Internet Engineering Task Force (IETF) T. Dahm
Request for Comments: 8907 A. Ota
Category: Informational Google Inc.
ISSN: 2070-1721 D.C. Medway Gash
Cisco Systems, Inc.
D. Carrel
IPsec Research
L. Grant
September 2020
The Terminal Access Controller Access-Control System Plus (TACACS+)
Protocol
Abstract
This document describes the Terminal Access Controller Access-Control
System Plus (TACACS+) protocol, which is widely deployed today to
provide Device Administration for routers, network access servers,
and other networked computing devices via one or more centralized
servers.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8907.
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Table of Contents
1. Introduction
2. Conventions
3. Technical Definitions
3.1. Client
3.2. Server
3.3. Packet
3.4. Connection
3.5. Session
3.6. Treatment of Enumerated Protocol Values
3.7. Treatment of Text Strings
4. TACACS+ Packets and Sessions
4.1. The TACACS+ Packet Header
4.2. The TACACS+ Packet Body
4.3. Single Connection Mode
4.4. Session Completion
4.5. Data Obfuscation
5. Authentication
5.1. The Authentication START Packet Body
5.2. The Authentication REPLY Packet Body
5.3. The Authentication CONTINUE Packet Body
5.4. Description of Authentication Process
5.4.1. Version Behavior
5.4.2. Common Authentication Flows
5.4.3. Aborting an Authentication Session
6. Authorization
6.1. The Authorization REQUEST Packet Body
6.2. The Authorization REPLY Packet Body
7. Accounting
7.1. The Account REQUEST Packet Body
7.2. The Accounting REPLY Packet Body
8. Argument-Value Pairs
8.1. Value Encoding
8.2. Authorization Arguments
8.3. Accounting Arguments
9. Privilege Levels
10. Security Considerations
10.1. General Security of the Protocol
10.2. Security of Authentication Sessions
10.3. Security of Authorization Sessions
10.4. Security of Accounting Sessions
10.5. TACACS+ Best Practices
10.5.1. Shared Secrets
10.5.2. Connections and Obfuscation
10.5.3. Authentication
10.5.4. Authorization
10.5.5. Redirection Mechanism
11. IANA Considerations
12. References
12.1. Normative References
12.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
This document describes the Terminal Access Controller Access-Control
System Plus (TACACS+) protocol. It was conceived initially as a
general Authentication, Authorization, and Accounting (AAA) protocol.
It is widely deployed today but is mainly confined for a specific
subset of AAA called Device Administration, which includes
authenticating access to network devices, providing central
authorization of operations, and auditing of those operations.
A wide range of TACACS+ clients and servers is already deployed in
the field. The TACACS+ protocol they are based on is defined in a
document that was originally intended for IETF publication, but was
never standardized. The document is known as "The Draft"
[THE-DRAFT].
This Draft was a product of its time and did not address all of the
key security concerns that are considered when designing modern
standards. Therefore, deployment must be executed with care. These
concerns are addressed in Section 10.
The primary intent of this informational document is to clarify the
subset of "The Draft", which is common to implementations supporting
Device Administration. It is intended that all implementations that
conform to this document will conform to "The Draft". However, it is
not intended that all implementations that conform to "The Draft"
will conform to this document. The following features from "The
Draft" have been removed:
* This document officially removes SENDPASS for security reasons.
* The normative description of legacy features such as the Apple
Remote Access Protocol (ARAP) and outbound authentication has been
removed.
* The Support for forwarding to an alternative daemon
(TAC_PLUS_AUTHEN_STATUS_FOLLOW) has been deprecated.
The TACACS+ protocol allows for arbitrary length and content
authentication exchanges to support alternative authentication
mechanisms. 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 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
key element of the design of TACACS+ protocol. Essentially, it makes
TACACS+ a suite of three protocols. This document will address each
one in separate sections. Although TACACS+ defines all three, an
implementation or deployment is not required to employ all three.
Separating the elements is useful for the Device Administration use
case, specifically, for authorization and accounting of individual
commands in a session. Note that there is no provision made at the
protocol level to associate authentication requests with
authorization requests.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Technical Definitions
This section provides a few basic definitions that are applicable to
this document.
3.1. Client
The client is any device that initiates TACACS+ protocol requests to
mediate access, mainly for the Device Administration use case.
3.2. Server
The server receives TACACS+ protocol requests and replies according
to its business model in accordance with the flows defined in this
document.
3.3. Packet
All uses of the word packet in this document refer to TACACS+
protocol data units unless explicitly noted otherwise. The informal
term "packet" has become an established part of the definition.
3.4. Connection
TACACS+ uses TCP for its transport. TCP Server port 49 is allocated
by IANA for TACACS+ traffic.
3.5. 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
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.
3.6. 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
listed in this document for a header field, then it should respond
with an 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.
3.7. Treatment of Text Strings
The TACACS+ protocol makes extensive use of text strings. "The
Draft" intended that these strings would be treated as byte arrays
where each byte would represent a US-ASCII character.
More recently, server implementations have been extended to interwork
with external identity services, and so a more nuanced approach is
needed. Usernames MUST be encoded and handled using the
UsernameCasePreserved Profile specified in [RFC8265]. The security
considerations in Section 8 of [RFC8265] apply.
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.
All other text fields in TACACS+ MUST be treated as printable byte
arrays of US-ASCII as defined by [RFC0020]. The term "printable"
used here means the fields MUST exclude the "Control Characters"
defined in Section 5.2 of [RFC0020].
4. TACACS+ Packets and Sessions
4.1. 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 |
+----------------+----------------+----------------+----------------+
The following general rules apply to all TACACS+ packet types:
* To signal that any variable-length data fields are unused, the
corresponding length values are set to zero. Such fields MUST be
ignored, and treated as if not present.
* The lengths of data and message fields in a packet are specified
by their corresponding length field (and are not null terminated).
* All length values are unsigned and in network byte order.
major_version
This is the major TACACS+ version number.
TAC_PLUS_MAJOR_VER := 0xc
minor_version
This is the minor TACACS+ version number.
TAC_PLUS_MINOR_VER_DEFAULT := 0x0
TAC_PLUS_MINOR_VER_ONE := 0x1
type
This is the packet type.
Options are:
TAC_PLUS_AUTHEN := 0x01 (Authentication)
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.
TACACS+ 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 body of
the packet. This option MUST NOT be used in production. The
application of this flag will be covered in "Security
Considerations" (Section 10).
This flag SHOULD be clear in all deployments. Modern network
traffic tools support encrypted traffic when configured with the
shared secret (see "Shared Secrets" (Section 10.5.1)), so
obfuscated mode can and SHOULD be used even during test.
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" (Section 4.3).
All other bits MUST be ignored when reading, and SHOULD be set to
zero when writing.
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 [RFC4086].
length
The total length of the packet body (not including the header).
Implementations MUST allow control over maximum packet sizes
accepted by TACACS+ Servers. The recommended maximum packet size
is 2^(16).
4.2. 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.
4.3. Single Connection Mode
Single Connection Mode is intended to improve performance where there
is a lot of traffic between a client and a server by allowing the
client to multiplex multiple sessions 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 Connection 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 Connection 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 Connection Mode.
No provision is made for changing Single Connection Mode after the
first two packets; the client and server MUST ignore the flag after
the second packet on a connection.
If Single Connection 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 be appropriate to allocate a long-lasting TCP
connection to a specific client in some deployments. 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
MUST 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 Connection Mode has been established.
The TCP connection underlying the Single Connection Mode will close
eventually either because of the timeout from the server or from an
intermediate link. If a session is in progress when the client
detects disconnect, then the client should handle it as described in
"Session Completion" (Section 4.4). If a session is not in progress,
then the client will need to detect this and restart the Single
Connection Mode when it initiates the next session.
4.4. Session Completion
The REPLY packets defined for the packet types in the sections below
(Authentication, Authorization, and Accounting) contain a status
field. The complete set of options for this field depend upon the
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 that is not aborted).
The server responds with a PASS or a FAIL to indicate that the
processing of the request completed and that the client can apply the
result (PASS or FAIL) to control the execution of the action that
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 cannot apply the result,
and it MUST behave as if the server could not be connected to. For
example, the client tries alternative methods, if they are available,
such as sending the request to a backup server or using local
configuration to determine whether the action that prompted the
request should be executed.
Refer to "Aborting an Authentication Session" (Section 5.4.3) for
details on handling additional status options.
When the session is complete, the TCP connection should be handled as
follows, according to whether Single Connection 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 "Single Connection Mode" (Section 4.3)) 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 4.5)), 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.
It is recommended that client implementations provide robust schemes
for dealing with servers that cannot be connected to. Options
include providing a list of servers for redundancy and an option for
a local fallback configuration if no servers can be reached. Details
will be implementation specific.
The client should manage connections and handle the case of a server
that establishes a connection but does not respond. The exact
behavior is implementation specific. It is recommended that the
client close the connection after a configurable timeout.
4.5. Data Obfuscation
The body of packets may be obfuscated. The following sections
describe the obfuscation method that is supported in the protocol.
In "The Draft", this process was actually referred to as Encryption,
but the algorithm would not meet modern standards and so will not be
termed as encryption in this document.
The obfuscation mechanism relies on a secret key, a shared secret
value that is known to both the client and the server. The secret
keys MUST remain secret.
Server implementations MUST allow a unique secret key to be
associated with each client. It is a site-dependent decision as to
whether or not the use of separate keys is appropriate.
The flag field MUST be configured with TAC_PLUS_UNENCRYPTED_FLAG set
to 0 so that the packet body is obfuscated by XORing it bytewise with
a pseudo-random pad:
ENCRYPTED {data} = data ^(pseudo_pad)
The packet body can then be de-obfuscated by XORing it bytewise with
a pseudo-random pad.
data = ENCRYPTED {data} ^(pseudo_pad)
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 [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 and session_id are extracted from the header.
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 secrets it has configured for the
device do not match, it MUST return ERROR. For details of TCP
connection handling on ERROR, refer to "Session Completion"
(Section 4.4).
TAC_PLUS_UNENCRYPTED_FLAG == 0x1
This option is deprecated and MUST NOT be used in production. In
this case, the entire packet body is in cleartext. A 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
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 signaled. For details of TCP connection
handling on ERROR, refer to "Session Completion" (Section 4.4).
Commonly, such failures are seen when the keys are mismatched between
the client and the TACACS+ server.
5. Authentication
Authentication is the action of determining who a user (or entity)
is. Authentication can take many forms. Traditional authentication
employs a name and a fixed password. However, fixed passwords are
vulnerable security, so many modern authentication mechanisms utilize
"one-time" passwords or a challenge-response query. TACACS+ is
designed to support all of these and be flexible 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" (Section 6)). 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.
5.1. The Authentication START 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
+----------------+----------------+----------------+----------------+
| 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.
Valid values are:
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 "Privilege Levels" (Section 9).
authen_type
The type of authentication. Please see "Common Authentication
Flows" (Section 5.4.2).
Valid values are:
TAC_PLUS_AUTHEN_TYPE_ASCII := 0x01
TAC_PLUS_AUTHEN_TYPE_PAP := 0x02
TAC_PLUS_AUTHEN_TYPE_CHAP := 0x03
TAC_PLUS_AUTHEN_TYPE_MSCHAP := 0x05
TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 := 0x06
authen_service
This is the service that is requesting the authentication.
Valid 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_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 indicates 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, which substitutes
the current user's identity with another. 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. If included, the user_len indicates the length of the user
field, in bytes.
port, port_len
The name of the client port on which the authentication is taking
place. The value of this field is free-format text and is client
specific. Examples of this argument include "tty10" to denote the
tenth tty line, and "async10" to denote the tenth async interface.
The client documentation SHOULD define the values and their
meanings for this field. For details of text encoding, see
"Treatment of Text Strings" (Section 3.7). The port_len indicates
the length of the port field, in bytes.
rem_addr, rem_addr_len
A string indicating the remote location from which the user has
connected to the client. For details of text encoding, see
"Treatment of Text Strings" (Section 3.7).
When TACACS+ was used for dial-up services, this value contained
the caller ID.
When TACACS+ is used for Device Administration, the user is
normally connected via a network, and in this case, the value is
intended to hold a network address, IPv4 or IPv6. For IPv6
address text representation defined, please see [RFC5952].
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
The data field is used to send data appropriate for the action and
authen_type. It is described in more detail in "Common
Authentication Flows" (Section 5.4.2). The data_len field
indicates the length of the data field, in bytes.
5.2. 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.
Valid values are:
TAC_PLUS_AUTHEN_STATUS_PASS := 0x01
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.
The server_msg_len indicates the length of the server_msg field,
in bytes. For details of text encoding, see "Treatment of Text
Strings" (Section 3.7).
data, data_len
A field that holds data that is a part of the authentication
exchange and is intended for client processing, not the user. It
is not a printable text encoding. Examples of its use are shown
in "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
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 ...
+----------------+
user_msg, user_msg_len
A field that 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. It is not a printable text encoding. 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
5.4. 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
authentication.
This document defines a core set of authentication flows to be
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, or RESTART. The actions and meanings when the server
sends a RESTART or ERROR 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,
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 a username and TAC_PLUS_AUTHEN_STATUS_GETPASS as a
request for a password. The TAC_PLUS_AUTHEN_STATUS_GETDATA is the
generic request for more information to flexibly support future
requirements.
If the information being requested by the server from 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 reflected in the user interface as it is
entered.
The data field is only used in the REPLY where explicitly defined
below.
5.4.1. Version Behavior
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
Challenge Handshake Authentication Protocol (CHAP) and Password
Authentication Protocol (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 |
+-------------+-------+--------+----------+
Table 1: TACACS+ Protocol Versioning
The '-' symbol represents that the option is not valid.
All authorization and accounting and ASCII authentication use
minor_version 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.
The removal of SENDPASS was prompted by security concerns and is no
longer considered part of the TACACS+ protocol.
5.4.2. 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.
5.4.2.1. ASCII Login
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. If the user does not provide a
username, then the server can send another
TAC_PLUS_AUTHEN_STATUS_GETUSER request, but the server MUST limit the
number of retries that are permitted; the recommended limit is three
attempts. 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.
5.4.2.2. 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 [RFC1334] (Obsolete). The REPLY
from the server MUST be either a PASS, FAIL, or ERROR.
5.4.2.3. 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 PPP hash as
defined in PPP Authentication [RFC1334] and then compares that value
with the response. The MD5 algorithm option is always used. The
REPLY from the server MUST be a PASS, FAIL, or ERROR.
The selection of the challenge and its length are not an aspect of
the TACACS+ protocol. However, it is strongly recommended that the
client/endstation interaction be configured with a secure challenge.
The TACACS+ server can help by rejecting authentications where the
challenge is below a minimum length (minimum recommended is 8 bytes).
5.4.2.4. 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
[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 [RFC2433]. The TACACS+ server
MUST reject authentications where the challenge deviates from 8 bytes
as defined in the RFC.
5.4.2.5. 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.
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 [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]. The
TACACS+ server MUST reject authentications where the challenge
deviates from 16 bytes as defined in the RFC.
5.4.2.6. 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 ASCII login (Section 5.4.2.1).
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.
5.4.2.7. 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.
5.4.3. 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 a text
explaining the reason for the abort. This text will be handled by
the server according to the requirements of the deployment. For
details of text encoding, see "Treatment of Text Strings"
(Section 3.7). For more details about session termination, refer to
"Session Completion" (Section 4.4).
In cases of PASS, FAIL, or ERROR, the server can insert a message
into server_msg to be displayed to the user.
"The Draft" [THE-DRAFT] defined a mechanism to direct authentication
requests to an alternative server. This mechanism is regarded as
insecure, is deprecated, and is not covered here. The client should
treat TAC_PLUS_AUTHEN_STATUS_FOLLOW as 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 a 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 the TAC_PLUS_AUTHEN_STATUS_RESTART
option, then it MUST process the response as if the status was
TAC_PLUS_AUTHEN_STATUS_FAIL.
6. 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
into 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 arguments used in
authorization, see "Authorization Arguments" (Section 8.2).
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
arguments that describe the services and options for which
authorization is requested.
The REPLY contains a variable set of response arguments (argument-
value pairs) that can restrict or modify the client's actions.
6.1. 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 field allows the client to indicate the authentication method
used to acquire 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
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
As this information is not always subject to verification, it MUST
NOT be used in policy evaluation. 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.
RADIUS is the RADIUS authentication protocol. RCMD refers to
authentication provided via the R-command protocols from Berkeley
Unix. KRB5 [RFC4120] and KRB4 [KRB4] are Kerberos versions 5 and
4.
As mentioned above, this field is used by the client to indicate
how it performed the authentication. One of the options
(TAC_PLUS_AUTHEN_METH_TACACSPLUS := 0x06) is TACACS+ itself, and
so the detail of how the client performed this option is given in
"Authentication" (Section 5). For all other options, such as KRB
and RADIUS, the TACACS+ protocol did not play any part in the
authentication phase; as those interactions were not conducted
using the TACACS+ protocol, they will not be documented here. For
implementers of clients who need details of the other protocols,
please refer to the respective Kerberos [RFC4120] and RADIUS
[RFC3579] RFCs.
priv_lvl
This field is used in the same way as the priv_lvl field in
authentication request and is described in "Privilege Levels"
(Section 9). It indicates the user's current privilege level.
authen_type
This field corresponds to the authen_type field in
"Authentication" (Section 5). 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 is the same as the authen_service field in
"Authentication" (Section 5). 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
This field matches the port field in "Authentication" (Section 5).
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 "Authentication"
(Section 5). The rem_addr_len indicates the length of the port
field, in bytes.
arg_cnt
This represents the number of authorization arguments to follow.
arg_1 ... arg_N, arg_1_len .... arg_N_len
These 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 field (arg_1... arg_N) with
a corresponding length field (which indicates the length of each
argument in bytes).
The authorization arguments in both the REQUEST and the REPLY are
argument-value pairs. The argument and the value are in a single
string and are separated by either a "=" (0X3D) or a "*" (0X2A).
The equals sign indicates a mandatory argument. The asterisk
indicates an optional one. For details of text encoding, see
"Treatment of Text Strings" (Section 3.7).
An argument name MUST NOT contain either of the separators. An
argument value MAY contain the separators. This means that the
arguments must be parsed until the first separator is encountered;
all characters in the argument, after this separator, are
interpreted as the argument value.
Optional arguments are ones that may be disregarded by either
client or server. Mandatory arguments require that the receiving
side can handle the argument, 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. The value part of
an argument-value pair may be empty, that is, the length of the
value may be zero.
Argument-value strings are not NULL terminated; rather, their
length value indicates their end. The maximum length of an
argument-value string is 255 characters. The minimum is two
characters (one name-value character and the separator).
Though the arguments allow extensibility, a common core set of
authorization arguments SHOULD be supported by clients and
servers; these are listed in "Authorization Arguments"
(Section 8.2).
6.2. 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
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.
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.
TAC_PLUS_AUTHOR_STATUS_PASS_REPL := 0x02
If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_REPL, then
the client MUST use the authorization argument-value pairs (if
any) in the response instead of the authorization argument-
value pairs from the request.
TAC_PLUS_AUTHOR_STATUS_FAIL := 0x10
If the status equals TAC_PLUS_AUTHOR_STATUS_FAIL, then the
requested authorization MUST be denied.
TAC_PLUS_AUTHOR_STATUS_ERROR := 0x11
A status of TAC_PLUS_AUTHOR_STATUS_ERROR indicates an error
occurred on the server. For the differences between ERROR and
FAIL, refer to "Session Completion" (Section 4.4). None of
the arg values have any relevance if an ERROR is set and must
be ignored.
TAC_PLUS_AUTHOR_STATUS_FOLLOW := 0x21
When the status equals TAC_PLUS_AUTHOR_STATUS_FOLLOW, 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.
server_msg, server_msg_len
This is a string that may be presented to the user. The
server_msg_len indicates the length of the server_msg field, in
bytes. For details of text encoding, see "Treatment of Text
Strings" (Section 3.7).
data, data_len
This is a 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. For details of text encoding, see "Treatment of
Text Strings" (Section 3.7).
arg_cnt
This represents 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 Arguments" (Section 8.2). Each argument is encoded
in the packet as a single arg field (arg_1... arg_N) with a
corresponding length field (which indicates the length of each
argument in bytes).
7. 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, and it may
also be used to account for services used such as in a billing
environment. To this end, TACACS+ supports three types of 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
arguments is defined in "Accounting Arguments" (Section 8.3).
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
+----------------+----------------+----------------+----------------+
| 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.
Valid values are:
TAC_PLUS_ACCT_FLAG_START := 0x02
TAC_PLUS_ACCT_FLAG_STOP := 0x04
TAC_PLUS_ACCT_FLAG_WATCHDOG := 0x08
All other fields are defined in "Authentication" (Section 5) and
"Authorization" (Section 6) and have the same semantics. They
provide details for the conditions on the client, and authentication
context, so that these details may be logged for accounting purposes.
See "Accounting Arguments" (Section 8.3) for the dictionary of
arguments relevant to accounting.
7.2. 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
When the status equals TAC_PLUS_ACCT_STATUS_FOLLOW, the
actions to be taken and the contents of the data field are
identical to the TAC_PLUS_AUTHEN_STATUS_FOLLOW status for
authentication.
server_msg, server_msg_len
This is a string that may be presented to the user. The
server_msg_len indicates the length of the server_msg field, in
bytes. For details of text encoding, see "Treatment of Text
Strings" (Section 3.7).
data, data_len
This is a 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. For details of text encoding, see "Treatment of
Text Strings" (Section 3.7).
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" [THE-DRAFT] 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.
+==========+======+=======+=============+=========================+
| 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 |
+----------+------+-------+-------------+-------------------------+
Table 2: Summary of Accounting Packets
The START and STOP flags are mutually exclusive.
The WATCHDOG flag is used by the client to communicate ongoing status
of a long-running task. Update records are sent at the client's
discretion. The frequency of the update depends upon the intended
application: a watchdog to provide progress indication will require
higher frequency than a daily keep-alive. When the WATCHDOG flag is
set along with the START flag, it indicates that the update record
provides additional or updated arguments from the original START
record. If the START flag is not set, then this indicates only that
task is still running, and no new information is provided (servers
MUST ignore any arguments). 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.
8. Argument-Value Pairs
TACACS+ is intended to be an extensible protocol. The arguments used
in Authorization and Accounting are not limited by this document.
Some arguments are defined below for common use cases. Clients MUST
use these arguments when supporting the corresponding use cases.
8.1. Value Encoding
All argument values are encoded as strings. For details of text
encoding, see "Treatment of Text Strings" (Section 3.7). The
following type representations SHOULD be followed.
Numeric
All numeric values in an argument-value string are provided as
decimal numbers, unless otherwise stated. All arguments include a
length field, and TACACS+ implementations MUST verify that they
can accommodate the lengths of numeric arguments before attempting
to process them. If the length cannot be accommodated, then the
argument MUST be regarded as not handled and the logic in
"Authorization" (Section 6.1) regarding the processing of
arguments MUST be applied.
Boolean
All Boolean arguments are encoded with values "true" or "false".
IP-Address
It is recommended that hosts be specified as an IP address so as
to avoid any ambiguities. For details of text encoding, see
"Treatment of Text Strings" (Section 3.7). IPv4 addresses are
specified as octet numerics separated by dots ('.'). IPv6 address
text representation is defined in [RFC5952].
Date Time
Absolute date/times are specified in seconds since the epoch,
12:00am, January 1, 1970. The time zone MUST be UTC unless a time
zone argument is specified.
String
Many values have no specific type representation and are
interpreted as plain strings.
Empty Values
Arguments may be submitted with no value, in which case they
consist of the name and the mandatory or optional separator. For
example, the argument "cmd", which has no value, is transmitted as
a string of four characters "cmd=".
8.2. Authorization Arguments
service (String)
The primary service. Specifying a service argument indicates that
this is a request for authorization or accounting of that service.
For example: "shell", "tty-server", "connection", "system" and
"firewall"; others may be chosen for the required application.
This argument MUST always be included.
protocol (String)
A field that may be used to indicate a subset of a service.
cmd (String)
A shell (exec) command. This indicates the command name of the
command that is to be run. The "cmd" argument 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 or 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.
cmd-arg (String)
An argument to a shell (exec) command. This indicates an argument
for the shell command that is to be run. Multiple cmd-arg
arguments may be specified, and they are order dependent.
acl (Numeric)
A number representing a connection access list. Applicable only
to session-based shell authorization. For details of text
encoding, see "Treatment of Text Strings" (Section 3.7).
inacl (String)
The identifier (name) of an interface input access list. For
details of text encoding, see "Treatment of Text Strings"
(Section 3.7).
outacl (String)
The identifier (name) of an interface output access list. For
details of text encoding, see "Treatment of Text Strings"
(Section 3.7).
addr (IP-Address)
A network address.
addr-pool (String)
The identifier of an address pool from which the client can assign
an address.
timeout (Numeric)
An absolute timer for the connection (in minutes). A value of
zero indicates no timeout.
idletime (Numeric)
An idle-timeout for the connection (in minutes). A value of zero
indicates no timeout.
autocmd (String)
An auto-command to run. Applicable only to session-based shell
authorization.
noescape (Boolean)
Prevents the user from using an escape character. Applicable only
to session-based shell authorization.
nohangup (Boolean)
Boolean. Do not disconnect after an automatic command.
Applicable only to session-based shell authorization.
priv-lvl (Numeric)
The privilege level to be assigned. Please refer to "Privilege
Levels" (Section 9).
8.3. Accounting Arguments
The following arguments are defined for TACACS+ accounting only.
They MUST precede any argument-value pairs that are defined in
"Authorization" (Section 6).
task_id (String)
Start and stop records for the same event MUST have matching
task_id argument values. The client MUST ensure that active
task_ids are not duplicated; a client MUST NOT reuse a task_id in
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 (Date Time)
The time the action started (in seconds since the epoch).
stop_time (Date Time)
The time the action stopped (in seconds since the epoch).
elapsed_time (Numeric)
The elapsed time in seconds for the action.
timezone (String)
The time zone abbreviation for all timestamps included in this
packet. A database of time zones is maintained in [TZDB].
event (String)
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 (String)
Accompanies an event argument. It describes why the event
occurred.
bytes (Numeric)
The number of bytes transferred by this action.
bytes_in (Numeric)
The number of bytes transferred by this action from the endstation
to the client port.
bytes_out (Numeric)
The number of bytes transferred by this action from the client to
the endstation port.
paks (Numeric)
The number of packets transferred by this action.
paks_in (Numeric)
The number of input packets transferred by this action from the
endstation to the client port.
paks_out (Numeric)
The number of output packets transferred by this action from the
client port to the endstation.
err_msg (String)
A string describing the status of the action. For details of text
encoding, see "Treatment of Text Strings" (Section 3.7).
Where the TACACS+ deployment is used to support the Device
Administration use case, it is often required to log all commands
entered into client devices. To support this mode of operation,
TACACS+ client devices MUST be configured to send an accounting start
packet for every command entered, irrespective of how the commands
were authorized. These "Command Accounting" packets MUST include the
"service" and "cmd" arguments, and if needed, the "cmd-arg" arguments
detailed in Section 8.2.
9. Privilege Levels
The TACACS+ protocol supports flexible authorization schemes through
the extensible arguments.
The privilege levels scheme is built into the protocol and has been
extensively used as an option for Session-based shell authorization.
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 specific commands) to different privilege levels. The
allocation of commands to privilege levels is highly dependent upon
the deployment. Common allocations are as follows:
TAC_PLUS_PRIV_LVL_MIN := 0x00. The level normally allocated to an
unauthenticated session.
TAC_PLUS_PRIV_LVL_USER := 0x01. The level normally allocated to a
regular authenticated session.
TAC_PLUS_PRIV_LVL_ROOT := 0x0f. The level normally allocated to a
session authenticated by a highly privileged user to allow
commands with significant system impact.
TAC_PLUS_PRIV_LVL_MAX := 0x0f. The highest privilege level.
A privilege level can be assigned to a shell (exec) session when it
starts. 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 is required to perform actions that
are mapped to a higher privilege level, an ENABLE-type
reauthentication can be initiated by the client. The client will
insert the required privilege level into the authentication header
for ENABLE authentication requests.
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.
However, it is still common enough that it SHOULD be supported by
servers.
10. Security Considerations
"The Draft" [THE-DRAFT] from 1998 did not address all of the key
security concerns that are considered when designing modern
standards. This section addresses known limitations and concerns
that will impact overall security of the protocol and systems where
this protocol is deployed to manage central authentication,
authorization, or accounting for network Device Administration.
Multiple implementations of the protocol described in "The Draft"
[THE-DRAFT] have been deployed. As the protocol was never
standardized, current implementations may be incompatible in non-
obvious ways, giving rise to additional security risks. This section
does not claim to enumerate all possible security vulnerabilities.
10.1. General Security of the Protocol
The TACACS+ protocol does not include a security mechanism that would
meet modern-day requirements. These security mechanisms would be
best referred to as "obfuscation" and not "encryption", since they
provide no meaningful integrity, privacy, or replay protection. An
attacker with access to the data stream should be assumed to be able
to read and modify all TACACS+ packets. Without mitigation, a range
of risks such as the following are possible:
* Accounting information may be modified by the man-in-the-middle
attacker, making such logs unsuitable and not trustable for
auditing purposes.
* 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 obfuscated 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 that may decrease the cost of brute-force
attacks.
* Chosen plaintext attacks that may decrease the cost of a brute-
force attacks.
* No forward secrecy.
Even though, to the best knowledge of the authors, this method of
encryption wasn't rigorously tested, enough information is available
that it is best referred to as "obfuscation" and not "encryption".
For these reasons, users deploying the TACACS+ protocol in their
environments MUST limit access to known clients and MUST control the
security of the entire transmission path. Attackers who can guess
the key or otherwise break the obfuscation will gain unrestricted and
undetected access to all TACACS+ traffic. Ensuring that a
centralized AAA system like TACACS+ is deployed on a secured
transport is essential to managing the security risk of such an
attack.
The following parts of this section enumerate only the session-
specific risks that are in addition to general risk associated with
bare obfuscation and lack of integrity checking.
10.2. Security of Authentication Sessions
Authentication sessions SHOULD be used via a secure transport (see
"TACACS+ Best Practices" (Section 10.5)) as the man-in-the-middle
attack may completely subvert them. Even CHAP, which may be
considered resistant to password interception, is unsafe as it does
not protect the username from a trivial man-in-the-middle attack.
This document deprecates the redirection mechanism using the
TAC_PLUS_AUTHEN_STATUS_FOLLOW option, which was included in "The
Draft". As part of this process, the secret key for a new server was
sent to the client. This public exchange of secret keys means that
once one session is broken, it may be possible to leverage that key
to attacking connections to other servers. This mechanism MUST NOT
be used in modern deployments. It MUST NOT be used outside a secured
deployment.
10.3. Security of Authorization Sessions
Authorization sessions SHOULD be used via a secure transport (see
"TACACS+ Best Practices" (Section 10.5)) as it's trivial to execute a
successful man-in-the-middle attack that changes well-known plaintext
in either requests or responses.
As an example, take the field "authen_method". It's not unusual in
actual deployments to authorize all commands received via the device
local serial port (a console port), as that one is usually considered
secure by virtue of the device located in a physically secure
location. If an administrator would configure the authorization
system to allow all commands entered by the user on a local console
to aid in troubleshooting, that would give all access to all commands
to any attacker that would be able to change the "authen_method" from
TAC_PLUS_AUTHEN_METH_TACACSPLUS to TAC_PLUS_AUTHEN_METH_LINE. In
this regard, the obfuscation provided by the protocol itself wouldn't
help much, because:
* A lack of integrity means that any byte in the payload may be
changed without either side detecting the change.
* Known plaintext means that an attacker would know with certainty
which octet is the target of the attack (in this case, first octet
after the header).
* In combination with known plaintext, the attacker can determine
with certainty the value of the crypto-pad octet used to obfuscate
the original octet.
10.4. Security of Accounting Sessions
Accounting sessions SHOULD be used via a secure transport (see
"TACACS+ Best Practices" (Section 10.5)). Although Accounting
sessions are not directly involved in authentication or authorizing
operations on the device, man-in-the-middle attackers may do any of
the following:
* Replace accounting data with new valid values or garbage that can
confuse auditors or hide information related to their
authentication and/or authorization attack attempts.
* Try and poison an accounting log with entries designed to make
systems behave in unintended ways (these systems could be TACACS+
servers and any other systems that would manage accounting
entries).
In addition to these direct manipulations, different client
implementations pass a different fidelity of accounting data. Some
vendors have been observed in the wild that pass sensitive data like
passwords, encryption keys, and the like as part of the accounting
log. Due to a lack of strong encryption with perfect forward
secrecy, this data may be revealed in the future, leading to a
security incident.
10.5. TACACS+ Best Practices
With respect to the observations about the security issues described
above, a network administrator MUST NOT rely on the obfuscation of
the TACACS+ protocol. TACACS+ MUST be used within a secure
deployment; TACACS+ MUST be deployed over networks that ensure
privacy and integrity of the communication and MUST be deployed over
a network that is separated from other traffic. Failure to do so
will impact overall network security.
The following recommendations impose restrictions on how the protocol
is applied. These restrictions were not imposed in "The Draft". New
implementations, and upgrades of current implementations, MUST
implement these recommendations. Vendors SHOULD provide mechanisms
to assist the administrator to achieve these best practices.
10.5.1. Shared Secrets
TACACS+ servers and clients MUST treat shared secrets as sensitive
data to be managed securely, as would be expected for other sensitive
data such as identity credential information. TACACS+ servers MUST
NOT leak sensitive data.
For example:
* TACACS+ servers MUST NOT expose shared secrets in logs.
* TACACS+ servers MUST allow a dedicated secret key to be defined
for each client.
* TACACS+ server management systems MUST provide a mechanism to
track secret key lifetimes and notify administrators to update
them periodically. TACACS+ server administrators SHOULD change
secret keys at regular intervals.
* TACACS+ servers SHOULD warn administrators if secret keys are not
unique per client.
* TACACS+ server administrators SHOULD always define a secret for
each client.
* TACACS+ servers and clients MUST support shared keys that are at
least 32 characters long.
* TACACS+ servers MUST support policy to define minimum complexity
for shared keys.
* TACACS+ clients SHOULD NOT allow servers to be configured without
a shared secret key or shared key that is less than 16 characters
long.
* TACACS+ server administrators SHOULD configure secret keys of a
minimum of 16 characters in length.
10.5.2. Connections and Obfuscation
TACACS+ servers MUST allow the definition of individual clients. The
servers MUST only accept network connection attempts from these
defined known clients.
TACACS+ servers MUST reject connections that have
TAC_PLUS_UNENCRYPTED_FLAG set. There MUST always be a shared secret
set on the server for the client requesting the connection.
If an invalid shared secret is detected when processing packets for a
client, TACACS+ servers MUST NOT accept any new sessions on that
connection. TACACS+ servers MUST terminate the connection on
completion of any sessions that were previously established with a
valid shared secret on that connection.
TACACS+ clients MUST NOT set TAC_PLUS_UNENCRYPTED_FLAG. Clients MUST
be implemented in a way that requires explicit configuration to
enable the use of TAC_PLUS_UNENCRYPTED_FLAG. This option MUST NOT be
used when the client is in production.
When a TACACS+ client receives responses from servers where:
* the response packet was received from the server configured with a
shared key, but the packet has TAC_PLUS_UNENCRYPTED_FLAG set, and
* the response packet was received from the server configured not to
use obfuscation, but the packet has TAC_PLUS_UNENCRYPTED_FLAG not
set,
the TACACS+ client MUST close the TCP session, and process the
response in the same way that a TAC_PLUS_AUTHEN_STATUS_FAIL
(authentication sessions) or TAC_PLUS_AUTHOR_STATUS_FAIL
(authorization sessions) was received.
10.5.3. Authentication
To help TACACS+ administrators select stronger authentication
options, TACACS+ servers MUST allow the administrator to configure
the server to only accept challenge/response options for
authentication (TAC_PLUS_AUTHEN_TYPE_CHAP or
TAC_PLUS_AUTHEN_TYPE_MSCHAP or TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 for
authen_type).
TACACS+ server administrators SHOULD enable the option mentioned in
the previous paragraph. TACACS+ server deployments SHOULD only
enable other options (such as TAC_PLUS_AUTHEN_TYPE_ASCII or
TAC_PLUS_AUTHEN_TYPE_PAP) when unavoidable due to requirements of
identity/password systems.
TACACS+ server administrators SHOULD NOT allow the same credentials
to be applied in challenge-based (TAC_PLUS_AUTHEN_TYPE_CHAP or
TAC_PLUS_AUTHEN_TYPE_MSCHAP or TAC_PLUS_AUTHEN_TYPE_MSCHAPV2) and
non-challenge-based authen_type options, as the insecurity of the
latter will compromise the security of the former.
TAC_PLUS_AUTHEN_SENDAUTH and TAC_PLUS_AUTHEN_SENDPASS options
mentioned in "The Draft" SHOULD NOT be used due to their security
implications. TACACS+ servers SHOULD NOT implement them. If they
must be implemented, the servers MUST default to the options being
disabled and MUST warn the administrator that these options are not
secure.
10.5.4. Authorization
The authorization and accounting features are intended to provide
extensibility and flexibility. There is a base dictionary defined in
this document, but it may be extended in deployments by using new
argument names. The cost of the flexibility is that administrators
and implementers MUST ensure that the argument and value pairs shared
between the clients and servers have consistent interpretation.
TACACS+ clients that receive an unrecognized mandatory argument MUST
evaluate server response as if they received
TAC_PLUS_AUTHOR_STATUS_FAIL.
10.5.5. Redirection Mechanism
"The Draft" described a redirection mechanism
(TAC_PLUS_AUTHEN_STATUS_FOLLOW). This feature is difficult to
secure. The option to send secret keys in the server list is
particularly insecure, as it can reveal client shared secrets.
TACACS+ servers MUST deprecate the redirection mechanism.
If the redirection mechanism is implemented, then TACACS+ servers
MUST disable it by default and MUST warn TACACS+ server
administrators that it must only be enabled within a secure
deployment due to the risks of revealing shared secrets.
TACACS+ clients SHOULD deprecate this feature by treating
TAC_PLUS_AUTHEN_STATUS_FOLLOW as TAC_PLUS_AUTHEN_STATUS_FAIL.
11. IANA Considerations
This document has no IANA actions.
12. References
12.1. 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,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.
[RFC1334] Lloyd, B. and W. Simpson, "PPP Authentication Protocols",
RFC 1334, DOI 10.17487/RFC1334, October 1992,
<https://www.rfc-editor.org/info/rfc1334>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions",
RFC 2433, DOI 10.17487/RFC2433, October 1998,
<https://www.rfc-editor.org/info/rfc2433>.
[RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2",
RFC 2759, DOI 10.17487/RFC2759, January 2000,
<https://www.rfc-editor.org/info/rfc2759>.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579,
DOI 10.17487/RFC3579, September 2003,
<https://www.rfc-editor.org/info/rfc3579>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
DOI 10.17487/RFC4120, July 2005,
<https://www.rfc-editor.org/info/rfc4120>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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>.
12.2. Informative References
[KRB4] Miller, S., Neuman, C., Schiller, J., and J. Saltzer,
"Section E.2.1: Kerberos Authentication and Authorization
System", MIT Project Athena, Cambridge, Massachusetts,
December 1987.
[THE-DRAFT]
Carrel, D. and L. Grant, "The TACACS+ Protocol Version
1.78", Work in Progress, Internet-Draft, draft-grant-
tacacs-02, January 1997,
<https://tools.ietf.org/html/draft-grant-tacacs-02>.
[TZDB] Eggert, P. and A. Olson, "Sources for Time Zone and
Daylight Saving Time Data", 1987,
<https://www.iana.org/time-zones>.
Acknowledgements
The authors would like to thank the following reviewers whose
comments and contributions made considerable improvements to this
document: Alan DeKok, Alexander Clouter, Chris Janicki, Tom Petch,
Robert Drake, and John Heasley, among many others.
The authors would particularly like to thank Alan DeKok, who provided
significant insights and recommendations on all aspects of the
document and the protocol. Alan DeKok has dedicated considerable
time and effort to help improve the document, identifying weaknesses
and providing remediation.
The authors would also like to thank the support from the OPSAWG
Chairs and advisors, especially Joe Clarke.
Authors' Addresses
Thorsten Dahm
Google Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Email: thorstendlux@google.com
Andrej Ota
Google Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Email: andrej@ota.si
Douglas C. Medway Gash
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
United States of America
Email: dcmgash@cisco.com
David Carrel
IPsec Research
Email: carrel@ipsec.org
Lol Grant
Email: lol.grant@gmail.com