Operations T. Dahm
Internet-Draft A. Ota
Intended status: Informational Google Inc
Expires: February 22, 2018 D. Medway Gash
Cisco Systems, Inc.
D. Carrel
vIPtela, Inc.
L. Grant
August 21, 2017
The TACACS+ Protocol
draft-ietf-opsawg-tacacs-07
Abstract
TACACS+ provides Device Administration for routers, network access
servers and other networked computing devices via one or more
centralized servers. This document describes the protocol that is
used by TACACS+.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on February 22, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
Dahm, et al. Expires February 22, 2018 [Page 1]
Internet-Draft The TACACS+ Protocol August 2017
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Technical Definitions . . . . . . . . . . . . . . . . . . . . 4
3. TACACS+ Connections and Sessions . . . . . . . . . . . . . . 4
3.1. Connection . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Session . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Single Connect Mode . . . . . . . . . . . . . . . . . . . 5
3.4. Session Completion . . . . . . . . . . . . . . . . . . . 5
3.5. Treatment of Enumerated Protocol Values . . . . . . . . . 6
3.6. Text Encoding . . . . . . . . . . . . . . . . . . . . . . 7
3.7. Data Obfuscation . . . . . . . . . . . . . . . . . . . . 7
3.8. The TACACS+ Packet Header . . . . . . . . . . . . . . . . 9
3.9. The TACACS+ Packet Body . . . . . . . . . . . . . . . . . 11
4. Authentication . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. The Authentication START Packet Body . . . . . . . . . . 11
4.2. The Authentication REPLY Packet Body . . . . . . . . . . 14
4.3. The Authentication CONTINUE Packet Body . . . . . . . . . 15
4.4. Description of Authentication Process . . . . . . . . . . 16
4.4.1. Version Behaviour . . . . . . . . . . . . . . . . . . 17
4.4.2. Common Authentication Flows . . . . . . . . . . . . . 17
4.4.3. Aborting an Authentication Session . . . . . . . . . 21
5. Authorization . . . . . . . . . . . . . . . . . . . . . . . . 22
5.1. The Authorization REQUEST Packet Body . . . . . . . . . . 23
5.2. The Authorization REPLY Packet Body . . . . . . . . . . . 26
6. Accounting . . . . . . . . . . . . . . . . . . . . . . . . . 27
6.1. The Account REQUEST Packet Body . . . . . . . . . . . . . 28
6.2. The Accounting REPLY Packet Body . . . . . . . . . . . . 29
7. Attribute-Value Pairs . . . . . . . . . . . . . . . . . . . . 30
7.1. Authorization Attributes . . . . . . . . . . . . . . . . 31
7.2. Accounting Attributes . . . . . . . . . . . . . . . . . . 34
Dahm, et al. Expires February 22, 2018 [Page 2]
Internet-Draft The TACACS+ Protocol August 2017
8. Privilege Levels . . . . . . . . . . . . . . . . . . . . . . 35
9. TACACS+ Security Considerations . . . . . . . . . . . . . . . 36
9.1. Overall Security of The Protocol . . . . . . . . . . . . 36
9.2. Security of Authentication Sessions . . . . . . . . . . . 38
9.3. Security of Authorization Sessions . . . . . . . . . . . 38
9.4. Security of Accounting Sessions . . . . . . . . . . . . . 38
9.5. TACACS+ Deployment Recommendations . . . . . . . . . . . 39
9.6. TACACS+ Client Implementation Recommendations . . . . . . 39
9.7. TACACS+ Server Implementation Recommendations . . . . . . 40
9.8. TACACS+ Security and Operational Concerns . . . . . . . . 40
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction
Terminal Access Controller Access-Control System Plus (TACACS+) was
originally conceived as a general Authentication, Authorization and
Accounting protocol. It is primarily used today for Device
Administration: authenticating access to network devices, providing
central authorization of operations, and audit of those operations.
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
draft document that was originally intended for IETF publication.
This document is known as `The Draft' [TheDraft] .
It is intended that all implementations which conform to this
document will conform to `The Draft'. However, attention is drawn to
the following specific adjustments of the protocol specification from
'The Draft':
This document officially removes SENDPASS for security reasons.
The normative description of Legacy features such as ARAP and
outbound authentication have been removed, however the required
enumerations are kept.
The TACACS+ protocol separates the functions of Authentication,
Authorization and Accounting. It allows for arbitrary length and
content authentication exchanges, which will support any
authentication mechanism to be utilized with TACACS+ clients. It is
extensible to provide for site customization and future development
features, and it uses TCP to ensure reliable delivery. The protocol
allows the TACACS+ client to request very fine-grained access control
and allows the server to respond to each component of that request.
The separation of authentication, authorization and accounting is a
fundamental component of the design of TACACS+. The distinction
Dahm, et al. Expires February 22, 2018 [Page 3]
Internet-Draft The TACACS+ Protocol August 2017
between them is very important so this document will address each one
separately. It is important to note that TACACS+ provides for all
three, but an implementation or configuration is not required to
employ all three. Each one serves a unique purpose that alone is
useful, and together can be quite powerful.
This document restricts itself to a description of the protocol that
is used by TACACS+. It does not cover deployment or best practices.
2. Technical Definitions
This section provides a few basic definitions that are applicable to
this document
Client
The client is any device, (often a Network Access Server) that
provides access services. The clients usually provide a character
mode front end and then allow the user to telnet or rlogin to another
host. A client may also support protocol based access services.
Server
The server receives TACACS+ protocol requests, and replies according
to its business model, in accordance with the flows defined in this
document.
Packet
All uses of the word packet in this document refer to TACACS+
protocol packets unless explicitly noted otherwise.
3. TACACS+ Connections and Sessions
3.1. Connection
TACACS+ uses TCP for its transport. Server port 49 is allocated for
TACACS+ traffic.
3.2. Session
The concept of a session is used throughout this document. A TACACS+
session is a single authentication sequence, a single authorization
exchange, or a single accounting exchange.
An accounting and authorization session will consist of a single pair
of packets (the request and its reply). An authentication session
may involve an arbitrary number of packets being exchanged. The
Dahm, et al. Expires February 22, 2018 [Page 4]
Internet-Draft The TACACS+ Protocol August 2017
session is an operational concept that is maintained between the
TACACS+ client and server. It does not necessarily correspond to a
given user or user action.
3.3. Single Connect Mode
Single Connection Mode is intended to improve performance by allowing
a client to multiplex multiple session on a single TCP connection.
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 sets this flag, to indicate that it supports multiplexing
TACACS+ sessions over a single TCP connection. The client MUST NOT
send a second packet on a connection until single-connect status has
been established.
To indicate it will support Single Connect Mode, the server sets this
flag in the first reply packet in response to the first request from
a client. The server may set this flag even if the client does not
set it, but the client may ignore the flag and close the connection
after the session completes.
The flag is only relevant for the first two packets on a connection,
to allow the client and server to establish Single Connect Mode.
This protocol does not define a procedure for changing Single Connect
Mode after the first two packets.
If single Connect Mode has not been established in the first two
packets of a TCP connection, then both the client and the server
close the connection at the end of the first session.
The client negotiates single Connection Mode to improve efficiency.
The server may refuse to allow Single connection Mode for the client.
For example it may not fit the specific deployment to allocate a long
lasting TCP connection to a specific client. Even if the server is
configured to permit single Connection Mode for a specific client,
the server may close the connection. For example: a server may be
configured to time out a Single Connection Mode TCP Connection after
a specific period of inactivity to preserve its resources. The
client MUST accommodate such closures on a TCP session even after
Single Conenction Mode has been established.
3.4. Session Completion
The REPLY packets defined for the packets types in the sections below
(Authentication, Authorization and Accounting) contain a status
field. The complete set of options for this field depend upon the
Dahm, et al. Expires February 22, 2018 [Page 5]
Internet-Draft The TACACS+ Protocol August 2017
packet type, but all three REPLY packet types define values
representing PASS, ERROR and FAIL, which indicate the last packet of
a regular session (one which is not aborted).
The server responds with a PASS or a FAIL to indicate that the
processing of the request completed and the client can apply the
result (PASS or FAIL) to control the execution of the action which
prompted the request to be sent to the server.
The server responds with an ERROR to indicate that the processing of
the request did not complete. The client can not apply the result
and it MUST behave as if the server could not be connected to. For
example, the client try alternative methods, if they are available,
such as sending the request to a backup server, or using local
configuration to determine whether the action which prompted the
request should be executed.
Refer to the section (Section 4.4.3) on Aborting Authentication
Sessions for details on handling additional status options .
When the session is complete, then the TCP connection should be
handled as follows, according to whether Single Connect Mode was
negotiated:
If Single Connection Mode was not negotiated, then the connection
should be closed
If Single Connection Mode was enabled, then the connection SHOULD be
left open (see section (Section 3.3) ), but may still be closed after
a timeout period to preserve deployment resources
If Single Connection Mode was enabled, but an ERROR occurred due to
connection issues (such as an incorrect secret, see section
(Section 3.7) ), then any further new sessions MUST NOT be accepted
on the connection. If there are any sessions that have already been
established then they MAY be completed. Once all active sessions are
completed then the connection MUST be closed.
3.5. Treatment of Enumerated Protocol Values
This document describes various enumerated values in the packet
header and the headers for specific packet types. for example in the
Authentication start packet type, this document defines the action
field with three values TAC_PLUS_AUTHEN_LOGIN, TAC_PLUS_AUTHEN_CHPASS
and TAC_PLUS_AUTHEN_SENDAUTH.
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
Dahm, et al. Expires February 22, 2018 [Page 6]
Internet-Draft The TACACS+ Protocol August 2017
listed in this document for a header field, then it should respond
with a ERROR and terminate the session. This will allow the client
to try a different option.
If an error occurs but the type of the incoming packet cannot be
determined, a packet with the identical cleartext header but with a
sequence number incremented by one and the length set to zero MUST be
returned to indicate an error.
3.6. Text Encoding
All text fields in TACACS+ MUST be US-ASCII, excepting special
consideration given to user field and data fields used for passwords.
To ensure interoperability of current deployments, the TACACS+ client
and server MUST handle user fields and those data fields used for
passwords as 8 bit octet strings. The deployment operator MUST
ensure that consistent character encoding is applied. The encoding
SHOULD be UTF-8, and other encodings outside US-ASCII SHOULD be
deprecated.
3.7. Data Obfuscation
The body of packets may be obfuscated. The following sections
describe the obfuscation mechanism that is supported in the protocol.
In 'The Draft' this process was actually referred to as Encryption,
but by modern day standards the mechanims would not meet the
requirements of an encryption mechanism.
The obfuscation mechanism relies on a secret key, it is referring to
a shared secret value that is known to both the client and the
server. This document does not discuss the management and storage of
those keys. It is an implementation detail of the server and client,
as to whether they will maintain only one key, or a different key for
each client or server with which they communicate. For security
reasons, the latter options MUST be available, but it is a site
dependent decision as to whether the use of separate keys is
appropriate.
The flag field may be set as follows:
TAC_PLUS_UNENCRYPTED_FLAG == 0x0
In this case, the packet body is obfuscated by XOR-ing it byte-wise
with a pseudo random pad.
ENCRYPTED {data} == data ^ pseudo_pad
Dahm, et al. Expires February 22, 2018 [Page 7]
Internet-Draft The TACACS+ Protocol August 2017
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.
Whenever used in this document, MD5 refers to the "RSA Data Security,
Inc. MD5 Message-Digest Algorithm" as specified in RFC 1321 [RFC1321]
.
pseudo_pad = {MD5_1 [,MD5_2 [ ... ,MD5_n]]} truncated to len(data)
The first MD5 hash is generated by concatenating the session_id, the
secret key, the version number and the sequence number and then
running MD5 over that stream. All of those input values are
available in the packet header, except for the secret key which is a
shared secret between the TACACS+ client and server.
The version number is the one byte concatenation of the major and
minor version numbers.
The session id is used in network byte order.
Subsequent hashes are generated by using the same input stream, but
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,
key, version, seq_no, MD5_1} .... MD5_n = MD5{session_id, key,
version, seq_no, MD5_n-1}
When a server detects that the secret(s) it has configured for the
device mismatch, it MUST return ERROR. The handling of the TCP
connection by the server is implementation independent.
TAC_PLUS_UNENCRYPTED_FLAG == 0x1
In this case, the entire packet body is in cleartext. Obfuscation
and de-obfuscation are null operations. This method should be
avoided unless absolutely required for debug purposes, when tooling
does not permit de-obfuscation.
If deployment is configured for obfuscating a connection then do no
skip de-obfuscation simply because an incoming packet indicates that
it is not obfuscated. If the flag is not set when expected, then it
must be dropped.
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
cleartext datalength value from the header, the packet MUST be
discarded, and an error signalled. The underlying TCP connection MAY
Dahm, et al. Expires February 22, 2018 [Page 8]
Internet-Draft The TACACS+ Protocol August 2017
also be closed, if it is not being used for other sessions in single-
connect mode.
Commonly such failures are seen when the keys are mismatched between
the client and the TACACS+ server.
If an error must be declared but the type of the incoming packet
cannot be determined, a packet with the identical cleartext header
but with a sequence number incremented by one and the length set to
zero MUST be returned to indicate an error.
3.8. The TACACS+ Packet Header
All TACACS+ packets begin with the following 12 byte header. The
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
+----------------+----------------+----------------+----------------+
|major | minor | | | |
|version| version| type | seq_no | flags |
+----------------+----------------+----------------+----------------+
| |
| session_id |
+----------------+----------------+----------------+----------------+
| |
| length |
+----------------+----------------+----------------+----------------+
major_version
This is the major TACACS+ version number.
TAC_PLUS_MAJOR_VER := 0xc
minor_version
The minor TACACS+ version number.
TAC_PLUS_MINOR_VER_DEFAULT := 0x0
TAC_PLUS_MINOR_VER_ONE := 0x1
type
This is the packet type. Legal values are:
TAC_PLUS_AUTHEN := 0x01 (Authentication)
Dahm, et al. Expires February 22, 2018 [Page 9]
Internet-Draft The TACACS+ Protocol August 2017
TAC_PLUS_AUTHOR := 0x02 (Authorization)
TAC_PLUS_ACCT := 0x03 (Accounting)
seq_no
This is the sequence number of the current packet. The first packet
in a session MUST have the sequence number 1 and each subsequent
packet will increment the sequence number by one. Thus clients only
send packets containing odd sequence numbers, and TACACS+ servers
only send packets containing even sequence numbers.
The sequence number must never wrap i.e. if the sequence number 2^8-1
is ever reached, that session must terminate and be restarted with a
sequence number of 1.
flags
This field contains various bitmapped flags.
The flag bit:
TAC_PLUS_UNENCRYPTED_FLAG := 0x01
This flag indicates that the sender did not obfuscate the bode of the
packet. The application of this flag will be covered in the security
section (Section 9) . section.
This flag SHOULD be clear in all deployments. Modern network traffic
tools easily support encryted traffic when configured with the shared
secret (see section below), so even in test scenarios, the obfuscated
mode SHOULD be used.
The single-connection flag:
TAC_PLUS_SINGLE_CONNECT_FLAG := 0x04
This flag is used to allow a client and server to negotiate Single
Connection Mode.
session_id
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
cryptographically strong random number generation method. Failure to
do so will compromise security of the session. For more details
refer to RFC 1750 [RFC1750]
Dahm, et al. Expires February 22, 2018 [Page 10]
Internet-Draft The TACACS+ Protocol August 2017
length
The total length of the packet body (not including the header).
3.9. The TACACS+ Packet Body
The TACACS+ body types are defined in the packet header. The next
sections of this document will address the contents of the different
TACACS+ bodies. The following general rules apply to all TACACS+
body types:
- To signal that any variable length data fields are unused, their
length value is set to zero.
- the lengths of data and message fields in a packet are specified
by their corresponding length fields, (and are not null
terminated.)
- All length values are unsigned and in network byte order.
4. Authentication
Authentication is the action of determining who a user (or entity)
is. Authentication can take many forms. Traditional authentication
utilizes a name and a fixed password. However, fixed passwords have
limitations, mainly in the area of security. Many modern
authentication mechanisms utilize "one-time" passwords or a
challenge-response query. TACACS+ is designed to support all of
these, and be powerful enough to handle any future mechanisms.
Authentication generally takes place when the user first logs in to a
machine or requests a service of it.
Authentication is not mandatory; it is a site-configured option.
Some sites do not require it. Others require it only for certain
services (see authorization below). Authentication may also take
place when a user attempts to gain extra privileges, and must
identify himself or herself as someone who possesses the required
information (passwords, etc.) for those privileges.
4.1. The Authentication START Packet Body
Dahm, et al. Expires February 22, 2018 [Page 11]
Internet-Draft The TACACS+ Protocol August 2017
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
+----------------+----------------+----------------+----------------+
| action | priv_lvl | authen_type | authen_service |
+----------------+----------------+----------------+----------------+
| user_len | port_len | rem_addr_len | data_len |
+----------------+----------------+----------------+----------------+
| user ...
+----------------+----------------+----------------+----------------+
| port ...
+----------------+----------------+----------------+----------------+
| rem_addr ...
+----------------+----------------+----------------+----------------+
| data...
+----------------+----------------+----------------+----------------+
Packet fields are as follows:
action
This indicates the authentication action. Legal values are listed
below.
TAC_PLUS_AUTHEN_LOGIN := 0x01
TAC_PLUS_AUTHEN_CHPASS := 0x02
TAC_PLUS_AUTHEN_SENDAUTH := 0x04
priv_lvl
This indicates the privilege level that the user is authenticating
as. Please refer to the Privilege Level section (Section 8) below.
authen_type
The type of authentication. Legal values are:
TAC_PLUS_AUTHEN_TYPE_ASCII := 0x01
TAC_PLUS_AUTHEN_TYPE_PAP := 0x02
TAC_PLUS_AUTHEN_TYPE_CHAP := 0x03
TAC_PLUS_AUTHEN_TYPE_ARAP := 0x04 (deprecated)
TAC_PLUS_AUTHEN_TYPE_MSCHAP := 0x05
TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 := 0x06
Dahm, et al. Expires February 22, 2018 [Page 12]
Internet-Draft The TACACS+ Protocol August 2017
authen_service
This is the service that is requesting the authentication. Legal
values are:
TAC_PLUS_AUTHEN_SVC_NONE := 0x00
TAC_PLUS_AUTHEN_SVC_LOGIN := 0x01
TAC_PLUS_AUTHEN_SVC_ENABLE := 0x02
TAC_PLUS_AUTHEN_SVC_PPP := 0x03
TAC_PLUS_AUTHEN_SVC_ARAP := 0x04
TAC_PLUS_AUTHEN_SVC_PT := 0x05
TAC_PLUS_AUTHEN_SVC_RCMD := 0x06
TAC_PLUS_AUTHEN_SVC_X25 := 0x07
TAC_PLUS_AUTHEN_SVC_NASI := 0x08
TAC_PLUS_AUTHEN_SVC_FWPROXY := 0x09
The TAC_PLUS_AUTHEN_SVC_NONE option is intended for the authorization
application of this field that indicates that no authentication was
performed by the device.
The TAC_PLUS_AUTHEN_SVC_LOGIN option is identifies regular login (as
opposed to ENABLE) to a client device.
The TAC_PLUS_AUTHEN_SVC_ENABLE option identifies the ENABLE
authen_service, which refers to a service requesting authentication
in order to grant the user different privileges. This is comparable
to the Unix "su(1)" command. An authen_service value of NONE is only
to be used when none of the other authen_service values are
appropriate. ENABLE may be requested independently, no requirements
for previous authentications or authorizations are imposed by the
protocol.
Other options are included for legacy/backwards compatibility.
user, user_len
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.
Dahm, et al. Expires February 22, 2018 [Page 13]
Internet-Draft The TACACS+ Protocol August 2017
If included, the user_len indicates the length of the user field, in
bytes.
port, port_len
The US-ASCII name of the client port on which the authentication is
taking place, and its length in bytes. The value of this field is
client specific. (For example, Cisco uses "tty10" to denote the
tenth tty line and "Async10" to denote the tenth async interface).
The port_len indicates the length of the port field, in bytes.
rem_addr, rem_addr_len
An US-ASCII string indicating the remote location from which the user
has connected to the client. It is intended to hold a network
address if the user is connected via a network, a caller ID is the
user is connected via ISDN or a POTS, or any other remote location
information that is 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.
data, data_len
This field is used to send data appropriate for the action and
authen_type. It is described in more detail in the section Common
Authentication flows (Section 4.4.2) . The data_len indicates the
length of the data field, in bytes.
4.2. The Authentication REPLY Packet Body
The TACACS+ server sends only one type of authentication packet (a
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
+----------------+----------------+----------------+----------------+
| status | flags | server_msg_len |
+----------------+----------------+----------------+----------------+
| data_len | server_msg ...
+----------------+----------------+----------------+----------------+
| data ...
+----------------+----------------+
status
The current status of the authentication. Legal values are:
TAC_PLUS_AUTHEN_STATUS_PASS := 0x01
Dahm, et al. Expires February 22, 2018 [Page 14]
Internet-Draft The TACACS+ Protocol August 2017
TAC_PLUS_AUTHEN_STATUS_FAIL := 0x02
TAC_PLUS_AUTHEN_STATUS_GETDATA := 0x03
TAC_PLUS_AUTHEN_STATUS_GETUSER := 0x04
TAC_PLUS_AUTHEN_STATUS_GETPASS := 0x05
TAC_PLUS_AUTHEN_STATUS_RESTART := 0x06
TAC_PLUS_AUTHEN_STATUS_ERROR := 0x07
TAC_PLUS_AUTHEN_STATUS_FOLLOW := 0x21
flags
Bitmapped flags that modify the action to be taken. The following
values are defined:
TAC_PLUS_REPLY_FLAG_NOECHO := 0x01
server_msg, server_msg_len
A message to be displayed to the user. This field is optional. If
it exists, it is intended to be presented to the user. US-ASCII
charset MUST be used. The server_msg_len indicates the length of the
server_msg field, in bytes.
data, data_len
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
are shown in the section Common Authentication flows (Section 4.4.2)
. The data_len indicates the length of the data field, in bytes.
4.3. The Authentication CONTINUE Packet Body
This packet is sent from the client to the server following the
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
+----------------+----------------+----------------+----------------+
| user_msg len | data_len |
+----------------+----------------+----------------+----------------+
| flags | user_msg ...
+----------------+----------------+----------------+----------------+
| data ...
+----------------+
Dahm, et al. Expires February 22, 2018 [Page 15]
Internet-Draft The TACACS+ Protocol August 2017
user_msg, user_msg_len
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
REPLY packet. The user_len indicates the length of the user field,
in bytes.
data, data_len
This field carries information that is specific to the action and the
authen_type for this session. Valid uses of this field are described
below. The data_len indicates the length of the data field, in
bytes.
flags
This holds the bitmapped flags that modify the action to be taken.
The following values are defined:
TAC_PLUS_CONTINUE_FLAG_ABORT := 0x01
4.4. Description of Authentication Process
The action, authen_type and authen_service fields (described above)
combine to indicate what kind of authentication is to be performed.
Every authentication START, REPLY and CONTINUE packet includes a data
field. The use of this field is dependent upon the kind of the
Authentication.
This document defines a standard set of the kinds of authentication
supported by TACACS+. Each authentication flow consists of a START
packet. The server responds either with a request for more
information (GETDATA, GETUSER or GETPASS) or a termination PASS,
FAIL, ERROR, RESTART or FOLLOW. The actions and meanings when the
server sends a RESTART, ERROR or FOLLOW are common and are described
further below.
When the REPLY status equals TAC_PLUS_AUTHEN_STATUS_GETDATA,
TAC_PLUS_AUTHEN_STATUS_GETUSER or TAC_PLUS_AUTHEN_STATUS_GETPASS,
then authentication continues and the server SHOULD provide
server_msg content for the client to prompt the user for more
information. The client MUST then return a CONTINUE packet
containing the requested information in the user_msg field.
The client should interpret TAC_PLUS_AUTHEN_STATUS_GETUSER as a
request for username and TAC_PLUS_AUTHEN_STATUS_GETPASS as a request
for password. The TAC_PLUS_AUTHEN_STATUS_GETDATA is the generic
request for more information to flexibly support future requirements.
Dahm, et al. Expires February 22, 2018 [Page 16]
Internet-Draft The TACACS+ Protocol August 2017
If the information being requested by the server form the client is
sensitive, then the server should set the TAC_PLUS_REPLY_FLAG_NOECHO
flag. When the client queries the user for the information, the
response MUST NOT be echoed as it is entered.
The data field is only used in the REPLY where explicitly defined
below.
4.4.1. Version Behaviour
The TACACS+ protocol is versioned to allow revisions while
maintaining backwards compatibility. The version number is in every
packet header. The changes between minor_version 0 and 1 apply only
to the authentication process, and all deal with the way that CHAP
and PAP authentications are handled. minor_version 1 may only be used
for authentication kinds that explicitly call for it in the table
below:
LOGIN CHPASS SENDAUTH
ASCII v0 v0 -
PAP v1 - v1
CHAP v1 - v1
MS-CHAPv1/2 v1 - v1
The '-' symbol represents that the option is not valid.
All authorisation and accounting and ASCII authentication use
minor_version number of 0.
PAP, CHAP and MS-CHAP login use minor_version 1. The normal exchange
is a single START packet from the client and a single REPLY from the
server.
SENDAUTH is only used for PPP when performing outbound
authentication.
The removal of SENDPASS was prompted by security concerns, and is no
longer considered part of the TACACS+ protocol.
4.4.2. Common Authentication Flows
This section describes common authentication flows. If the server
does not implement an option, it MUST respond with
TAC_PLUS_AUTHEN_STATUS_FAIL.
Inbound ASCII Login
Dahm, et al. Expires February 22, 2018 [Page 17]
Internet-Draft The TACACS+ Protocol August 2017
action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII
minor_version = 0x0
This is a standard ASCII authentication. The START packet MAY
contain the username. If the user does not include the username then
the server MUST obtain it from the client with a CONTINUE
TAC_PLUS_AUTHEN_STATUS_GETUSER. When the server has the username, it
will obtain the password using a continue with
TAC_PLUS_AUTHEN_STATUS_GETPASS. ASCII login uses the user_msg field
for both the username and password. The data fields in both the
START and CONTINUE packets are not used for ASCII logins, any content
MUST be ignored. The session is composed of a single START followed
by zero or more pairs of REPLYs and CONTINUEs, followed by a final
REPLY indicating PASS, FAIL or ERROR.
Inbound PAP Login
action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_PAP
minor_version = 0x1
The entire exchange MUST consist of a single START packet and a
single REPLY. The START packet MUST contain a username and the data
field MUST contain the PAP ASCII password. A PAP authentication only
consists of a username and password RFC 1334 [RFC1334] . The REPLY
from the server MUST be either a PASS, FAIL or ERROR.
Inbound CHAP login
action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_CHAP
minor_version = 0x1
The entire exchange MUST consist of a single START packet and a
single REPLY. The START packet MUST contain the username in the user
field and the data field is a concatenation of the PPP id, the
challenge and the response.
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
length of the response field (always 16 octets).
To perform the authentication, the server calculates the PAP hash as
defined in the PPP Authentication RFC RFC 1334 [RFC1334] and then
Dahm, et al. Expires February 22, 2018 [Page 18]
Internet-Draft The TACACS+ Protocol August 2017
compare that value with the response. The REPLY from the server MUST
be a PASS, FAIL or ERROR.
The client condcuts the exchange with the endstation and then sends
the resulting materials (challenge and responsee) to the server. So
although the selection of the challenge and its length are not an
aspect of the TACACS+ protocol, it is strongly recommended that the
client/endstation interaction is configured with a secure challenge
in mind, and the TACACS+ server can help by rejecting authentications
where the challenge is below a minimum length (for example, 8 bytes).
Inbound MS-CHAP v1 login
action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_MSCHAP
minor_version = 0x1
The entire exchange MUST consist of a single START packet and a
single REPLY. The START packet MUST contain the username in the user
field and the data field will be a concatenation of the PPP id, the
MS-CHAP challenge and the MS-CHAP response.
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
length of the response field (always 49 octets).
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
2433 [RFC2433] and then compare the resulting value with the
response. The REPLY from the server MUST be a PASS or FAIL.
For best practices, please refer to RFC 2433 [RFC2433]
Inbound MS-CHAP v2 login
action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_MSCHAPV2
minor_version = 0x1
The entire exchange MUST consist of a single START packet and a
single REPLY. The START packet MUST contain the username in the user
field and the data field will be a concatenation of the PPP id, the
MS-CHAP challenge and the MS-CHAP response.
Dahm, et al. Expires February 22, 2018 [Page 19]
Internet-Draft The TACACS+ Protocol August 2017
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
length of the response field (always 49 octets).
To perform the authentication, the server will use the algorithm
specified RFC 2759 [RFC2759] on the user's secret and challenge and
then compare the resulting value with the response. The REPLY from
the server MUST be a PASS or FAIL.
For best practices for MS-CHAP v2, please refer to RFC2759 [RFC2759]
Enable Requests
action = TAC_PLUS_AUTHEN_LOGIN
priv_lvl = implementation dependent
authen_type = not used
service = TAC_PLUS_AUTHEN_SVC_ENABLE
This is an ENABLE request, used to change the current running
privilege level of a user. The exchange MAY consist of multiple
messages while the server collects the information it requires in
order to allow changing the principal's privilege level. This
exchange is very similar to an Inbound ASCII login.
In order to readily distinguish enable requests from other types of
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
set to this value when requesting any other operation.
ASCII change password request
action = TAC_PLUS_AUTHEN_CHPASS
authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII
This exchange consists of multiple messages while the server collects
the information it requires in order to change the user's password.
It is very similar to an ASCII login. The status value
TAC_PLUS_AUTHEN_STATUS_GETPASS MUST only be used when requesting the
"new" password. It MAY be sent multiple times. When requesting the
"old" password, the status value MUST be set to
TAC_PLUS_AUTHEN_STATUS_GETDATA.
Dahm, et al. Expires February 22, 2018 [Page 20]
Internet-Draft The TACACS+ Protocol August 2017
4.4.3. Aborting an Authentication Session
The client may prematurely terminate a session by setting the
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
message explaining the reason for the abort. This information will
be handled by the server according to the requirements of the
deployment. The session is terminated, for more details about
session temrination, oplease refer to section (Section 3.4)
In the case of PALL, FAIL or ERROR, the server can insert a message
into server_msg to be displayed to the user.
The Draft `The Draft' [TheDraft] defined a mechanism to direct
authentication requests to an alternative server. This mechanism is
regarded as legacy and its implementation is optional.
If this feature is not implemented, then the client should treat
TAC_PLUS_AUTHEN_STATUS_FOLLOW as TAC_PLUS_AUTHEN_STATUS_FAIL
When the status equals TAC_PLUS_AUTHEN_STATUS_FOLLOW the packet
indicates that the TACACS+ server requests that authentication is
performed with an alternate server. The data field MUST contain
ASCII text describing one or more servers. A server description
appears like this:
[@<protocol>@]<host>>[@<key>]
If more than one host is specified, they MUST be separated into rows
by an ASCII Carriage Return (0x0D).
The protocol and key are optional, and apply only to host in the same
row. The protocol can describe an alternate way of performing the
authentication, other than TACACS+. If the protocol is not present,
then TACACS+ is assumed.
Protocols are ASCII numbers corresponding to the methods listed in
the authen_method field of authorization packets (defined below).
The host is specified as either a fully qualified domain name, or an
ASCII numeric IPV4 address specified as octets separated by dots
('.'), or IPV6 address text representation defined in RFC 4291.
If a key is supplied, the client MAY use the key in order to
authenticate to that host. The client may use a preconfigured key
for the host if it has one.
Use of the hosts in a TAC_PLUS_AUTHEN_STATUS_FOLLOW packet is at the
discretion of the TACACS+ client. It may choose to use any one, all
Dahm, et al. Expires February 22, 2018 [Page 21]
Internet-Draft The TACACS+ Protocol August 2017
or none of these hosts. If it chooses to use none, then it MUST
treat the authentication as if the return status was
TAC_PLUS_AUTHEN_STATUS_FAIL.
If the status equals TAC_PLUS_AUTHEN_STATUS_ERROR, then the host is
indicating that it is experiencing an unrecoverable error and the
authentication will proceed as if that host could not be contacted.
The data field may contain a message to be printed on an
administrative console or log.
If the status equals TAC_PLUS_AUTHEN_STATUS_RESTART, then the
authentication sequence is restarted with a new START packet from the
client, with new session Id, and seq_no set to 1. This REPLY packet
indicates that the current authen_type value (as specified in the
START packet) is not acceptable for this session. The client may try
an alternative authen_type.
If a client does not implement TAC_PLUS_AUTHEN_STATUS_RESTART option,
then it MUST process the response as if the status was
TAC_PLUS_AUTHEN_STATUS_FAIL.
5. Authorization
In the TACACS+ Protocol, authorization is the action of determining
what a user is allowed to do. Generally authentication precedes
authorization, though it is not mandatory that a client use the same
service for authentication that it will use for authorization. An
authorization request may indicate that the user is not authenticated
(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
is allowed the services in question.
Authorization does not merely provide yes or no answers, but it may
also customize the service for the particular user. A common use of
authorization is to provision a shell session when a user first logs
in to a device to administer it. The TACACS+ server might respond to
the request by allowing the service, but placing a time restriction
on the login shell. For a list of common attributes used in
authorization, see the Authorization Attributes section (Section 7.1)
.
In the TACACS+ protocol an authorization is always a single pair of
messages: a REQUEST from the client followed by a REPLY from the
server.
The authorization REQUEST message contains a fixed set of fields that
indicate how the user was authenticated and a variable set of
Dahm, et al. Expires February 22, 2018 [Page 22]
Internet-Draft The TACACS+ Protocol August 2017
arguments that describe the services and options for which
authorization is requested.
The REPLY contains a variable set of response arguments (attribute-
value pairs) that can restrict or modify the clients actions.
5.1. The Authorization 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
+----------------+----------------+----------------+----------------+
| authen_method | priv_lvl | authen_type | authen_service |
+----------------+----------------+----------------+----------------+
| user_len | port_len | rem_addr_len | arg_cnt |
+----------------+----------------+----------------+----------------+
| arg_1_len | arg_2_len | ... | arg_N_len |
+----------------+----------------+----------------+----------------+
| user ...
+----------------+----------------+----------------+----------------+
| port ...
+----------------+----------------+----------------+----------------+
| rem_addr ...
+----------------+----------------+----------------+----------------+
| arg_1 ...
+----------------+----------------+----------------+----------------+
| arg_2 ...
+----------------+----------------+----------------+----------------+
| ...
+----------------+----------------+----------------+----------------+
| arg_N ...
+----------------+----------------+----------------+----------------+
authen_method
This indicates the authentication method used by the client to
acquire the user information.
TAC_PLUS_AUTHEN_METH_NOT_SET := 0x00
TAC_PLUS_AUTHEN_METH_NONE := 0x01
TAC_PLUS_AUTHEN_METH_KRB5 := 0x02
TAC_PLUS_AUTHEN_METH_LINE := 0x03
TAC_PLUS_AUTHEN_METH_ENABLE := 0x04
TAC_PLUS_AUTHEN_METH_LOCAL := 0x05
Dahm, et al. Expires February 22, 2018 [Page 23]
Internet-Draft The TACACS+ Protocol August 2017
TAC_PLUS_AUTHEN_METH_TACACSPLUS := 0x06
TAC_PLUS_AUTHEN_METH_GUEST := 0x08
TAC_PLUS_AUTHEN_METH_RADIUS := 0x10
TAC_PLUS_AUTHEN_METH_KRB4 := 0x11
TAC_PLUS_AUTHEN_METH_RCMD := 0x20
KRB5 and KRB4 are Kerberos version 5 and 4. LINE refers to a fixed
password associated with the terminal line used to gain access.
LOCAL is a client local user database. ENABLE is a command that
authenticates in order to grant new privileges. TACACSPLUS is, of
course, TACACS+. GUEST is an unqualified guest authentication, such
as an ARAP guest login. RADIUS is the Radius authentication
protocol. RCMD refers to authentication provided via the R-command
protocols from Berkeley Unix.
priv_lvl
This field is used in the same way as the priv_lvl field in
authentication request and is described in the Privilege Level
section (Section 8) below. It indicates the users current privilege
level.
authen_type
This field corrsponds to the authen_type field in the authentication
section (Section 4) above. It indicates the type of authentication
that was performed. If this information is not available, then the
client will set authen_type to: TAC_PLUS_AUTHEN_TYPE_NOT_SET := 0x00.
This value is valid only in authorization and accounting requests.
authen_service
This field matches the authen_service field in the authentication
section (Section 4) above. It indicates the service through which
the user authenticated.
user, user_len
This field contains the user's account name. The user_len MUST
indicate the length of the user field, in bytes.
port, port_len
Dahm, et al. Expires February 22, 2018 [Page 24]
Internet-Draft The TACACS+ Protocol August 2017
This field matches the port field in the authentication section
(Section 4) above. The port_len indicates the length of the port
field, in bytes.
rem_addr, rem_addr_len
This field matches the rem_addr field in the authentication section
(Section 4) above. The rem_addr_len indicates the length of the port
field, in bytes.
arg_cnt
The number of authorization arguments to follow
arg_1 ... arg_N, arg_1_len .... arg_N_len
The arguments are the primary elements of the authorization
interaction. In the request packet they describe the specifics of
the authorization that is being requested. Each argument is encoded
in the packet as a single arg filed (arg_1... arg_N) with a
corresponding length fields (which indicates the length of each
argument in bytes).
The authorization arguments in both the REQUEST and the REPLY are
attribute-value pairs. The attribute and the value are in a single
US-ASCII string and are separated by either a "=" (0X3D) or a "*"
(0X2A). The equals sign indicates a mandatory argument. The
asterisk indicates an optional one.
It is not legal for an attribute name to contain either of the
separators. It is legal for attribute values to contain the
separators.
Optional arguments are ones that may be disregarded by either client
or server. Mandatory arguments require that the receiving side can
handle the attribute, that is: its implementation and configuration
includes the details of how to act on it. If the client receives a
mandatory argument that it cannot handle, it MUST consider the
authorization to have failed. It is legal to send an attribute-value
pair with a zero length value.
Attribute-value strings are not NULL terminated, rather their length
value indicates their end. The maximum length of an attribute-value
string is 255 characters. The minimum is two characters (one name
value character and the separator)
Though the attributes allow extensibility, a common core set of
authorization attributes SHOULD be supported by clients and servers,
Dahm, et al. Expires February 22, 2018 [Page 25]
Internet-Draft The TACACS+ Protocol August 2017
these are listed in the Authorization Attributes (Section 7.1)
section below.
5.2. The Authorization REPLY 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
+----------------+----------------+----------------+----------------+
| status | arg_cnt | server_msg len |
+----------------+----------------+----------------+----------------+
+ data_len | arg_1_len | arg_2_len |
+----------------+----------------+----------------+----------------+
| ... | arg_N_len | server_msg ...
+----------------+----------------+----------------+----------------+
| data ...
+----------------+----------------+----------------+----------------+
| arg_1 ...
+----------------+----------------+----------------+----------------+
| arg_2 ...
+----------------+----------------+----------------+----------------+
| ...
+----------------+----------------+----------------+----------------+
| arg_N ...
+----------------+----------------+----------------+----------------+
status This field indicates the authorization status
TAC_PLUS_AUTHOR_STATUS_PASS_ADD := 0x01
TAC_PLUS_AUTHOR_STATUS_PASS_REPL := 0x02
TAC_PLUS_AUTHOR_STATUS_FAIL := 0x10
TAC_PLUS_AUTHOR_STATUS_ERROR := 0x11
TAC_PLUS_AUTHOR_STATUS_FOLLOW := 0x21
server_msg, server_msg_len
This is an US-ASCII string that may be presented to the user. The
server_msg_len indicates the length of the server_msg field, in
bytes.
data, data_len
This is an US-ASCII string that may be presented on an administrative
display, console or log. The decision to present this message is
client specific. The data_len indicates the length of the data
field, in bytes.
Dahm, et al. Expires February 22, 2018 [Page 26]
Internet-Draft The TACACS+ Protocol August 2017
arg_cnt
The number of authorization arguments to follow.
arg_1 ... arg_N, arg_1_len .... arg_N_len
The arguments describe the specifics of the authorization that is
being requested. For details of the content of the args, refer to:
Authorization Attributes (Section 7.1) section below. Each argument
is encoded in the packet as a single arg field (arg_1... arg_N) with
a corresponding length fields (which indicates the length of each
argument in bytes).
If the status equals TAC_PLUS_AUTHOR_STATUS_FAIL, then the requested
authorization MUST be denied.
If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_ADD, then the
arguments specified in the request are authorized and the arguments
in the response MUST be applied according to the rules described
above.
If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_REPL then the client
MUST use the authorization attribute-value pairs (if any) in the
response, instead of the authorization attribute-value pairs from the
request.
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.
A status of TAC_PLUS_AUTHOR_STATUS_ERROR indicates an error occurred
on the server. For the differences between ERROR and FAIL, refer to
section Session Completion (Section 3.4) . None of the arg values
have any relevance if an ERROR is set, and must be ignored.
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
contents of the data field are identical to the
TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication.
6. Accounting
Accounting is typically the third action after authentication and
authorization. But again, neither authentication nor authorization
is required. Accounting is the action of recording what a user is
doing, and/or has done. Accounting in TACACS+ can serve two
purposes: It may be used as an auditing tool for security services.
It may also be used to account for services used, such as in a
billing environment. To this end, TACACS+ supports three types of
Dahm, et al. Expires February 22, 2018 [Page 27]
Internet-Draft The TACACS+ Protocol August 2017
accounting records. Start records indicate that a service is about
to begin. Stop records indicate that a service has just terminated,
and Update records are intermediate notices that indicate that a
service is still being performed. TACACS+ accounting records contain
all the information used in the authorization records, and also
contain accounting specific information such as start and stop times
(when appropriate) and resource usage information. A list of
accounting attributes is defined in the accounting section
(Section 6) .
6.1. 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
+----------------+----------------+----------------+----------------+
| flags | authen_method | priv_lvl | authen_type |
+----------------+----------------+----------------+----------------+
| authen_service | user_len | port_len | rem_addr_len |
+----------------+----------------+----------------+----------------+
| arg_cnt | arg_1_len | arg_2_len | ... |
+----------------+----------------+----------------+----------------+
| arg_N_len | user ...
+----------------+----------------+----------------+----------------+
| port ...
+----------------+----------------+----------------+----------------+
| rem_addr ...
+----------------+----------------+----------------+----------------+
| arg_1 ...
+----------------+----------------+----------------+----------------+
| arg_2 ...
+----------------+----------------+----------------+----------------+
| ...
+----------------+----------------+----------------+----------------+
| arg_N ...
+----------------+----------------+----------------+----------------+
flags
This holds bitmapped flags.
TAC_PLUS_ACCT_FLAG_START := 0x02
TAC_PLUS_ACCT_FLAG_STOP := 0x04
TAC_PLUS_ACCT_FLAG_WATCHDOG := 0x08
All other fields are defined in the authorization and authentication
sections above and have the same semantics. They provide details for
Dahm, et al. Expires February 22, 2018 [Page 28]
Internet-Draft The TACACS+ Protocol August 2017
the conditions on the client, and authentication context, so that
these details may be logged for accounting purposes.
See section 12 Accounting Attribute-value Pairs for the dictionary of
attributes relevant to accounting.
6.2. The Accounting REPLY Packet Body
The purpose of accounting is to record the action that has occurred
on the client. The server MUST reply with success only when the
accounting request has been recorded. If the server did not record
the accounting request then it MUST reply with ERROR.
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
+----------------+----------------+----------------+----------------+
| server_msg len | data_len |
+----------------+----------------+----------------+----------------+
| status | server_msg ...
+----------------+----------------+----------------+----------------+
| data ...
+----------------+
status
This is the return status. Values are:
TAC_PLUS_ACCT_STATUS_SUCCESS := 0x01
TAC_PLUS_ACCT_STATUS_ERROR := 0x02
TAC_PLUS_ACCT_STATUS_FOLLOW := 0x21
server_msg, server_msg_len
This is a US-ASCII string that may be presented to the user. The
server_msg_len indicates the length of the server_msg field, in
bytes.
data, data_len
This is a US-ASCII string that may be presented on an administrative
display, console or log. The decision to present this message is
client specific. The data_len indicates the length of the data
field, in bytes.
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
TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication.
Dahm, et al. Expires February 22, 2018 [Page 29]
Internet-Draft The TACACS+ Protocol August 2017
TACACS+ accounting is intended to record various types of events on
clients, for example: login sessions, command entry, and others as
required by the client implementation. These events are collectively
referred to in `The Draft' [TheDraft] as "tasks".
The TAC_PLUS_ACCT_FLAG_START flag indicates that this is a start
accounting message. Start messages will only be sent once when a
task is started. The TAC_PLUS_ACCT_FLAG_STOP indicates that this is
a stop record and that the task has terminated. The
TAC_PLUS_ACCT_FLAG_WATCHDOG flag means that this is an update record.
Update records are sent at the client's discretion if the task has
not finished.
Summary of Accounting Packets
+----------+-------+-------+-------------+-------------------------+
| Watchdog | Stop | Start | Flags & 0xE | Meaning |
+----------+-------+-------+-------------+-------------------------+
| 0 | 0 | 0 | 0 | INVALID |
| 0 | 0 | 1 | 2 | Start Accounting Record |
| 0 | 1 | 0 | 4 | Stop Accounting Record |
| 0 | 1 | 1 | 6 | INVALID |
| 1 | 0 | 0 | 8 | Watchdog, no update |
| 1 | 0 | 1 | A | Watchdog, with update |
| 1 | 1 | 0 | C | INVALID |
| 1 | 1 | 1 | E | INVALID |
+----------+-------+-------+-------------+-------------------------+
The START and STOP flags are mutually exclusive. When the WATCHDOG
flag is set along with the START flag, it indicates that the update
record is a duplicate of the original START record. If the START
flag is not set, then this indicates only that task is still running.
The STOP flag MUST NOT be set in conjunction with the WATCHDOG flag.
The Server MUST respond with TAC_PLUS_ACCT_STATUS_ERROR if the client
requests an INVALID option.
7. Attribute-Value Pairs
TACACS+ is intended to be an extensible protocol. The attributes
used in Authorization and Accounting are not limited by thsi
document. Some attributes are defined below for common use cases,
clients MUST use these attributes when supporting the corresponding
use cases.
All numeric values in an attribute-value string are provided as
decimal US-ASCII numbers, unless otherwise stated.
Dahm, et al. Expires February 22, 2018 [Page 30]
Internet-Draft The TACACS+ Protocol August 2017
All boolean attributes are encoded with values "true" or "false".
It is recommended that hosts be specified as a IP address so as to
avoid any ambiguities. ASCII numeric IPV4 address are specified as
octets separated by dots ('.'), IPV6 address text representation
defined in RFC 4291.
Absolute times are specified in seconds since the epoch, 12:00am Jan
1 1970. The timezone MUST be UTC unless a timezone attribute is
specified.
Attributes may be submitted with no value, in which case they consist
of the name and the mandatory or optional separator. For example,
the attribute "cmd" which has no value is transmitted as a string of
four characters "cmd="
7.1. Authorization Attributes
service
The primary service. Specifying a service attribute indicates that
this is a request for authorization or accounting of that service.
For example: "shell", "tty-server", "connection", "system" and
"firewall". This attribute MUST always be included.
protocol
the ptotocol field may be used to indicate a subset of a setvice.
cmd
a shell (exec) command. This indicates the command name of the
command that is to be run. The "cmd" attribute MUST be specified if
service equals "shell".
Authorization of shell commands is a common use-case for the TACACS+
protocol. Command Authorization generally takes one of two forms:
session-based and command-based.
For session-based shell authorization, the "cmd" argument will have
an empty value. The client determines which commands are allowed in
a session according to the arguments present in the authorization.
In command-based authorization, the client requests that the server
determine whether a command is allowed by making an authorization
request for each command. The "cmd" argument will have the command
name as its value.
Dahm, et al. Expires February 22, 2018 [Page 31]
Internet-Draft The TACACS+ Protocol August 2017
cmd-arg
an argument to a shell (exec) command. This indicates an argument
for the shell command that is to be run. Multiple cmd-arg attributes
may be specified, and they are order dependent.
acl
US-ASCII number representing a connection access list. Applicable
only to session-based shell authorization.
inacl
US-ASCII identifier for an interface input access list.
outacl
US-ASCII identifier for an interface output access list.
addr
a network address
addr-pool
The identifier of an address pool from which the client can assign an
address.
routing
Boolean. Specifies whether routing information is to be propagated
to, and accepted from this interface.
route
Indicates a route that is to be applied to this interface. Values
MUST be of the form "<dst_address> <mask> [<routing_addr>]". If a
<routing_addr> is not specified, the resulting route is via the
requesting peer.
timeout
an absolute timer for the connection (in minutes). A value of zero
indicates no timeout.
idletime
Dahm, et al. Expires February 22, 2018 [Page 32]
Internet-Draft The TACACS+ Protocol August 2017
an idle-timeout for the connection (in minutes). A value of zero
indicates no timeout.
autocmd
an auto-command to run. Applicable only to session-based shell
authorization.
noescape
Boolean. Prevents user from using an escape character. Applicable
only to session-based shell authorization.
nohangup
Boolean. Do not disconnect after an automatic command. Applicable
only to session-based shell authorization.y.
priv-lvl
privilege level to be assigned. Please refer to the Privilege Level
section (Section 8) below.
remote_user
remote userid (authen_method must have the value
TAC_PLUS_AUTHEN_METH_RCMD). In the case of rcmd authorizations, the
authen_method will be set to TAC_PLUS_AUTHEN_METH_RCMD and the
remote_user and remote_host attributes will provide the remote user
and host information to enable rhost style authorization. The
response may request that a privilege level be set for the user.
remote_host
remote host (authen_method must have the value
TAC_PLUS_AUTHEN_METH_RCMD)
callback-dialstring
Indicates that callback is to be done. Value is a dialstring, or
empty. Empty value indicates that the service MAY choose to get the
dialstring through other means.
callback-line
The line number to use for a callback.
callback-rotary
Dahm, et al. Expires February 22, 2018 [Page 33]
Internet-Draft The TACACS+ Protocol August 2017
The rotary number to use for a callback.
nocallback-verify
Do not require authentication after callback.
7.2. Accounting Attributes
The following attributes are defined for TACACS+ accounting only.
They MUST precede any attribute-value pairs that are defined in the
authorization section (Section 5) above.
task_id
Start and stop records for the same event MUST have matching task_id
attribute values. The client MUST ensure that active task_ids are
not duplicated: a client MUST NOT reuse a task_id a start record
until it has sent a stop record for that task_id. Servers MUST not
make assumptions about the format of a task_id.
start_time
The time the action started (in seconds since the epoch.).
stop_time
The time the action stopped (in seconds since the epoch.)
elapsed_time
The elapsed time in seconds for the action.
timezone
The timezone abbreviation for all timestamps included in this packet.
event
Used only when "service=system". Current values are "net_acct",
"cmd_acct", "conn_acct", "shell_acct" "sys_acct" and "clock_change".
These indicate system level changes. The flags field SHOULD indicate
whether the service started or stopped.
reason
Accompanies an event attribute. It describes why the event occurred.
bytes
Dahm, et al. Expires February 22, 2018 [Page 34]
Internet-Draft The TACACS+ Protocol August 2017
The number of bytes transferred by this action
bytes_in
The number of input bytes transferred by this action to the port
bytes_out
The number of output bytes transferred by this action from the port
paks
The number of packets transferred by this action.
paks_in
The number of input packets transferred by this action to the port.
paks_out
The number of output packets transferred by this action from the
port.
status
The numeric status value associated with the action. This is a
signed four (4) byte word in network byte order. 0 is defined as
success. Negative numbers indicate errors. Positive numbers
indicate non-error failures. The exact status values may be defined
by the client.
err_msg
An US-ASCII string describing the status of the action.
8. Privilege Levels
The TACACS+ Protocol supports flexible authorization schemes through
the extensible attributes.
One scheme is built in to the protocol and has been extensively used
for Session-based shell authorization: Privilege Levels. Privilege
Levels are ordered values from 0 to 15 with each level being a
superset of the next lower value. Configuration and implementation
of the client will map actions ()such as the permission to execute of
specific commands) to different privilege levels. Pre-defined values
are:
Dahm, et al. Expires February 22, 2018 [Page 35]
Internet-Draft The TACACS+ Protocol August 2017
TAC_PLUS_PRIV_LVL_MAX := 0x0f
TAC_PLUS_PRIV_LVL_ROOT := 0x0f
TAC_PLUS_PRIV_LVL_USER := 0x01
TAC_PLUS_PRIV_LVL_MIN := 0x00
A Privilege level can be assigned to a shell (EXEC) session when it
starts starts (for example, TAC_PLUS_PRIV_LVL_USER). The client will
permit the actions associated with this level to be executed. This
privilege level is returned by the Server in a session-based shell
authorization (when "service" equals "shell" and "cmd" is empty).
When a user required to perfrom actions that are mapped to a higher
privilege level, then an ENABLE type reuthentication can be initiated
by the client, in a way similar to su in unix. The client will
insert the required privilege level into the authentication header
for enable authentication request.
The use of Privilege levels to determine session-based access to
commands and resources is not mandatory for clients. Although the
privilege level scheme is widely supported, its lack of flexibility
in requiring a single monotonic hierarchy of permissions means that
other session-based command authorization schemes have evolved, and
so it is no longer mandatory for clients to use it. However, it is
still common enough that it SHOULD be supported by servers.
9. TACACS+ Security Considerations
Although in widespread use, the TACACS+ protocol (as defined in "the
Draft") does not meet modern security standards on its own. For this
reason, the authors intend to follow up this document with a more
secure version of the protocol.
TACACS+ was originally specified in "The Draft" (1998) is incomplete,
and leaves key points unspecified. As a result, software authors
have had to make implementation choices about what should, or should
not, be done in certain situations. These implementation choices are
somewhat constrained by ad hoc interoperability tests. That is, all
TACACS+ clients and servers interoperate, so there is a rough
consensus on how the protocol works.
9.1. Overall Security of The Protocol
TACACS+ protocol does not include a security mechanism that would
meet modern day requirements. Support for MD5-based crypto pad
encryption fails to provide any kind of transport integrity, which
presents at least the following risks:
Dahm, et al. Expires February 22, 2018 [Page 36]
Internet-Draft The TACACS+ Protocol August 2017
Accounting information may be modified by the man-in-the-middle
attacker, making such logs unsuitable and untrustable for auditing
purposes.
Only the body of the request is encrypted which leaves all header
fields open to trivial modification by the man-in-the-middle
attacker.
Invalid or misleading values may be inserted by the man-in-the-
middle attacker in various fields at known offsets to try and
circumvent the authentication or authorization checks even inside
the encrypted body.
While the protocol provides some measure of transport privacy, it is
vulnerable to at least the following attacks:
Brute force attacks exploiting increased efficiency of MD5 digest
computation.
Known plaintext attacks which may decrease the cost of brute force
attack.
Chosen plaintext attacks which may decrease the cost of a brute
force attack.
No forward secrecy means that original data may be revealed at the
later time and still provide valuable information to the attacker.
Even though, to the best knowledge of authors, this method of
encryption wasn't rigorously tested, authors feel that enough
information is available that it is best referred to as "obfuscation"
and not "encryption" and as such it MUST NOT BE relied upon to
provide privacy.
For example, a "session_id" can be replaced by an alternate one,
which could allow an unprivileged administrator to "steal" the
authorization from a session for a privileged administrator. An
attacker could also update the "flags" field to indicate that one or
the other end of a connection requires TAC_PLUS_UNENCRYPTED_FLAG,
which would subvert the obfuscation mechanism.
For this reasons, users deploying TACACS+ protocol in their
environments MUST limit access to known clients and MUST control the
security of the entire transmission path. Attacks who can guess the
key or otherwise break the obfuscation WILL gain unrestricted and
undetected access to all TACACS+ traffic. The security risk of such
attack succeeding against a centralised AAA system like TACACS+
cannot be overstated.
Dahm, et al. Expires February 22, 2018 [Page 37]
Internet-Draft The TACACS+ Protocol August 2017
9.2. Security of Authentication Sessions
The authentication options include options which MUST NOT be used
outside a secured deployment. Specifically, options which permit the
exchange of clear-text passwords or MSCHAPv1 and MS-CHAPv2. As of
the publication of this document, there has been no similar attacks
on the CHAP protocol.
Section 4.4.3 permits the redirection of a session to another server
via the TAC_PLUS_AUTHEN_STATUS_FOLLOW mechanism. As part of this
process, the secret key for a new server can be sent to the client.
This public exchange of secret keys means that once one session is
broken, it may be possible to leverage that attack to attacking
connections to other servers. This option MUST NOT be used outside a
secured deployment.
9.3. Security of Authorization Sessions
TACACS+ authorization is specifically separate from authentication.
Careful consideration must be given to whether this mode is
appropriate for the target deployment. Authorization sessions are
not cryptographically linked to any authentication sessions.
Instead, sessions are tied together implicitly by the contents of the
other fields, such as "use", "port", "rem_addr", etc.
The specification allows for the exchange of attribute-value pairs.
While a few such attributes are defined here, the protocol is
extensible, and vendors can define their own attributes. There is no
registry for such attributes, and in the absence of a published
specification, no way for a client or server to know the meaning of a
new attribute.
As a result, implemetors MUST ensure that new attribute-value pairs
are used consistently to communicate between client and server
implementations.
9.4. Security of Accounting Sessions
The security considerations for accounting sessions are largely the
same as for authorization sessions. This section describes
additional issues specific to accounting sessions.
There is no way in TACACS+ to signal that accounting is required.
There is no way for a server to signal a client how often accounting
is required. The accounting packets are received solely at the
clients discretion. Adding such functionality would assist with
auditing of user actions.
Dahm, et al. Expires February 22, 2018 [Page 38]
Internet-Draft The TACACS+ Protocol August 2017
The "task_id" field is defined only for accounting packets, and not
for authentication or authorization packets. As such, it is
difficult to correlate accounting data with a previous authentication
or authorization request.
9.5. TACACS+ Deployment Recommendations
Due to the above concerns with the protocol, it is critical that it
be deployed in a secure manner. The following recommendations are
made for those deploying and configuring TACACS+ as a solution for
Device Administration:
Secure the Deployment: TACACS+ does not provide modern security so
TACACS+ MUST BE employed over networks which ensure privacy and
integrity of the communication. The way this is ensured will
depend upon the organisational means: a dedicated and secure
management network where available in enterprise deployments, or
IPsec where dedicated networks are not available.
Always set a secret key (recommended minimum 14 characters) on the
client and server when configuring the connection between them.
Servers MUST be configured with a list of known clients. Servers
MUST be configured to reject requests from clients not on the
list. A unique secret key SHOULD be configured for every
individual client.
Restrict to TAC_PLUS_AUTHEN_TYPE_CHAP for authen_type where
possible. Use other options only when unavoidable due to
requirements of identity/password systems.
Servers SHOULD be restricted to requiring TACACS+ authentication
for authorization requests (i.e. TAC_PLUS_AUTHEN_METH_TACACSPLUS
is used).
Avoid the use of the redirection mechanism.
TAC_PLUS_AUTHEN_STATUS_FOLLOW, specifically avoid the option to
send send secret keys in the server list.
Take case when applying extensions to the dictionary of
authorization/accounting arguments. Ensure that the client and
server use of new argument names are consistent.
9.6. TACACS+ Client Implementation Recommendations
When implementing TACACS+ Clients it is recommended:
Dahm, et al. Expires February 22, 2018 [Page 39]
Internet-Draft The TACACS+ Protocol August 2017
Clients SHOULD not use TAC_PLUS_UNENCRYPTED_FLAG, even on networks
that are considered secure.
Ignore redirects to hosts which are outside of the pre-configured
range or list. A client SHOULD ignore any key provided via
TAC_PLUS_AUTHEN_STATUS_FOLLOW, and SHOULD instead use a
preconfigured key for that host.
If receiving an unknown mandatory authorization attribute, behave
as if it had received TAC_PLUS_AUTHOR_STATUS_FAIL. For full
details, refer to (Authorization attributes section).
9.7. TACACS+ Server Implementation Recommendations
When implementing TACACS+ Servers, it is recommended:
Server SHOULD reject all connections which have the
TAC_PLUS_UNENCRYPTED_FLAG with applicable ERROR response for type
of packet.
Servers MUST permit configuration of secret keys per individual
client. Servers SHOULD warn administrators if secret keys are not
unique per client.
On detection of an invalid shared secret: Servers SHOULD NOT
accept any new sessions on a connection, and terminate the
connection on completion of any sessions previously established
with a valid shared secret.
Allow the administrator to mandate : - TAC_PLUS_AUTHEN_TYPE_CHAP
for authen_type - TAC_PLUS_AUTHEN_METH_TACACSPLUS for
authen_method in authorization - Minimum length for shared secrets
9.8. TACACS+ Security and Operational Concerns
This section identifies some of the known security and operational
concerns. It is important to acknowledge that TACACS+ on its own
does not provide modern levels of security, and that it MUST be used
within a secure deployment.
The "encryption" is based upon MD5. In modern terms this can be
regarded merely as "obfuscation".
Only the packet body (not header) is obfuscated. For example,
session_id, flags containing TAC_PLUS_UNENCRYPTED_FLAG is exposed
in cleartext.
Dahm, et al. Expires February 22, 2018 [Page 40]
Internet-Draft The TACACS+ Protocol August 2017
Support of insecure authentication protocols such as plaintext,
MS-CHAP.
Difficulty to correlate authentication, authorization, and
accounting requests for a single unit of end client activity.
Potential confusion between clients and servers from different
vendors of the meaning of specific argument attributes.
Potential confusion between clients and servers from different
vendors of the meaning of specific commands.
In summary: It is strongly advised that TACACS+ MUST be used within a
secure deployment. Failure to do so may impact overall network
security.
10. References
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC1334] Lloyd, B. and W. Simpson, "PPP Authentication Protocols",
RFC 1334, DOI 10.17487/RFC1334, October 1992,
<http://www.rfc-editor.org/info/rfc1334>.
[RFC1750] Eastlake 3rd, D., Crocker, S., and J. Schiller,
"Randomness Recommendations for Security", RFC 1750,
DOI 10.17487/RFC1750, December 1994,
<http://www.rfc-editor.org/info/rfc1750>.
[RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions",
RFC 2433, DOI 10.17487/RFC2433, October 1998,
<http://www.rfc-editor.org/info/rfc2433>.
[RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2",
RFC 2759, DOI 10.17487/RFC2759, January 2000,
<http://www.rfc-editor.org/info/rfc2759>.
[TheDraft]
Carrel, D. and L. Grant, "The TACACS+ Protocol Version
1.78", June 1997, <https://tools.ietf.org/html/draft-
grant-tacacs-02>.
Authors' Addresses
Dahm, et al. Expires February 22, 2018 [Page 41]
Internet-Draft The TACACS+ Protocol August 2017
Thorsten Dahm
Google Inc
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
EMail: thorstendlux@google.com
Andrej Ota
Google Inc
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
EMail: aota@google.com
Douglas C. Medway Gash
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
US
Phone: +44 0208 8244508
EMail: dcmgash@cisco.com
David Carrel
vIPtela, Inc.
1732 North First St.
San Jose, CA 95112
US
EMail: dcarrel@viptela.com
Lol Grant
Dahm, et al. Expires February 22, 2018 [Page 42]