One document matched: draft-kzm-snmpv2-sec-alt-00.txt
Internet Draft User-based Security Model for SNMPv2 June 1995
User-based Security Model
for Version 2 of the
Simple Network Management Protocol (SNMPv2)
Fri Jun 30 23:59:59 1995
draft-kzm-snmpv2-sec-alt-00.txt
Keith McCloghrie
Cisco Systems, Inc.
kzm@cisco.com
Marshall T. Rose
Dover Beach Consulting, Inc.
mrose@dbc.mtview.ca.us
Glenn W. Waters
Bell-Northern Research Ltd.
gwaters@bnr.ca
James M. Galvin
Trusted Information Systems
galvin@tis.com
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as ``work in progress.''
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To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow
Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe),
ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
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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 the 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 the administrative framework.
The 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. A Note on Terminology
For the purpose of exposition, the original Internet-standard Network
Management Framework, as described in RFCs 1155, 1157, and 1212, is
termed the SNMP version 1 framework (SNMPv1). The current framework is
termed the SNMP version 2 framework (SNMPv2).
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1.2. 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
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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.
1.3. 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 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.
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(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.
1.4. 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.
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1.5. 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 inserted into the SNMPv2 message prior to computing
the digest and then overwritten prior to transmission. 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 indicators are included in each message generated. A
recipient evaluates the time indicators to determine if the message
is recent. 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 message replay (e.g., the request-id [12]), or for set
operations, the re-ordering, replay, deletion, or suppression of
messages (e.g., the MIB variable snmpSetSerialNo [14]).
- In support of data confidentiality, an encryption algorithm is
required. An appropriate portion of the message is encrypted prior
to being transmitted.
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1.5.1. 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 inserted into the message prior
to the digest computation. Thus, the verification of the user is
implicit with the verification of the digest.
- verifying that a message sent to/from one agent cannot be replayed
to/as-if-from another agent.
Included in each message is an identifier unique to the SNMPv2
agent which is the sender or intended recipient of the message.
- detecting messages which were not recently generated.
A set of time indicators are included in the message, indicating
the time of generation. Messages without recent time indicators
are not considered authentic.
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, and is included in each authenticated
message sent to/from that 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 for that agent.
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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. 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 an agent is
authentic, that manager must first achieve time synchronization with
that agent.
1.5.2. 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.
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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.
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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.
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 may 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 may be different at
different agents.
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2.2. Context Selectors
An SNMPv2 context is a collection of management information accessible
(locally or via proxy) by an SNMPv2 agent. An SNMPv2 agent potentially
has access to many contexts. Each SNMPv2 message contains a context
selector which uniquely identifies an SNMPv2 context accessible by the
SNMPv2 agent to which the message is directed. At the option of the
administrator, a context selector may uniquely identify an SNMPv2
context among all SNMPv2 contexts within the administrative domain.
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.
Orthogonal to its indication of authentication and privacy, the qoS of
an SNMPv2 message may also indicate that the message relates to a
maintenance function (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
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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 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 are replayed as if from a
different agent.
When an agent 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 from a manager are
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replayed to the same agent at a time 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 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 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.
If an agent 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. Maintenance Functions
In order to facilitate communication between SNMPv2 entities, certain
"maintenance" functions are defined. An operation relating to a
maintenance function is carried in an SNMPv2 message with the
appropriate qoS indicator. For example, error reporting and time
synchronization are achieved by performing SNMP operations on behalf of
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the user "usec" and accessing the context for which the context selector
is the zero-length string.
When processing a maintenance function, an SNMPv2 entity utilizes the
same mechanisms defined for normal operations. However, normal
operations execute with respect to an administration's security policy
(which may vary between administrations). In contrast, maintenance
functions always execute within a fixed, standardized security policy.
This is advantageous in that it allows code re-use within an SNMPv2
entity, while also not allowing an administration's policy to impair the
proper operation of essential maintenance functions. However, not all
of the rules applicable to normal users and contexts specified in this
document necessarily apply to these maintenance functions.
The sole purpose of maintenance functions is to ensure that all SNMPv2
entities provide essential maintenance functionality within a well-
known, standardized, security environment. Maintenance functions are
intended for use only by the internal operations of an SNMPv2 entity.
Thus, their scope is intentionally restricted to be the minimum
necessary to fulfill their purpose.
2.6.1. The Well-known Maintenance User
All implementations must support the well-known user named "usec", which
has no authentication nor privacy capabilities. The access policy for
this maintenance user allows read-only access to the local context which
has the empty string as its selector, and which contains all objects in
the snmpStatsGroup [14] and usecBasicGroup object groups.
2.6.2. 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 snmpStatsGroup [14] or
usecBasicGroup 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,
then after incrementing the appropriate counter, it is required to
generate a report PDU and to send it to the transport address which
originated the received message.
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Report-PDU ::=
[8]
IMPLICIT PDU
If the agent is able to determine the request-id field of the received
PDU, then it uses that value for the request-id field of the report PDU.
Otherwise, the value 2147483647 is used.
The error-status and error-index fields of the report PDU are always set
to zero.
The variable-bindings field contains a single variable: the identity of
the statistics counter which was incremented and its new value.
A report PDU is never sent by an SNMPv2 entity acting in a manager role,
nor by any SNMPv2 entity as a result of receiving a message containing a
report PDU.
2.6.3. Time Synchronization
An SNMPv2 manager achieves time synchronization with an agent by issuing
a maintenance function to retrieve agentID.0, agentBoots.0, and,
agentTime.0.
It is recommended that after these values are retrieved, that the
manager attempt an authenticated retrieval using the new values before
updating its local configuration datastore.
2.6.4. Proxy Error Propagation
When a proxy SNMPv2 agent receives a report PDU from a proxied agent, it
invokes the appropriate maintenance procedures. If the result of such
maintenance procedures determines that a proxy-forwarded request cannot
be delivered to the proxied agent, then the snmpStatsProxyDrops counter
[14] 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 snmpStatsProxyDrops as a
varbind, is included among the reasons why a proxy-forwarded request
cannot be delivered.)
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2.7. SNMPv2 Messages
The syntax of an SNMPv2 message differs from that of an SNMPv1 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=0>
-- <qoS><agentID><agentBoots><agentTime>
-- <userLen><userName><authLen><authDigest>
-- <maxSize><contextSelector>
data
CHOICE {
plaintext
PDUs,
encrypted
OCTET STRING
}
}
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where:
parameters
if the first octet of this component (<model>) is zero, 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... .... maintenance function
where bit 7 is the most significant bit.
<agentID> = 12 octets
a unique identifier for the agent.
<agentBoots> = 32-bits
an unsigned quantity (0..4294967295) in network-byte order.
<agentTime> = 32-bits
an unsigned quantity (0..2147483647) in network-byte order.
<userLen> = 1 octet
the length of following <userName> field.
<userName> = 1..32 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.
<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.
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<contextSelector> = 0..64 octets
the context selector by which the agent 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.
2.8. 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.
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.
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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.
(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) 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 string is used as the value of
the data component.
- 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:
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- the requested qoS, userName, 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 agentID, agentBoots, and agentTime from the LCD are
used. Otherwise, the <agentID>, <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.
(6) The constructed Message value is serialized (i.e., encoded)
according to the conventions of [13] and [12].
(7) If the qoS specifies that the message is to be authenticated, then
an MD5 digest value is computed over the octet sequence
representing the serialized Message value. The <authDigest> field
is then set to the computed digest value.
(8) The serialized Message value is transmitted to the determined
transport address.
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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.
(1) The snmpStatsPackets counter [14] is incremented. If the received
message is not the serialization (according to the conventions of
[13]) of a Message value, then the snmpStatsEncodingErrors counter
[14] 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 snmpStatsEncodingErrors counter
[14] is incremented, and the message is discarded without further
processing.
(3) The values of the individual fields are extracted from the
parameters component of the Message value.
(4) If the value of the <model> field is not 0, then either the message
is processed according to some other security model, or the
snmpStatsEncodingErrors counter [14] is incremented, and the
message is discarded without further processing.
(5) 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 is
generated, and the received message is discarded without further
processing.
(6) 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.
(7) The LCD is consulted for information about the SNMPv2 context
identified by the <contextSelector> field. If information about
this SNMPv2 context is absent from the LCD, then the
usecStatsUnknownContextSelectors counter is incremented, a report
PDU is generated, and the received message is discarded without
further processing.
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(8) If the <qoS> field indicates an authenticated message, the user's
authentication protocol is the Digest Authentication Protocol
described in this memo, and <contextSelector> indicates a context
realized by the local SNMPv2 entity (i.e., a context for which it
will accept authorized retrieval and/or modification operations),
then:
- if the <agentID> field is not equal to the local value of agentID
then the usecStatsNotInWindows counter is incremented, a report
PDU is generated, and the received message is discarded without
further processing.
- if the value of the <agentBoots> field differs from the local
value of agentBoots, or the value of the <agentTime> field
differs from the local value of agentTime by more than +/- 150
seconds, then the usecStatsNotInWindows counter is incremented, a
report PDU is generated, and the received message is discarded
without further processing. However, if the
snmpV2EnableAuthenTraps object [14] is enabled, then the SNMPv2
entity sends authorizationFailure traps [14] according to its
configuration.
- the value of the <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
new serialized Message.
- 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
snmpV2EnableAuthenTraps object [14] is enabled, then the SNMPv2
entity sends authorizationFailure traps [14] according to its
configuration.
(9) If the <qoS> field indicates an authenticated message, the user's
authentication protocol is the Digest Authentication Protocol
described in this memo, and the <contextSelector> field indicates a
context not realized by the local SNMPv2 entity, then:
- the local values of agentBoots and agentTime corresponding to the
value of the <agentID> field are extracted from the LCD.
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- if the local value of agentBoots is 0xffffffff, or the
<agentBoots> field differs from the local value of agentBoots, or
the <agentTime> differs from the local value of agentTime by more
than +/- 150 seconds, then the usecStatsNotInWindows counter is
incremented, and the received message is discarded without
further processing; however, time synchronization procedures may
be invoked.
- the value of the <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
new serialized Message.
- 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.
- the LCD is updated by setting the local value of agentTime
corresponding to the value of <agentID> field, to the value of
the <agentTime> field.
(10) If the <qoS> field indicates use of a privacy protocol, then the
octet sequence representing the data component is decrypted
according to the user's privacy protocol to obtain a serialized
PDUs value. Otherwise the data component is assumed to directly
contain the PDUs value.
(11) The SNMPv2 operation type is determined from the ASN.1 tag value
associated with the PDUs component.
(12) If the SNMPv2 message contains a report PDU, then the request-id in
the PDU is correlated to an outstanding request, and if the
correlation is successful, the appropriate maintenance function
(e.g., time synchronization, proxy error propagation, etc.) is
invoked. Otherwise, the snmpStatsBadOperations counter [14] is
incremented, and the received message is discarded without further
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processing.
(13) If the SNMPv2 operation type is either a Get, GetNext, GetBulk, or
Set operation, then:
a) if the LCD information indicates that the SNMPv2 context is of
type remote, then the usecStatsUnknownContextSelectors counter
is incremented, a report PDU is generated, and the received
message is discarded without further processing.
b) the LCD is consulted for access rights authorized for
communications on behalf of the user concerning management
information in the indicated SNMPv2 context for the particular
SNMPv2 operation type.
c) if the SNMPv2 operation type is not among the authorized
access rights, then the received message is discarded without
further processing after generation and transmission of a
response message. This response message is sent on behalf of
the same user. Its context, var-bind-list and request-id
components are identical to those of the received request.
Its error-index component is zero and its error-status
component is authorizationError [12].
d) The information extracted from the LCD concerning the user and
the SNMPv2 context, together with the sending transport
address of the received message is cached for later use in
generating a response message.
e) if the LCD information indicates the SNMPv2 context is of type
local, then the management operation represented by the PDUs
value is performed by the receiving SNMPv2 entity with respect
to the relevant MIB view within the SNMPv2 context according
to the procedures set forth in [12], where the relevant MIB
view is determined according to the user, the contextSelector,
the qoS values and the type of operation requested.
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f) if the LCD information indicates the SNMPv2 context is of type
proxy, then:
i. the user and context to be used to forward the request
are extracted from the LCD. If either or both of these
values are not currently available for use, then
snmpStatsProxyDrops [14] is incremented, a report PDU is
generated, and the received message is discarded.
ii. a new SNMPv2 message is constructed: its PDUs component
is copied from that in the received message except that
the contained request-id is replaced by a unique value
(this value will enable a subsequent response message to
be correlated with this request); and the <userName> and
<contextSelector> fields are set to the values extracted
from the LCD.
iii. the information cached in Step 13d above is augmented
with the request-id of the received message as well as
the request-id and context of the constructed message.
iv. the constructed message is forwarded to the appropriate
transport address.
(14) If the SNMPv2 operation type is either a SNMPv2-Trap, Inform, or
Response operation, then:
a) if the LCD information indicates the SNMPv2 context is of type
local, then the usecStatsUnknownContextSelectors counter is
incremented, a report PDU is generated, and the received
message is discarded without further processing.
b) if the LCD information indicates the SNMPv2 context is of type
remote, then the management operation represented by the PDUs
value is performed by the receiving SNMPv2 entity according to
the procedures set forth in [12].
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c) if the LCD information indicates the SNMPv2 context is of type
proxy and the SNMPv2 operation type is a Response, then:
i. the request-id is extracted from the PDUs component of
the received message. The SNMPv2 context and extracted
request-id are used to correlate this response message to
the corresponding values for a previously forwarded
request by inspecting the cache of information as
augmented in Substep iii of Step 13f above. If no such
correlated information is found, then the received
message is discarded without further processing.
ii. a new SNMPv2 message is constructed: its PDUs component
is copied from that in the received message except that
the contained request-id is replaced by the value saved
in the correlated information from the original request;
its <userName> and <contextSelector> fields are set to
the values saved from the original request.
iii. the constructed message is forwarded to the transport
address saved in the correlated information as the
sending transport address of the original request.
iv. the correlated information is deleted from the cache of
information.
d) if the LCD information indicates the SNMPv2 context is of type
proxy, and the SNMPv2 operation type is an Inform or SNMPv2-
Trap, then:
i. a unique request-id is selected for use by all forwarded
copies of this request. This value will enable a
subsequent response message to be correlated with this
request.
ii. information is extracted from the LCD concerning all
combinations of userName and contextSelector with which
the received message is to be forwarded.
iii. for each such combination whose access rights permit
Inform or SNMPv2-Trap operations (as appropriate) to be
forwarded, a new SNMPv2 message is constructed: its PDUs
component is copied from that in the received message
except that the contained request-id is replaced by the
value selected in Step i above; its <userName> and
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<contextSelector> fields are set to the values extracted
in Step ii above.
iv. if the SNMPv2 operation type of the received message is
an Inform, then for each constructed SNMPv2 message,
information concerning the <userName>, <contextSelector>,
request-id and sending transport address of the received
message, as well as the request-id and contextSelector of
the constructed message, is cached for later use in
generating a response message.
v. each constructed message is forwarded to the appropriate
transport address.
3.2.1. Additional Details
For the sake of clarity and to prevent the above procedure from being
even longer, the following details were omitted from the above
procedure.
3.2.1.1. ASN.1 Parsing Errors
For ASN.1 parsing errors, the snmpStatsEncodingErrors counter [14] is
incremented and a report PDU is generated whenever such an ASN.1 parsing
error is discovered.
3.2.1.2. Incorrectly Encoded Parameters
For an incorrectly encoded parameters component of the Message value
(e.g., incorrect or inconsistent value of the <userLen> or <authLen>
fields), the snmpStatsEncodingErrors counter [14] is incremented and a
report PDU is generated whenever such an encoding error is discovered.
3.2.1.3. Generation of a Report PDU
Some steps specify that the received message is discarded without
further processing whenever a report PDU is generated. However, first,
an SNMPv2 manager never generates a report PDU; second, a report PDU
must not be generated unless and until the SNMPv2 operation type can be
determined, so as to ensure that a report PDU is not generated due to
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the receipt of a report PDU.
In addition, a generated report PDU must whenever possible contain the
same request-id value as in the PDU contained in the received message.
Meeting these constraints normally requires the message to be further
processed just enough so as to extract its SNMPv2 operation type and
request-id. Even in the case where the userName is unknown, an attempt
must be made to extract the SNMPv2 operation type and request-id by
assuming the message is not encrypted. With this assumption, the only
situation in which the SNMPv2 operation type and request-id cannot be
extracted is when an ASN.1 parsing error occurs.
3.3. Generating a Response
The procedure for generating a response to an SNMPv2 management request
is identical to the procedure for transmitting a request (see Section
3.1), with these exceptions:
- The response is sent on behalf of the same user and with the same
value of the contextSelector as the request.
- The PDUs value of the responding Message value is the response
which results from performing the operation specified in the
original PDUs value.
- The authentication protocol and other relevant information for the
user is obtained, not from the LCD, but rather from information
cached (in Step 13d) when processing the original message.
- The serialized Message value is transmitted using the transport
address and transport domain from which its corresponding request
originated - even if that is different from any transport
information obtained from the LCD.
- If the qoS specifies that the message is to be authenticated, then
the current values of agentID, agentBoots, and agentTime from the
LCD are used.
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4. Definitions
SNMPv2-USEC-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE, Counter32, Unsigned32,
snmpModules
FROM SNMPv2-SMI
MODULE-COMPLIANCE, OBJECT-GROUP
FROM SNMPv2-CONF;
usecMIB MODULE-IDENTITY
LAST-UPDATED "9506300000Z"
ORGANIZATION "IETF SNMPv2 Working Group"
CONTACT-INFO
" Keith McCloghrie
Postal: Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
US
Tel: +1 408 526 5260
E-mail: kzm@cisco.com"
DESCRIPTION
"The MIB module for SNMPv2 entities implementing the user-
based security model."
::= { snmpModules 6 }
usecMIBObjects OBJECT IDENTIFIER ::= { usecMIB 1 }
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-- the USEC Basic group
--
-- a collection of objects providing basic instrumentation of
-- the SNMPv2 entity implementing the user-based security model
usecAgent OBJECT IDENTIFIER ::= { usecMIBObjects 1 }
agentID OBJECT-TYPE
SYNTAX OCTET STRING (SIZE (12))
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The agent's administratively-unique identifier.
The initial value for this object may be configured via an
operator console entry or via an algorithmic function. In
the later case, the following guidelines are recommended:
1) The first four octets are set to the binary equivalent
of the agent's SNMP network management private
enterprise number as assigned by the Internet Assigned
Numbers Authority (IANA). For example, if Acme
Networks has been assigned { enterprises 696 }, the
first four octets would be assigned '000002b8'H.
2) The remaining eight octets are the cookie whose
contents are determined via one or more enterprise-
specific methods. Such methods must be designed so as
to maximize the possibility that the value of this
object will be unique in the agent's administrative
domain. For example, the cookie may be the IP address
of the agent, or the MAC address of one of the
interfaces, with each address suitably padded with
random octets. If multiple methods are defined, then
it is recommended that the cookie be further divided
into one octet that indicates the method being used and
seven octets which are a function of the method."
::= { usecAgent 1 }
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agentBoots OBJECT-TYPE
SYNTAX Unsigned32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times that the agent has re-initialized
itself since its initial configuration."
::= { usecAgent 2 }
agentTime OBJECT-TYPE
SYNTAX Unsigned32 (0..2147483647)
UNITS "seconds"
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of seconds since the agent last incremented the
agentBoots object."
::= { usecAgent 3 }
agentSize OBJECT-TYPE
SYNTAX INTEGER (484..65507)
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The maximum length in octets of an SNMPv2 message which
this agent will accept using any transport mapping."
::= { usecAgent 4 }
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-- USEC statistics
--
-- a collection of objects providing basic instrumentation of
-- the SNMPv2 entity implementing the user-based security model
usecStats OBJECT IDENTIFIER ::= { usecMIBObjects 2 }
usecStatsUnsupportedQoS OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The total number of packets received by the SNMPv2 entity
which were dropped because they requested a quality-of-
service that was unknown to the agent or otherwise
unavailable."
::= { usecStats 1 }
usecStatsNotInWindows OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The total number of packets received by the SNMPv2 entity
which were dropped because they appeared outside of the
agent's window."
::= { usecStats 2 }
usecStatsUnknownUserNames OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The total number of packets received by the SNMPv2 entity
which were dropped because they referenced a user that was
not known to the agent."
::= { usecStats 3 }
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usecStatsWrongDigestValues OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The total number of packets received by the SNMPv2 entity
which were dropped because they didn't contain the expected
digest value."
::= { usecStats 4 }
usecStatsUnknownContextSelectors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The total number of packets received by the SNMPv2 entity
which were dropped because they referenced a context that
was not known to the agent."
::= { usecStats 5 }
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-- conformance information
usecMIBConformance
OBJECT IDENTIFIER ::= { usecMIB 2 }
usecMIBCompliances
OBJECT IDENTIFIER ::= { usecMIBConformance 1 }
usecMIBGroups OBJECT IDENTIFIER ::= { usecMIBConformance 2 }
-- compliance statements
usecMIBCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION
"The compliance statement for SNMPv2 entities which
implement the SNMPv2 USEC MIB."
MODULE -- this module
MANDATORY-GROUPS { usecBasicGroup }
::= { usecMIBCompliances 1 }
-- units of conformance
usecBasicGroup OBJECT-GROUP
OBJECTS { agentID, agentBoots, agentTime, agentSize,
usecStatsUnsupportedQoS,
usecStatsNotInWindows,
usecStatsUnknownUserNames,
usecStatsWrongDigestValues,
usecStatsUnknownContextSelectors }
STATUS current
DESCRIPTION
"A collection of objects providing basic instrumentation of
an SNMPv2 entity which implements the SNMPv2 USEC MIB."
::= { usecMIBGroups 1 }
END
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5. Security Considerations
5.1. Recommended Practices
This section describes practices that contribute to the secure,
effective operation of the mechanisms defined in this memo.
- A management station must discard SNMPv2 responses for which
neither the request-id component nor the represented management
information corresponds to any currently outstanding request.
Although it would be typical for a management station to do this as
a matter of course, when using these security protocols it is
significant due to the possibility of message duplication
(malicious or otherwise).
- When sending state altering messages to a managed agent, a
management station should delay sending successive messages to the
managed agent until a positive acknowledgement is received for the
previous message or until the previous message expires.
No message ordering is imposed by the SNMPv2. Messages may be
received in any order relative to their time of generation and each
will be processed in the ordered received. Note that when an
authenticated message is sent to a managed agent, it will be valid
for a period of time of approximately 150 seconds under normal
circumstances, and is subject to replay during this period.
Indeed, a management station must cope with the loss and re-
ordering of messages resulting from anomalies in the network as a
matter of course.
However, a managed object, snmpSetSerialNo [14], is specifically
defined for use with SNMPv2 set operations in order to provide a
mechanism to ensure the processing of SNMPv2 messages occurs in a
specific order.
- The frequency with which the secrets of an SNMPv2 user should be
changed is indirectly related to the frequency of their use.
Protecting the secrets from disclosure is critical to the overall
security of the protocols. Frequent use of a secret provides a
continued source of data that may be useful to a cryptanalyst in
exploiting known or perceived weaknesses in an algorithm. Frequent
changes to the secret avoid this vulnerability.
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Changing a secret after each use is generally regarded as the most
secure practice, but a significant amount of overhead may be
associated with that approach.
Note, too, in a local environment the threat of disclosure may be
less significant, and as such the changing of secrets may be less
frequent. However, when public data networks are the communication
paths, more caution is prudent.
5.2. Defining Users
The mechanisms defined in this document employ the notion of "users"
having access rights. How "users" are defined is subject to the
security policy of the network administration. For example, users could
be individuals (e.g., "joe" or "jane"), or a particular role (e.g.,
"operator" or "administrator"), or a combination (e.g., "joe-operator",
"jane-operator" or "joe-admin"). Furthermore, a "user" may be a logical
entity, such as a manager station application or set of manager station
applications, acting on behalf of a individual or role, or set of
individuals, or set of roles, including combinations.
Appendix A describes an algorithm for mapping a user "password" to a 16
octet value for use as either a user's authentication key or privacy key
(or both). Passwords are often generated, remembered, and input by a
human. Human-generated passwords may be less than the 16 octets
required by the authentication and privacy protocols, and brute force
attacks can be quite easy on a relatively short ASCII character set.
Therefore, the algorithm is Appendix A performs a transformation on the
password. If the Appendix A algorithm is used, agent implementations
(and agent configuration applications) must ensure that passwords are at
least 8 characters in length.
Because the Appendix A algorithm uses such passwords (nearly) directly,
it is very important that they not be easily guessed. It is suggested
that they be composed of mixed-case alphanumeric and punctuation
characters that don't form words or phrases that might be found in a
dictionary. Longer passwords improve the security of the system. Users
may wish to input multiword phrases to make their password string longer
while ensuring that it is memorable.
Note that there is security risk in configuring the same "user" on
multiple systems where the same password is used on each system, since
the compromise of that user's secrets on one system results in the
compromise of that user on all other systems having the same password.
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There is also greater security risk and less accountability in allowing
multiple humans to know the password for a given "user".
5.3. Conformance
To claim conformance to this memo, an SNMPv2 implementation:
- must recognize and perform maintenance functions on behalf of the
well-known maintenance user "usec".
- must implement the Digest Authentication Protocol. Otherwise, it is
termed an "insecure SNMPv2 implementation" -- one which adheres to
the SNMPv2 Administrative Framework but does not support secure
operations.
- must, to the maximal extent possible, prohibit access to the
secret(s) of each user about which it maintains information in a LCD,
under all circumstances except as required to generate and/or
validate SNMPv2 messages with respect to that user.
- must implement the SNMPv2 USEC MIB.
Implementation of the Symmetric Encryption Protocol is optional.
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6. Acknowledgements
The authors wish to acknowledge the contributions of the SNMPv2 Working
Group in general. In particular, the authors extend a special thanks
for the contributions of:
Dave Arneson (Cabletron)
Uri Blumenthal (IBM)
Doug Book (Chipcom)
Kim Curran (Bell-Northern Research)
Maria Greene (Ascom Timeplex)
Deirdre Kostick (Bellcore)
Dave Harrington (Cabletron)
Jeff Johnson (Cisco Systems)
David Levi (SNMP Research)
Brian O'Keefe (Hewlett Packard)
Andrew Pearson (SNMP Research)
Dave Perkins (Bay Networks)
Randy Presuhn (Peer Networks)
Shawn Routhier (Epilogue)
Bob Stewart (Cisco Systems)
Kaj Tesink (Bellcore)
Bert Wijnen (IBM)
7. References
[1] Case, J., Galvin, J., McCloghrie, K., Rose, M., and Waldbusser, S.,
"Administrative Infrastructure for Version 2 of the Simple Network
Management Protocol (SNMPv2)", Internet Draft, SNMP Research, Inc.,
Trusted Information Systems, Cisco Systems, Dover Beach Consulting,
Inc., Carnegie Mellon University, May 1995.
[2] Case, J., Fedor, M., Schoffstall, M., Davin, J., "Simple Network
Management Protocol", STD 15, RFC 1157, SNMP Research, Performance
Systems International, MIT Laboratory for Computer Science, May
1990.
[3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT
Laboratory for Computer Science, April 1992.
[4] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S.,
"Coexistence between Version 1 and Version 2 of the Internet-
standard" Network Management Framework", Internet Draft, SNMP
Research, Inc., Cisco Systems, Dover Beach Consulting, Inc.,
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Carnegie Mellon University, May 1995.
[5] Data Encryption Standard, National Institute of Standards and
Technology. Federal Information Processing Standard (FIPS)
Publication 46-1. Supersedes FIPS Publication 46, (January, 1977;
reaffirmed January, 1988).
[6] Data Encryption Algorithm, American National Standards Institute.
ANSI X3.92-1981, (December, 1980).
[7] DES Modes of Operation, National Institute of Standards and
Technology. Federal Information Processing Standard (FIPS)
Publication 81, (December, 1980).
[8] Data Encryption Algorithm - Modes of Operation, American National
Standards Institute. ANSI X3.106-1983, (May 1983).
[9] Guidelines for Implementing and Using the NBS Data Encryption
Standard, National Institute of Standards and Technology. Federal
Information Processing Standard (FIPS) Publication 74, (April,
1981).
[10] Validating the Correctness of Hardware Implementations of the NBS
Data Encryption Standard, National Institute of Standards and
Technology. Special Publication 500-20.
[11] Maintenance Testing for the Data Encryption Standard, National
Institute of Standards and Technology. Special Publication 500-61,
(August, 1980).
[12] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S., "Protocol
Operations for Version 2 of the Simple Network Management Protocol
(SNMPv2)", Internet Draft, SNMP Research, Inc., Cisco Systems,
Dover Beach Consulting, Inc., Carnegie Mellon University, May 1995.
[13] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S., "Transport
Mappings for Version 2 of the Simple Network Management Protocol
(SNMPv2)", Internet Draft, SNMP Research, Inc., Cisco Systems,
Dover Beach Consulting, Inc., Carnegie Mellon University, May 1995.
[14] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S., "Management
Information Base for Version 2 of the Simple Network Management
Protocol (SNMPv2)", Internet Draft, SNMP Research, Inc., Cisco
Systems, Dover Beach Consulting, Inc., Carnegie Mellon University,
May 1995.
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APPENDIX A - Installation
A.1. Agent Installation Parameters
During installation, an agent is configured with several parameters.
These include:
(1) a security posture
The choice of security posture determines the extent of the view
configured for unauthenticated access. One of three possible
choices is selected:
minimum-secure,
semi-secure, or
very-secure.
(2) one or more transport service addresses
These parameters may be specified explicitly, or they may be
specified implicitly as the same set of network-layer addresses
configured for other uses by the device together with the well-
known transport-layer "port" information for the appropriate
transport domain [13]. The agent listens on each of these
transport service addresses for messages sent on behalf of any user
it knows about.
(3) one or more secrets
These are the authentication/privacy secrets for the first user to
be configured.
One way to accomplish this is to have the installer enter a
"password" for each required secret. The password is then
algorithmically converted into the required secret by: forming a
string of length 1,048,576 octets by repeating the value of the
password as often as necessary, truncating accordingly, and using
the resulting string as the input to the MD5 algorithm. The
resulting digest is the required secret (see Appendix A.2).
With these configured parameters, the agent instantiates the following
user, context, views and access rights. This configuration information
should be persistent.
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- One user:
privacy not supported privacy supported
--------------------- -----------------
<userName> "public" "public"
<authProtocol> Digest Auth. Protocol Digest Auth. Protocol
<authPrivateKey> authentication key authentication key
<privProtocol> none Symmetric Privacy Protocol
<privPrivateKey> -- privacy key
- One local context with its <contextSelector> as the empty-string.
- One view for authenticated access:
- the <all> MIB view is the "internet" subtree.
- A second view for unauthenticated access. This view is configured
according to the selected security posture. For the "very-secure"
posture:
- the <restricted> MIB view is the union of the "snmpStats"
and "usecStats" subtrees.
For the "semi-secure" posture:
- the <restricted> MIB view is the union of the "snmpStats",
"usecStats" and "system" subtrees.
For the "minimum-secure" posture:
- the <restricted> MIB view is the "internet" subtree.
- Access rights to allow:
- read-only access for unauthenticated messages on behalf of the
user "public" to the <restricted> MIB view of contextSelector
"".
- read-write access for authenticated but not private messages
on behalf of the user "public" to the <all> MIB view of
contextSelector "".
- if privacy is supported, read-write access for authenticated
and private messages on behalf of the user "public" to the
<all> MIB view of contextSelector "".
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A.2. Password to Key Algorithm
The following code fragment demonstrates the password to key algorithm
which can be used when mapping a password to an authentication or
privacy key. (The calls to MD5 are as documented in RFC 1321.)
void password_to_key(password, passwordlen, key)
u_char *password; /* IN */
u_int passwordlen; /* IN */
u_char *key; /* OUT - caller supplies pointer to 16
octet buffer */ {
MD5_CTX MD;
u_char *cp, password_buf[64];
u_long password_index = 0;
u_long count = 0, i;
MD5Init (&MD); /* initialize MD5 */
/* loop until we've done 1 Megabyte */
while (count < 1048576) {
cp = password_buf;
for(i = 0; i < 64; i++) {
*cp++ = password[ password_index++ % passwordlen ];
/*
* Take the next byte of the password, wrapping to the
* beginning of the password as necessary.
*/
}
MDupdate (&MD, password_buf, 64);
count += 64;
}
MD5Final (key, &MD); /* tell MD5 we're done */
return; }
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APPENDIX B - Compatibility with SNMPv1 Community Profiles
For an insecure SNMPv2 implementation (see Section 5.3) of an agent, for
which the configuration of access rights is specified via SNMPv1
community profiles, it is mandatory that it treat SNMPv2 messages having
a zero-valued qoS field and a zero-length contextSelector field as
referring to an SNMPv1 community profile; in particular, to the SNMPv1
community profile identified by treating the value of the <userName>
field as if it were an SNMPv1 community string,
For example, the 5-octet community string hexadecimal value of
x"0001020304" would be encoded as a parmeters component value of:
field hex value
----- ---------
<model> 00
<qos> 00
<agentID> 00000000 00000000 00000000
<agentBoots> 00000000
<agentTime> 00000000
<userLen> 05
<userName> 0001020304
<authLen> 00
<authDigest> --
<maxSize> 01e4 (= 484 decimal)
<contextSelector> --
or:
x"0000 00000000 00000000 00000000 00000000 00000000 05 0001020304
0001e4"
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Authors' Addresses
Keith McCloghrie
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
US
Phone: +1 408 526 5260
Email: kzm@cisco.com
Marshall T. Rose
Dover Beach Consulting, Inc.
420 Whisman Court
Mountain View, CA 94043-2186
US
Phone: +1 415 968 1052
Email: mrose@dbc.mtview.ca.us
Glenn W. Waters
Bell-Northern Research Ltd.
P.O. Box 3511, Station C
Ottawa, Ontario K1Y 4H7
CA
Phone: +1 613 763 3933
Email: gwaters@bnr.ca
James M. Galvin
Trusted Information Systems, Inc.
3060 Washington Road, Route 97
Glenwood, MD 21738
US
Phone: +1 301 854 6889
EMail: galvin@tis.com
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Table of Contents
1 Introduction .................................................... 3
1.1 A Note on Terminology ......................................... 3
1.2 Threats ....................................................... 4
1.3 Goals and Constraints ......................................... 5
1.4 Security Services ............................................. 6
1.5 Mechanisms .................................................... 7
1.5.1 Digest Authentication Protocol .............................. 8
1.5.2 Symmetric Encryption Protocol ............................... 9
2 Elements of the Model ........................................... 11
2.1 SNMPv2 Users .................................................. 11
2.2 Context Selectors ............................................. 12
2.3 Quality of Service (qoS) ...................................... 12
2.4 Access Policy ................................................. 12
2.5 Replay Protection ............................................. 13
2.5.1 agentID ..................................................... 13
2.5.2 agentBoots and agentTime .................................... 13
2.5.3 Time Window ................................................. 14
2.6 Maintenance Functions ......................................... 14
2.6.1 The Well-known Maintenance User ............................. 15
2.6.2 Error Reporting ............................................. 15
2.6.3 Time Synchronization ........................................ 16
2.6.4 Proxy Error Propagation ..................................... 16
2.7 SNMPv2 Messages ............................................... 17
2.8 Local Configuration Datastore (LCD) ........................... 19
3 Elements of Procedure ........................................... 20
3.1 Generating a Request or Notification .......................... 20
3.2 Processing a Received Communication ........................... 22
3.2.1 Additional Details .......................................... 28
3.2.1.1 ASN.1 Parsing Errors ...................................... 28
3.2.1.2 Incorrectly Encoded Parameters ............................ 28
3.2.1.3 Generation of a Report PDU ................................ 28
3.3 Generating a Response ......................................... 29
4 Definitions ..................................................... 30
4.1 The USEC Basic Group .......................................... 31
4.2 Conformance Information ....................................... 35
4.2.1 Compliance Statements ....................................... 35
4.2.2 Units of Conformance ........................................ 35
5 Security Considerations ......................................... 36
5.1 Recommended Practices ......................................... 36
5.2 Defining Users ................................................ 37
5.3 Conformance ................................................... 38
6 Acknowledgements ................................................ 39
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7 References ...................................................... 39
Appendix A Installation ........................................... 41
Appendix A.1 Agent Installation Parameters ........................ 41
Appendix A.2 Password to Key Algorithm ............................ 43
Appendix B Compatibility with SNMPv1 Community Profiles ........... 44
Authors' Addresses ................................................ 45
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