To retrieve or manipulate management information using the third
version of the Simple Network Management Protocol (SNMPv3) [RFC3410],
it is necessary to know the identifier of the remote SNMP protocol
engine, the so-called snmpEngineID [RFC3411]. While an appropriate
snmpEngineID can in principle be configured on each management
application for each SNMP agent, it is often desirable to discover
the snmpEngineID automatically.
This document introduces a discovery mechanism that can be used to
learn the snmpEngineID of a remote SNMP protocol engine. The
proposed mechanism is independent of the features provided by SNMP
security models. The mechanism has been designed to coexist with
discovery mechanisms that may exist in SNMP security models, such as
the authoritative engine identifier discovery of the User-based
Security Model (USM) of SNMP [RFC3414].
This document updates RFC 3411 [RFC3411] by clarifying the IANA rules
for the maintenance of the SnmpEngineID format registry.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Within an administrative domain, an SNMP engine is uniquely
identified by an snmpEngineID value [RFC3411]. An SNMP entity, which
consists of an SNMP engine and several SNMP applications, may provide
access to multiple contexts.
An SNMP context is a collection of management information accessible
by an SNMP entity. An item of management information may exist in
more than one context and an SNMP entity potentially has access to
many contexts [RFC3411]. A context is identified by the snmpEngineID
value of the entity hosting the management information (also called a
contextEngineID) and a context name that identifies the specific
context (also called a contextName).
To identify an individual item of management information within an
administrative domain, a four tuple is used consisting of
1. a contextEngineID,
2. a contextName,
3. an object type, and
4. its instance identification.
The last two elements are encoded in an object identifier (OID)
value. The contextName is a character string (following the
SnmpAdminString textual convention of the SNMP-FRAMEWORK-MIB
[RFC3411]) while the contextEngineID is an octet string constructed
according to the rules defined as part of the SnmpEngineID textual
convention of the SNMP-FRAMEWORK-MIB [RFC3411].
The SNMP protocol operations and the protocol data units (PDUs)
operate on OIDs and thus deal with object types and instances
[RFC3416]. The SNMP architecture [RFC3411] introduces the concept of
a scopedPDU as a data structure containing a contextEngineID, a
contextName, and a PDU. The SNMP version 3 (SNMPv3) message format
uses ScopedPDUs to exchange management information [RFC3412].
Within the SNMP framework, contextEngineIDs serve as end-to-end
identifiers. This becomes important in situations where SNMP proxies
are deployed to translate between protocol versions or to cross
middleboxes such as network address translators. In addition,
snmpEngineIDs separate the identification of an SNMP engine from the
transport addresses used to communicate with an SNMP engine. This
property can be used to correlate management information easily, even
in situations where multiple different transports were used to
retrieve the information or where transport addresses can change
To retrieve data from an SNMPv3 agent, it is necessary to know the
appropriate contextEngineID. The User-based Security Model (USM) of
SNMPv3 provides a mechanism to discover the snmpEngineID of the
remote SNMP engine, since this is needed for security processing
reasons. The discovered snmpEngineID can subsequently be used as a
contextEngineID in a ScopedPDU to access management information local
to the remote SNMP engine. Other security models, such as the
Transport Security Model (TSM) [TSM], lack such a procedure and may
use the discovery mechanism defined in this memo.
The proposed discovery mechanism consists of two parts, namely (i)
the definition of a special well-known snmpEngineID value, called the
localEngineID, which always refers to a local default context, and
(ii) the definition of a procedure to acquire the snmpEngineID scalar
of the SNMP-FRAMEWORK-MIB [RFC3411] using the special well-known
local localEngineID value.
3.1. Local EngineID
An SNMP command responder implementing this specification MUST
register their pduTypes using the localEngineID snmpEngineID value
(defined below) by invoking the registerContextEngineID() Abstract
Service Interface (ASI) defined in RFC 3412 [RFC3412]. This
registration is done in addition to the normal registration under the
SNMP engine's snmpEngineID. This is consistent with the SNMPv3
specifications since they explicitly allow registration of multiple
engineIDs and multiple pduTypes [RFC3412].
The SnmpEngineID textual convention [RFC3411] defines that an
snmpEngineID value MUST be between 5 and 32 octets long. This
specification proposes to use the variable length format 3) of the
SnmpEngineID textual convention and to allocate the reserved, unused
format value 6, using the enterprise ID 0 for the localEngineID. An
ASN.1 definition for localEngineID would look like this:
localEngineID OCTET STRING ::= '8000000006'H
The localEngineID value always provides access to the default context
of an SNMP engine. Note that the localEngineID value is intended to
be used as a special value for the contextEngineID field in the
ScopedPDU. It MUST NOT be used as a value to identify an SNMP
engine; that is, this value MUST NOT be used in the snmpEngineID.0
scalar [RFC3418] or in the msgAuthoritativeEngineID field in the
securityParameters of the User-based Security Model (USM) [RFC3414].
3.2. EngineID Discovery
Discovery of the snmpEngineID is done by sending a Read Class
protocol operation (see Section 2.8 of [RFC3411]) to retrieve the
snmpEngineID scalar using the localEngineID defined above as a
contextEngineID value. Implementations SHOULD only perform this
discovery step when it is needed. In particular, if security models
are used that already discover the remote snmpEngineID (such as USM),
then no further discovery is necessary. The same is true in
situations where the application already knows a suitable
The procedure to discover the snmpEngineID of a remote SNMP engine
can be described as follows:
1. Check whether a suitable contextEngineID value is already known.
If yes, use the provided contextEngineID value and stop the
2. Check whether the selected security model supports discovery of
the remote snmpEngineID (e.g., USM with its discovery mechanism).
If yes, let the security model perform the discovery. If the
remote snmpEngineID value has been successfully determined,
assign it to the contextEngineID and stop the discovery
3. Send a Read Class operation to the remote SNMP engine using the
localEngineID value as the contextEngineID in order to retrieve
the scalar snmpEngineID.0 of the SNMP-FRAMEWORK-MIB [RFC3411].
If successful, set the contextEngineID to the retrieved value and
stop the discovery procedure.
4. Return an error indication that a suitable contextEngineID could
not be discovered.
The procedure outlined above is an example and can be modified to
retrieve more variables in step 3, such as the sysObjectID.0 scalar
or the snmpSetSerialNo.0 scalar of the SNMPv2-MIB [RFC3418].
4. IANA Considerations
RFC 3411 requested that IANA create a registry for SnmpEngineID
formats. However, RFC 3411 did not ask IANA to record the initial
assignments made by RFC 3411 nor did RFC 3411 spell out the precise
allocation rules. To address this issue, the following rules are
IANA maintains a registry for SnmpEngineID formats. The first four
octets of an SnmpEngineID carry an enterprise number, while the fifth
octet in a variable length SnmpEngineID value, called the format
octet, indicates how the following octets are formed. The following
format values were allocated in [RFC3411]:
Format Description References
------- ----------- ----------
0 reserved, unused [RFC3411]
1 IPv4 address [RFC3411]
2 IPv6 address [RFC3411]
3 MAC address [RFC3411]
4 administratively assigned text [RFC3411]
5 administratively assigned octets [RFC3411]
6-127 reserved, unused [RFC3411]
128-255 enterprise specific [RFC3411]
IANA can assign new format values out of the originally assigned and
reserved number space 1-127. For new assignments in this number
space, a specification is required as per [RFC5226]. The number
space 128-255 is enterprise specific and is not controlled by IANA.
Per this document, IANA has made the following assignment:
Format Description References
------- ----------- ----------
6 local engine [RFC5343]
5. Security Considerations
SNMP version 3 (SNMPv3) provides cryptographic security to protect
devices from unauthorized access. This specification recommends use
of the security services provided by SNMPv3. In particular, it is
RECOMMENDED to protect the discovery exchange.
An snmpEngineID can contain information such as a device's MAC
address, IPv4 address, IPv6 address, or administratively assigned
text. An attacker located behind a router / firewall / network
address translator may not be able to obtain this information
directly, and he therefore might discover snmpEngineID values in
order to obtain this kind of device information.
In many environments, making snmpEngineID values accessible via a
security level of noAuthNoPriv will benefit legitimate tools that try
to algorithmically determine some basic information about a device.
For this reason, the default View-based Access Control Model (VACM)
configuration in Appendix A of RFC 3415 [RFC3415] gives noAuthNoPriv
read access to the snmpEngineID. Furthermore, the USM discovery
mechanism defined in RFC 3414 [RFC3414] uses unprotected messages and
reveals snmpEngineID values.
In highly secure environments, snmpEngineID values can be protected
by using the discovery mechanism described in this document together
with a security model that does not exchange cleartext SNMP messages,
such as the Transport Security Model (TSM) [TSM].
The isAccessAllowed() abstract service primitive of the SNMP access
control subsystem does not take the contextEngineID into account when
checking access rights [RFC3411]. As a consequence, it is not
possible to define a special view for context engineID discovery. A
request with a localEngineID is thus treated like a request with the
correct snmpEngineID by the access control subsystem. This is inline
with the SNMPv3 design where the authenticated identity is the
securityName (together with the securityModel and securityLevel
information), and transport addresses or knowledge of contextEngineID
values do not impact the access-control decision.
Dave Perkins suggested the introduction of a "local" contextEngineID
during the interim meeting of the ISMS (Integrated Security Model for
SNMP) working group in Boston, 2006. Joe Fernandez, David
Harrington, Dan Romascanu, and Bert Wijnen provided helpful review
and feedback, which helped to improve this document.
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
[RFC3412] Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
"Message Processing and Dispatching for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3412,
[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.
[RFC3416] Presuhn, R., "Version 2 of the Protocol Operations for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3416, December 2002.
[RFC3418] Presuhn, R., "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3418, December 2002.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
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.
[RFC3415] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based
Access Control Model (VACM) for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3415,
[TSM] Harrington, D., "Transport Security Model for SNMP", Work
in Progress, July 2008.
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