Network Working Group D. McDonald Request for Comments: 2367 C. Metz Category: Informational B. Phan July 1998 PF_KEY Key Management API, Version 2 Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1998). All Rights Reserved. Abstract A generic key management API that can be used not only for IP Security [Atk95a] [Atk95b] [Atk95c] but also for other network security services is presented in this document. Version 1 of this API was implemented inside 4.4-Lite BSD as part of the U. S. Naval Research Laboratory's freely distributable and usable IPv6 and IPsec implementation[AMPMC96]. It is documented here for the benefit of others who might also adopt and use the API, thus providing increased portability of key management applications (e.g. a manual keying application, an ISAKMP daemon, a GKMP daemon [HM97a][HM97b], a Photuris daemon, or a SKIP certificate discovery protocol daemon). Table of Contents 1 Introduction ............................................. 3 1.1 Terminology .............................................. 3 1.2 Conceptual Model ......................................... 4 1.3 PF_KEY Socket Definition ................................. 8 1.4 Overview of PF_KEY Messaging Behavior .................... 8 1.5 Common PF_KEY Operations ................................. 9 1.6 Differences Between PF_KEY and PF_ROUTE .................. 10 1.7 Name Space ............................................... 11 1.8 On Manual Keying ..........................................11 2 PF_KEY Message Format .................................... 11 2.1 Base Message Header Format ............................... 12 2.2 Alignment of Headers and Extension Headers ............... 14 2.3 Additional Message Fields ................................ 14 2.3.1 Association Extension .................................... 15 2.3.2 Lifetime Extension ....................................... 16 McDonald, et. al. Informational [Page 1] RFC 2367 PF_KEY Key Management API July 1998 2.3.3 Address Extension ........................................ 18 2.3.4 Key Extension ............................................ 19 2.3.5 Identity Extension ....................................... 21 2.3.6 Sensitivity Extension .................................... 21 2.3.7 Proposal Extension ....................................... 22 2.3.8 Supported Algorithms Extension ........................... 25 2.3.9 SPI Range Extension ...................................... 26 2.4 Illustration of Message Layout ........................... 27 3 Symbolic Names ........................................... 30 3.1 Message Types ............................................ 31 3.1.1 SADB_GETSPI .............................................. 32 3.1.2 SADB_UPDATE .............................................. 33 3.1.3 SADB_ADD ................................................. 34 3.1.4 SADB_DELETE .............................................. 35 3.1.5 SADB_GET ................................................. 36 3.1.6 SADB_ACQUIRE ............................................. 36 3.1.7 SADB_REGISTER ............................................ 38 3.1.8 SADB_EXPIRE .............................................. 39 3.1.9 SADB_FLUSH ............................................... 40 3.1.10 SADB_DUMP ................................................ 40 3.2 Security Association Flags ............................... 41 3.3 Security Association States .............................. 41 3.4 Security Association Types ............................... 41 3.5 Algorithm Types .......................................... 42 3.6 Extension Header Values .................................. 43 3.7 Identity Extension Values ................................ 44 3.8 Sensitivity Extension Values ............................. 45 3.9 Proposal Extension Values ................................ 45 4 Future Directions ........................................ 45 5 Examples ................................................. 45 5.1 Simple IP Security Example ............................... 46 5.2 Proxy IP Security Example ................................ 47 5.3 OSPF Security Example .................................... 50 5.4 Miscellaneous ............................................ 50 6 Security Considerations .................................. 51 Acknowledgments ............,............................. 52 References ............................................... 52 Disclaimer ............................................... 54 Authors' Addresses ....................................... 54 A Promiscuous Send/Receive Extension ....................... 55 B Passive Change Message Extension ......................... 57 C Key Management Private Data Extension .................... 58 D Sample Header File ....................................... 59 E Change Log ............................................... 64 F Full Copyright Statement ................................. 68 McDonald, et. al. Informational [Page 2] RFC 2367 PF_KEY Key Management API July 1998 1 Introduction PF_KEY is a new socket protocol family used by trusted privileged key management applications to communicate with an operating system's key management internals (referred to here as the "Key Engine" or the Security Association Database (SADB)). The Key Engine and its structures incorporate the required security attributes for a session and are instances of the "Security Association" (SA) concept described in [Atk95a]. The names PF_KEY and Key Engine thus refer to more than cryptographic keys and are retained for consistency with the traditional phrase, "Key Management". PF_KEY is derived in part from the BSD routing socket, PF_ROUTE. [Skl91] This document describes Version 2 of PF_KEY. Version 1 was implemented in the first five alpha test versions of the NRL IPv6+IPsec Software Distribution for 4.4-Lite BSD UNIX and the Cisco ISAKMP/Oakley key management daemon. Version 2 extends and refines this interface. Theoretically, the messages defined in this document could be used in a non-socket context (e.g. between two directly communicating user-level processes), but this document will not discuss in detail such possibilities. Security policy is deliberately omitted from this interface. PF_KEY is not a mechanism for tuning systemwide security policy, nor is it intended to enforce any sort of key management policy. The developers of PF_KEY believe that it is important to separate security mechanisms (such as PF_KEY) from security policies. This permits a single mechanism to more easily support multiple policies. 1.1 Terminology Even though this document is not intended to be an actual Internet standard, the words that are used to define the significance of particular features of this interface are usually capitalized. Some of these words, including MUST, MAY, and SHOULD, are detailed in [Bra97]. - CONFORMANCE and COMPLIANCE Conformance to this specification has the same meaning as compliance to this specification. In either case, the mandatory-to-implement, or MUST, items MUST be fully implemented as specified here. If any mandatory item is not implemented as specified here, that implementation is not conforming and not compliant with this specification. McDonald, et. al. Informational [Page 3] RFC 2367 PF_KEY Key Management API July 1998 This specification also uses many terms that are commonly used in the context of network security. Other documents provide more definitions and background information on these [VK83, HA94, Atk95a]. Two terms deserve special mention: - (Encryption/Authentication) Algorithm For PF_KEY purposes, an algorithm, whether encryption or authentication, is the set of operations performed on a packet to complete authentication or encryption as indicated by the SA type. A PF_KEY algorithm MAY consist of more than one cryptographic algorithm. Another possibility is that the same basic cryptographic algorithm may be applied with different modes of operation or some other implementation difference. These differences, henceforth called _algorithm differentiators_, distinguish between different PF_KEY algorithms, and options to the same algorithm. Algorithm differentiators will often cause fundamentally different security properties. For example, both DES and 3DES use the same cryptographic algorithm, but they are used differently and have different security properties. The triple-application of DES is considered an algorithm differentiator. There are therefore separate PF_KEY algorithms for DES and 3DES. Keyed-MD5 and HMAC-MD5 use the same hash function, but construct their message authentication codes differently. The use of HMAC is an algorithm differentiator. DES-ECB and DES-CBC are the same cryptographic algorithm, but use a different mode. Mode (e.g., chaining vs. code-book) is an algorithm differentiator. Blowfish with a 128-bit key, however, is similar to Blowfish with a 384-bit key, because the algorithm's workings are otherwise the same and therefore the key length is not an algorithm differentiator. In terms of IP Security, a general rule of thumb is that whatever might be labeled the "encryption" part of an ESP transform is probably a PF_KEY encryption algorithm. Whatever might be labelled the "authentication" part of an AH or ESP transform is probably a PF_KEY authentication algorithm. 1.2 Conceptual Model This section describes the conceptual model of an operating system that implements the PF_KEY key management application programming interface. This section is intended to provide background material useful to understand the rest of this document. Presentation of this conceptual model does not constrain a PF_KEY implementation to strictly adhere to the conceptual components discussed in this subsection. McDonald, et. al. Informational [Page 4] RFC 2367 PF_KEY Key Management API July 1998 Key management is most commonly implemented in whole or in part at the application layer. For example, the ISAKMP/Oakley, GKMP, and Photuris proposals for IPsec key management are all application-layer protocols. Manual keying is also done at the application layer. Even parts of the SKIP IP-layer keying proposal can be implemented at the application layer. Figure 1 shows the relationship between a Key Management daemon and PF_KEY. Key management daemons use PF_KEY to communicate with the Key Engine and use PF_INET (or PF_INET6 in the case of IPv6) to communicate, via the network, with a remote key management entity. The "Key Engine" or "Security Association Database (SADB)" is a logical entity in the kernel that stores, updates, and deletes Security Association data for various security protocols. There are logical interfaces within the kernel (e.g. getassocbyspi(), getassocbysocket()) that security protocols inside the kernel (e.g. IP Security, aka IPsec) use to request and obtain Security Associations. In the case of IPsec, if by policy a particular outbound packet needs processing, then the IPsec implementation requests an appropriate Security Association from the Key Engine via the kernel-internal interface. If the Key Engine has an appropriate SA, it allocates the SA to this session (marking it as used) and returns the SA to the IPsec implementation for use. If the Key Engine has no such SA but a key management application has previously indicated (via a PF_KEY SADB_REGISTER message) that it can obtain such SAs, then the Key Engine requests that such an SA be created (via a PF_KEY SADB_ACQUIRE message). When the key management daemon creates a new SA, it places it into the Key Engine for future use. McDonald, et. al. Informational [Page 5] RFC 2367 PF_KEY Key Management API July 1998 +---------------+ |Key Mgmt Daemon| +---------------+ | | | | | | Applications ======[PF_KEY]====[PF_INET]========================== | | OS Kernel +------------+ +-----------------+ | Key Engine | | TCP/IP, | | or SADB |---| including IPsec | +------------+ | | +-----------------+ | +-----------+ | Network | | Interface | +-----------+ Figure 1: Relationship of Key Mgmt to PF_KEY For performance reasons, some security protocols (e.g. IP Security) are usually implemented inside the operating system kernel. Other security protocols (e.g. OSPFv2 Cryptographic Authentication) are implemented in trusted privileged applications outside the kernel. Figure 2 shows a trusted, privileged routing daemon using PF_INET to communicate routing information with a remote routing daemon and using PF_KEY to request, obtain, and delete Security Associations used with a routing protocol. McDonald, et. al. Informational [Page 6] RFC 2367 PF_KEY Key Management API July 1998 +---------------+ |Routing Daemon| +---------------+ | | | | | | Applications ======[PF_KEY]====[PF_INET]========================== | | OS Kernel +------------+ +---------+ | Key Engine | | TCP/IP | | or SADB |---| | +------------+ +---------+ | +-----------+ | Network | | Interface | +-----------+ Figure 2: Relationship of Trusted Application to PF_KEY When a trusted privileged application is using the Key Engine but implements the security protocol within itself, then operation varies slightly. In this case, the application needing an SA sends a PF_KEY SADB_ACQUIRE message down to the Key Engine, which then either returns an error or sends a similar SADB_ACQUIRE message up to one or more key management applications capable of creating such SAs. As before, the key management daemon stores the SA into the Key Engine. Then, the trusted privileged application uses an SADB_GET message to obtain the SA from the Key Engine. In some implementations, policy may be implemented in user-space, even though the actual cryptographic processing takes place in the kernel. Such policy communication between the kernel mechanisms and the user-space policy MAY be implemented by PF_KEY extensions, or other such mechanism. This document does not specify such extensions. A PF_KEY implementation specified by the memo does NOT have to support configuring systemwide policy using PF_KEY. Untrusted clients, for example a user's web browser or telnet client, do not need to use PF_KEY. Mechanisms not specified here are used by such untrusted client applications to request security services (e.g. IPsec) from an operating system. For security reasons, only trusted, privileged applications are permitted to open a PF_KEY socket. McDonald, et. al. Informational [Page 7] RFC 2367 PF_KEY Key Management API July 1998 1.3 PF_KEY Socket Definition The PF_KEY protocol family (PF_KEY) symbol is defined in in the same manner that other protocol families are defined. PF_KEY does not use any socket addresses. Applications using PF_KEY MUST NOT depend on the availability of a symbol named AF_KEY, but kernel implementations are encouraged to define that symbol for completeness. The key management socket is created as follows: #include #include #include int s; s = socket(PF_KEY, SOCK_RAW, PF_KEY_V2); The PF_KEY domain currently supports only the SOCK_RAW socket type. The protocol field MUST be set to PF_KEY_V2, or else EPROTONOSUPPORT will be returned. Only a trusted, privileged process can create a PF_KEY socket. On conventional UNIX systems, a privileged process is a process with an effective userid of zero. On non-MLS proprietary operating systems, the notion of a "privileged process" is implementation-defined. On Compartmented Mode Workstations (CMWs) or other systems that claim to provide Multi-Level Security (MLS), a process MUST have the "key management privilege" in order to open a PF_KEY socket[DIA]. MLS systems that don't currently have such a specific privilege MUST add that special privilege and enforce it with PF_KEY in order to comply and conform with this specification. Some systems, most notably some popular personal computers, do not have the concept of an unprivileged user. These systems SHOULD take steps to restrict the programs allowed to access the PF_KEY API. 1.4 Overview of PF_KEY Messaging Behavior A process interacts with the key engine by sending and receiving messages using the PF_KEY socket. Security association information can be inserted into and retrieved from the kernel's security association table using a set of predefined messages. In the normal case, all properly-formed messages sent to the kernel are returned to all open PF_KEY sockets, including the sender. Improperly formed messages will result in errors, and an implementation MUST check for a properly formed message before returning it to the appropriate listeners. Unlike the routing socket, most errors are sent in reply messages, not the errno field when write() or send() fails. PF_KEY message delivery is not guaranteed, especially in cases where kernel or socket buffers are exhausted and messages are dropped. McDonald, et. al. Informational [Page 8] RFC 2367 PF_KEY Key Management API July 1998 Some messages are generated by the operating system to indicate that actions need to be taken, and are not necessarily in response to any message sent down by the user. Such messages are not received by all PF_KEY sockets, but by sockets which have indicated that kernel- originated messages are to be received. These messages are special because of the expected frequency at which they will occur. Also, an implementation may further wish to restrict return messages from the kernel, in cases where not all PF_KEY sockets are in the same trust domain. Many of the normal BSD socket calls have undefined behavior on PF_KEY sockets. These include: bind(), connect(), socketpair(), accept(), getpeername(), getsockname(), ioctl(), and listen(). 1.5 Common PF_KEY Operations There are two basic ways to add a new Security Association into the kernel. The simplest is to send a single SADB_ADD message, containing all of the SA information, from the application into the kernel's Key Engine. This approach works particularly well with manual key management, which is required for IPsec, and other security protocols. The second approach to add a new Security Association into the kernel is for the application to first request a Security Parameters Index (SPI) value from the kernel using the SADB_GETSPI message and then send an SADB_UPDATE message with the complete Security Association data. This second approach works well with key management daemons when the SPI values need to be known before the entire Security Association data is known (e.g. so the SPI value can be indicated to the remote end of the key management session). An individual Security Association can be deleted using the SADB_DELETE message. Categories of SAs or the entire kernel SA table can be deleted using the SADB_FLUSH message. The SADB_GET message is used by a trusted application-layer process (e.g. routed(8) or gated(8)) to retrieve an SA (e.g. RIP SA or OSPF SA) from the kernel's Key Engine. The kernel or an application-layer can use the SADB_ACQUIRE message to request that a Security Association be created by some application-layer key management process that has registered with the kernel via an SADB_REGISTER message. This ACQUIRE message will have a sequence number associated with it. This sequence number MUST be used by followup SADB_GETSPI, SADB_UPDATE, and SADB_ADD messages, in order to keep track of which request gets its keying material. The sequence number (described below) is similar to a transaction ID in a McDonald, et. al. Informational [Page 9] RFC 2367 PF_KEY Key Management API July 1998 remote procedure call. The SADB_EXPIRE message is sent from the kernel to key management applications when the "soft lifetime" or "hard lifetime" of a Security Association has expired. Key management applications should use receipt of a soft lifetime SADB_EXPIRE message as a hint to negotiate a replacement SA so the replacement SA will be ready and in the kernel before it is needed. A SADB_DUMP message is also defined, but this is primarily intended for PF_KEY implementor debugging and is not used in ordinary operation of PF_KEY. 1.6 Differences Between PF_KEY and PF_ROUTE The following bullets are points of difference between the routing socket and PF_KEY. Programmers who are used to the routing socket semantics will find some differences in PF_KEY. * PF_KEY message errors are usually returned in PF_KEY messages instead of causing write() operations to fail and returning the error number in errno. This means that other listeners on a PF_KEY socket can be aware that requests from another process failed, which can be useful for auditing purposes. This also means that applications that fail to read PF_KEY messages cannot do error checking. An implementation MAY return the errors EINVAL, ENOMEM, and ENOBUFS by causing write() operations to fail and returning the error number in errno. This is an optimization for common error cases in which it does not make sense for any other process to receive the error. An application MUST NOT depend on such errors being set by the write() call, but it SHOULD check for such errors, and handle them in an appropriate manner. * The entire message isn't always reflected in the reply. A SADB_ADD message is an example of this. * The PID is not set by the kernel. The process that originates the message MUST set the sadb_msg_pid to its own PID. If the kernel ORIGINATES a message, it MUST set the sadb_msg_pid to 0. A reply to an original message SHOULD have the pid of the original message. (E.g. the kernel's response to an SADB_ADD SHOULD have its pid set to the pid value of the original SADB_ADD message.) McDonald, et. al. Informational [Page 10] RFC 2367 PF_KEY Key Management API July 1998 1.7 Name Space All PF_KEYv2 preprocessor symbols and structure definitions are defined as a result of including the header file . There is exactly one exception to this rule: the symbol "PF_KEY" (two exceptions if "AF_KEY" is also counted), which is defined as a result of including the header file . All PF_KEYv2 preprocessor symbols start with the prefix "SADB_" and all structure names start with "sadb_". There are exactly two exceptions to this rule: the symbol "PF_KEY_V2" and the symbol "PFKEYV2_REVISION". The symbol "PFKEYV2_REVISION" is a date-encoded value not unlike certain values defined by POSIX and X/Open. The current value for PFKEYV2_REVISION is 199806L, where 1998 is the year and 06 is the month. Inclusion of the file MUST NOT define symbols or structures in the PF_KEYv2 name space that are not described in this document without the explicit prior permission of the authors. Any symbols or structures in the PF_KEYv2 name space that are not described in this document MUST start with "SADB_X_" or "sadb_x_". An implementation that fails to obey these rules IS NOT COMPLIANT WITH THIS SPECIFICATION and MUST NOT make any claim to be. These rules also apply to any files that might be included as a result of including the file . This rule provides implementors with some assurance that they will not encounter namespace-related surprises. 1.8 On Manual Keying Not unlike the 4.4-Lite BSD PF_ROUTE socket, this interface allows an application full-reign over the security associations in a kernel that implements PF_KEY. A PF_KEY implementation MUST have some sort of manual interface to PF_KEY, which SHOULD allow all of the functionality of the programmatic interface described here. 2. PF_KEY Message Format PF_KEY messages consist of a base header followed by additional data fields, some of which may be optional. The format of the additional data is dependent on the type of message. PF_KEY messages currently do not mandate any specific ordering for non-network multi-octet fields. Unless otherwise specified (e.g. SPI values), fields MUST be in host-specific byte order. McDonald, et. al. Informational [Page 11] RFC 2367 PF_KEY Key Management API July 1998 2.1 Base Message Header Format PF_KEY messages consist of the base message header followed by security association specific data whose types and lengths are specified by a generic type-length encoding. This base header is shown below, using POSIX types. The fields are arranged primarily for alignment, and where possible, for reasons of clarity. struct sadb_msg { uint8_t sadb_msg_version; uint8_t sadb_msg_type; uint8_t sadb_msg_errno; uint8_t sadb_msg_satype; uint16_t sadb_msg_len; uint16_t sadb_msg_reserved; uint32_t sadb_msg_seq; uint32_t sadb_msg_pid; }; /* sizeof(struct sadb_msg) == 16 */ sadb_msg_version The version field of this PF_KEY message. This MUST be set to PF_KEY_V2. If this is not set to PF_KEY_V2, the write() call MAY fail and return EINVAL. Otherwise, the behavior is undetermined, given that the application might not understand the formatting of the messages arriving from the kernel. sadb_msg_type Identifies the type of message. The valid message types are described later in this document. sadb_msg_errno Should be set to zero by the sender. The responder stores the error code in this field if an error has occurred. This includes the case where the responder is in user space. (e.g. user-space negotiation fails, an errno can be returned.) sadb_msg_satype Indicates the type of security association(s). Valid Security Association types are declared in the file . The current set of Security Association types is enumerated later in this document. McDonald, et. al. Informational [Page 12] RFC 2367 PF_KEY Key Management API July 1998 sadb_msg_len Contains the total length, in 64-bit words, of all data in the PF_KEY message including the base header length and additional data after the base header, if any. This length includes any padding or extra space that might exist. Unless otherwise stated, all other length fields are also measured in 64-bit words. On user to kernel messages, this field MUST be verified against the length of the inbound message. EMSGSIZE MUST be returned if the verification fails. On kernel to user messages, a size mismatch is most likely the result of the user not providing a large enough buffer for the message. In these cases, the user application SHOULD drop the message, but it MAY try and extract what information it can out of the message. sadb_msg_reserved Reserved value. It MUST be zeroed by the sender. All fields labeled reserved later in the document have the same semantics as this field. sadb_msg_seq Contains the sequence number of this message. This field, along with sadb_msg_pid, MUST be used to uniquely identify requests to a process. The sender is responsible for filling in this field. This responsibility also includes matching the sadb_msg_seq of a request (e.g. SADB_ACQUIRE). This field is similar to a transaction ID in a remote procedure call implementation. sadb_msg_pid Identifies the process which originated this message, or which process a message is bound for. For example, if process id 2112 sends an SADB_UPDATE message to the kernel, the process MUST set this field to 2112 and the kernel will set this field to 2112 in its reply to that SADB_UPDATE message. This field, along with sadb_msg_seq, can be used to uniquely identify requests to a process. It is currently assumed that a 32-bit quantity will hold an operating system's process ID space. McDonald, et. al. Informational [Page 13] RFC 2367 PF_KEY Key Management API July 1998 2.2 Alignment of Headers and Extension Headers The base message header is a multiple of 64 bits and fields after it in memory will be 64 bit aligned if the base itself is 64 bit aligned. Some of the subsequent extension headers have 64 bit fields in them, and as a consequence need to be 64 bit aligned in an environment where 64 bit quantities need to be 64 bit aligned. The basic unit of alignment and length in PF_KEY Version 2 is 64 bits. Therefore: * All extension headers, inclusive of the sadb_ext overlay fields, MUST be a multiple of 64 bits long. * All variable length data MUST be padded appropriately such that its length in a message is a multiple of 64 bits. * All length fields are, unless otherwise specified, in units of 64 bits. * Implementations may safely access quantities of between 8 and 64 bits directly within a message without risk of alignment faults. All PF_KEYv2 structures are packed and already have all intended padding. Implementations MUST NOT insert any extra fields, including hidden padding, into any structure in this document. This forbids implementations from "extending" or "enhancing" existing headers without changing the extension header type. As a guard against such insertion of silent padding, each structure in this document is labeled with its size in bytes. The size of these structures in an implementation MUST match the size listed. 2.3 Additional Message Fields The additional data following the base header consists of various length-type-values fields. The first 32-bits are of a constant form: struct sadb_ext { uint16_t sadb_ext_len; uint16_t sadb_ext_type; }; /* sizeof(struct sadb_ext) == 4 */ sadb_ext_len Length of the extension header in 64 bit words, inclusive. McDonald, et. al. Informational [Page 14] RFC 2367 PF_KEY Key Management API July 1998 sadb_ext_type The type of extension header that follows. Values for this field are detailed later. The value zero is reserved. Types of extension headers include: Association, Lifetime(s), Address(s), Key(s), Identity(ies), Sensitivity, Proposal, and Supported. There MUST be only one instance of a extension type in a message. (e.g. Base, Key, Lifetime, Key is forbidden). An EINVAL will be returned if there are duplicate extensions within a message. Implementations MAY enforce ordering of extensions in the order presented in the EXTENSION HEADER VALUES section. If an unknown extension type is encountered, it MUST be ignored. Applications using extension headers not specified in this document MUST be prepared to work around other system components not processing those headers. Likewise, if an application encounters an unknown extension from the kernel, it must be prepared to work around it. Also, a kernel that generates extra extension header types MUST NOT _depend_ on applications also understanding extra extension header types. All extension definitions include these two fields (len and exttype) because they are instances of a generic extension (not unlike sockaddr_in and sockaddr_in6 are instances of a generic sockaddr). The sadb_ext header MUST NOT ever be present in a message without at least four bytes of extension header data following it, and, therefore, there is no problem with it being only four bytes long. All extensions documented in this section MUST be implemented by a PF_KEY implementation. 2.3.1 Association Extension The Association extension specifies data specific to a single security association. The only times this extension is not present is when control messages (e.g. SADB_FLUSH or SADB_REGISTER) are being passed and on the SADB_ACQUIRE message. struct sadb_sa { uint16_t sadb_sa_len; uint16_t sadb_sa_exttype; uint32_t sadb_sa_spi; uint8_t sadb_sa_replay; uint8_t sadb_sa_state; uint8_t sadb_sa_auth; uint8_t sadb_sa_encrypt; uint32_t sadb_sa_flags; }; McDonald, et. al. Informational [Page 15] RFC 2367 PF_KEY Key Management API July 1998 /* sizeof(struct sadb_sa) == 16 */ sadb_sa_spi The Security Parameters Index value for the security association. Although this is a 32-bit field, some types of security associations might have an SPI or key identifier that is less than 32-bits long. In this case, the smaller value shall be stored in the least significant bits of this field and the unneeded bits shall be zero. This field MUST be in network byte order. sadb_sa_replay The size of the replay window, if not zero. If zero, then no replay window is in use. sadb_sa_state The state of the security association. The currently defined states are described later in this document. sadb_sa_auth The authentication algorithm to be used with this security association. The valid authentication algorithms are described later in this document. A value of zero means that no authentication is used for this security association. sadb_sa_encrypt The encryption algorithm to be used with this security association. The valid encryption algorithms are described later in this document. A value of zero means that no encryption is used for this security association. sadb_sa_flags A bitmap of options defined for the security association. The currently defined flags are described later in this document. The kernel MUST check these values where appropriate. For example, IPsec AH with no authentication algorithm is probably an error. When used with some messages, the values in some fields in this header should be ignored. 2.3.2 Lifetime Extension The Lifetime extension specifies one or more lifetime variants for this security association. If no Lifetime extension is present the association has an infinite lifetime. An association SHOULD have a lifetime of some sort associated with it. Lifetime variants come in three varieties, HARD - indicating the hard-limit expiration, SOFT - indicating the soft-limit expiration, and CURRENT - indicating the current state of a given security association. The Lifetime McDonald, et. al. Informational [Page 16] RFC 2367 PF_KEY Key Management API July 1998 extension looks like: struct sadb_lifetime { uint16_t sadb_lifetime_len; uint16_t sadb_lifetime_exttype; uint32_t sadb_lifetime_allocations; uint64_t sadb_lifetime_bytes; uint64_t sadb_lifetime_addtime; uint64_t sadb_lifetime_usetime; }; /* sizeof(struct sadb_lifetime) == 32 */ sadb_lifetime_allocations For CURRENT, the number of different connections, endpoints, or flows that the association has been allocated towards. For HARD and SOFT, the number of these the association may be allocated towards before it expires. The concept of a connection, flow, or endpoint is system specific. sadb_lifetime_bytes For CURRENT, how many bytes have been processed using this security association. For HARD and SOFT, the number of bytes that may be processed using this security association before it expires. sadb_lifetime_addtime For CURRENT, the time, in seconds, when the association was created. For HARD and SOFT, the number of seconds after the creation of the association until it expires. For such time fields, it is assumed that 64-bits is sufficiently large to hold the POSIX time_t value. If this assumption is wrong, this field will have to be revisited. sadb_lifetime_usetime For CURRENT, the time, in seconds, when association was first used. For HARD and SOFT, the number of seconds after the first use of the association until it expires. The semantics of lifetimes are inclusive-OR, first-to-expire. This means that if values for bytes and time, or multiple times, are passed in, the first of these values to be reached will cause a lifetime expiration. McDonald, et. al. Informational [Page 17] RFC 2367 PF_KEY Key Management API July 1998 2.3.3 Address Extension The Address extension specifies one or more addresses that are associated with a security association. Address extensions for both source and destination MUST be present when an Association extension is present. The format of an Address extension is: struct sadb_address { uint16_t sadb_address_len; uint16_t sadb_address_exttype; uint8_t sadb_address_proto; uint8_t sadb_address_prefixlen; uint16_t sadb_address_reserved; }; /* sizeof(struct sadb_address) == 8 */ /* followed by some form of struct sockaddr */ The sockaddr structure SHOULD conform to the sockaddr structure of the system implementing PF_KEY. If the system has an sa_len field, so SHOULD the sockaddrs in the message. If the system has NO sa_len field, the sockaddrs SHOULD NOT have an sa_len field. All non-address information in the sockaddrs, such as sin_zero for AF_INET sockaddrs, and sin6_flowinfo for AF_INET6 sockaddrs, MUST be zeroed out. The zeroing of ports (e.g. sin_port and sin6_port) MUST be done for all messages except for originating SADB_ACQUIRE messages, which SHOULD fill them in with ports from the relevant TCP or UDP session which generates the ACQUIRE message. If the ports are non-zero, then the sadb_address_proto field, normally zero, MUST be filled in with the transport protocol's number. If the sadb_address_prefixlen is non- zero, then the address has a prefix (often used in KM access control decisions), with length specified in sadb_address_prefixlen. These additional fields may be useful to KM applications. The SRC and DST addresses for a security association MUST be in the same protocol family and MUST always be present or absent together in a message. The PROXY address MAY be in a different protocol family, and for most security protocols, represents an actual originator of a packet. (For example, the inner-packets's source address in a tunnel.) The SRC address MUST be a unicast or unspecified (e.g., INADDR_ANY) address. The DST address can be any valid destination address (unicast, multicast, or even broadcast). The PROXY address SHOULD be a unicast address (there are experimental security protocols where PROXY semantics may be different than described above). McDonald, et. al. Informational [Page 18] RFC 2367 PF_KEY Key Management API July 1998 2.3.4 Key Extension The Key extension specifies one or more keys that are associated with a security association. A Key extension will not always be present with messages, because of security risks. The format of a Key extension is: struct sadb_key { uint16_t sadb_key_len; uint16_t sadb_key_exttype; uint16_t sadb_key_bits; uint16_t sadb_key_reserved; }; /* sizeof(struct sadb_key) == 8 */ /* followed by the key data */ sadb_key_bits The length of the valid key data, in bits. A value of zero in sadb_key_bits MUST cause an error. The key extension comes in two varieties. The AUTH version is used with authentication keys (e.g. IPsec AH, OSPF MD5) and the ENCRYPT version is used with encryption keys (e.g. IPsec ESP). PF_KEY deals only with fully formed cryptographic keys, not with "raw key material". For example, when ISAKMP/Oakley is in use, the key management daemon is always responsible for transforming the result of the Diffie-Hellman computation into distinct fully formed keys PRIOR to sending those keys into the kernel via PF_KEY. This rule is made because PF_KEY is designed to support multiple security protocols (not just IP Security) and also multiple key management schemes including manual keying, which does not have the concept of "raw key material". A clean, protocol-independent interface is important for portability to different operating systems as well as for portability to different security protocols. If an algorithm defines its key to include parity bits (e.g. DES) then the key used with PF_KEY MUST also include those parity bits. For example, this means that a single DES key is always a 64-bit quantity. When a particular security protocol only requires one authentication and/or one encryption key, the fully formed key is transmitted using the appropriate key extension. When a particular security protocol requires more than one key for the same function (e.g. Triple-DES using 2 or 3 keys, and asymmetric algorithms), then those two fully formed keys MUST be concatenated together in the order used for outbound packet processing. In the case of multiple keys, the algorithm MUST be able to determine the lengths of the individual McDonald, et. al. Informational [Page 19] RFC 2367 PF_KEY Key Management API July 1998 keys based on the information provided. The total key length (when combined with knowledge of the algorithm in use) usually provides sufficient information to make this determination. Keys are always passed through the PF_KEY interface in the order that they are used for outbound packet processing. For inbound processing, the correct order that keys are used might be different from this canonical concatenation order used with the PF_KEY interface. It is the responsibility of the implementation to use the keys in the correct order for both inbound and outbound processing. For example, consider a pair of nodes communicating unicast using an ESP three-key Triple-DES Security Association. Both the outbound SA on the sender node, and the inbound SA on the receiver node will contain key-A, followed by key-B, followed by key-C in their respective ENCRYPT key extensions. The outbound SA will use key-A first, followed by key-B, then key-C when encrypting. The inbound SA will use key-C, followed by key-B, then key-A when decrypting. (NOTE: We are aware that 3DES is actually encrypt-decrypt-encrypt.) The canonical ordering of key-A, key-B, key-C is used for 3DES, and should be documented. The order of "encryption" is the canonical order for this example. [Sch96] The key data bits are arranged most-significant to least significant. For example, a 22-bit key would take up three octets, with the least significant two bits not containing key material. Five additional octets would then be used for padding to the next 64-bit boundary. While not directly related to PF_KEY, there is a user interface issue regarding odd-digit hexadecimal representation of keys. Consider the example of the 16-bit number: 0x123 That will require two octets of storage. In the absence of other information, however, unclear whether the value shown is stored as: 01 23 OR 12 30 It is the opinion of the authors that the former (0x123 == 0x0123) is the better way to interpret this ambiguity. Extra information (for example, specifying 0x0123 or 0x1230, or specifying that this is only a twelve-bit number) would solve this problem. McDonald, et. al. Informational [Page 20] RFC 2367 PF_KEY Key Management API July 1998 2.3.5 Identity Extension The Identity extension contains endpoint identities. This information is used by key management to select the identity certificate that is used in negotiations. This information may also be provided by a kernel to network security aware applications to identify the remote entity, possibly for access control purposes. If this extension is not present, key management MUST assume that the addresses in the Address extension are the only identities for this Security Association. The Identity extension looks like: struct sadb_ident { uint16_t sadb_ident_len; uint16_t sadb_ident_exttype; uint16_t sadb_ident_type; uint16_t sadb_ident_reserved; uint64_t sadb_ident_id; }; /* sizeof(struct sadb_ident) == 16 */ /* followed by the identity string, if present */ sadb_ident_type The type of identity information that follows. Currently defined identity types are described later in this document. sadb_ident_id An identifier used to aid in the construction of an identity string if none is present. A POSIX user id value is one such identifier that will be used in this field. Use of this field is described later in this document. A C string containing a textual representation of the identity information optionally follows the sadb_ident extension. The format of this string is determined by the value in sadb_ident_type, and is described later in this document. 2.3.6 Sensitivity Extension The Sensitivity extension contains security labeling information for a security association. If this extension is not present, no sensitivity-related data can be obtained from this security association. If this extension is present, then the need for explicit security labeling on the packet is obviated. struct sadb_sens { uint16_t sadb_sens_len; uint16_t sadb_sens_exttype; McDonald, et. al. Informational [Page 21] RFC 2367 PF_KEY Key Management API July 1998 uint32_t sadb_sens_dpd; uint8_t sadb_sens_sens_level; uint8_t sadb_sens_sens_len; uint8_t sadb_sens_integ_level; uint8_t sadb_sens_integ_len; uint32_t sadb_sens_reserved; }; /* sizeof(struct sadb_sens) == 16 */ /* followed by: uint64_t sadb_sens_bitmap[sens_len]; uint64_t sadb_integ_bitmap[integ_len]; */ sadb_sens_dpd Describes the protection domain, which allows interpretation of the levels and compartment bitmaps. sadb_sens_sens_level The sensitivity level. sadb_sens_sens_len The length, in 64 bit words, of the sensitivity bitmap. sadb_sens_integ_level The integrity level. sadb_sens_integ_len The length, in 64 bit words, of the integrity bitmap. This sensitivity extension is designed to support the Bell-LaPadula [BL74] security model used in compartmented-mode or multi-level secure systems, the Clark-Wilson [CW87] commercial security model, and/or the Biba integrity model [Biba77]. These formal models can be used to implement a wide variety of security policies. The definition of a particular security policy is outside the scope of this document. Each of the bitmaps MUST be padded to a 64-bit boundary if they are not implicitly 64-bit aligned. 2.3.7 Proposal Extension The Proposal extension contains a "proposed situation" of algorithm preferences. It looks like: struct sadb_prop { uint16_t sadb_prop_len; uint16_t sadb_prop_exttype; uint8_t sadb_prop_replay; uint8_t sadb_prop_reserved[3]; }; /* sizeof(struct sadb_prop) == 8 */ McDonald, et. al. Informational [Page 22] RFC 2367 PF_KEY Key Management API July 1998 /* followed by: struct sadb_comb sadb_combs[(sadb_prop_len * sizeof(uint64_t) - sizeof(struct sadb_prop)) / sizeof(struct sadb_comb)]; */ Following the header is a list of proposed parameter combinations in preferential order. The values in these fields have the same definition as the fields those values will move into if the combination is chosen. NOTE: Some algorithms in some security protocols will have variable IV lengths per algorithm. Variable length IVs are not supported by PF_KEY v2. If they were, however, proposed IV lengths would go in the Proposal Extension. These combinations look like: struct sadb_comb { uint8_t sadb_comb_auth; uint8_t sadb_comb_encrypt; uint16_t sadb_comb_flags; uint16_t sadb_comb_auth_minbits; uint16_t sadb_comb_auth_maxbits; uint16_t sadb_comb_encrypt_minbits; uint16_t sadb_comb_encrypt_maxbits; uint32_t sadb_comb_reserved; uint32_t sadb_comb_soft_allocations; uint32_t sadb_comb_hard_allocations; uint64_t sadb_comb_soft_bytes; uint64_t sadb_comb_hard_bytes; uint64_t sadb_comb_soft_addtime; uint64_t sadb_comb_hard_addtime; uint64_t sadb_comb_soft_usetime; uint64_t sadb_comb_hard_usetime; }; /* sizeof(struct sadb_comb) == 72 */ sadb_comb_auth If this combination is accepted, this will be the value of sadb_sa_auth. sadb_comb_encrypt If this combination is accepted, this will be the value of sadb_sa_encrypt. McDonald, et. al. Informational [Page 23] RFC 2367 PF_KEY Key Management API July 1998 sadb_comb_auth_minbits; sadb_comb_auth_maxbits; The minimum and maximum acceptable authentication key lengths, respectably, in bits. If sadb_comb_auth is zero, both of these values MUST be zero. If sadb_comb_auth is nonzero, both of these values MUST be nonzero. If this combination is accepted, a value between these (inclusive) will be stored in the sadb_key_bits field of KEY_AUTH. The minimum MUST NOT be greater than the maximum. sadb_comb_encrypt_minbits; sadb_comb_encrypt_maxbits; The minimum and maximum acceptable encryption key lengths, respectably, in bits. If sadb_comb_encrypt is zero, both of these values MUST be zero. If sadb_comb_encrypt is nonzero, both of these values MUST be nonzero. If this combination is accepted, a value between these (inclusive) will be stored in the sadb_key_bits field of KEY_ENCRYPT. The minimum MUST NOT be greater than the maximum. sadb_comb_soft_allocations sadb_comb_hard_allocations If this combination is accepted, these are proposed values of sadb_lifetime_allocations in the SOFT and HARD lifetimes, respectively. sadb_comb_soft_bytes sadb_comb_hard_bytes If this combination is accepted, these are proposed values of sadb_lifetime_bytes in the SOFT and HARD lifetimes, respectively. sadb_comb_soft_addtime sadb_comb_hard_addtime If this combination is accepted, these are proposed values of sadb_lifetime_addtime in the SOFT and HARD lifetimes, respectively. sadb_comb_soft_usetime sadb_comb_hard_usetime If this combination is accepted, these are proposed values of sadb_lifetime_usetime in the SOFT and HARD lifetimes, respectively. McDonald, et. al. Informational [Page 24] RFC 2367 PF_KEY Key Management API July 1998 Each combination has an authentication and encryption algorithm, which may be 0, indicating none. A combination's flags are the same as the flags in the Association extension. The minimum and maximum key lengths (which are in bits) are derived from possible a priori policy decisions, along with basic properties of the algorithm. Lifetime attributes are also included in a combination, as some algorithms may know something about their lifetimes and can suggest lifetime limits. 2.3.8 Supported Algorithms Extension The Supported Algorithms extension contains a list of all algorithms supported by the system. This tells key management what algorithms it can negotiate. Available authentication algorithms are listed in the SUPPORTED_AUTH extension and available encryption algorithms are listed in the SUPPORTED_ENCRYPT extension. The format of these extensions is: struct sadb_supported { uint16_t sadb_supported_len; uint16_t sadb_supported_exttype; uint32_t sadb_supported_reserved; }; /* sizeof(struct sadb_supported) == 8 */ /* followed by: struct sadb_alg sadb_algs[(sadb_supported_len * sizeof(uint64_t) - sizeof(struct sadb_supported)) / sizeof(struct sadb_alg)]; */ This header is followed by one or more algorithm descriptions. An algorithm description looks like: struct sadb_alg { uint8_t sadb_alg_id; uint8_t sadb_alg_ivlen; uint16_t sadb_alg_minbits; uint16_t sadb_alg_maxbits; uint16_t sadb_alg_reserved; }; /* sizeof(struct sadb_alg) == 8 */ sadb_alg_id The algorithm identification value for this algorithm. This is the value that is stored in sadb_sa_auth or sadb_sa_encrypt if this algorithm is selected. McDonald, et. al. Informational [Page 25] RFC 2367 PF_KEY Key Management API July 1998 sadb_alg_ivlen The length of the initialization vector to be used for the algorithm. If an IV is not needed, this value MUST be set to zero. sadb_alg_minbits The minimum acceptable key length, in bits. A value of zero is invalid. sadb_alg_maxbits The maximum acceptable key length, in bits. A value of zero is invalid. The minimum MUST NOT be greater than the maximum. 2.3.9 SPI Range Extension One PF_KEY message, SADB_GETSPI, might need a range of acceptable SPI values. This extension performs such a function. struct sadb_spirange { uint16_t sadb_spirange_len; uint16_t sadb_spirange_exttype; uint32_t sadb_spirange_min; uint32_t sadb_spirange_max; uint32_t sadb_spirange_reserved; }; /* sizeof(struct sadb_spirange) == 16 */ sadb_spirange_min The minimum acceptable SPI value. sadb_spirange_max The maximum acceptable SPI value. The maximum MUST be greater than or equal to the minimum. McDonald, et. al. Informational [Page 26] RFC 2367 PF_KEY Key Management API July 1998 2.4 Illustration of Message Layout The following shows how the octets are laid out in a PF_KEY message. Optional fields are indicated as such. The base header is as follows: 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 +---------------+---------------+---------------+---------------+ | ...version | sadb_msg_type | sadb_msg_errno| ...msg_satype | +---------------+---------------+---------------+---------------+ | sadb_msg_len | sadb_msg_reserved | +---------------+---------------+---------------+---------------+ | sadb_msg_seq | +---------------+---------------+---------------+---------------+ | sadb_msg_pid | +---------------+---------------+---------------+---------------+ The base header may be followed by one or more of the following extension fields, depending on the values of various base header fields. The following fields are ordered such that if they appear, they SHOULD appear in the order presented below. An extension field MUST not be repeated. If there is a situation where an extension MUST be repeated, it should be brought to the attention of the authors. The Association extension 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 +---------------+---------------+---------------+---------------+ | sadb_sa_len | sadb_sa_exttype | +---------------+---------------+---------------+---------------+ | sadb_sa_spi | +---------------+---------------+---------------+---------------+ | ...replay | sadb_sa_state | sadb_sa_auth |sadb_sa_encrypt| +---------------+---------------+---------------+---------------+ | sadb_sa_flags | +---------------+---------------+---------------+---------------+ The Lifetime extension +---------------+---------------+---------------+---------------+ | sadb_lifetime_len | sadb_lifetime_exttype | +---------------+---------------+---------------+---------------+ | sadb_lifetime_allocations | +---------------+---------------+---------------+---------------+ McDonald, et. al. Informational [Page 27] RFC 2367 PF_KEY Key Management API July 1998 +---------------+---------------+---------------+---------------+ | sadb_lifetime_bytes | | (64 bits) | +---------------+---------------+---------------+---------------+ | sadb_lifetime_addtime | | (64 bits) | +---------------+---------------+---------------+---------------+ | sadb_lifetime_usetime | | (64 bits) | +---------------+---------------+---------------+---------------+ The Address extension +---------------+---------------+---------------+---------------+ | sadb_address_len | sadb_address_exttype | +---------------+---------------+---------------+---------------+ | _address_proto| ..._prefixlen | sadb_address_reserved | +---------------+---------------+----