 4000}, then a new latch will be created in ESTABLISHED state. Sometime later, TCP will send a SYN packet protected by the A-to-B child SA, per the SPD.
When an application running on B creates a TCP listener "socket" on port 4000, B will create a LISTENER connection latch for the 3-tuple {TCP, B, 4000}. When B receives A's TCP SYN packet, it will then create a connection latch for {TCP, B, 4000, A, 32800}. Since, by this point, child SAs have been created whose traffic selectors encompass this 5-tuple and there are no other conflicting SAs in the SAD, this connection latch will be created in the ESTABLISHED state.
If C attempts to mount a man-in-the-middle attack on A (i.e., pretends to have B's address(es)) any time after A created its connection latch, then C's SAs with A will cause the connection latch to break, and the TCP connection to be reset (since we assume no APIs by which TCP could notify the application of the connection latch break). If C attempts to impersonate A to B, then the same thing will happen on B.
If A does not support connection latching, then C will be able to impersonate B to A at any time. Without having seen the cleartext of traffic between A and B, C will be limited by the TCP sequence numbers to attacks such as RST attacks. Similarly, if B does not support connection latching, then C will be able to impersonate A to B.
In this section, we describe connection latching in terms of interfaces between ULPs and IPsec based on tagging packets as they go up and down the IP stack.
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This section is INFORMATIVE.
In this model, the ULPs maintain connection latch objects and state, rather than the IPsec key manager, as well as effectively caching a subset of the decorrelated SPD in ULP TCBs. Tagging packets, as they move up and down the stack, with SA identifiers then allows the ULPs to enforce connection latching semantics. These tags, of course, don't appear on the wire.
The interface between the ULPs and IPsec interface is as follows:
o The IPsec layer tags all inbound protected packets addressed to the host with the index of the SAD entry corresponding to the SA that protected the packet.
o The IPsec layer understands two types of tags on outbound packets:
* a tag specifying a set of latched parameters (peer ID, quality of protection, etc.) that the IPsec layer will use to find or acquire an appropriate SA for protecting the outbound packet (else IPsec will inform the ULP and drop the packet);
* a tag requesting feedback about the SA used to protect the outgoing packet, if any.
ULPs create latched connections by interfacing with IPsec as follows:
o When the ULP passes a connection's initiating packet to IP, the ULP requests feedback about the SA used to protect the outgoing packet, if any, and may specify latching parameters requested by the application. If the packet is protected by IPsec, then the ULP records certain parameters of the SA used to protect it in the connection's TCB.
o When a ULP receives a connection's initiating packet, it processes the IPsec tag of the packet, and it records in the connection's TCB the parameters of the SA that should be latched.
Once SA parameters are recorded in a connection's TCB, the ULP enforces the connection's latch, or binding, to these parameters as follows:
o The ULP processes the IPsec tag of all inbound packets for a given connection and checks that the SAs used to protect input packets match the connection latches recorded in the TCBs. Packets that are not so protected are dropped (this corresponds to transitioning the connection latch to the BROKEN state until the
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next acceptable packet arrives, but in this model, this transition is imaginary) or cause the ULP to break the connection latch and inform the application.
o The ULP always requests that outgoing packets be protected by SAs that match the latched connection by appropriately tagging outbound packets.
By effectively caching a subset of the decorrelated SPD in ULP TCBs and through its packet tagging nature, this method of connection latching can also optimize processing of the SPD by obviating the need to search it, both, on input and output, for packets intended for the host or originated by the host. This makes implementation of the OPTIONAL "logical SPD" updates described in Sections 2.3 and 3.1 an incidental side effect of this approach.
This model of connection latching may not be workable with ESP/AH offload hardware that does not support the packet tagging scheme described above.
Note that this model has no explicit BROKEN connection latch state.
Extending the ULP/IPsec packet-tagging interface to the application for use with connection-less datagram transports should enable applications to use such transports and implement connection latching at the application layer.
Non-native IPsec implementations, primarily BITS and SG, can implement connection latching, too. One major distinction between native IPsec and BITS, bump-in-the-wire (BITW), or SG IPsec is the lack of APIs for applications at the end-points in the case of the latter. As a result, there can be no uses of the latch management interfaces as described in Section 2.3: not at the ULP end-points. Therefore, BITS/BITW/SG implementations must discern ULP connection state from packet inspection (which many firewalls can do) and emulate calls to the key manager accordingly.
When a connection latch is broken, a BITS/BITW/SG implementation may have to fake a connection reset by sending appropriate packets (e.g., TCP RST packets), for the affected connections.
As with all stateful middleboxes, this scheme suffers from the inability of the middlebox to interact with the applications. For example, connection death may be difficult to ascertain. Nor can
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channel binding applications work with channels maintained by proxy without being able to communicate (securely) about it with the middlebox.
One of the recommendations for connection latching implementors is to make peer CERT payloads (certificates) available to the applications.
Additionally, raw public keys are likely to be used in the construction of channel bindings for IPsec channels (see [IPSEC-CB]), and they must be available, in any case, in order to implement leap- of-faith at the application layer (see [RFC5386] and [RFC5387]).
Certificates and raw public keys are large bit strings, too large to be reasonably kept in kernel-mode implementations of connection latching (which will likely be the typical case). Such implementations should intern peer IDs in a user-mode database and use small integers to refer to them from the kernel-mode SAD and LD. Corruption of such a database is akin to corruption of the SAD/LD; in the event of corruption, the implementation MUST act as though all ESTABLISHED and BROKEN connection latches are administratively transitioned to the CLOSED state. Implementations without IPsec APIs MAY hash peer IDs and use the hash to refer to them, preferably using a strong hash algorithm.
At its bare minimum, connection latching is a passive layer atop IPsec that warns ULPs of SPD and SAD changes that are incompatible with the SPD/SAD state that was applicable to a connection when it was established.
There are some optional features, such as (abstract) APIs. Some of these features make connection latching a somewhat more active feature. Specifically, the optional logical SPD updates described in Section 2.3 and the optional protection/bypass feature described in the following sub-section.
Given IPsec APIs, an application could request that a connection's packets be protected where they would otherwise be bypassed; that is, applications could override BYPASS policy. Locally privileged applications could request that their connections' packets be bypassed rather than protected; that is, privileged applications could override PROTECT policy. We call this "optional protection".
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Both native IPsec models of connection latching can be extended to support optional protection. With the model described in Section 2.4, optional protection comes naturally: the IPsec layer need only check that the protection requested for outbound packets meets or exceeds (as determined by local or system policy) the quality of protection, if any, required by the SPD. In the case of the model described in Section 2.3, enforcement of minimum protection requirements would be done by the IPsec key manager via the connection latch state machine.
When an application requests, and local policy permits, either additional protection or bypassing protection, then the SPD MUST be logically updated such that there exists a suitable SPD entry protecting or bypassing the exact 5-tuple recorded by the corresponding connection latch. Such logical SPD updates MUST be made at connection latch creation time, and MUST be made atomically (see the note about race conditions in Section 2.3). Such updates of the SPD MUST NOT survive system crashes or reboots.
Some connection-oriented ULPs, specifically TCP, support simultaneous connections (where two clients connect to each other, using the same 5-tuple, at the same time). Connection latching supports simultaneous latching as well, provided that the key exchange protocol does not make it impossible.
Consider two applications doing a simultaneous TCP connect to each other and requesting an IPsec channel. If they request the same connection latching parameters, then the connection and channel should be established as usual. Even if the key exchange protocol in use doesn't support simultaneous IKE_SA and/or child SA establishment, provided one peer's attempt to create the necessary child SAs succeeds, then the other peer should be able to notice the new SAs immediately upon failure of its attempts to create the same.
If, however, the two peer applications were to request different connection latching parameters, then the connection latch must fail on one end or on both ends.
The following sub-sections describe connection latching for each of three transport protocols. Note that for TCP and UDP, there is nothing in the following sections that should not already be obvious from the remainder of this document. The section on SCTP, however, specifies details related to SCTP multi-homing, that may not be as obvious.
IPsec connection latch creation/release for TCP [RFC0793] connections is triggered when:
o a TCP listener end-point is created (e.g., when the BSD Sockets listen() function is called on a socket). This should cause the creation of a LISTENER connection latch.
o a TCP SYN packet is received on an IP address and port number for which there is a listener. This should cause the creation of an ESTABLISHED or BROKEN connection latch.
o a TCP SYN packet is sent (e.g., as the result of a call to the BSD Sockets connect() function). This should cause the creation of an ESTABLISHED or BROKEN connection latch.
o any state transition of a TCP connection to the CLOSED state will cause a corresponding transition for any associated connection latch to the CLOSED state as well.
See Section 5.5 for how to handle latch transitions to the BROKEN state.
5.2. Connection Latching to IPsec for UDP with Simulated Connections
UDP [RFC0768] is a connection-less transport protocol. However, some networking APIs (e.g., the BSD Sockets API) allow for emulation of UDP connections. In this case, connection latching can be supported using either model given above. We ignore, in this section, the fact that the connection latching model described in Section 2.4 can support per-datagram latching by extending its packet tagging interfaces to the application.
IPsec connection latch creation/release for UDP connections is triggered when:
o an application creates a UDP "connection". This should cause the creation of an ESTABLISHED or BROKEN connection latch.
o an application destroys a UDP "connection". This should cause the creation of an ESTABLISHED or BROKEN connection latch.
When a connection latch transitions to the BROKEN state and the application requested (or system policy dictates it) that the connection be broken, then UDP should inform the application, if
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there is a way to do so, or else it should wait, allowing the application-layer keepalive/timeout strategy, if any, to time out the connection.
What constitutes an appropriate action in the face of administrative transitions of connection latches to the CLOSED state depends on whether the implementation's "connected" UDP sockets API provides a way for the socket to return an error indicating that it has been closed.
5.3. Connection Latching to IPsec for UDP with Datagram-Tagging APIs
Implementations based on either model of connection latching can provide applications with datagram-tagging APIs based on those described in Section 2.4. Implementations UDP with of the normative model of IPsec connection latching have to confirm, on output, that the application provided 5-tuple agrees with the application-provided connection latch; on input, UDP can derive the tag by searching for a connection latch matching incoming datagram's 5-tuple.
SCTP [RFC4960], a connection-oriented protocol is similar, in some ways, to TCP. The salient difference, with respect to connection latching, between SCTP and TCP is that SCTP allows each end-point to be identified by a set of IP addresses, though, like TCP, each end- point of an SCTP connection (or, rather, SCTP association) can only have one port number.
We can represent the multiplicity of SCTP association end-point addresses as a multiplicity of 5-tuples, each of which with its own connection latch. Alternatively, we can extend the connection latch object to support a multiplicity of addresses for each end-point. The first approach is used throughout this document; therefore, we will assume that representation.
Of course, this approach results in N x M connection latches for any SCTP associations (where one end-point has N addresses and the other has M); whereas the alternative requires one connection latch per SCTP association (with N + M addresses). Implementors may choose either approach.
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IPsec connection latch creation/release for SCTP connections is triggered when:
o an SCTP listener end-point is created (e.g., when the SCTP sockets listen() function is called on a socket). This should cause the creation of a LISTENER connection latch for each address of the listener.
o an SCTP INIT chunk is received on an IP address and port number for which there is a listener. This should cause the creation of one or more ESTABLISHED or BROKEN connection latches, one for each distinct 5-tuple given the client and server's addresses.
o an SCTP INIT chunk is sent (e.g., as the result of a call to the SCTP sockets connect() function). This should cause the creation of one or more ESTABLISHED or BROKEN connection latches.
o an SCTP Address Configuration Change Chunk (ASCONF) [RFC5061] adding an end-point IP address is sent or received. This should cause the creation of one or more ESTABLISHED or BROKEN connection latches.
o any state transition of an SCTP association to the CLOSED state will cause a corresponding transition for any associated connection latches to the CLOSED state as well.
o an SCTP ASCONF chunk [RFC5061] deleting an end-point IP address is sent or received. This should cause one or more associated connection latches to be CLOSED.
See Section 5.5 for how to handle latch transitions to the BROKEN state.
There are several ways to handle connection latch transitions to the BROKEN state in the case of connection-oriented ULPs like TCP or SCTP:
a. Wait for a possible future transition back to the ESTABLISHED state, until which time the ULP will not move data between the two end-points of the connection. ULP and application timeout mechanisms will, of course, be triggered in the event of too lengthy a stay in the BROKEN state. SCTP can detect these timeouts and initiate failover, in the case of multi-homed associations.
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b. Act as though the connection has been reset (RST message received, in TCP, or ABORT message received, in SCTP).
c. Act as though an ICMP destination unreachable message had been received (in SCTP such messages can trigger path failover in the case of multi-homed associations).
Implementations SHOULD provide APIs that allow applications either 1) to be informed (asynchronously or otherwise) of latch breaks so that they may choose a disposition, and/or 2) to select a specific disposition a priori (before a latch break happens). The options for disposition are wait, close, or proceed with path failover.
Implementations MUST provide a default disposition in the event of a connection latch break. Though (a) is clearly the purist default, we RECOMMEND (b) for TCP and SCTP associations where only a single path remains (one 5-tuple), and (c) for multi-homed SCTP associations. The rationale for this recommendation is as follows: a conflicting SA most likely indicates that the original peer is gone and has been replaced by another, and it's not likely that the original peer will return; thus, failing faster seems reasonable.
Note that our recommended default behavior does not create off-path reset denial-of-service (DoS) attacks. To break a connection latch, an attacker would first have to successfully establish an SA, with one of the connection's end-points, that conflicts with the connection latch and that requires multiple messages to be exchanged between that end-point and the attacker. Unless the attacker's chosen victim end-point allows the attacker to claim IP address ranges for its SAs, then the attacker would have to actually take over the other end-point's addresses, which rules out off-path attacks.
Connection latching effectively adds a mechanism for dealing with the existence, in the SAD, of multiple non-equivalent child SAs with overlapping traffic selectors. This mechanism consists of, at minimum, a local notification of transport protocols (and, through them, applications) of the existence of such a conflict that affects a transport layer's connections. Affected transports are also notified when the conflict is cleared. The transports must drop inbound packets, and must not send outbound packets for connections that are affected by a conflict. In this minimal form, connection latching is a passive, local feature layered atop IPsec.
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We achieve this by adding a new type of IPsec database, the Latch Database (LD), containing objects that represent a transport protocol's interest in protecting a given packet flow from such conflicts. The LD is managed in conjunction with updates to the SAD and the SPD, so that updates to either that conflict with established connection latches can be detected. For some IPsec implementations, this may imply significant changes to their internals. However, two different models of connection latching are given, and we hope that most native IPsec implementors will find at least one model to be simple enough to implement in their stack.
This notion of conflicting SAs and how to deal with the situation does not modify the basic IPsec architecture -- the feature of IPsec that allows such conflicts to arise remains, and it is up to the transport protocols and applications to select whether and how to respond to them.
There are, however, interesting corner cases in the normative model of connection latching that implementors must be aware of. The notes in Section 2.3.1 are particularly relevant.
Section 3 describes optional features of connection latching where the key manager takes on a somewhat more active, though still local, role. There are two such features: optional protect/bypass and preservation of "logical" SPD entries to allow latched connections to remain in the ESTABLISHED state in the face of adverse administrative SPD (but not SAD) changes. These two features interact with administrative interfaces to IPsec; administrators must be made aware of these features, and they SHOULD be given a way to break ESTABLISHED connection latches. Also, given recent trends toward centralizing parts of IPsec policy, these two features can be said to have non-local effects where they prevent distributed policy changes from taking effect completely.
Connection latching is not negotiated. It is therefore possible for one end of a connection to be using connection latching while the other does not; in which case, it's possible for policy changes local to the non-latched end to cause packets to be sent unprotected. The end doing connection latching will reject unprotected packets, but if they bear sensitive data, then the damage may already be done. Therefore, applications SHOULD check that both ends of a connection are latched (such a check is implicit for applications that use channel binding to IPsec).
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Connection latching protects individual connections from weak peer ID<->address binding, IPsec configuration changes, and from configurations that allow multiple peers to assert the same addresses. But connection latching does not ensure that any two connections with the same end-point addresses will have the same latched peer IDs. In other words, applications that use multiple concurrent connections between two given nodes may not be protected any more or less by use of IPsec connection latching than by use of IPsec alone without connection latching. Such multi-connection applications can, however, examine the latched SA parameters of each connection to ensure that all concurrent connections with the same end-point addresses also have the same end-point IPsec IDs.
Connection latching protects against TCP RST attacks. It does not help, however, if the original peer of a TCP connection is no longer available (e.g., if an attacker has been able to interrupt the network connection between the two peers).
IPsec channels are a prerequisite for channel binding [RFC5056] to IPsec. Connection latching provides such channels, but the channel bindings for IPsec channels (latched connections) are not specified herein -- that is a work in progress [IPSEC-CB].
Without IPsec APIs, connection latching provides marginal security benefits over traditional IPsec. Such APIs are not described herein; see [ABSTRACT-API].
Connection latch state transitions to the BROKEN state can be triggered by on-path attackers and any off-path attackers that can attack routers or cause an end-point to accept an ICMP Redirect message. Connection latching protects applications against on- and off-path attackers in general, but not against on-path denial of service specifically.
Attackers can break latches if they can trigger DPD on one or both end-points and if they cause packets to not move between two end- points. Such attacks generally require that the attacker be on-path; therefore, we consider it acceptable to break latches when DPD concludes that a peer is dead or rebooted.
Attackers can also break latches if IPsec policy on a node allows the attacker to use the IP address of a peer of that node. Such
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configurations are expected to be used in conjunction with BTNS in general. Such attacks generally require that the attacker be on- path.
The author thanks Michael Richardson for all his help, as well as Stephen Kent, Sam Hartman, Bill Sommerfeld, Dan McDonald, Daniel Migault, and many others who've participated in the BTNS WG or who've answered questions about IPsec, connection latching implementations, etc.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, September 2007.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. Kozuka, "Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration", RFC 5061, September 2007.
[RFC5386] Williams, N. and M. Richardson, "Better-Than- Nothing Security: An Unauthenticated Mode of IPsec", RFC 5386, November 2008.