Network Working Group G. Waters, Editor
Request for Comments: 1910 Bell-Northern Research Ltd.
Category: Experimental February 1996
User-based Security Model for SNMPv2
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Table of Contents
1. Introduction ................................................ 2
1.1 Threats .................................................... 3
1.2 Goals and Constraints ...................................... 4
1.3 Security Services .......................................... 5
1.4 Mechanisms ................................................. 5
1.4.1 Digest Authentication Protocol ........................... 7
1.4.2 Symmetric Encryption Protocol ............................ 8
2. Elements of the Model ....................................... 10
2.1 SNMPv2 Users ............................................... 10
2.2 Contexts and Context Selectors ............................. 11
2.3 Quality of Service (qoS) ................................... 13
2.4 Access Policy .............................................. 13
2.5 Replay Protection .......................................... 13
2.5.1 agentID .................................................. 14
2.5.2 agentBoots and agentTime ................................. 14
2.5.3 Time Window .............................................. 15
2.6 Error Reporting ............................................ 15
2.7 Time Synchronization ....................................... 16
2.8 Proxy Error Propagation .................................... 16
2.9 SNMPv2 Messages Using this Model ........................... 16
2.10 Local Configuration Datastore (LCD) ....................... 18
3. Elements of Procedure ....................................... 19
3.1 Generating a Request or Notification ....................... 19
3.2 Processing a Received Communication ........................ 20
3.2.1 Additional Details ....................................... 28
3.2.1.1 ASN.1 Parsing Errors ................................... 28
3.2.1.2 Incorrectly Encoded Parameters ......................... 29
3.2.1.3 Generation of a Report PDU ............................. 29
3.2.1.4 Cache Timeout .......................................... 29
3.3 Generating a Response ...................................... 30
4. Discovery ................................................... 30
5. Definitions ................................................. 31
4.1 The USEC Basic Group ....................................... 32
4.2 Conformance Information .................................... 35
4.2.1 Compliance Statements .................................... 35
4.2.2 Units of Conformance ..................................... 35
6. Security Considerations ..................................... 36
6.1 Recommended Practices ...................................... 36
6.2 Defining Users ............................................. 37
6.3 Conformance ................................................ 38
7. Editor's Address ............................................ 38
8. Acknowledgements ............................................ 39
9. References .................................................. 39
Appendix A Installation ........................................ 41
Appendix A.1 Agent Installation Parameters ..................... 41
Appendix A.2 Password to Key Algorithm ......................... 43
Appendix A.3 Password to Key Sample ............................ 44
1. Introduction
A management system contains: several (potentially many) nodes, each
with a processing entity, termed an agent, which has access to
management instrumentation; at least one management station; and, a
management protocol, used to convey management information between
the agents and management stations. Operations of the protocol are
carried out under an administrative framework which defines
authentication, authorization, access control, and privacy policies.
Management stations execute management applications which monitor and
control managed elements. Managed elements are devices such as
hosts, routers, terminal servers, etc., which are monitored and
controlled via access to their management information.
The Administrative Infrastructure for SNMPv2 document [1] defines an
administrative framework which realizes effective management in a
variety of configurations and environments.
In this administrative framework, a security model defines the
mechanisms used to achieve an administratively-defined level of
security for protocol interactions. Although many such security
models might be defined, it is the purpose of this document, User-
based Security Model for SNMPv2, to define the first, and, as of this
writing, only, security model for this administrative framework.
This administrative framework includes the provision of an access
control model. The enforcement of access rights requires the means
to identify the entity on whose behalf a request is generated. This
SNMPv2 security model identifies an entity on whose behalf an SNMPv2
message is generated as a "user".
1.1. Threats
Several of the classical threats to network protocols are applicable
to the network management problem and therefore would be applicable
to any SNMPv2 security model. Other threats are not applicable to
the network management problem. This section discusses principal
threats, secondary threats, and threats which are of lesser
importance.
The principal threats against which this SNMPv2 security model should
provide protection are:
Modification of Information
The modification threat is the danger that some unauthorized entity
may alter in-transit SNMPv2 messages generated on behalf of an
authorized user in such a way as to effect unauthorized management
operations, including falsifying the value of an object.
Masquerade
The masquerade threat is the danger that management operations not
authorized for some user may be attempted by assuming the identity
of another user that has the appropriate authorizations.
Two secondary threats are also identified. The security protocols
defined in this memo do provide protection against:
Message Stream Modification
The SNMPv2 protocol is typically based upon a connectionless
transport service which may operate over any subnetwork service.
The re-ordering, delay or replay of messages can and does occur
through the natural operation of many such subnetwork services.
The message stream modification threat is the danger that messages
may be maliciously re-ordered, delayed or replayed to an extent
which is greater than can occur through the natural operation of a
subnetwork service, in order to effect unauthorized management
operations.
Disclosure
The disclosure threat is the danger of eavesdropping on the
exchanges between managed agents and a management station.
Protecting against this threat may be required as a matter of local
policy.
There are at least two threats that an SNMPv2 security protocol need
not protect against. The security protocols defined in this memo do
not provide protection against:
Denial of Service
An SNMPv2 security protocol need not attempt to address the broad
range of attacks by which service on behalf of authorized users is
denied. Indeed, such denial-of-service attacks are in many cases
indistinguishable from the type of network failures with which any
viable network management protocol must cope as a matter of course.
Traffic Analysis
In addition, an SNMPv2 security protocol need not attempt to
address traffic analysis attacks. Indeed, many traffic patterns
are predictable - agents may be managed on a regular basis by a
relatively small number of management stations - and therefore
there is no significant advantage afforded by protecting against
traffic analysis.
1.2. Goals and Constraints
Based on the foregoing account of threats in the SNMP network
management environment, the goals of this SNMPv2 security model are
as follows.
(1) The protocol should provide for verification that each received
SNMPv2 message has not been modified during its transmission
through the network in such a way that an unauthorized management
operation might result.
(2) The protocol should provide for verification of the identity of the
user on whose behalf a received SNMPv2 message claims to have been
generated.
(3) The protocol should provide for detection of received SNMPv2
messages, which request or contain management information, whose
time of generation was not recent.
(4) The protocol should provide, when necessary, that the contents of
each received SNMPv2 message are protected from disclosure.
In addition to the principal goal of supporting secure network
management, the design of this SNMPv2 security model is also
influenced by the following constraints:
(1) When the requirements of effective management in times of network
stress are inconsistent with those of security, the design should
prefer the former.
(2) Neither the security protocol nor its underlying security
mechanisms should depend upon the ready availability of other
network services (e.g., Network Time Protocol (NTP) or key
management protocols).
(3) A security mechanism should entail no changes to the basic SNMP
network management philosophy.
1.3. Security Services
The security services necessary to support the goals of an SNMPv2
security model are as follows.
Data Integrity
is the provision of the property that data has not been altered or
destroyed in an unauthorized manner, nor have data sequences been
altered to an extent greater than can occur non-maliciously.
Data Origin Authentication
is the provision of the property that the claimed identity of the
user on whose behalf received data was originated is corroborated.
Data Confidentiality
is the provision of the property that information is not made
available or disclosed to unauthorized individuals, entities, or
processes.
For the protocols specified in this memo, it is not possible to
assure the specific originator of a received SNMPv2 message; rather,
it is the user on whose behalf the message was originated that is
authenticated.
For these protocols, it not possible to obtain data integrity without
data origin authentication, nor is it possible to obtain data origin
authentication without data integrity. Further, there is no
provision for data confidentiality without both data integrity and
data origin authentication.
The security protocols used in this memo are considered acceptably
secure at the time of writing. However, the procedures allow for new
authentication and privacy methods to be specified at a future time
if the need arises.
1.4. Mechanisms
The security protocols defined in this memo employ several types of
mechanisms in order to realize the goals and security services
described above:
- In support of data integrity, a message digest algorithm is
required. A digest is calculated over an appropriate portion of an
SNMPv2 message and included as part of the message sent to the
recipient.
- In support of data origin authentication and data integrity, a
secret value is both inserted into, and appended to, the SNMPv2
message prior to computing the digest; the inserted value
overwritten prior to transmission, and the appended value is not
transmitted. The secret value is shared by all SNMPv2 entities
authorized to originate messages on behalf of the appropriate user.
- To protect against the threat of message delay or replay (to an
extent greater than can occur through normal operation), a set of
time (at the agent) indicators and a request-id are included in
each message generated. An SNMPv2 agent evaluates the time
indicators to determine if a received message is recent. An SNMPv2
manager evaluates the time indicators to ensure that a received
message is at least as recent as the last message it received from
the same source. An SNMPv2 manager uses received authentic
messages to advance its notion of time (at the agent). An SNMPv2
manager also evaluates the request-id in received Response messages
and discards messages which do not correspond to outstanding
requests.
These mechanisms provide for the detection of messages whose time
of generation was not recent in all but one circumstance; this
circumstance is the delay or replay of a Report message (sent to a
manager) when the manager has has not recently communicated with
the source of the Report message. In this circumstance, the
detection guarantees only that the Report message is more recent
than the last communication between source and destination of the
Report message. However, Report messages do not request or contain
management information, and thus, goal #3 in Section 1.2 above is
met; further, Report messages can at most cause the manager to
advance its notion of time (at the agent) by less than the proper
amount.
This protection against the threat of message delay or replay does
not imply nor provide any protection against unauthorized deletion
or suppression of messages. Other mechanisms defined independently
of the security protocol can also be used to detect the re-
ordering, replay, deletion, or suppression of messages containing
set operations (e.g., the MIB variable snmpSetSerialNo [15]).
- In support of data confidentiality, an encryption algorithm is
required. An appropriate portion of the message is encrypted prior
to being transmitted.
1.4.1. Digest Authentication Protocol
The Digest Authentication Protocol defined in this memo provides for:
- verifying the integrity of a received message (i.e., the message
received is the message sent).
The integrity of the message is protected by computing a digest
over an appropriate portion of a message. The digest is computed
by the originator of the message, transmitted with the message, and
verified by the recipient of the message.
- verifying the user on whose behalf the message was generated.
A secret value known only to SNMPv2 entities authorized to generate
messages on behalf of this user is both inserted into, and appended
to, the message prior to the digest computation. Thus, the
verification of the user is implicit with the verification of the
digest. (Note that the use of two copies of the secret, one near
the start and one at the end, is recommended by [14].)
- verifying that a message sent to/from one SNMPv2 entity cannot be
replayed to/as-if-from another SNMPv2 entity.
Included in each message is an identifier unique to the SNMPv2
agent associated with the sender or intended recipient of the
message. Also, each message containing a Response PDU contains a
request-id which associates the message to a recently generated
request.
A Report message sent by one SNMPv2 agent to one SNMPv2 manager can
potentially be replayed to another SNMPv2 manager. However, Report
messages do not request or contain management information, and
thus, goal #3 in Section 1.2 above is met; further, Report messages
can at most cause the manager to advance its notion of time (at the
agent) by less than the correct amount.
- detecting messages which were not recently generated.
A set of time indicators are included in the message, indicating
the time of generation. Messages (other than those containing
Report PDUs) without recent time indicators are not considered
authentic. In addition, messages containing Response PDUs have a
request-id; if the request-id does not match that of a recently
generated request, then the message is not considered to be
authentic.
A Report message sent by an SNMPv2 agent can potentially be
replayed at a later time to an SNMPv2 manager which has not
recently communicated with that agent. However, Report messages do
not request or contain management information, and thus, goal #3 in
Section 1.2 above is met; further, Report messages can at most
cause the manager to advance its notion of time (at the agent) by
less than the correct amount.
This protocol uses the MD5 [3] message digest algorithm. A 128-bit
digest is calculated over the designated portion of an SNMPv2 message
and included as part of the message sent to the recipient. The size
of both the digest carried in a message and the private
authentication key is 16 octets.
This memo allows the same user to be defined on multiple SNMPv2
agents and managers. Each SNMPv2 agent maintains a value, agentID,
which uniquely identifies the agent. This value is included in each
message sent to/from that agent. Messages sent from a SNMPv2 dual-
role entity [1] to a SNMPv2 manager include the agentID value
maintained by the dual-role entity's agent. On receipt of a message,
an agent checks the value to ensure it is the intended recipient, and
a manager uses the value to ensure that the message is processed
using the correct state information.
Each SNMPv2 agent maintains two values, agentBoots and agentTime,
which taken together provide an indication of time at that agent.
Both of these values are included in an authenticated message sent
to/received from that agent. Authenticated messages sent from a
SNMPv2 dual-role entity to a SNMPv2 manager include the agentBoots
and agentTime values maintained by the dual-role entity's agent. On
receipt, the values are checked to ensure that the indicated time is
within a time window of the current time. The time window represents
an administrative upper bound on acceptable delivery delay for
protocol messages.
For an SNMPv2 manager to generate a message which an agent will
accept as authentic, and to verify that a message received from that
agent is authentic, that manager must first achieve time
synchronization with that agent. Similarly, for a manger to verify
that a message received from an SNMPv2 dual-role entity is authentic,
that manager must first achieve time synchronization with the dual-
role entity's agent.
1.4.2. Symmetric Encryption Protocol
The Symmetric Encryption Protocol defined in this memo provides
support for data confidentiality through the use of the Data
Encryption Standard (DES) in the Cipher Block Chaining mode of
operation. The designated portion of an SNMPv2 message is encrypted
and included as part of the message sent to the recipient.
Two organizations have published specifications defining the DES: the
National Institute of Standards and Technology (NIST) [5] and the
American National Standards Institute [6]. There is a companion
Modes of Operation specification for each definition (see [7] and
[8], respectively).
The NIST has published three additional documents that implementors
may find useful.
- There is a document with guidelines for implementing and using the
DES, including functional specifications for the DES and its modes
of operation [9].
- There is a specification of a validation test suite for the DES
[10]. The suite is designed to test all aspects of the DES and is
useful for pinpointing specific problems.
- There is a specification of a maintenance test for the DES [11].
The test utilizes a minimal amount of data and processing to test
all components of the DES. It provides a simple yes-or-no
indication of correct operation and is useful to run as part of an
initialization step, e.g., when a computer reboots.
This Symmetric Encryption Protocol specifies that the size of the
privacy key is 16 octets, of which the first 8 octets are a DES key
and the second 8 octets are a DES Initialization Vector. The 64-bit
DES key in the first 8 octets of the private key is a 56 bit quantity
used directly by the algorithm plus 8 parity bits - arranged so that
one parity bit is the least significant bit of each octet. The
setting of the parity bits is ignored by this protocol.
The length of an octet sequence to be encrypted by the DES must be an
integral multiple of 8. When encrypting, the data is padded at the
end as necessary; the actual pad value is irrelevant.
If the length of the octet sequence to be decrypted is not an
integral multiple of 8 octets, the processing of the octet sequence
is halted and an appropriate exception noted. When decrypting, the
padding is ignored.
2. Elements of the Model
This section contains definitions required to realize the security
model defined by this memo.
2.1. SNMPv2 Users
Management operations using this security model make use of a defined
set of user identities. For any SNMPv2 user on whose behalf
management operations are authorized at a particular SNMPv2 agent,
that agent must have knowledge of that user. A SNMPv2 manager that
wishes to communicate with a particular agent must also have
knowledge of a user known to that agent, including knowledge of the
applicable attributes of that user. Similarly, a SNMPv2 manager that
wishes to receive messages from a SNMPv2 dual-role entity must have
knowledge of the user on whose behalf the dual-role entity sends the
message.
A user and its attributes are defined as follows:
<userName>
An octet string representing the name of the user.
<authProtocol>
An indication of whether messages sent on behalf of this user can
be authenticated, and if so, the type of authentication protocol
which is used. One such protocol is defined in this memo: the
Digest Authentication Protocol.
<authPrivateKey>
If messages sent on behalf of this user can be authenticated, the
(private) authentication key for use with the authentication
protocol. Note that a user's authentication key will normally be
different at different agents.
<privProtocol>
An indication of whether messages sent on behalf of this user can
be protected from disclosure, and if so, the type of privacy
protocol which is used. One such protocol is defined in this memo:
the Symmetric Encryption Protocol.
<privPrivateKey>
If messages sent on behalf of this user can be protected from
disclosure, the (private) privacy key for use with the privacy
protocol. Note that a user's privacy key will normally be
different at different agents.
2.2. Contexts and Context Selectors
An SNMPv2 context is a collection of management information
accessible (locally or via proxy) by an SNMPv2 agent. An item of
management information may exist in more than one context. An SNMPv2
agent potentially has access to many contexts. Each SNMPv2 message
contains a context selector which unambiguously identifies an SNMPv2
context accessible by the SNMPv2 agent to which the message is
directed or by the SNMPv2 agent associated with the sender of the
message.
For a local SNMPv2 context which is realized by an SNMPv2 entity,
that SNMPv2 entity uses locally-defined mechanisms to access the
management information identified by the SNMPv2 context.
For a proxy SNMPv2 context, the SNMPv2 entity acts as a proxy SNMPv2
agent to access the management information identified by the SNMPv2
context.
The term remote SNMPv2 context is used at an SNMPv2 manager to
indicate a SNMPv2 context (either local or proxy) which is not
realized by the local SNMPv2 entity (i.e., the local SNMPv2 entity
uses neither locally-defined mechanisms, nor acts as a proxy SNMPv2
agent to access the management information identified by the SNMPv2
context).
Proxy SNMPv2 contexts are further categorized as either local-proxy
contexts or remote-proxy contexts. A proxy SNMPv2 agent receives
Get/GetNext/GetBulk/Set operations for a local-proxy context, and
forwards them with a remote-proxy context; it receives SNMPv2-Trap
and Inform operations for a remote-proxy context, and forwards them
with a local-proxy context; for Response operations, a proxy SNMPv2
agent receives them with either a local-proxy or remote-proxy
context, and forwards them with a remote-proxy or local-proxy
context, respectively.
For the non-proxy situation:
context-A
Manager <----------------> Agent
the type of context is:
+-----------------+
| context-A |
+-----------------+-----------------+
| Manager | remote |
+-----------------+-----------------+
| Agent | local |
+-----------------+-----------------+
| agentID | of Agent |
+-----------------+-----------------+
| contextSelector | locally unique |
+-----------------+-----------------+
For proxy:
context-B context-C
Manager <----------------> Proxy <----------------> Agent
Agent
the type and identity of the contexts are:
+-----------------+-----------------+
| context-B | context-C |
+-----------------+-----------------+-----------------+
| Manager | remote | -- |
+-----------------+-----------------+-----------------+
| Proxy-Agent | local-proxy | remote-proxy |
+-----------------+-----------------+-----------------+
| Agent | -- | local |
+-----------------+-----------------+-----------------+
| agentID | of Proxy agent | of Agent |
+-----------------+-----------------+-----------------+
| contextSelector | locally unique | locally unique |
+-----------------+-----------------+-----------------+
The combination of an agentID value and a context selector provides a
globally-unique identification of a context. When a context is
accessible by multiple agents (e.g., including by proxy SNMPv2
agents), it has multiple such globally-unique identifications, one
associated with each agent which can access it. In the example above,
"context-B" and "context-C" are different names for the same context.
2.3. Quality of Service (qoS)
Messages are generated with a particular Quality of Service (qoS),
either:
- without authentication and privacy,
- with authentication but not privacy,
- with authentication and privacy.
All users are capable of having messages without authentication and
privacy generated on their behalf. Users having an authentication
protocol and an authentication key can have messages with
authentication but not privacy generated on their behalf. Users
having an authentication protocol, an authentication key, a privacy
protocol and a privacy key can have messages with authentication and
privacy generated on their behalf.
In addition to its indications of authentication and privacy, the qoS
may also indicate that the message contains an operation that may
result in a report PDU being generated (see Section 2.6 below).
2.4. Access Policy
An administration's access policy determines the access rights of
users. For a particular SNMPv2 context to which a user has access
using a particular qoS, that user's access rights are given by a list
of authorized operations, and for a local context, a read-view and a
write-view. The read-view is the set of object instances authorized
for the user when reading objects. Reading objects occurs when
processing a retrieval (get, get-next, get-bulk) operation and when
sending a notification. The write-view is the set of object
instances authorized for the user when writing objects. Writing
objects occurs when processing a set operation. A user's access
rights may be different at different agents.
2.5. Replay Protection
Each SNMPv2 agent (or dual-role entity) maintains three objects:
- agentID, which is an identifier unique among all agents in (at
least) an administrative domain;
- agentBoots, which is a count of the number of times the agent has
rebooted/re-initialized since agentID was last configured; and,
- agentTime, which is the number of seconds since agentBoots was last
incremented.
An SNMPv2 agent is always authoritative with respect to these
variables. It is the responsibility of an SNMPv2 manager to
synchronize with the agent, as appropriate. In the case of an SNMPv2
dual-role entity sending an Inform-Request, it is that entity acting
in an agent role which is authoritative with respect to these
variables for the Inform-Request.
An agent is required to maintain the values of agentID and agentBoots
in non-volatile storage.
2.5.1. agentID
The agentID value contained in an authenticated message is used to
defeat attacks in which messages from a manager are replayed to a
different agent and/or messages from one agent (or dual-role entity)
are replayed as if from a different agent (or dual-role entity).
When an agent (or dual-role entity) is first installed, it sets its
local value of agentID according to a enterprise-specific algorithm
(see the definition of agentID in Section 4.1).
2.5.2. agentBoots and agentTime
The agentBoots and agentTime values contained in an authenticated
message are used to defeat attacks in which messages are replayed
when they are no longer valid. Through use of agentBoots and
agentTime, there is no requirement for an SNMPv2 agent to have a
non-volatile clock which ticks (i.e., increases with the passage of
time) even when the agent is powered off. Rather, each time an
SNMPv2 agent reboots, it retrieves, increments, and then stores
agentBoots in non-volatile storage, and resets agentTime to zero.
When an agent (or dual-role entity) is first installed, it sets its
local values of agentBoots and agentTime to zero. If agentTime ever
reaches its maximum value (2147483647), then agentBoots is
incremented as if the agent has rebooted and agentTime is reset to
zero and starts incrementing again.
Each time an agent (or dual-role entity) reboots, any SNMPv2 managers
holding that agent's values of agentBoots and agentTime need to re-
synchronize prior to sending correctly authenticated messages to that
agent (see Section 2.7 for re-synchronization procedures). Note,
however, that the procedures do provide for a notification to be
accepted as authentic by a manager, when sent by an agent which has
rebooted since the manager last re-synchronized.
If an agent (or dual-role entity) is ever unable to determine its
latest agentBoots value, then it must set its agentBoots value to
0xffffffff.
Whenever the local value of agentBoots has the value 0xffffffff, it
latches at that value and an authenticated message always causes an
usecStatsNotInWindows authentication failure.
In order to reset an agent whose agentBoots value has reached the
value 0xffffffff, manual intervention is required. The agent must be
physically visited and re-configured, either with a new agentID
value, or with new secret values for the authentication and privacy
keys of all users known to that agent.
2.5.3. Time Window
The Time Window is a value that specifies the window of time in which
a message generated on behalf of any user is valid. This memo
specifies that the same value of the Time Window, 150 seconds, is
used for all users.
2.6. Error Reporting
While processing a received communication, an SNMPv2 entity may
determine that the message is unacceptable (see Section 3.2). In
this case, the appropriate counter from the snmpGroup [15] or
usecStatsGroup object groups is incremented and the received message
is discarded without further processing.
If an SNMPv2 entity acting in the agent role makes such a
determination and the qoS indicates that a report may be generated,
then after incrementing the appropriate counter, it is required to
generate a message containing a report PDU, with the same user and
context as the received message, and to send it to the transport
address which originated the received message. For all report PDUs,
except those generated due to incrementing the usecStatsNotInWindows
counter, the report PDU is unauthenticated. For those generated due
to incrementing usecStatsNotInWindows, the report PDU is
authenticated only if the received message was authenticated.
The report flag in the qoS may only be set if the message contains a
Get, GetNext, GetBulk, Set operation. The report flag should never
be set for a message that contains a Response, Inform, SNMPv2-Trap or
Report operation. Furthermore, a report PDU is never sent by an
SNMPv2 entity acting in a manager role.
2.7. Time Synchronization
Time synchronization, required by a management entity in order to
proceed with authentic communications, has occurred when the
management entity has obtained local values of agentBoots and
agentTime from the agent that are within the agent's time window. To
remain synchronized, the local values must remain within the agent's
time window and thus must be kept loosely synchronized with the
values stored at the agent. In addition to keeping a local version
of agentBoots and agentTime, a manager must also keep one other local
variable, latestReceivedAgentTime. This value records the highest
value of agentTime that was received by the manager from the agent
and is used to eliminate the possibility of replaying messages that
would prevent the manager's notion of the agentTime from advancing.
Time synchronization occurs as part of the procedures of receiving a
message (Section 3.2, step 9d). As such, no explicit time
synchronization procedure is required by a management entity. Note,
that whenever the local value of agentID is changed (e.g., through
discovery) or when a new secret is configured, the local values of
agentBoots and latestReceivedAgentTime should be set to zero. This
will cause the time synchronization to occur when the next authentic
message is received.
2.8. Proxy Error Propagation
When a proxy SNMPv2 agent receives a report PDU from a proxied agent
and it is determined that a proxy-forwarded request cannot be
delivered to the proxied agent, then the snmpProxyDrops counter [15]
is incremented and a report PDU is generated and transmitted to the
transport address from which the original request was received.
(Note that the receipt of a report PDU containing snmpProxyDrops as a
VarBind, is included among the reasons why a proxy-forwarded request
cannot be delivered.)
2.9. SNMPv2 Messages Using this Model
The syntax of an SNMPv2 message using this security model differs
from that of an SNMPv1 [2] message as follows:
- The version component is changed to 2.
- The data component contains either a PDU or an OCTET STRING
containing an encrypted PDU.
The SNMPv1 community string is now termed the "parameters" component
and contains a set of administrative information for the message.
Only the PDU is protected from disclosure by the privacy protocol.
This exposes the administrative information to eavesdroppers.
However, malicious use of this information is considered to be a
Traffic Analysis attack against which protection is not provided.
For an authenticated SNMPv2 message, the message digest is applied to
the entire message given to the transport service. As such, message
generation first privatizes the PDU, then adds the message wrapper,
and then authenticates the message.
An SNMPv2 message is an ASN.1 value with the following syntax:
Message ::=
SEQUENCE {
version
INTEGER { v2 (2) },
parameters
OCTET STRING,
-- <model=1>
-- <qoS><agentID><agentBoots><agentTime><maxSize>
-- <userLen><userName><authLen><authDigest>
-- <contextSelector>
data
CHOICE {
plaintext
PDUs,
encrypted
OCTET STRING
}
}
where:
parameters
a concatenation of the following values in network-byte order. If
the first octet (<model>) is one, then
<qoS> = 8-bits of quality-of-service
bitnumber
7654 3210 meaning
---- ---- --------------------------------
.... ..00 no authentication nor privacy
.... ..01 authentication, no privacy
.... ..1. authentication and privacy
.... .1.. generation of report PDU allowed
where bit 7 is the most significant bit.
<agentID> = 12 octets
a unique identifier for the agent (or dual-role entity).
<agentBoots> = 32-bits
an unsigned quantity (0..4294967295) in network-byte order.
<agentTime> = 32-bits
an unsigned quantity (0..2147483647) in network-byte order.
<maxSize> = 16-bits
an unsigned quantity (484..65507) in network-byte order, which
identifies the maximum message size which the sender of this
message can receive using the same transport domain as used
for this message.
<userLen> = 1 octet
the length of following <userName> field.
<userName> = 1..16 arbitrary octets
the user on whose behalf this message is sent.
<authLen> = 1 octet
the length of following <authDigest> field.
<authDigest> = 0..255 octets
for authenticated messages, the authentication digest.
Otherwise, the value has zero-length on transmission and is
ignored on receipt.
<contextSelector> = 0..40 arbitrary octets
the context selector which in combination with agentID
identifies the SNMPv2 context containing the management
information referenced by the SNMPv2 message.
plaintext
an SNMPv2 PDU as defined in [12].
encrypted
the encrypted form of an SNMPv2 PDU.
2.10. Local Configuration Datastore (LCD)
Each SNMPv2 entity maintains a local conceptually database, called
the Local Configuration Datastore (LCD), which holds its known set of
information about SNMPv2 users and other associated (e.g., access
control) information. An LCD may potentially be required to hold
information about multiple SNMPv2 agent entities. As such, the
<agentID> should be used to identify a particular agent entity in the
LCD.
It is a local implementation issue as to whether information in the
LCD is stored information or whether it is obtained dynamically
(e.g., as a part of an SNMPv2 manager's API) on an as-needed basis.
3. Elements of Procedure
This section describes the procedures followed by an SNMPv2 entity in
processing SNMPv2 messages.
3.1. Generating a Request or Notification
This section describes the procedure followed by an SNMPv2 entity
whenever it generates a message containing a management operation
(either a request or a notification) on behalf of a user, for a
particular context and with a particular qoS value.
(1) Information concerning the user is extracted from the LCD. The
transport domain and transport address to which the operation is to
be sent is determined. The context is resolved into an agentID
value and a contextSelector value.
(2) If the qoS specifies that the message is to be protected from
disclosure, but the user does not support both an authentication
and a privacy protocol, or does not have configured authentication
and privacy keys, then the operation cannot be sent.
(3) If the qoS specifies that the message is to be authenticated, but
the user does not support an authentication protocol, or does not
have a configured authentication key, then the operation cannot be
sent.
(4) The operation is serialized (i.e., encoded) according to the
conventions of [13] and [12] into a PDUs value.
(5) If the operation is a Get, GetNext, GetBulk, or Set then the report
flag in the qoS is set to the value 1.
(6) An SNMPv2 message is constructed using the ASN.1 Message syntax:
- the version component is set to the value 2.
- if the qoS specifies that the message is to be protected from
disclosure, then the octet sequence representing the serialized
PDUs value is encrypted according to the user's privacy protocol
and privacy key, and the encrypted data is encoded as an octet
string and is used as the data component of the message.
- if the qoS specifies that the message is not to be protected from
disclosure, then the serialized PDUs value is used directly as
the value of the data component.
- the parameters component is constructed using:
- the requested qoS, userName, agentID and context selector,
- if the qoS specifies that the message is to be authenticated or
the management operation is a notification, then the current
values of agentBoots, and agentTime corresponding to agentID
from the LCD are used. Otherwise, the <agentBoots> and
<agentTime> fields are set to zero-filled octets.
- the <maxSize> field is set to the maximum message size which
the local SNMPv2 entity can receive using the transport domain
which will be used to send this message.
- if the qoS specifies that the message is to be authenticated,
then the <authDigest> field is temporarily set to the user's
authentication key. Otherwise, the <authDigest> field is set
to the zero-length string.
(7) The constructed Message value is serialized (i.e., encoded)
according to the conventions of [13] and [12].
(8) If the qoS specifies that the message is to be authenticated, then
an MD5 digest value is computed over the octet sequence
representing the concatenation of the serialized Message value and
the user's authentication key. The <authDigest> field is then set
to the computed digest value.
(9) The serialized Message value is transmitted to the determined
transport address.
3.2. Processing a Received Communication
This section describes the procedure followed by an SNMPv2 entity
whenever it receives an SNMPv2 message. This procedure is
independent of the transport service address at which the message was
received. For clarity, some of the details of this procedure are
left out and are described in Section 3.2.1 and its sub-sections.
(1) The snmpInPkts counter [15] is incremented. If the received
message is not the serialization (according to the conventions of
[13]) of a Message value, then the snmpInASNParseErrs counter [15]
is incremented, and the message is discarded without further
processing.
(2) If the value of the version component has a value other than 2,
then the message is either processed according to some other
version of this protocol, or the snmpInBadVersions counter [15] is
incremented, and the message is discarded without further
processing.
(3) The value of the <model> field is extracted from the parameters
component of the Message value. If the value of the <model> field
is not 1, then either the message is processed according to some
other security model, or the usecStatsBadParameters counter is
incremented, and the message is discarded without further
processing.
(4) The values of the rest of the fields are extracted from the
parameters component of the Message value.
(5) If the <agentID> field contained in the parameters is unknown then:
- a manager that performs discovery may optionally create a new LCD
entry and continue processing; or
- the usecStatsUnknownContexts counter is incremented, a report PDU
is generated, and the received message is discarded without
further processing.
(6) The LCD is consulted for information about the SNMPv2 context
identified by the combination of the <agentID> and
<contextSelector> fields. If information about this SNMPv2 context
is absent from the LCD, then the usecStatsUnknownContexts counter
is incremented, a report PDU is generated, and the received message
is discarded without further processing.
(7) Information about the value of the <userName> field is extracted
from the LCD. If no information is available, then the
usecStatsUnknownUserNames counter is incremented, a report PDU [1]
is generated, and the received message is discarded without further
processing.
(8) If the information about the user indicates that it does not
support the quality of service indicated by the <qoS> field, then
the usecStatsUnsupportedQoS counter is incremented, a report PDU is
generated, and the received message is discarded without further
processing.
(9) If the <qoS> field indicates an authenticated message and the
user's authentication protocol is the Digest Authentication
Protocol described in this memo, then:
a) the local values of agentBoots and agentTime corresponding to
the value of the <agentID> field are extracted from the LCD.
b) the value of <authDigest> field is temporarily saved. A new
serialized Message is constructed which differs from that
received in exactly one respect: that the <authDigest> field
within it has the value of the user's authentication key. An
MD5 digest value is computed over the octet sequence
representing the concatenation of the new serialized Message and
the user's authentication key.
c) if the LCD information indicates the SNMPv2 context is of type
local (i.e., an agent), then:
- if the computed digest differs from the saved authDigest
value, then the usecStatsWrongDigestValues counter is
incremented, a report PDU is generated, and the received
message is discarded without further processing. However, if
the snmpEnableAuthenTraps object [15] is enabled, then the
SNMPv2 entity sends authenticationFailure traps [15] according
to its configuration.
- if any of the following conditions is true, then the message
is considered to be outside of the Time Window:
- the local value of agentBoots is 0xffffffff;
- the <agentBoots> field differs from the local value of
agentBoots; or,
- the value of the <agentTime> field differs from the local
notion of agentTime by more than +/- 150 seconds.
- if the message is considered to be outside of the Time Window
then the usecStatsNotInWindows counter is incremented, an
authenticated report PDU is generated (see section 2.7), and
the received message is discarded without further processing.
d) if the LCD information indicates the SNMPv2 context is not
realized by the local SNMPv2 entity (i.e., a manager), then:
- if the computed digest differs from the saved authDigest
value, then the usecStatsWrongDigestValues counter is
incremented and the received message is discarded without
further processing.
- if all of the following conditions are true:
- if the <qoS> field indicates that privacy is not in use;
- the SNMPv2 operation type determined from the ASN.1 tag
value associated with the PDU's component is a Report;
- the Report was generated due to a usecStatsNotInWindows
error condition; and,
- the <agentBoots> field is greater than the local value of
agentBoots, or the <agentBoots> field is equal to the
local value of agentBoots and the <agentTime> field is
greater than the value of latestReceivedAgentTime,
then the LCD entry corresponding to the value of the <agentID>
field is updated, by setting the local value of agentBoots
from the <agentBoots> field, the value latestReceivedAgentTime
from the <agentTime> field, and the local value of agentTime
from the <agentTime> field.
- if any of the following conditions is true, then the message
is considered to be outside of the Time Window:
- the local value of agentBoots is 0xffffffff;
- the <agentBoots> field is less than the local value of
agentBoots; or,
- the <agentBoots> field is equal to the local value of
agentBoots and the <agentTime> field is more than 150
seconds less than the local notion of agentTime.
- if the message is considered to be outside of the Time Window
then the usecStatsNotInWindows counter is incremented, and the
received message is discarded without further processing;
however, time synchronization procedures may be invoked. Note
that this procedure allows for <agentBoots> to be greater than
the local value of agentBoots to allow for received messages
to be accepted as authentic when received from an agent that
has rebooted since the manager last re-synchronized.
- if at least one of the following conditions is true:
- the <agentBoots> field is greater than the local value of
agentBoots; or,
- the <agentBoots> field is equal to the local value of
agentBoots and the <agentTime> field is greater than the