Network Working Group D. Harrington Internet-Draft Independent Expires: April 1, 2005 J. Schoenwaelder International University Bremen October 2004 Transport Mapping Security Model (TMSM) for the Simple Network Management Protocol version 3 (SNMPv3) draft-schoenw-snmp-tlsm-01.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. 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 April 1, 2005. Copyright Notice Copyright (C) The Internet Society (2004). Abstract This document describes a Transport Mapping Security Model (TMSM) for the Simple Network Management Protocol (SNMP) architecture defined in RFC3411. At this stage, this document does not provide a complete solution - it rather identifies and discusses some key aspects that need discussion and future work. Harrington & Schoenwaelder Expires April 1, 2005 [Page 1] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Requirements of a Transport Mapping Security Model . . . . . . 4 3.1 Security Requirements . . . . . . . . . . . . . . . . . . 4 3.2 Architectural Modularity Requirements . . . . . . . . . . 5 3.3 Passing messages between Dispatchers . . . . . . . . . . . 5 3.4 Security Parameter Passing Requirement . . . . . . . . . . 6 3.4.1 Using an ASI . . . . . . . . . . . . . . . . . . . . . 6 3.4.2 Using a cache . . . . . . . . . . . . . . . . . . . . 6 3.4.3 Using an encapsulating header . . . . . . . . . . . . 7 3.4.4 Using existing fields in a message . . . . . . . . . . 7 3.5 Access Control Requirements . . . . . . . . . . . . . . . 8 3.5.1 Architectural securityName Binding Requirement . . . . 8 4. Fields in the SNMPv3 message . . . . . . . . . . . . . . . . . 8 4.1 msgVersion . . . . . . . . . . . . . . . . . . . . . . . . 8 4.2 msgGlobalData . . . . . . . . . . . . . . . . . . . . . . 9 4.3 securityLevel and msgFlags . . . . . . . . . . . . . . . . 9 4.4 The tmStateReference for Passing Security Parameters . . . 11 4.5 securityStateReference Cached Security Data . . . . . . . 11 4.5.1 Prepare an Outgoing SNMP Message . . . . . . . . . . . 12 4.5.2 Prepare Data Elements from an Incoming SNMP Message . 12 4.6 Notifications . . . . . . . . . . . . . . . . . . . . . . 13 5. Transport Mapping Security Model Samples . . . . . . . . . . . 13 5.1 TLS/TCP Transport Mapping Security Model . . . . . . . . . 13 5.1.1 tmStateReference for TLS . . . . . . . . . . . . . . . 13 5.1.2 MP portion for TLS TM-Security Model . . . . . . . . . 14 5.1.3 MIB Module for TLS Security . . . . . . . . . . . . . 14 5.2 DTLS/UDP Transport Mapping Security Model . . . . . . . . 14 5.2.1 tmStateReference for DTLS . . . . . . . . . . . . . . 15 5.3 SASL Transport Mapping Security Model . . . . . . . . . . 16 5.3.1 tmStateReference for SASL DIGEST-MD5 . . . . . . . . 16 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.1 Normative References . . . . . . . . . . . . . . . . . . . . 17 7.2 Informative References . . . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18 A. Message security versus session security . . . . . . . . . . . 19 A.1 msgFlags versus actual security . . . . . . . . . . . . . 19 A.2 Message security versus session security . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . . 21 Harrington & Schoenwaelder Expires April 1, 2005 [Page 2] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 1. Introduction There are multiple ways to secure one's home or business, but they largely boil down to a continuum of alternatives. Let's consider three general approaches. In the first approach, an individual/ company could buy a gun, learn to use it, and sit on your front porch waiting for intruders. In the second approach, one could hire an employee with a gun, schedule the employee, position the employee to guard what you want protected, hire a second guard to cover if the first gets sick, and so on. In the third approach, you could hire a security company, tell them what you want protected, and they could hire employees, train them, buy the guns, position the guards, schedule the guards, send a replacement when a guard cannot make it, etc., thus providing the security you want, with no significant effort on your part other than identifying requirements and verifying the quality of the service being provided. The User-based Security Model (USM) as defined in [RFC3414] largely uses the first approach - it provides its own security. It utilizes existing mechanisms (MD5=the gun), but provides all the coordination. USM provides for the authentication of a principal, message encryption, data integrity checking, timeliness checking, etc. USM was designed to be independent of other existing security infrastructures. USM therefore requires a separate user and key management infrastructure. Operators have reported that deploying another user and key management infrastructure in order to use SNMPv3 is a reason for not deploying SNMPv3 at this point in time. It is possible but difficult to define external mechanisms that handle the distribution of keys for use by the USM approach. A solution based on the second approach might use a USM-compliant architecture, but replace the authentication mechanism with an external mechanism, such as RADIUS, to provide the authentication service. It might be possible to utilize an external protocol to encrypt a message, to check timeliness, to check data integrity, etc. It is difficult to cobble together a number of subcontracted services and coordinate them however, because it is difficult to build solid security bindings between the various services, and potential for gaps in the security is significant. A solution based on the third approach might utilize one or more lower-layer security mechanisms to provide the message-oriented security services required. These would include authentication of the sender, encryption, timeliness checking, and data integrity checking. There are a number of IETF standards available or in development to address these problems at lower layers, frequently at the transport layer. A solution based on this approach might also Harrington & Schoenwaelder Expires April 1, 2005 [Page 3] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 utilize a "transport" that is actually another application operating at the application layer, such as SSH [SSHauth] This document proposes a Transport Mapping Security Model (TMSM), as an extension of the SNMPv3 architecture, that would allow security to be provided an external protocol connected to the SNMP engine through an SNMP transport-mapping. Such a TMSM would then enable the use of existing security mechanisms such as (TLS) [RFC2246], Kerberos [RFC1510] or SASL [RFC2222] within the SNMPv3 architecture. As pointed out in the EUSM proposal [EUSM], it is desirable to use mechanisms that could "unify the approach for administrative security for SNMPv3 and CLI" and other management interfaces. The use of security services provided by lower layers or other applications is the approach commonly used for the CLI, and is the approach being proposed for NETCONF This document provides the motivation for leveraging transport layer security mechanisms for secure SNMP communication, identifies some key issues and provides some proposals for design choices that may be made to provide a workable solution that meets operational requirements and fits into the SNMP architecture defined in [RFC3411] 2. Motivation There are a number of Internet security protocols and mechanisms that are in wide spread use. Many of them try to provide a generic infrastructure to be used by many different application layer protocols. The motivation behind TMSM is to leverage these protocols where it seems useful. There are a number of challenges to be addressed to map the security provided by a secure transport into the SNMP architecture so that SNMP continues to work without any surprises. These are discussed in detail below. 3. Requirements of a Transport Mapping Security Model 3.1 Security Requirements Transport mapping security protocols SHOULD ideally provide the protection against the following message-oriented threats [RFC3411]: 1. modification of information 2. masquerade 3. message stream modification 4. disclosure Harrington & Schoenwaelder Expires April 1, 2005 [Page 4] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 According to [RFC3411], it is not required to protect against denial of service or traffic analysis. 3.2 Architectural Modularity Requirements [RFC3411] section 3 describes a modular architecture to allow the evolution of the SNMP protocol standards over time. This architecture includes a Security Subsystem which is responsible for realizing security services. Transport mapping security is by its very nature a security layer which is plugged in between the transport layer and the dispatcher. Conceptually, transport mapping security models will be called from within the Transport Mapping portion of an SNMP engine, or will be positioned between the transport mapping subsystem and the dispatcher. The design of a transport mapping security model must abide the goals of the RFC3411 architecture, section 1. To that end, this transport mapping security model proposal focuses on a modular subsystem that can be advanced through the standards process independently of other proposals, and independent of other subsystems as much as possible. This subsystem is designed as an architectural extension that permits different transport mapping security protocols to be "plugged into" this subsystem, to support supplemental transport mapping security models in addition to those described here. IETF standards typically require one mandatory-to-implement solution, with the capability of adding new security mechanisms in the future. Any transport mapping security model should define one minimum-compliance mechanism, preferably one which is already widely deployed within the transport layer security protocol used. This architectural extension is illustrated by the following diagram, which is a modified version of the diagram taken from the SNMP architecture document. TODO: Insert drawing here... 3.3 Passing messages between Dispatchers Typically, with a TMSM model, the transport mapping will establish an encrypted tunnel between the transport mappings of two SNMP engines, without passing anything to the SNMP dispatcher. One transport mapping security model instance encrypts all messages, and the other transport mapping security model instance decrypts the messages. Harrington & Schoenwaelder Expires April 1, 2005 [Page 5] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 After the transport layer tunnel is established, then SNMP messages can conceptually be sent through the tunnel from one SNMP engine dispatcher to another SNMP engine dispatcher. SNMP messages are passed unencrypted from the source dispatcher to its own TMSM, and presented unencrypted to the destination SNMP dispatcher. Once the tunnel is established, multiple SNMP messages may be able to be passed through the same tunnel. 3.4 Security Parameter Passing Requirement [RFC3411] section 4 describes primitives to describe the abstract service interfaces used to conceptually pass information between the various subsystems, models and applications within the architecture. A Transport mapping Security Model must pass information between subsystems as well. The RFC3411 architecture has no ASI parameters for passing security information between the transport mapping and the dispatcher, and between the dispatcher and the message processing model. Since the TM portion of the security model and the MP portion of the security model are co-resident within an implementation, it is assumed there is a trust relationship that exists within the implementation. There are four approaches that could be used for passing information between the TM portion of the securitymodel and the MP portion of the security model : we could define an ASI to supplement the existing ASIs, or the TMSM could pass the information in an implementation-specific cache, or the TMSM could add a header to encapsulate the SNMP message, or the TMSM could utilize fields already defined in the existing SNMPv3 message. 3.4.1 Using an ASI RFC3411 discusses the purpose, and an explicit non-purpose, of the ASI approach: "This modularity of specification is not meant to be interpreted as imposing any specific requirements on implementation." An ASI is not an API, and following a defined ASI is not required for interoperability, so implementors are really free to use any method they choose. However, defining an ASI has the advantage of being consistent with existing RFC3411/3412 practice. 3.4.2 Using a cache A cache mechanism could be used, into which the TM portion of the security model puts information about the security applied to an incoming message, and an MP portion of the security model extracts Harrington & Schoenwaelder Expires April 1, 2005 [Page 6] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 that information from the cache. The cache is not passed via an explicit ASI. Given that there may be multiple TM-security caches, a cache ID probably needs to be passed in the message in the ASI so the MP portion of the security model knows which cache to consult. This approach would be consistent with the securityStateReference cache already being passed around in the ASI. The cache could be thought of as an additional parameter in the ASI. The ASI would not need to be changed since the SNMPv3 WG expected that additional parameters could be passed for value-add features of specific implementations. 3.4.3 Using an encapsulating header A header could encapsulate the SNMP message to pass necessary information from the TM portion of the security model to the dispatcher and then to the MP portion of the security model. The message header would be included in the wholeMessage ASI parameter, and would be removed by a corresponding messaging model. This would imply a new messaging model would need to be specified as well. The other approaches may be able to use the standard SNMPv3 messaging model, with a new MP-security model. 3.4.4 Using existing fields in a message [RFC3412] describes the SNMPv3 message, which contains fields to pass security related parameters. The TMSM could use these fields in an SNMPv3 message, or comparable fields in other message formats to pass information between transport mapping security models in different SNMP engines, and to pass information between a TM security model and the corresponding MP security model. It is importnat to understand that SNMP messages are ASN.1 encoded, and the SNMP architecture places no constraints on how the ASN.1 gets decoded - it might be decoded in one massive decode, or individual portions of the message, such as individual varbinds, may be decoded only as needed. This is an implementation decision. If the fields in an incoming SNMPv3 message are changed by the TM portion before passing it to the MP portion, then the TM portion will need to encode its parameters in ASN.1 or the message model would need to be modified to permit non-encoded data to be added to the message in a manner that would not impact the existing ASN.1 encoding/decoding of the message. In addition, the MP portion may not be able to perform a transport-independent message integrity check, and transport-independent encryption may not be able to be performed by the MP portion of the model. While it may be desirable for most TMSM models to perform those services through the TM portion Harrington & Schoenwaelder Expires April 1, 2005 [Page 7] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 of the model, assuming the use of a cache or an encapsulating header would not impose such constraints on future models. This document will describe a cache approach, but an encapsulating header or other mechanisms could also be used if preferred for specific TM security models. 3.5 Access Control Requirements 3.5.1 Architectural securityName Binding Requirement For SNMP access control to function properly, the security mechanism must establish a securityName, which is the security model independent identifier for a principal, a security model identifier, and a securityLevel. The SNMPv3 message processing architecture subsystem relies on a message model based security model, such as USM, to play an role in security that goes beyond protecting the message - it ties various security models for the same principal to a security-model independent securityName which can be used for subsequent processing, such as for access control. The TMSM assumes two portions to a security model, one tied to the transport mapping and another tied to the message processing model. and will be referred to here as a TM-portion and an MP-portion of the security model. Depending on the specific design of the security model, different features might be provided by the TM portion or by the MP portion. For example, the binding of a mechanism-specific authenticated identity to a securityName might be done by the TM portion or by the MP portion. The SNMP architecture distinguishes between messages with no authentication and no privacy (noAuthNoPriv), authentication without privacy (authNoPriv) and authentication with privacy (authPriv). Hence, the authentication of a transport layer identity plays an important role and must be considered by any transport layer security mechanism used. However, it is also possible that a second level of authentication, one provided by a AAA server, for example, may be used to provide the authentication identity which is bound to the securityName, if the type of authentication provided by the transport layer (e.g. host-based or anonymous) is considered adequate to secure and/or encrypt the message, but inadequate to provide the desired granularity of access control (e.g. user-based). 4. Fields in the SNMPv3 message 4.1 msgVersion For proposals that reuse the SNMPv3 message format, this field should Harrington & Schoenwaelder Expires April 1, 2005 [Page 8] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 contain the value 3. 4.2 msgGlobalData msgID and msgMaxSize are used identically for the TMSM models as for the USM model. msgSecurityModel should be set to a value from the SnmpSecurityModel enumeration [RFC3410] to identify the specific TMSM model. msgSecurityParameters is used identically for the TMSM models as for the USM model. msgFlags have the same values for the TMSM models as for the USM model. "The authFlag and privFlag fields indicate the securityLevel that was applied to the message before it was sent on the wire." 4.3 securityLevel and msgFlags For an outgoing message, msgFlags is the requested security for the message; if a TMSM cannot provide the requested securityLevel, the model MUST describe a standard behavior that is followed for that situation. If the TMSM cannot provide at least the requested level of security, the TMSM MUST discard the request and SHOULD notify the message processing model that the request failed. [dbh: how is yet to be determined, and may be model-specific or implementation-specific.] For an outgoing message, if the TMSM is able to provide stronger than requested security, that may be acceptable. The transport layer protocol would need to indicate to the receiver what security has been applied to the actual message. To avoid the need to mess with the ASN.1 encoding, the SNMPv3 message carries the requested msgFlags, not the actual securityLevel applied to the message. If a message format other than SNMPv3 is used, then the new message may carry the more accurate securityLevel in the SNMP message. For an incoming message, the receiving TMSM knows what must be done to process the message based on the transport layer mechanisms. If the underlying transport security mechanisms for the receiver cannot provide the matching securityLevel, then the message should follow the standard behaviors for the transport security mechanism, or be discarded silently. Part of the responsibility of the TMSM is to ensure that the actual security provided by the underlying transport layer security mechanisms is configured to meet or exceed the securityLevel required by the msgFlags in the SNMP message. When the MP portion of the Harrington & Schoenwaelder Expires April 1, 2005 [Page 9] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 security model processes the incoming message, it should compare the msgFlags field to the securityLevel actually provided for the message by the transport layer security. If they differ, the MP portion of the security model should determine whether the changed securityLevel is acceptable. If not, it should discard the message. Depending on the model, the MP portion may issue a reportPDU with the XXXXXXX model-specific counter. Questions about msgFlags: Is the securityLevel looked at before the security model gets to it.? No. the security model has two parts - the TM portion and the MP portion. The securityLevel is looked at by the TM portion before it gets to the MP piece, but both are parts of the same security model. Would it be legal for the security model to ignore the incoming flags and change them before passing them back up? If it changed them, it wouldn't necessarily be ignoring them. The TM portion should pass both an actual securityLevel applied to the message, and the msgFlags in the SNMP message to the MP piece for consideration related to access control.. The msgFlags parameter in the SNMP message is never changed when processing an incoming message. Would it be legal for the security model to ignore the outgoing flags and change them before passing them out? no; because the two portions are parts of the same security model, either the MP piece should recognize that a securityLevel cannot be met or exceeded, and reject the message during the message-build phase, or the TM piece should determine if it is possible to honor the request. It is possible to apply an increased securityLevel for an outgoing request, but the procedure to do so must be spelled out clearly in the model design. The security model would need to (MUST) check the incoming security level flags to make sure they matched the TLS/whatever session setup and if not drop the message. Yes, mostly. Depending on the model, either the TM portion or the MP portion MUST verify that the actual processing met or exceeded the securityLevel requested by the msgFlags and that it is acceptable to the specific-model processing (or operator configuration) for this different securityLevel to be applied to the message. This is also true (especially) for outgoing messages. You might legally be able to have a authNoPriv message that is actually encrypted via the transport (but not the other way around of course). Yes, a TMSM could define that as the behavior (or permit an operator to specify that is acceptable behavior) when a requested securityLevel cannot be provided, but a stronger securityLevel can be provided. Harrington & Schoenwaelder Expires April 1, 2005 [Page 10] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 See the Appendix A appendix for further discussion of the msgFlags field versus the actual securityLevel provided. [dbh: it may be a good thing to merge the Question and Answer with the appendix, either here or there.] 4.4 The tmStateReference for Passing Security Parameters A tmStateReference is used to pass data between the TM portion and the MP portion of the security model, similar to the securityStateReference described in RFC3412. This can be envisioned as being appended to the ASIs between the TM and the MP or as being passed in an encapsulating header. The TM portion of the security model may provide only some aspects of security, and leave some aspects to the MP portion of the model. tmStateReference should be used to pass any parameters, in a model- and mechanism-specific format, that will be needed to coordinate the activities of the TM and MP portions of the model, and the parameters subsequently passed in securityStateReference . For example, the TM portion may provide privacy and data integrity and authentication and authorization policy retrievals, or some subset of these features, depending on the features available in the transport mechanisms. A field in tmStateReference should identify which services were provided for each received message by the TM portion, the securityLevel applied to the received message, the model-specific security identity, the session identifier for session based transport security, and so on. 4.5 securityStateReference Cached Security Data From RFC3411: "For each message received, the Security Model caches the state information such that a Response message can be generated using the same security information, even if the Local Configuration Datastore is altered between the time of the incoming request and the outgoing response. A Message Processing Model has the responsibility for explicitly releasing the cached data if such data is no longer needed. To enable this, an abstract securityStateReference data element is passed from the Security Model to the Message Processing Model. The cached security data may be implicitly released via the generation of a response, or explicitly released by using the stateRelease primitive, as described in section 4.5.1." To differentiate what information needs to be provided to the MP portion by the TM portion, and vice-versa, this document will differentiate the tmStateReference from the securityStateReference. An implementation MAY use one cache and one reference to serve both Harrington & Schoenwaelder Expires April 1, 2005 [Page 11] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 functions, but an implementor must be aware of the cache-release issues to prevent the cache from being released before the TM portion has had an opportunity to extract the information it needs. 4.5.1 Prepare an Outgoing SNMP Message According to RFC3412, section 7.1, the SNMPv3 message processing model calls the MP portion of the TM security model using the generateResponseMsg() or generateRequestMsg(). The MP portion of the model may need to put information into the tmStateReference cache for use by the TM portion of the model, such as: tmSecurityStateReference - the unique identifier for the cached information tmTransportDomain tmTransportAddress tmSecurityModel - an indicator of which mechanisms to use tmSecurityName - a model-specific identifier of the security principal tmSecurityLevel - an indicator of which security services are requested and may contain additional information such as tmSessionID tmSessionKey tmSessionMsgID 4.5.2 Prepare Data Elements from an Incoming SNMP Message For an incoming message, the TM portion of a model will need to put information from the transport mechanisms used into the tmStateReference so the MP portion of the model can extract the information and add it conceptually to the securityStateReference. The tmStateReference cache will likely contain at least the following information: tmStateReference - a unique identifier for the cached information tmSecurityStateReference - the unique identifier for the cached information tmTransportDomain tmTransportAddress tmSecurityModel - an indicator of which mechanisms to use tmSecurityName - a model-specific identifier of the security principal tmSecurityLevel - an indicator of which security services are requested tmAuthProtocol tmPrivProtocol and may contain additional information such as Harrington & Schoenwaelder Expires April 1, 2005 [Page 12] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 tmSessionID tmSessionKey tmSessionMsgID 4.6 Notifications For notifications, if the cache has been released and then session closed, then the MP portion of the security model will request the TM portion of the security model to establish a session, populate the cache, and pass the securityStateReference to the MP portion of the security model. TODO: We need to determine what state needs to be saved here. 5. Transport Mapping Security Model Samples 5.1 TLS/TCP Transport Mapping Security Model SNMP supports multiple transports. The preferred transport for SNMP over IP is UDP [RFC3417]. An experimental transport for SNMP over TCP is defined in [RFC3430]. TLS/TCP will create an association between the TMSM of one SNMP entity and the TMSM of another SNMP entity. The created "tunnel" may provide encryption and data integrity. Both encryption and data integrity are optional features in TLS. The TLS TM portion of the security model MUST provide authentication if auth is requested in the securityLevel of the SNMP message request (RFC3412 4.1.1). The TLS TM-security model MUST specify that the messages be encrypted if priv is requested in the securityLevel parameter of the SNMP message request (RFC3412 4.1.1). The TLS TM-security model SHOULD use the TLS Handshake Protocol with mutual authentication. 5.1.1 tmStateReference for TLS Upon establishment of a TLS session, the TM-security model will cache the state information. A tmStateReference that is unique within the SNMP entity will be stored in the cache, and passed to the corresponding MP portion of the security model, to enable lookup. The MP security model will pass the securityStateReference to the Message Processing Model for memory management. The tmStateReference cache: tmStateReference tmSecurityStateReference Harrington & Schoenwaelder Expires April 1, 2005 [Page 13] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 tmTransportDomain = TCP/IPv4 tmTransportAddress = x.x.x.x:y tmSecurityModel - TLS TMSM tmSecurityName = "dbharrington" tmSecurityLevel = "authPriv" tmAuthProtocol = Handshake MD5 tmPrivProtocol = Handshake DES tmSessionID = Handshake session identifier tmSessionKey = Handshake peer certificate tmSessionMasterSecret = master secret tmSessionParameters = compression method, cipher spec, is-resumable 5.1.2 MP portion for TLS TM-Security Model messageProcessingModel = SNMPv3 securityModel = TLS TMSM securityName = tmSecurityName securityLevel = msgSecurityLevel 5.1.3 MIB Module for TLS Security Each security model should use its own MIB module, rather than utilizing the USM MIB, to eliminate dependencies on a model that could be replaced some day. See RFC3411 section 4.1.1. The TLS MIB module needs to provide the mapping from model-specific identity to a model independent securityName. TODO: Module needs to be worked out once things become stable... 5.2 DTLS/UDP Transport Mapping Security Model DTLS has been proposed as a UDP-based TLS. Transport Layer Security (TLS) [RFC2246] traditionally requires a connection-oriented transport and is usually used over TCP. Datagram Transport Layer Security (DTLS) [DTLS] provides security services equivalent to TLS for connection-less transports such as UDP. DTLS provides all the security services needed from an SNMP architectural point of view. Although it is possible to derive a securityName from the public key certificates (e.g. the subject field), this approach requires to install certificates on agents and as well as managers, leading to a certificate management problem which again does not integrate well with established AAA systems. Another option is to run an authentication exchange which is integrated with TLS, such as Secure Remote Password with TLS Harrington & Schoenwaelder Expires April 1, 2005 [Page 14] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 [SRP-TLS]. A similar option would be to use Kerberos authentication with TLS as defined in [RFC2712]. It is important to stress that the authentication exchange must be integrated into the TLS mechanism to prevent man-in-the-middle attacks. While SASL [RFC2222] is often used on top of a TLS encrypted channel to authenticate users, this choice seems to be problematic until the mechanism to cryptographically bind SASL into the TLS mechanism has been defined. DTLS will create an association between the TMSM of one SNMP entity and the TMSM of another SNMP entity. The created "tunnel" may provide encryption and data integrity. Both encryption and data integrity are optional features in DTLS. The DTLS TM-security model MUST provide authentication if auth is requested in the securityLevel of the SNMP message request (RFC3412 4.1.1). The TLS TM-security model MUST specify that the messages be encrypted if priv is requested in the securityLevel parameter of the SNMP message request (RFC3412 4.1.1). The DTLS TM-security model SHOULD use the TLS Handshake Protocol with mutual authentication. 5.2.1 tmStateReference for DTLS Upon establishment of a DTLS session, the TM-security model will cache the state information. A tmStateReference that is unique within the SNMP entity will be stored in the cache, and passed to the corresponding MP portion of the security model, to enable lookup. The MP security model will pass the securityStateReference to the Message Processing Model for memory management. The tmStateReference cache: tmStateReference tmSecurityStateReference tmTransportDomain = UDP/IPv4 tmTransportAddress = x.x.x.x:y tmSecurityModel - DTLS TMSM tmSecurityName = "dbharrington" tmSecurityLevel = "authPriv" tmAuthProtocol = Handshake MD5 tmPrivProtocol = Handshake DES tmSessionID = Handshake session identifier tmSessionKey = Handshake peer certificate tmSessionMasterSecret = master secret tmSessionParameters = compression method, cipher spec, is-resumable Harrington & Schoenwaelder Expires April 1, 2005 [Page 15] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 tmSessionSequence = epoch, sequence TODO: Need to discuss to what extent DTLS is a reasonable choice for SNMP interactions. What is the status of the work to cryptographically bind SASL to DTLS? More details need to be worked out... 5.3 SASL Transport Mapping Security Model The Simple Authentication and Security Layer (SASL) [RFC2222] provides a hook for authentication and security mechanisms to be used in application protocols. SASL supports a number of authentication and security mechanisms, among them Kerberos via the GSSAPI mechanism. This sample will use DIGEST-MD5 because it supports authentication, integrity checking, and confidentiality. DIGEST-MD5 supports auth, auth with integrity, and auth with confidentiality. Since SNMPv3 assumes integrity checking is part of authentication, if msgFlags is set to authNoPriv, the qop-value should be set to auth-int; if msgFlags is authPriv, then qop-value should be auth-conf. Realm is optional, but can be utilized by the securityModel if desired. SNMP does not use this value, but a TMSM could map the realm into SNMP processing in various ways. For example, realm and username could be concatenated to be the securityName value, e.g. helpdesk::username", or the realm could be used to specify a groupname to use in the VACM access control. This would be similar to the EUSM's approach to having the securityName-to-group mapping done by the external AAA server. 5.3.1 tmStateReference for SASL DIGEST-MD5 The tmStateReference cache: tmStateReference tmSecurityStateReference tmTransportDomain = TCP/IPv4 tmTransportAddress = x.x.x.x:y tmSecurityModel - SASL TMSM tmSecurityName = username tmSecurityLevel = [auth-conf] tmAuthProtocol = md5-sess tmPrivProtocol = 3des Harrington & Schoenwaelder Expires April 1, 2005 [Page 16] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 tmServicesProvided = mutual authentication, reauthentication, integrity, encryption tmParameters = "realm=helpdesk, serv-type=SNMP 6. Acknowledgments The authors would like to thank Ira McDonald, Ken Hornstein, and Nagendra Modadugu for their comments and suggestions. 7. References 7.1 Normative References [RFC3411] Harrington, D., Presuhn, R. and B. Wijnen, "An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks", STD 62, RFC 3411, December 2002. [RFC3412] Case, J., Harrington, D., Presuhn, R. and B. Wijnen, "Message processing and Dispatching for SNMP", STD 62, RFC 3412, December 2002. [RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)", STD 62, RFC 3414, December 2002. [RFC3417] Presuhn (Editor), R., "Transport Mappings for the Simple Network Management Protocol (SNMP)", STD 62, RFC 3417, December 2002. [RFC3430] Schoenwaelder, J., "Simple Network Management Protocol (SNMP) over Transmission Control Protocol (TCP) Transport Mapping", RFC 3430, December 2002. [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network Authentication Service (V5)", RFC 1510, September 1993. [RFC2222] Myers, J., "Simple Authentication and Security Layer (SASL)", STD 62, RFC RFC2222, October 1997. [DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security", ID draft-rescorla-dtls-01.txt, July 2004. Harrington & Schoenwaelder Expires April 1, 2005 [Page 17] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 7.2 Informative References [RFC3410] Case, J., Mundy, R., Partain, D. and B. Stewart, "Introduction and Applicability Statements for Internet-Standard Management Framework", RFC 3410, December 2002. [RFC2712] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)", RFC 2712, October 1999. [SRP-TLS] Taylor, D., Wu, T., Mavroyanopoulos, M. and T. Perrin, "Using SRP for TLS Authentication", ID draft-ietf-tls-srp-08.txt, August 2004. [EUSM] Narayan, D., McCloghrie, K., Salowey, J. and C. Elliot, "External USM for SNMPv3", ID draft-kaushik-snmp-external-usm-00.txt, July 2004. [NETCONF] Enns, R., "NETCONF Configuration Protocol", ID draft-ietf-netconf-prot-04.txt, October 2004. [SSHauth] Lonvick, C., "SSH Authentication Protocol", ID draft-ietf-secsh-userauth-21.txt, June 2004. Authors' Addresses David Harrington Independent Harding Rd Portsmouth NH USA Phone: +1 603 436 8634 EMail: dbharrington@comcast.net Juergen Schoenwaelder International University Bremen Campus Ring 1 28725 Bremen Germany Phone: +49 421 200-3587 EMail: j.schoenwaelder@iu-bremen.de Harrington & Schoenwaelder Expires April 1, 2005 [Page 18] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 Appendix A. Message security versus session security A.1 msgFlags versus actual security Using IPSEC, SSH, or SSL/TLS to provide security services "below" the SNMP message, the use of securityName and securityLevel will differ from the USM/VACM approach to SNMP access control. VACM uses the "securityName" and the "securityLevel" to determine if access is allowed. With the SNMPv3 message and USM security model, both securityLevel and securityName are contained in every SNMPv3 message. Any proposal for a security model using IPSEC, SSH, or SSL/TLS needs to specify how this info is made available to the SNMPv3 message processing, and how it is used. One specific case to consider is the relationship between the msgFlags of an SNMPv3 message, and the actual services provided by the lower layer security. For example, if a session is set up with encryption, is the priv bit always (or never) set in the msgFlags field, and is the PDU never (or always) encrypted? Do msgFlags have to match the security services provided by the lower layer, or are the msgFlags ignored and the values from the lower layer used? A.2 Message security versus session security For SBSM, and for many TMSM models, securityName is specified during session setup, and associated with the session identifier. Is it possible for the request (and notification) originator to specify per message auth and encryption services, or are they are "fixed" by the transport/session model? If a session is created as 'authPriv', then keys for encryption would still be negotiated once at the beginning of the session. But if a message is presented to the session with a security level of authNoPriv, then that message could simply be authenticated and not encrypted. Wouldn't that also have some security benefit, in that it reduces the encrypted data available to an attacker gathering packets to try and discover the encryption keys? Agents are often resource-constrained. Adding sessions increases the need for resources, we shouldn't require two sessions when one can suffice. 2 bytes per session structure and a compare or two is much less of a resource burden on an agent than two separate sessions. It's not just about CPU power of the device but the percentage of CPU cycles that are spent on network management. There isn't much value in using encryption for a performance management system polling PEs for performance data on thousands of interfaces every ten minutes,it Harrington & Schoenwaelder Expires April 1, 2005 [Page 19] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 just adds significant overhead to processing of the packet. Using an encrypted TLS channel for everything may not work for use cases in performance management wherein we collect massive amounts of non sensitive data at periodic intervals. Each SNMP "session" would have to negotiate two separate protection channels (authPriv and authNoPriv) and for every packet the SNMP engine will use the appropriate channel based on the desired securityLevel. If the underlying transport layer security was configurable on a per-message basis, a TMSM could have a MIB module with configurable maxSecurityLevel and a minSecurityLevel objects to identify the range of possible levels, and not all messages sent via that session are of the same level. A session's maxSecurityLevel would identify the maximum security it could provide, and a session created with a minSecurityLevel of authPriv would reject an attempt to send an authNoPriv message. Harrington & Schoenwaelder Expires April 1, 2005 [Page 20] Internet-Draft SNMPv3 Transport Mapping Security Model October 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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