draft-ietf-opsawg-tacacs-14.txt   draft-ietf-opsawg-tacacs-15.txt 
Operations T. Dahm Operations T. Dahm
Internet-Draft A. Ota Internet-Draft A. Ota
Intended status: Informational Google Inc Intended status: Informational Google Inc
Expires: March 11, 2020 D. Medway Gash Expires: March 25, 2020 D. Medway Gash
Cisco Systems, Inc. Cisco Systems, Inc.
D. Carrel D. Carrel
vIPtela, Inc. vIPtela, Inc.
L. Grant L. Grant
September 8, 2019 September 22, 2019
The TACACS+ Protocol The TACACS+ Protocol
draft-ietf-opsawg-tacacs-14 draft-ietf-opsawg-tacacs-15
Abstract Abstract
Terminal Access Controller Access-Control System Plus (TACACS+) Terminal Access Controller Access-Control System Plus (TACACS+)
provides Device Administration for routers, network access servers provides Device Administration for routers, network access servers
and other networked computing devices via one or more centralized and other networked computing devices via one or more centralized
servers. This document describes the protocol that is used by servers. This document describes the protocol that is used by
TACACS+. TACACS+.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 11, 2020. This Internet-Draft will expire on March 25, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Technical Definitions . . . . . . . . . . . . . . . . . . . . 4 3. Technical Definitions . . . . . . . . . . . . . . . . . . . . 4
4. TACACS+ Connections and Sessions . . . . . . . . . . . . . . 5 3.1. Client . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Connection . . . . . . . . . . . . . . . . . . . . . . . 5 3.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Session . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.3. Packet . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Treatment of Enumerated Protocol Values . . . . . . . . . 5 3.4. Connection . . . . . . . . . . . . . . . . . . . . . . . 5
4.4. Text Encoding . . . . . . . . . . . . . . . . . . . . . . 6 3.5. Session . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.5. The TACACS+ Packet Header . . . . . . . . . . . . . . . . 6 3.6. Treatment of Enumerated Protocol Values . . . . . . . . . 5
4.6. The TACACS+ Packet Body . . . . . . . . . . . . . . . . . 8 3.7. Treatment of Text Strings . . . . . . . . . . . . . . . . 5
4.7. Single Connection Mode . . . . . . . . . . . . . . . . . 8 4. TACACS+ Packets and Sessions . . . . . . . . . . . . . . . . 6
4.8. Session Completion . . . . . . . . . . . . . . . . . . . 9 4.1. The TACACS+ Packet Header . . . . . . . . . . . . . . . . 6
4.9. Data Obfuscation . . . . . . . . . . . . . . . . . . . . 10 4.2. The TACACS+ Packet Body . . . . . . . . . . . . . . . . . 8
4.3. Single Connection Mode . . . . . . . . . . . . . . . . . 9
4.4. Session Completion . . . . . . . . . . . . . . . . . . . 9
4.5. Data Obfuscation . . . . . . . . . . . . . . . . . . . . 11
5. Authentication . . . . . . . . . . . . . . . . . . . . . . . 12 5. Authentication . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. The Authentication START Packet Body . . . . . . . . . . 12 5.1. The Authentication START Packet Body . . . . . . . . . . 13
5.2. The Authentication REPLY Packet Body . . . . . . . . . . 15 5.2. The Authentication REPLY Packet Body . . . . . . . . . . 15
5.3. The Authentication CONTINUE Packet Body . . . . . . . . . 16 5.3. The Authentication CONTINUE Packet Body . . . . . . . . . 16
5.4. Description of Authentication Process . . . . . . . . . . 17 5.4. Description of Authentication Process . . . . . . . . . . 17
5.4.1. Version Behavior . . . . . . . . . . . . . . . . . . 17 5.4.1. Version Behavior . . . . . . . . . . . . . . . . . . 18
5.4.2. Common Authentication Flows . . . . . . . . . . . . . 18 5.4.2. Common Authentication Flows . . . . . . . . . . . . . 19
5.4.3. Aborting an Authentication Session . . . . . . . . . 21 5.4.3. Aborting an Authentication Session . . . . . . . . . 22
6. Authorization . . . . . . . . . . . . . . . . . . . . . . . . 22 6. Authorization . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1. The Authorization REQUEST Packet Body . . . . . . . . . . 23 6.1. The Authorization REQUEST Packet Body . . . . . . . . . . 23
6.2. The Authorization REPLY Packet Body . . . . . . . . . . . 26 6.2. The Authorization REPLY Packet Body . . . . . . . . . . . 27
7. Accounting . . . . . . . . . . . . . . . . . . . . . . . . . 27 7. Accounting . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.1. The Account REQUEST Packet Body . . . . . . . . . . . . . 28 7.1. The Account REQUEST Packet Body . . . . . . . . . . . . . 29
7.2. The Accounting REPLY Packet Body . . . . . . . . . . . . 29 7.2. The Accounting REPLY Packet Body . . . . . . . . . . . . 30
8. Attribute-Value Pairs . . . . . . . . . . . . . . . . . . . . 30 8. Attribute-Value Pairs . . . . . . . . . . . . . . . . . . . . 31
8.1. Value Encoding . . . . . . . . . . . . . . . . . . . . . 31 8.1. Value Encoding . . . . . . . . . . . . . . . . . . . . . 32
8.2. Authorization Attributes . . . . . . . . . . . . . . . . 31 8.2. Authorization Attributes . . . . . . . . . . . . . . . . 32
8.3. Accounting Attributes . . . . . . . . . . . . . . . . . . 33 8.3. Accounting Attributes . . . . . . . . . . . . . . . . . . 34
9. Privilege Levels . . . . . . . . . . . . . . . . . . . . . . 35 9. Privilege Levels . . . . . . . . . . . . . . . . . . . . . . 36
10. Security Considerations . . . . . . . . . . . . . . . . . . . 36 10. Security Considerations . . . . . . . . . . . . . . . . . . . 37
10.1. General Security of the Protocol . . . . . . . . . . . . 36 10.1. General Security of the Protocol . . . . . . . . . . . . 37
10.2. Security of Authentication Sessions . . . . . . . . . . 37 10.2. Security of Authentication Sessions . . . . . . . . . . 38
10.3. Security of Authorization Sessions . . . . . . . . . . . 38 10.3. Security of Authorization Sessions . . . . . . . . . . . 39
10.4. Security of Accounting Sessions . . . . . . . . . . . . 38 10.4. Security of Accounting Sessions . . . . . . . . . . . . 39
10.5. TACACS+ Best Practices . . . . . . . . . . . . . . . . . 39 10.5. TACACS+ Best Practices . . . . . . . . . . . . . . . . . 40
10.5.1. Shared Secrets . . . . . . . . . . . . . . . . . . . 39 10.5.1. Shared Secrets . . . . . . . . . . . . . . . . . . . 40
10.5.2. Connections and Obfuscation . . . . . . . . . . . . 40 10.5.2. Connections and Obfuscation . . . . . . . . . . . . 41
10.5.3. Authentication . . . . . . . . . . . . . . . . . . . 41 10.5.3. Authentication . . . . . . . . . . . . . . . . . . . 42
10.5.4. Authorization . . . . . . . . . . . . . . . . . . . 41 10.5.4. Authorization . . . . . . . . . . . . . . . . . . . 42
10.5.5. Redirection Mechanism . . . . . . . . . . . . . . . 41 10.5.5. Redirection Mechanism . . . . . . . . . . . . . . . 42
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 43
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 42 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 43
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 42 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 43
13.1. Normative References . . . . . . . . . . . . . . . . . . 42 13.1. Normative References . . . . . . . . . . . . . . . . . . 43
13.2. Informative References . . . . . . . . . . . . . . . . . 43 13.2. Informative References . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction 1. Introduction
Terminal Access Controller Access-Control System Plus (TACACS+) was Terminal Access Controller Access-Control System Plus (TACACS+) was
conceived initially as a general Authentication, Authorization and conceived initially as a general Authentication, Authorization and
Accounting protocol. It's use today is mainly confined to Device Accounting protocol. It's use today is mainly confined to Device
Administration: authenticating access to network devices, providing Administration: authenticating access to network devices, providing
central authorization of operations, and audit of those operations. central authorization of operations, and audit of those operations.
A wide range of TACACS+ clients and servers are already deployed in A wide range of TACACS+ clients and servers are already deployed in
the field. The TACACS+ protocol they are based on is defined in a the field. The TACACS+ protocol they are based on is defined in a
draft document that was originally intended for IETF publication, and draft document that was originally intended for IETF publication, and
is known as `The Draft' [TheDraft]. This did not address all of the is known as `The Draft' [TheDraft] . This did not address all of the
key security concerns which are considered when designing modern key security concerns which are considered when designing modern
standards. For more details please refer to security section standards. For more details please refer to security section
(Section 10). (Section 10) .
This is intended to document the TACACS+ protocol as it is currently This is intended to document the TACACS+ protocol as it is currently
deployed. It is intended that all implementations which conform to deployed. It is intended that all implementations which conform to
this document will conform to `The Draft'. However, attention is this document will conform to `The Draft'. However, attention is
drawn to the following specific adjustments of the protocol drawn to the following specific adjustments of the protocol
specification from 'The Draft': specification from 'The Draft':
This document officially removes SENDPASS for security reasons. This document officially removes SENDPASS for security reasons.
The normative description of Legacy features such as ARAP and The normative description of Legacy features such as ARAP and
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
3. Technical Definitions 3. Technical Definitions
This section provides a few basic definitions that are applicable to This section provides a few basic definitions that are applicable to
this document this document
Client 3.1. Client
The client is any device which initiates TACACS+ protocol requests to The client is any device which initiates TACACS+ protocol requests to
mediate access, mainly for the Device Administration use case. mediate access, mainly for the Device Administration use case.
Server 3.2. Server
The server receives TACACS+ protocol requests, and replies according The server receives TACACS+ protocol requests, and replies according
to its business model, in accordance with the flows defined in this to its business model, in accordance with the flows defined in this
document. document.
Packet 3.3. Packet
All uses of the word packet in this document refer to TACACS+ All uses of the word packet in this document refer to TACACS+
protocol data units unless explicitly noted otherwise. The informal protocol data units unless explicitly noted otherwise. The informal
term "Packet" has become an established part of the definition. term "Packet" has become an established part of the definition.
Session 3.4. Connection
A TACACS+ Session refers to a single authentication, authorization,
or accounting transaction between the client and the server. These
frequently consist of one packet from the client and one response
from the server. TACACS+ incorporates features to extend sessions to
multiple packets, if needed to support the flow, as described in
later parts of this document.
4. TACACS+ Connections and Sessions
4.1. Connection
TACACS+ uses TCP for its transport. TCP Server port 49 is allocated TACACS+ uses TCP for its transport. TCP Server port 49 is allocated
by IANA for TACACS+ traffic. by IANA for TACACS+ traffic.
4.2. Session 3.5. Session
The concept of a session is used throughout this document. A TACACS+ The concept of a session is used throughout this document. A TACACS+
session is a single authentication sequence, a single authorization session is a single authentication sequence, a single authorization
exchange, or a single accounting exchange. exchange, or a single accounting exchange.
An accounting and authorization session will consist of a single pair An accounting and authorization session will consist of a single pair
of packets (the request and its reply). An authentication session of packets (the request and its reply). An authentication session
may involve an arbitrary number of packets being exchanged. The may involve an arbitrary number of packets being exchanged. The
session is an operational concept that is maintained between the session is an operational concept that is maintained between the
TACACS+ client and server. It does not necessarily correspond to a TACACS+ client and server. It does not necessarily correspond to a
given user or user action. given user or user action.
4.3. Treatment of Enumerated Protocol Values 3.6. Treatment of Enumerated Protocol Values
This document describes various enumerated values in the packet This document describes various enumerated values in the packet
header and the headers for specific packet types. For example, in header and the headers for specific packet types. For example, in
the Authentication start packet type, this document defines the the Authentication start packet type, this document defines the
action field with three values TAC_PLUS_AUTHEN_LOGIN, action field with three values TAC_PLUS_AUTHEN_LOGIN,
TAC_PLUS_AUTHEN_CHPASS and TAC_PLUS_AUTHEN_SENDAUTH. TAC_PLUS_AUTHEN_CHPASS and TAC_PLUS_AUTHEN_SENDAUTH.
If the server does not implement one of the defined options in a If the server does not implement one of the defined options in a
packet that it receives, or it encounters an option that is not packet that it receives, or it encounters an option that is not
listed in this document for a header field, then it should respond listed in this document for a header field, then it should respond
with an ERROR and terminate the session. This will allow the client with an ERROR and terminate the session. This will allow the client
to try a different option. to try a different option.
If an error occurs but the type of the incoming packet cannot be If an error occurs but the type of the incoming packet cannot be
determined, a packet with the identical cleartext header but with a determined, a packet with the identical cleartext header but with a
sequence number incremented by one and the length set to zero MUST be sequence number incremented by one and the length set to zero MUST be
returned to indicate an error. returned to indicate an error.
4.4. Text Encoding 3.7. Treatment of Text Strings
All text fields in TACACS+ MUST be printable US-ASCII, excepting The TACACS+ protocol makes extensive use of text strings. The
special consideration given to user field and data fields used for original draft intended that these strings would be treated as byte
passwords. arrays where each byte would represent a US-ASCII character.
To ensure interoperability of current deployments, the TACACS+ client More recently, server implementations have been extended to interwork
and server MUST handle user fields and those data fields used for with external identity services, and so a more nuanced approach is
passwords as 8-bit octet strings. The deployment operator MUST needed. Text Strings in the TACACS+ protocol MUST be handled in the
ensure that consistent character encoding is applied from the end following way:
client to the server. The encoding SHOULD be UTF-8, and other
encodings outside printable US-ASCII SHOULD be deprecated.
4.5. The TACACS+ Packet Header Usernames
Usernames MUST be encoded and handled using the UsernameCasePreserved
Profile specified in RFC 8265 [RFC8265]. The security considerations
in Section 8 of that RFC apply.
Passwords
Passwords MUST be handled and processed using the OpaqueString
Profile specified in RFC 8265 [RFC8265]. The security considerations
in Section 8 of that RFC apply.
Binary Data Strings
Where specifically mentioned, data fields contain arrays of arbitrary
bytes as required for protocol processing. These are not intended to
be made visible through user interface to users.
Printable Text String
All other text fields in TACACS+ MUST be treated as printable byte
arrays of US-ASCII as defined by RFC 20 [RFC0020]. The term
"printable" used here means the fields MUST exclude the "Control
Characters" defined in section 5.2 of RFC 20 [RFC0020].
4. TACACS+ Packets and Sessions
4.1. The TACACS+ Packet Header
All TACACS+ packets begin with the following 12-byte header. The All TACACS+ packets begin with the following 12-byte header. The
header describes the remainder of the packet: header describes the remainder of the packet:
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+ +----------------+----------------+----------------+----------------+
|major | minor | | | | |major | minor | | | |
|version| version| type | seq_no | flags | |version| version| type | seq_no | flags |
+----------------+----------------+----------------+----------------+ +----------------+----------------+----------------+----------------+
| | | |
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flags flags
This field contains various bitmapped flags. This field contains various bitmapped flags.
The flag bit: The flag bit:
TAC_PLUS_UNENCRYPTED_FLAG := 0x01 TAC_PLUS_UNENCRYPTED_FLAG := 0x01
This flag indicates that the sender did not obfuscate the body of the This flag indicates that the sender did not obfuscate the body of the
packet. The application of this flag will be covered in the security packet. The application of this flag will be covered in the security
section (Section 10). section (Section 10) .
This flag SHOULD be clear in all deployments. Modern network traffic This flag SHOULD be clear in all deployments. Modern network traffic
tools support encrypted traffic when configured with the shared tools support encrypted traffic when configured with the shared
secret (see section below), so obfuscated mode can and SHOULD be used secret (see section below), so obfuscated mode can and SHOULD be used
even during test. even during test.
The single-connection flag: The single-connection flag:
TAC_PLUS_SINGLE_CONNECT_FLAG := 0x04 TAC_PLUS_SINGLE_CONNECT_FLAG := 0x04
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The Id for this TACACS+ session. This field does not change for the The Id for this TACACS+ session. This field does not change for the
duration of the TACACS+ session. This number MUST be generated by a duration of the TACACS+ session. This number MUST be generated by a
cryptographically strong random number generation method. Failure to cryptographically strong random number generation method. Failure to
do so will compromise security of the session. For more details do so will compromise security of the session. For more details
refer to RFC 4086 [RFC4086] refer to RFC 4086 [RFC4086]
length length
The total length of the packet body (not including the header). The total length of the packet body (not including the header).
4.6. The TACACS+ Packet Body 4.2. The TACACS+ Packet Body
The TACACS+ body types are defined in the packet header. The next The TACACS+ body types are defined in the packet header. The next
sections of this document will address the contents of the different sections of this document will address the contents of the different
TACACS+ bodies. TACACS+ bodies.
4.7. Single Connection Mode 4.3. Single Connection Mode
Single Connection Mode is intended to improve performance by allowing Single Connection Mode is intended to improve performance by allowing
a client to multiplex multiple session on a single TCP connection. a client to multiplex multiple session on a single TCP connection.
The packet header contains the TAC_PLUS_SINGLE_CONNECT_FLAG used by The packet header contains the TAC_PLUS_SINGLE_CONNECT_FLAG used by
the client and server to negotiate the use of Single Connect Mode. the client and server to negotiate the use of Single Connect Mode.
The client sets this flag, to indicate that it supports multiplexing The client sets this flag, to indicate that it supports multiplexing
TACACS+ sessions over a single TCP connection. The client MUST NOT TACACS+ sessions over a single TCP connection. The client MUST NOT
send a second packet on a connection until single-connect status has send a second packet on a connection until single-connect status has
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The server may refuse to allow Single Connection Mode for the client. The server may refuse to allow Single Connection Mode for the client.
For example, it may not be appropriate to allocate a long-lasting TCP For example, it may not be appropriate to allocate a long-lasting TCP
connection to a specific client in some deployments. Even if the connection to a specific client in some deployments. Even if the
server is configured to permit single Connection Mode for a specific server is configured to permit single Connection Mode for a specific
client, the server may close the connection. For example: a server client, the server may close the connection. For example: a server
may be configured to time out a Single Connection Mode TCP Connection may be configured to time out a Single Connection Mode TCP Connection
after a specific period of inactivity to preserve its resources. The after a specific period of inactivity to preserve its resources. The
client MUST accommodate such closures on a TCP session even after client MUST accommodate such closures on a TCP session even after
Single Connection Mode has been established. Single Connection Mode has been established.
4.8. Session Completion 4.4. Session Completion
The REPLY packets defined for the packets types in the sections below The REPLY packets defined for the packets types in the sections below
(Authentication, Authorization and Accounting) contain a status (Authentication, Authorization and Accounting) contain a status
field. The complete set of options for this field depend upon the field. The complete set of options for this field depend upon the
packet type, but all three REPLY packet types define values packet type, but all three REPLY packet types define values
representing PASS, ERROR and FAIL, which indicate the last packet of representing PASS, ERROR and FAIL, which indicate the last packet of
a regular session (one which is not aborted). a regular session (one which is not aborted).
The server responds with a PASS or a FAIL to indicate that the The server responds with a PASS or a FAIL to indicate that the
processing of the request completed and the client can apply the processing of the request completed and the client can apply the
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Sessions for details on handling additional status options. Sessions for details on handling additional status options.
When the session is complete, then the TCP connection should be When the session is complete, then the TCP connection should be
handled as follows, according to whether Single Connection Mode was handled as follows, according to whether Single Connection Mode was
negotiated: negotiated:
If Single Connection Mode was not negotiated, then the connection If Single Connection Mode was not negotiated, then the connection
should be closed should be closed
If Single Connection Mode was enabled, then the connection SHOULD be If Single Connection Mode was enabled, then the connection SHOULD be
left open (see section (Section 4.7)), but may still be closed after left open (see section (Section 4.3) ), but may still be closed after
a timeout period to preserve deployment resources. a timeout period to preserve deployment resources.
If Single Connection Mode was enabled, but an ERROR occurred due to If Single Connection Mode was enabled, but an ERROR occurred due to
connection issues (such as an incorrect secret, see section connection issues (such as an incorrect secret, see section
(Section 4.9)), then any further new sessions MUST NOT be accepted on (Section 4.5) ), then any further new sessions MUST NOT be accepted
the connection. If there are any sessions that have already been on the connection. If there are any sessions that have already been
established then they MAY be completed. Once all active sessions are established then they MAY be completed. Once all active sessions are
completed then the connection MUST be closed. completed then the connection MUST be closed.
It is recommended that client implementations provide robust schemes It is recommended that client implementations provide robust schemes
for dealing with servers which cannot be connected to. Options for dealing with servers which cannot be connected to. Options
include providing a list of servers for redundancy, and an option for include providing a list of servers for redundancy, and an option for
a local fallback configuration if no servers can be reached. Details a local fallback configuration if no servers can be reached. Details
will be implementation specific. will be implementation specific.
The client should manage connections and handle the case of a server The client should manage connections and handle the case of a server
which establishes a connection, but does not respond. The exact which establishes a connection, but does not respond. The exact
behavior is implementation specific. It is recommended that the behavior is implementation specific. It is recommended that the
client should close the connection after a configurable timeout. client should close the connection after a configurable timeout.
4.9. Data Obfuscation 4.5. Data Obfuscation
The body of packets may be obfuscated. The following sections The body of packets may be obfuscated. The following sections
describe the obfuscation method that is supported in the protocol. describe the obfuscation method that is supported in the protocol.
In 'The Draft' this process was actually referred to as Encryption, In 'The Draft' this process was actually referred to as Encryption,
but the algorithm would not meet modern standards, and so will not be but the algorithm would not meet modern standards, and so will not be
termed as encryption in this document. termed as encryption in this document.
The obfuscation mechanism relies on a secret key, a shared secret The obfuscation mechanism relies on a secret key, a shared secret
value that is known to both the client and the server. The secret value that is known to both the client and the server. The secret
keys MUST remain secret. keys MUST remain secret.
skipping to change at page 11, line 18 skipping to change at page 11, line 39
The packet body can then be de-obfuscated by XOR-ing it byte-wise The packet body can then be de-obfuscated by XOR-ing it byte-wise
with a pseudo random pad. with a pseudo random pad.
data = ENCRYPTED {data} ^ pseudo_pad data = ENCRYPTED {data} ^ pseudo_pad
The pad is generated by concatenating a series of MD5 hashes (each 16 The pad is generated by concatenating a series of MD5 hashes (each 16
bytes long) and truncating it to the length of the input data. bytes long) and truncating it to the length of the input data.
Whenever used in this document, MD5 refers to the "RSA Data Security, Whenever used in this document, MD5 refers to the "RSA Data Security,
Inc. MD5 Message-Digest Algorithm" as specified in RFC 1321 Inc. MD5 Message-Digest Algorithm" as specified in RFC 1321 [RFC1321]
[RFC1321]. .
pseudo_pad = {MD5_1 [,MD5_2 [ ... ,MD5_n]]} truncated to len(data) pseudo_pad = {MD5_1 [,MD5_2 [ ... ,MD5_n]]} truncated to len(data)
The first MD5 hash is generated by concatenating the session_id, the The first MD5 hash is generated by concatenating the session_id, the
secret key, the version number and the sequence number and then secret key, the version number and the sequence number and then
running MD5 over that stream. All of those input values are running MD5 over that stream. All of those input values are
available in the packet header, except for the secret key which is a available in the packet header, except for the secret key which is a
shared secret between the TACACS+ client and server. shared secret between the TACACS+ client and server.
The version number and session_id are extracted from the header The version number and session_id are extracted from the header
skipping to change at page 11, line 30 skipping to change at page 12, line 4
pseudo_pad = {MD5_1 [,MD5_2 [ ... ,MD5_n]]} truncated to len(data) pseudo_pad = {MD5_1 [,MD5_2 [ ... ,MD5_n]]} truncated to len(data)
The first MD5 hash is generated by concatenating the session_id, the The first MD5 hash is generated by concatenating the session_id, the
secret key, the version number and the sequence number and then secret key, the version number and the sequence number and then
running MD5 over that stream. All of those input values are running MD5 over that stream. All of those input values are
available in the packet header, except for the secret key which is a available in the packet header, except for the secret key which is a
shared secret between the TACACS+ client and server. shared secret between the TACACS+ client and server.
The version number and session_id are extracted from the header The version number and session_id are extracted from the header
Subsequent hashes are generated by using the same input stream, but Subsequent hashes are generated by using the same input stream, but
concatenating the previous hash value at the end of the input stream. concatenating the previous hash value at the end of the input stream.
MD5_1 = MD5{session_id, key, version, seq_no} MD5_2 = MD5{session_id, MD5_1 = MD5{session_id, key, version, seq_no} MD5_2 = MD5{session_id,
key, version, seq_no, MD5_1} .... MD5_n = MD5{session_id, key, key, version, seq_no, MD5_1} .... MD5_n = MD5{session_id, key,
version, seq_no, MD5_n-1} version, seq_no, MD5_n-1}
When a server detects that the secret(s) it has configured for the When a server detects that the secret(s) it has configured for the
device mismatch, it MUST return ERROR. For details of TCP connection device mismatch, it MUST return ERROR. For details of TCP connection
handling on ERROR, refer to section (Section 4.8). handling on ERROR, refer to section (Section 4.4) .
TAC_PLUS_UNENCRYPTED_FLAG == 0x1 TAC_PLUS_UNENCRYPTED_FLAG == 0x1
In this case, the entire packet body is in cleartext. Obfuscation In this case, the entire packet body is in cleartext. Obfuscation
and de-obfuscation are null operations. This method should be and de-obfuscation are null operations. This method should be
avoided unless absolutely required for debug purposes, when tooling avoided unless absolutely required for debug purposes, when tooling
does not permit de-obfuscation. does not permit de-obfuscation.
If deployment is configured for obfuscating a connection then the If deployment is configured for obfuscating a connection then the
request MUST be dropped if TAC_PLUS_UNENCRYPTED_FLAG is set to true. request MUST be dropped if TAC_PLUS_UNENCRYPTED_FLAG is set to true.
After a packet body is de-obfuscated, the lengths of the component After a packet body is de-obfuscated, the lengths of the component
values in the packet are summed. If the sum is not identical to the values in the packet are summed. If the sum is not identical to the
cleartext datalength value from the header, the packet MUST be cleartext datalength value from the header, the packet MUST be
discarded, and an ERROR signaled. For details of TCP connection discarded, and an ERROR signaled. For details of TCP connection
handling on ERROR, refer to section (Section 4.8). handling on ERROR, refer to section (Section 4.4) .
Commonly such failures are seen when the keys are mismatched between Commonly such failures are seen when the keys are mismatched between
the client and the TACACS+ server. the client and the TACACS+ server.
5. Authentication 5. Authentication
Authentication is the action of determining who a user (or entity) Authentication is the action of determining who a user (or entity)
is. Authentication can take many forms. Traditional authentication is. Authentication can take many forms. Traditional authentication
employs a name and a fixed password. However, fixed passwords are employs a name and a fixed password. However, fixed passwords are
vulnerable security, so many modern authentication mechanisms utilize vulnerable security, so many modern authentication mechanisms utilize
skipping to change at page 13, line 21 skipping to change at page 13, line 43
TAC_PLUS_AUTHEN_SENDAUTH := 0x04 TAC_PLUS_AUTHEN_SENDAUTH := 0x04
priv_lvl priv_lvl
This indicates the privilege level that the user is authenticating This indicates the privilege level that the user is authenticating
as. Please refer to the Privilege Level section (Section 9) below. as. Please refer to the Privilege Level section (Section 9) below.
authen_type authen_type
The type of authentication. Please see section Common Authentication The type of authentication. Please see section Common Authentication
Flows (Section 5.4.2). Valid values are: Flows (Section 5.4.2) . Valid values are:
TAC_PLUS_AUTHEN_TYPE_ASCII := 0x01 TAC_PLUS_AUTHEN_TYPE_ASCII := 0x01
TAC_PLUS_AUTHEN_TYPE_PAP := 0x02 TAC_PLUS_AUTHEN_TYPE_PAP := 0x02
TAC_PLUS_AUTHEN_TYPE_CHAP := 0x03 TAC_PLUS_AUTHEN_TYPE_CHAP := 0x03
TAC_PLUS_AUTHEN_TYPE_MSCHAP := 0x05 TAC_PLUS_AUTHEN_TYPE_MSCHAP := 0x05
TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 := 0x06 TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 := 0x06
authen_service authen_service
This is the service that is requesting the authentication. Valid This is the service that is requesting the authentication. Valid
values are: values are:
TAC_PLUS_AUTHEN_SVC_NONE := 0x00 TAC_PLUS_AUTHEN_SVC_NONE := 0x00
TAC_PLUS_AUTHEN_SVC_LOGIN := 0x01 TAC_PLUS_AUTHEN_SVC_LOGIN := 0x01
skipping to change at page 14, line 30 skipping to change at page 15, line 4
used when none of the other authen_service values are appropriate. used when none of the other authen_service values are appropriate.
ENABLE may be requested independently, no requirements for previous ENABLE may be requested independently, no requirements for previous
authentications or authorizations are imposed by the protocol. authentications or authorizations are imposed by the protocol.
Other options are included for legacy/backwards compatibility. Other options are included for legacy/backwards compatibility.
user, user_len user, user_len
The username is optional in this packet, depending upon the class of The username is optional in this packet, depending upon the class of
authentication. If it is absent, the client MUST set user_len to 0. authentication. If it is absent, the client MUST set user_len to 0.
If included, the user_len indicates the length of the user field, in If included, the user_len indicates the length of the user field, in
bytes. bytes.
port, port_len port, port_len
The printable US-ASCII name of the client port on which the The name of the client port on which the authentication is taking
authentication is taking place, and its length in bytes. The value place, and its length in bytes. The value of this field is client
of this field is client specific. The port_len indicates the length specific. The port_len indicates the length of the port field, in
of the port field, in bytes. bytes. For details of text encoding, see (Section 3.7) .
rem_addr, rem_addr_len rem_addr, rem_addr_len
A printable US-ASCII string indicating the remote location from which A string indicating the remote location from which the user has
the user has connected to the client. connected to the client. For details of text encoding, see
(Section 3.7) .
When TACACS+ was used for dial-up services, this value contained the When TACACS+ was used for dial-up services, this value contained the
caller ID caller ID
When TACACS+ is used for Device Administration, the user is normally When TACACS+ is used for Device Administration, the user is normally
connected via a network, and in this case the value is intended to connected via a network, and in this case the value is intended to
hold a network address, IPv4 or IPv6. For IPv6 address text hold a network address, IPv4 or IPv6. For IPv6 address text
representation defined please see RFC 5952 [RFC5952]. representation defined please see RFC 5952 [RFC5952] .
This field is optional (since the information may not be available). This field is optional (since the information may not be available).
The rem_addr_len indicates the length of the user field, in bytes. The rem_addr_len indicates the length of the user field, in bytes.
data, data_len data, data_len
This field is used to send data appropriate for the action and This field is used to send data appropriate for the action and
authen_type. It is described in more detail in the section Common authen_type. It is described in more detail in the section Common
Authentication flows (Section 5.4.2). The data_len indicates the Authentication flows (Section 5.4.2) . The data_len indicates the
length of the data field, in bytes. length of the data field, in bytes.
5.2. The Authentication REPLY Packet Body 5.2. The Authentication REPLY Packet Body
The TACACS+ server sends only one type of authentication packet (a The TACACS+ server sends only one type of authentication packet (a
REPLY packet) to the client. REPLY packet) to the client.
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+ +----------------+----------------+----------------+----------------+
| status | flags | server_msg_len | | status | flags | server_msg_len |
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flags flags
Bitmapped flags that modify the action to be taken. The following Bitmapped flags that modify the action to be taken. The following
values are defined: values are defined:
TAC_PLUS_REPLY_FLAG_NOECHO := 0x01 TAC_PLUS_REPLY_FLAG_NOECHO := 0x01
server_msg, server_msg_len server_msg, server_msg_len
A message to be displayed to the user. This field is optional. The A message to be displayed to the user. This field is optional. The
printable US-ASCII charset MUST be used. The server_msg_len server_msg_len indicates the length of the server_msg field, in
indicates the length of the server_msg field, in bytes. bytes. For details of text encoding, see (Section 3.7) .
data, data_len data, data_len
This field holds data that is a part of the authentication exchange This field holds data that is a part of the authentication exchange
and is intended for the client, not the user. Examples of its use and is intended for client processing, not the user. It is not a
are shown in the section Common Authentication flows (Section 5.4.2). printable text encoding. Examples of its use are shown in the
The data_len indicates the length of the data field, in bytes. section Common Authentication flows (Section 5.4.2) . The data_len
indicates the length of the data field, in bytes.
5.3. The Authentication CONTINUE Packet Body 5.3. The Authentication CONTINUE Packet Body
This packet is sent from the client to the server following the This packet is sent from the client to the server following the
receipt of a REPLY packet. receipt of a REPLY packet.
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+ +----------------+----------------+----------------+----------------+
| user_msg len | data_len | | user_msg len | data_len |
+----------------+----------------+----------------+----------------+ +----------------+----------------+----------------+----------------+
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This field is the string that the user entered, or the client This field is the string that the user entered, or the client
provided on behalf of the user, in response to the server_msg from a provided on behalf of the user, in response to the server_msg from a
REPLY packet. The user_len indicates the length of the user field, REPLY packet. The user_len indicates the length of the user field,
in bytes. in bytes.
data, data_len data, data_len
This field carries information that is specific to the action and the This field carries information that is specific to the action and the
authen_type for this session. Valid uses of this field are described authen_type for this session. Valid uses of this field are described
below. The data_len indicates the length of the data field, in below. It is not a printable text encoding. The data_len indicates
bytes. the length of the data field, in bytes.
flags flags
This holds the bitmapped flags that modify the action to be taken. This holds the bitmapped flags that modify the action to be taken.
The following values are defined: The following values are defined:
TAC_PLUS_CONTINUE_FLAG_ABORT := 0x01 TAC_PLUS_CONTINUE_FLAG_ABORT := 0x01
5.4. Description of Authentication Process 5.4. Description of Authentication Process
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5.4.2.2. PAP Login 5.4.2.2. PAP Login
action = TAC_PLUS_AUTHEN_LOGIN action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_PAP authen_type = TAC_PLUS_AUTHEN_TYPE_PAP
minor_version = 0x1 minor_version = 0x1
The entire exchange MUST consist of a single START packet and a The entire exchange MUST consist of a single START packet and a
single REPLY. The START packet MUST contain a username and the data single REPLY. The START packet MUST contain a username and the data
field MUST contain the PAP ASCII password. A PAP authentication only field MUST contain the PAP ASCII password. A PAP authentication only
consists of a username and password RFC 1334 [RFC1334](Obsolete). consists of a username and password RFC 1334 [RFC1334] (Obsolete).
The REPLY from the server MUST be either a PASS, FAIL or ERROR. The REPLY from the server MUST be either a PASS, FAIL or ERROR.
5.4.2.3. CHAP login 5.4.2.3. CHAP login
action = TAC_PLUS_AUTHEN_LOGIN action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_CHAP authen_type = TAC_PLUS_AUTHEN_TYPE_CHAP
minor_version = 0x1 minor_version = 0x1
The entire exchange MUST consist of a single START packet and a The entire exchange MUST consist of a single START packet and a
single REPLY. The START packet MUST contain the username in the user single REPLY. The START packet MUST contain the username in the user
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The length of the challenge value can be determined from the length The length of the challenge value can be determined from the length
of the data field minus the length of the id (always 1 octet) and the of the data field minus the length of the id (always 1 octet) and the
length of the response field (always 49 octets). length of the response field (always 49 octets).
To perform the authentication, the server will use a combination of To perform the authentication, the server will use a combination of
MD4 and DES on the user's secret and the challenge, as defined in RFC MD4 and DES on the user's secret and the challenge, as defined in RFC
2433 [RFC2433] and then compare the resulting value with the 2433 [RFC2433] and then compare the resulting value with the
response. The REPLY from the server MUST be a PASS or FAIL. response. The REPLY from the server MUST be a PASS or FAIL.
For best practices, please refer to RFC 2433 [RFC2433]. The TACACS+ For best practices, please refer to RFC 2433 [RFC2433] . The TACACS+
server MUST reject authentications where the challenge deviates from server MUST reject authentications where the challenge deviates from
8 bytes as defined in the RFC. 8 bytes as defined in the RFC.
5.4.2.5. MS-CHAP v2 login 5.4.2.5. MS-CHAP v2 login
action = TAC_PLUS_AUTHEN_LOGIN action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 authen_type = TAC_PLUS_AUTHEN_TYPE_MSCHAPV2
minor_version = 0x1 minor_version = 0x1
The entire exchange MUST consist of a single START packet and a The entire exchange MUST consist of a single START packet and a
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The length of the challenge value can be determined from the length The length of the challenge value can be determined from the length
of the data field minus the length of the id (always 1 octet) and the of the data field minus the length of the id (always 1 octet) and the
length of the response field (always 49 octets). length of the response field (always 49 octets).
To perform the authentication, the server will use the algorithm To perform the authentication, the server will use the algorithm
specified RFC 2759 [RFC2759] on the user's secret and challenge and specified RFC 2759 [RFC2759] on the user's secret and challenge and
then compare the resulting value with the response. The REPLY from then compare the resulting value with the response. The REPLY from
the server MUST be a PASS or FAIL. the server MUST be a PASS or FAIL.
For best practices for MS-CHAP v2, please refer to RFC2759 [RFC2759]. For best practices for MS-CHAP v2, please refer to RFC2759 [RFC2759]
The TACACS+ server MUST reject authentications where the challenge . The TACACS+ server MUST reject authentications where the challenge
deviates from 16 bytes as defined in the RFC. deviates from 16 bytes as defined in the RFC.
5.4.2.6. Enable Requests 5.4.2.6. Enable Requests
action = TAC_PLUS_AUTHEN_LOGIN action = TAC_PLUS_AUTHEN_LOGIN
priv_lvl = implementation dependent priv_lvl = implementation dependent
authen_type = not used authen_type = not used
service = TAC_PLUS_AUTHEN_SVC_ENABLE service = TAC_PLUS_AUTHEN_SVC_ENABLE
This is an ENABLE request, used to change the current running This is an ENABLE request, used to change the current running
privilege level of a user. The exchange MAY consist of multiple privilege level of a user. The exchange MAY consist of multiple
messages while the server collects the information it requires in messages while the server collects the information it requires in
order to allow changing the principal's privilege level. This order to allow changing the principal's privilege level. This
exchange is very similar to an ASCII login (Section 5.4.2.1). exchange is very similar to an ASCII login (Section 5.4.2.1) .
In order to readily distinguish enable requests from other types of In order to readily distinguish enable requests from other types of
request, the value of the authen_service field MUST be set to request, the value of the authen_service field MUST be set to
TAC_PLUS_AUTHEN_SVC_ENABLE when requesting an ENABLE. It MUST NOT be TAC_PLUS_AUTHEN_SVC_ENABLE when requesting an ENABLE. It MUST NOT be
set to this value when requesting any other operation. set to this value when requesting any other operation.
5.4.2.7. ASCII change password request 5.4.2.7. ASCII change password request
action = TAC_PLUS_AUTHEN_CHPASS action = TAC_PLUS_AUTHEN_CHPASS
authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII
skipping to change at page 21, line 37 skipping to change at page 22, line 22
It is very similar to an ASCII login. The status value It is very similar to an ASCII login. The status value
TAC_PLUS_AUTHEN_STATUS_GETPASS MUST only be used when requesting the TAC_PLUS_AUTHEN_STATUS_GETPASS MUST only be used when requesting the
"new" password. It MAY be sent multiple times. When requesting the "new" password. It MAY be sent multiple times. When requesting the
"old" password, the status value MUST be set to "old" password, the status value MUST be set to
TAC_PLUS_AUTHEN_STATUS_GETDATA. TAC_PLUS_AUTHEN_STATUS_GETDATA.
5.4.3. Aborting an Authentication Session 5.4.3. Aborting an Authentication Session
The client may prematurely terminate a session by setting the The client may prematurely terminate a session by setting the
TAC_PLUS_CONTINUE_FLAG_ABORT flag in the CONTINUE message. If this TAC_PLUS_CONTINUE_FLAG_ABORT flag in the CONTINUE message. If this
flag is set, the data portion of the message may contain an ASCII flag is set, the data portion of the message may contain an message
message explaining the reason for the abort. This information will explaining the reason for the abort. For details of text encoding,
be handled by the server according to the requirements of the see (Section 3.7) . This information will be handled by the server
deployment. The session is terminated, for more details about according to the requirements of the deployment. The session is
session termination, refer to section (Section 4.8). terminated, for more details about session termination, refer to
section (Section 4.4) .
In cases of PASS, FAIL or ERROR, the server can insert a message into In cases of PASS, FAIL or ERROR, the server can insert a message into
server_msg to be displayed to the user. server_msg to be displayed to the user.
The Draft `The Draft' [TheDraft] defined a mechanism to direct The Draft `The Draft' [TheDraft] defined a mechanism to direct
authentication requests to an alternative server. This mechanism is authentication requests to an alternative server. This mechanism is
regarded as insecure, is deprecated, and not covered here. The regarded as insecure, is deprecated, and not covered here. The
client should treat TAC_PLUS_AUTHEN_STATUS_FOLLOW as client should treat TAC_PLUS_AUTHEN_STATUS_FOLLOW as
TAC_PLUS_AUTHEN_STATUS_FAIL TAC_PLUS_AUTHEN_STATUS_FAIL
skipping to change at page 22, line 41 skipping to change at page 23, line 26
(we don't know who they are). In this case it is up to the server to (we don't know who they are). In this case it is up to the server to
determine, according to its configuration, if an unauthenticated user determine, according to its configuration, if an unauthenticated user
is allowed the services in question. is allowed the services in question.
Authorization does not merely provide yes or no answers, but it may Authorization does not merely provide yes or no answers, but it may
also customize the service for the particular user. A common use of also customize the service for the particular user. A common use of
authorization is to provision a shell session when a user first logs authorization is to provision a shell session when a user first logs
into a device to administer it. The TACACS+ server might respond to into a device to administer it. The TACACS+ server might respond to
the request by allowing the service, but placing a time restriction the request by allowing the service, but placing a time restriction
on the login shell. For a list of common attributes used in on the login shell. For a list of common attributes used in
authorization, see the Authorization Attributes section authorization, see the Authorization Attributes section (Section 8.2)
(Section 8.2). .
In the TACACS+ protocol an authorization is always a single pair of In the TACACS+ protocol an authorization is always a single pair of
messages: a REQUEST from the client followed by a REPLY from the messages: a REQUEST from the client followed by a REPLY from the
server. server.
The authorization REQUEST message contains a fixed set of fields that The authorization REQUEST message contains a fixed set of fields that
indicate how the user was authenticated and a variable set of indicate how the user was authenticated and a variable set of
arguments that describe the services and options for which arguments that describe the services and options for which
authorization is requested. authorization is requested.
skipping to change at page 25, line 26 skipping to change at page 26, line 23
The arguments are the primary elements of the authorization The arguments are the primary elements of the authorization
interaction. In the request packet, they describe the specifics of interaction. In the request packet, they describe the specifics of
the authorization that is being requested. Each argument is encoded the authorization that is being requested. Each argument is encoded
in the packet as a single arg field (arg_1... arg_N) with a in the packet as a single arg field (arg_1... arg_N) with a
corresponding length fields (which indicates the length of each corresponding length fields (which indicates the length of each
argument in bytes). argument in bytes).
The authorization arguments in both the REQUEST and the REPLY are The authorization arguments in both the REQUEST and the REPLY are
attribute-value pairs. The attribute and the value are in a single attribute-value pairs. The attribute and the value are in a single
printable US-ASCII string and are separated by either a "=" (0X3D) or string and are separated by either a "=" (0X3D) or a "*" (0X2A). The
a "*" (0X2A). The equals sign indicates a mandatory argument. The equals sign indicates a mandatory argument. The asterisk indicates
asterisk indicates an optional one. an optional one. For details of text encoding, see (Section 3.7) .
An attribute name MUST NOT contain either of the separators. An An attribute name MUST NOT contain either of the separators. An
attribute value MAY contain the separators. This means that the attribute value MAY contain the separators. This means that the
arguments must be parsed until the first separator is encountered, arguments must be parsed until the first separator is encountered,
all characters in the argument, after this separator, are interpreted all characters in the argument, after this separator, are interpreted
as the argument value. as the argument value.
Optional arguments are ones that may be disregarded by either client Optional arguments are ones that may be disregarded by either client
or server. Mandatory arguments require that the receiving side can or server. Mandatory arguments require that the receiving side can
handle the attribute, that is: its implementation and configuration handle the attribute, that is: its implementation and configuration
skipping to change at page 26, line 40 skipping to change at page 27, line 40
TAC_PLUS_AUTHOR_STATUS_PASS_REPL := 0x02 TAC_PLUS_AUTHOR_STATUS_PASS_REPL := 0x02
TAC_PLUS_AUTHOR_STATUS_FAIL := 0x10 TAC_PLUS_AUTHOR_STATUS_FAIL := 0x10
TAC_PLUS_AUTHOR_STATUS_ERROR := 0x11 TAC_PLUS_AUTHOR_STATUS_ERROR := 0x11
TAC_PLUS_AUTHOR_STATUS_FOLLOW := 0x21 TAC_PLUS_AUTHOR_STATUS_FOLLOW := 0x21
server_msg, server_msg_len server_msg, server_msg_len
This is a printable US-ASCII string that may be presented to the This is a string that may be presented to the user. The
user. The server_msg_len indicates the length of the server_msg server_msg_len indicates the length of the server_msg field, in
field, in bytes. bytes. For details of text encoding, see (Section 3.7) .
data, data_len data, data_len
This is a printable US-ASCII string that may be presented on an This is a string that may be presented on an administrative display,
administrative display, console or log. The decision to present this console or log. The decision to present this message is client
message is client specific. The data_len indicates the length of the specific. The data_len indicates the length of the data field, in
data field, in bytes. bytes. For details of text encoding, see (Section 3.7) .
arg_cnt arg_cnt
The number of authorization arguments to follow. The number of authorization arguments to follow.
arg_1 ... arg_N, arg_1_len .... arg_N_len arg_1 ... arg_N, arg_1_len .... arg_N_len
The arguments describe the specifics of the authorization that is The arguments describe the specifics of the authorization that is
being requested. For details of the content of the args, refer to: being requested. For details of the content of the args, refer to:
Authorization Attributes (Section 8.2) section below. Each argument Authorization Attributes (Section 8.2) section below. Each argument
is encoded in the packet as a single arg field (arg_1... arg_N) with is encoded in the packet as a single arg field (arg_1... arg_N) with
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If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_REPL then the client If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_REPL then the client
MUST use the authorization attribute-value pairs (if any) in the MUST use the authorization attribute-value pairs (if any) in the
response, instead of the authorization attribute-value pairs from the response, instead of the authorization attribute-value pairs from the
request. request.
To approve the authorization with no modifications, the server sets To approve the authorization with no modifications, the server sets
the status to TAC_PLUS_AUTHOR_STATUS_PASS_ADD and the arg_cnt to 0. the status to TAC_PLUS_AUTHOR_STATUS_PASS_ADD and the arg_cnt to 0.
A status of TAC_PLUS_AUTHOR_STATUS_ERROR indicates an error occurred A status of TAC_PLUS_AUTHOR_STATUS_ERROR indicates an error occurred
on the server. For the differences between ERROR and FAIL, refer to on the server. For the differences between ERROR and FAIL, refer to
section Session Completion (Section 4.8). None of the arg values section Session Completion (Section 4.4) . None of the arg values
have any relevance if an ERROR is set, and must be ignored. have any relevance if an ERROR is set, and must be ignored.
When the status equals TAC_PLUS_AUTHOR_STATUS_FOLLOW, then the When the status equals TAC_PLUS_AUTHOR_STATUS_FOLLOW, then the
arg_cnt MUST be 0. In that case, the actions to be taken and the arg_cnt MUST be 0. In that case, the actions to be taken and the
contents of the data field are identical to the contents of the data field are identical to the
TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication. TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication.
7. Accounting 7. Accounting
Accounting is typically the third action after authentication and Accounting is typically the third action after authentication and
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It may also be used to account for services used, such as in a It may also be used to account for services used, such as in a
billing environment. To this end, TACACS+ supports three types of billing environment. To this end, TACACS+ supports three types of
accounting records. Start records indicate that a service is about accounting records. Start records indicate that a service is about
to begin. Stop records indicate that a service has just terminated, to begin. Stop records indicate that a service has just terminated,
and Update records are intermediate notices that indicate that a and Update records are intermediate notices that indicate that a
service is still being performed. TACACS+ accounting records contain service is still being performed. TACACS+ accounting records contain
all the information used in the authorization records, and also all the information used in the authorization records, and also
contain accounting specific information such as start and stop times contain accounting specific information such as start and stop times
(when appropriate) and resource usage information. A list of (when appropriate) and resource usage information. A list of
accounting attributes is defined in the accounting section accounting attributes is defined in the accounting section
(Section 7). (Section 7) .
7.1. The Account REQUEST Packet Body 7.1. The Account REQUEST Packet Body
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+ +----------------+----------------+----------------+----------------+
| flags | authen_method | priv_lvl | authen_type | | flags | authen_method | priv_lvl | authen_type |
+----------------+----------------+----------------+----------------+ +----------------+----------------+----------------+----------------+
| authen_service | user_len | port_len | rem_addr_len | | authen_service | user_len | port_len | rem_addr_len |
+----------------+----------------+----------------+----------------+ +----------------+----------------+----------------+----------------+
| arg_cnt | arg_1_len | arg_2_len | ... | | arg_cnt | arg_1_len | arg_2_len | ... |
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This is the return status. Values are: This is the return status. Values are:
TAC_PLUS_ACCT_STATUS_SUCCESS := 0x01 TAC_PLUS_ACCT_STATUS_SUCCESS := 0x01
TAC_PLUS_ACCT_STATUS_ERROR := 0x02 TAC_PLUS_ACCT_STATUS_ERROR := 0x02
TAC_PLUS_ACCT_STATUS_FOLLOW := 0x21 TAC_PLUS_ACCT_STATUS_FOLLOW := 0x21
server_msg, server_msg_len server_msg, server_msg_len
This is a printable US-ASCII string that may be presented to the This is a string that may be presented to the user. The
user. The server_msg_len indicates the length of the server_msg server_msg_len indicates the length of the server_msg field, in
field, in bytes. bytes. For details of text encoding, see (Section 3.7) .
data, data_len data, data_len
This is a printable US-ASCII string that may be presented on an This is a string that may be presented on an administrative display,
administrative display, console or log. The decision to present this console or log. The decision to present this message is client
message is client specific. The data_len indicates the length of the specific. The data_len indicates the length of the data field, in
data field, in bytes. bytes. For details of text encoding, see (Section 3.7) .
When the status equals TAC_PLUS_ACCT_STATUS_FOLLOW, then the actions When the status equals TAC_PLUS_ACCT_STATUS_FOLLOW, then the actions
to be taken and the contents of the data field are identical to the to be taken and the contents of the data field are identical to the
TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication. TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication.
TACACS+ accounting is intended to record various types of events on TACACS+ accounting is intended to record various types of events on
clients, for example: login sessions, command entry, and others as clients, for example: login sessions, command entry, and others as
required by the client implementation. These events are collectively required by the client implementation. These events are collectively
referred to in `The Draft' [TheDraft] as "tasks". referred to in `The Draft' [TheDraft] as "tasks".
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8. Attribute-Value Pairs 8. Attribute-Value Pairs
TACACS+ is intended to be an extensible protocol. The attributes TACACS+ is intended to be an extensible protocol. The attributes
used in Authorization and Accounting are not limited by this used in Authorization and Accounting are not limited by this
document. Some attributes are defined below for common use cases, document. Some attributes are defined below for common use cases,
clients MUST use these attributes when supporting the corresponding clients MUST use these attributes when supporting the corresponding
use cases. use cases.
8.1. Value Encoding 8.1. Value Encoding
All attribute values are encoded as printable US-ASCII strings. The All attribute values are encoded as strings. For details of text
following type representations SHOULD be followed encoding, see (Section 3.7) . The following type representations
SHOULD be followed
Numeric Numeric
All numeric values in an attribute-value string are provided as All numeric values in an attribute-value string are provided as
decimal printable US-ASCII numbers, unless otherwise stated. decimal numbers, unless otherwise stated.
Boolean Boolean
All Boolean attributes are encoded as printable US-ASCII with values All Boolean attributes are encoded with values "true" or "false".
"true" or "false".
IP-Address IP-Address
It is recommended that hosts be specified as a IP address so as to It is recommended that hosts be specified as a IP address so as to
avoid any ambiguities. IPv4 address are specified as US-ASCII octet avoid any ambiguities. For details of text encoding, see
numerics separated by dots ('.'), IPv6 address text representation (Section 3.7) . IPv4 address are specified as octet numerics
defined in RFC 5952 [RFC5952]. separated by dots ('.'), IPv6 address text representation defined in
RFC 5952 [RFC5952] .
Date Time Date Time
Absolute date/times are specified in seconds since the epoch, 12:00am Absolute date/times are specified in seconds since the epoch, 12:00am
Jan 1 1970. The timezone MUST be UTC unless a timezone attribute is Jan 1 1970. The timezone MUST be UTC unless a timezone attribute is
specified. specified.
String String
Many values have no specific type representation and so are Many values have no specific type representation and so are
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name as its value. name as its value.
cmd-arg (String) cmd-arg (String)
an argument to a shell (exec) command. This indicates an argument an argument to a shell (exec) command. This indicates an argument
for the shell command that is to be run. Multiple cmd-arg attributes for the shell command that is to be run. Multiple cmd-arg attributes
may be specified, and they are order dependent. may be specified, and they are order dependent.
acl (Numeric) acl (Numeric)
printable US-ASCII number representing a connection access list. a number representing a connection access list. Applicable only to
Applicable only to session-based shell authorization. session-based shell authorization. For details of text encoding, see
(Section 3.7) .
inacl (String) inacl (String)
printable US-ASCII identifier (name) of an interface input access identifier (name) of an interface input access list. For details of
list. text encoding, see (Section 3.7) .
outacl (String) outacl (String)
printable US-ASCII identifier (name) of an interface output access identifier (name) of an interface output access list. For details of
list. text encoding, see (Section 3.7) .
addr (IP-Address) addr (IP-Address)
a network address a network address
addr-pool (String) addr-pool (String)
The identifier of an address pool from which the client can assign an The identifier of an address pool from which the client can assign an
address. address.
timeout (Numeric) timeout (Numeric)
an absolute timer for the connection (in minutes). A value of zero an absolute timer for the connection (in minutes). A value of zero
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The time the action stopped (in seconds since the epoch.) The time the action stopped (in seconds since the epoch.)
elapsed_time (Numeric) elapsed_time (Numeric)
The elapsed time in seconds for the action. The elapsed time in seconds for the action.
timezone (String) timezone (String)
The timezone abbreviation for all timestamps included in this packet. The timezone abbreviation for all timestamps included in this packet.
A database of timezones is maintained here: TZDB [TZDB]. A database of timezones is maintained here: TZDB [TZDB] .
event (String) event (String)
Used only when "service=system". Current values are "net_acct", Used only when "service=system". Current values are "net_acct",
"cmd_acct", "conn_acct", "shell_acct" "sys_acct" and "clock_change". "cmd_acct", "conn_acct", "shell_acct" "sys_acct" and "clock_change".
These indicate system-level changes. The flags field SHOULD indicate These indicate system-level changes. The flags field SHOULD indicate
whether the service started or stopped. whether the service started or stopped.
reason (String) reason (String)
skipping to change at page 35, line 17 skipping to change at page 36, line 20
The number of input packets transferred by this action from the The number of input packets transferred by this action from the
endstation to the client port. endstation to the client port.
paks_out (Numeric) paks_out (Numeric)
The number of output packets transferred by this action from the The number of output packets transferred by this action from the
client port to the endstation. client port to the endstation.
err_msg (String) err_msg (String)
A printable US-ASCII string describing the status of the action. string describing the status of the action. For details of text
encoding, see (Section 3.7) .
9. Privilege Levels 9. Privilege Levels
The TACACS+ Protocol supports flexible authorization schemes through The TACACS+ Protocol supports flexible authorization schemes through
the extensible attributes. the extensible attributes.
One scheme is built into the protocol and has been extensively used One scheme is built into the protocol and has been extensively used
for Session-based shell authorization: Privilege Levels. Privilege for Session-based shell authorization: Privilege Levels. Privilege
Levels are ordered values from 0 to 15 with each level being a Levels are ordered values from 0 to 15 with each level being a
superset of the next lower value. Configuration and implementation superset of the next lower value. Configuration and implementation
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time and effort to help improve the document, identifying weaknesses time and effort to help improve the document, identifying weaknesses
and providing remediation. and providing remediation.
The authors would also like to thank the support from the OPSAWG The authors would also like to thank the support from the OPSAWG
Chairs and advisors. Chairs and advisors.
13. References 13. References
13.1. Normative References 13.1. Normative References
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992. April 1992.
[RFC1334] Lloyd, B. and W. Simpson, "PPP Authentication Protocols", [RFC1334] Lloyd, B. and W. Simpson, "PPP Authentication Protocols",
RFC 1334, DOI 10.17487/RFC1334, October 1992, RFC 1334, DOI 10.17487/RFC1334, October 1992,
<http://www.rfc-editor.org/info/rfc1334>. <http://www.rfc-editor.org/info/rfc1334>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
skipping to change at page 43, line 23 skipping to change at page 44, line 28
[RFC4086] Eastlake 3rd, D., Crocker, S., and J. Schiller, [RFC4086] Eastlake 3rd, D., Crocker, S., and J. Schiller,
"Randomness Requirements for Security", RFC 4086, "Randomness Requirements for Security", RFC 4086,
DOI 10.17487/RFC4086, June 2005, DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>. <http://www.rfc-editor.org/info/rfc4086>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6 [RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010, DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>. <https://www.rfc-editor.org/info/rfc5952>.
[RFC8265] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 8265,
DOI 10.17487/RFC8265, October 2017,
<https://www.rfc-editor.org/info/rfc8265>.
13.2. Informative References 13.2. Informative References
[TheDraft] [TheDraft]
Carrel, D. and L. Grant, "The TACACS+ Protocol Version Carrel, D. and L. Grant, "The TACACS+ Protocol Version
1.78", June 1997, 1.78", June 1997,
<https://tools.ietf.org/html/draft-grant-tacacs-02>. <https://tools.ietf.org/html/draft-grant-tacacs-02>.
[TZDB] Eggert, P. and A. Olson, "Sources for Time Zone and [TZDB] Eggert, P. and A. Olson, "Sources for Time Zone and
Daylight Saving Time Data", 1987, Daylight Saving Time Data", 1987,
<https://www.iana.org/time-zones>. <https://www.iana.org/time-zones>.
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