Internet DRAFT - draft-ietf-opsawg-tacacs
draft-ietf-opsawg-tacacs
Operations T. Dahm
Internet-Draft A. Ota
Intended status: Informational Google Inc
Expires: September 21, 2020 D. Medway Gash
Cisco Systems, Inc.
D. Carrel
vIPtela, Inc.
L. Grant
March 20, 2020
The TACACS+ Protocol
draft-ietf-opsawg-tacacs-18
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 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
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This Internet-Draft will expire on September 21, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Technical Definitions . . . . . . . . . . . . . . . . . . . . 4
3.1. Client . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Packet . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.4. Connection . . . . . . . . . . . . . . . . . . . . . . . 5
3.5. Session . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.6. Treatment of Enumerated Protocol Values . . . . . . . . . 5
3.7. Treatment of Text Strings . . . . . . . . . . . . . . . . 6
4. TACACS+ Packets and Sessions . . . . . . . . . . . . . . . . 6
4.1. The TACACS+ Packet Header . . . . . . . . . . . . . . . . 6
4.2. The TACACS+ Packet Body . . . . . . . . . . . . . . . . . 8
4.3. Single Connection Mode . . . . . . . . . . . . . . . . . 8
4.4. Session Completion . . . . . . . . . . . . . . . . . . . 9
4.5. Data Obfuscation . . . . . . . . . . . . . . . . . . . . 11
5. Authentication . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. The Authentication START Packet Body . . . . . . . . . . 13
5.2. The Authentication REPLY Packet Body . . . . . . . . . . 15
5.3. The Authentication CONTINUE Packet Body . . . . . . . . . 17
5.4. Description of Authentication Process . . . . . . . . . . 17
5.4.1. Version Behavior . . . . . . . . . . . . . . . . . . 18
5.4.2. Common Authentication Flows . . . . . . . . . . . . . 19
5.4.3. Aborting an Authentication Session . . . . . . . . . 22
6. Authorization . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1. The Authorization REQUEST Packet Body . . . . . . . . . . 23
6.2. The Authorization REPLY Packet Body . . . . . . . . . . . 27
7. Accounting . . . . . . . . . . . . . . . . . . . . . . . . . 29
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7.1. The Account REQUEST Packet Body . . . . . . . . . . . . . 29
7.2. The Accounting REPLY Packet Body . . . . . . . . . . . . 30
8. Argument-Value Pairs . . . . . . . . . . . . . . . . . . . . 32
8.1. Value Encoding . . . . . . . . . . . . . . . . . . . . . 32
8.2. Authorization Arguments . . . . . . . . . . . . . . . . . 33
8.3. Accounting Arguments . . . . . . . . . . . . . . . . . . 35
9. Privilege Levels . . . . . . . . . . . . . . . . . . . . . . 36
10. Security Considerations . . . . . . . . . . . . . . . . . . . 37
10.1. General Security of the Protocol . . . . . . . . . . . . 38
10.2. Security of Authentication Sessions . . . . . . . . . . 39
10.3. Security of Authorization Sessions . . . . . . . . . . . 39
10.4. Security of Accounting Sessions . . . . . . . . . . . . 40
10.5. TACACS+ Best Practices . . . . . . . . . . . . . . . . . 40
10.5.1. Shared Secrets . . . . . . . . . . . . . . . . . . . 40
10.5.2. Connections and Obfuscation . . . . . . . . . . . . 41
10.5.3. Authentication . . . . . . . . . . . . . . . . . . . 42
10.5.4. Authorization . . . . . . . . . . . . . . . . . . . 43
10.5.5. Redirection Mechanism . . . . . . . . . . . . . . . 43
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 43
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 43
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 44
13.1. Normative References . . . . . . . . . . . . . . . . . . 44
13.2. Informative References . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
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: Device Administration, that is: 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, but
was never standardized. The draft document is known as `The Draft'
[TheDraft].
This Draft was a product of its time, and did not address all of the
key security concerns which are considered when designing modern
standards. Deployment must therefore be executed with care. These
concerns are addressed in the security section (Section 10).
The primary intent of this informational document is to clarify the
subset of `The Draft' which is common to implementations supporting
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Device Administration. It is intended that all implementations which
conform to this document will conform to `The Draft'. However, it is
not intended that all implementations which 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 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 Device Administration use case,
specifically, for authorization of individual commands in a session.
Note that there is no provision made at the protocol level for
association of an authentication to 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
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3.1. Client
The client is any device which 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.
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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
original 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 RFC 8265 [RFC8265]. The
security considerations in Section 8 of that RFC 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 RFC 20 [RFC0020]. The term
"printable" used here means the fields MUST exclude the "Control
Characters" defined in section 5.2 of RFC 20 [RFC0020].
4. TACACS+ Packets and Sessions
4.1. The TACACS+ Packet Header
All TACACS+ packets begin with the following 12-byte header. The
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:
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- 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 fields, (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
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
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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 the security section (Section 10).
This flag SHOULD be clear in all deployments. Modern network traffic
tools support encrypted traffic when configured with the shared
secret (see section below), 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 RFC 4086 [RFC4086].
length
The total length of the packet body (not including the header).
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 session on a single TCP connection.
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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 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
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 the
it initiates the next session.
4.4. 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
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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 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 which prompted the
request should be executed.
Refer to Section 5.4.3 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 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 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 which 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
which establishes a connection, but does not respond. The exact
behavior is implementation specific. It is recommended that the
client should close the connection after a configurable timeout.
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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 the use of separate keys is appropriate.
The flag field MUST be configured with the following bit as follows:
TAC_PLUS_UNENCRYPTED_FLAG = 0x0
So that the packet body is obfuscated by XOR-ing it byte-wise with a
pseudo-random pad.
ENCRYPTED {data} = data ^ pseudo_pad
The packet body can then be de-obfuscated by XOR-ing it byte-wise
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 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 and session_id are extracted from the header
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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. For details of TCP connection
handling on ERROR, refer to 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 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 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.
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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 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 9) below.
authen_type
The type of authentication. Please see section 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
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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.
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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 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 (Section 3.7).
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
(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 RFC 5952 [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
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 5.4.2). The data_len 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.
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+----------------+----------------+----------------+----------------+
| 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 (Section 3.7).
data, data_len
This field 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 the
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section Common Authentication flows (Section 5.4.2). The data_len
indicates the length of the data field, in bytes.
5.3. The Authentication CONTINUE Packet Body
This packet is sent from the client to the server following the
receipt of a REPLY packet.
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+----------------+----------------+----------------+----------------+
| user_msg len | data_len |
+----------------+----------------+----------------+----------------+
| flags | user_msg ...
+----------------+----------------+----------------+----------------+
| data ...
+----------------+
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. 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 the
Authentication.
This document defines a core set of authentication flows to be
supported by TACACS+. Each authentication flow consists of a START
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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,
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.
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 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 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 authorization and accounting and ASCII authentication use
minor_version number of 0.
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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, 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 RFC 1334 [RFC1334] (Obsolete).
The REPLY from the server MUST be either a PASS, FAIL or ERROR.
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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 the PPP Authentication RFC 1334 [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 is 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 RFC
2433 [RFC2433] and then compare the resulting value with the
response. The REPLY from the server MUST be a PASS or FAIL.
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For best practices, please refer to RFC 2433 [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 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].
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.
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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 message
explaining the reason for the abort. For details of text encoding,
see (Section 3.7). This information will be handled by the server
according to the requirements of the deployment. The session is
terminated, for more details about session termination, refer to
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' [TheDraft] defined a mechanism to direct
authentication requests to an alternative server. This mechanism is
regarded as insecure, is deprecated, and 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 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.
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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.
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 the Authorization Arguments section (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
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+----------------+----------------+----------------+----------------+
| 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 filed allows the client to indicate the authentication method
used by the 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
TAC_PLUS_AUTHEN_METH_TACACSPLUS := 0x06
TAC_PLUS_AUTHEN_METH_GUEST := 0x08
TAC_PLUS_AUTHEN_METH_RADIUS := 0x10
TAC_PLUS_AUTHEN_METH_KRB4 := 0x11
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TAC_PLUS_AUTHEN_METH_RCMD := 0x20
As this information is not always subject to verification, it is
recommended that this field is in policy evaluastion. 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 and KRB4 are Kerberos version 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 (Section 5). For all other options, such as
KRB and RADIUS, then 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 the Privilege Level
section (Section 9) below. It indicates the users current privilege
level.
authen_type
This field corresponds to the authen_type field in the authentication
section (Section 5) 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 is the same as the authen_service field in the
authentication section (Section 5) above. It indicates the service
through which the user authenticated.
user, user_len
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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 the authentication section
(Section 5) 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 5) 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 field (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
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 (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.
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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 the Authorization Arguments (Section 8.2) section
below.
6.2. The Authorization REPLY Packet Body
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+----------------+----------------+----------------+----------------+
| 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 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 (Section 3.7).
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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 (Section 3.7).
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 Arguments (Section 8.2) 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 argument-value pairs (if any) in the
response, instead of the authorization argument-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
Session Completion (Section 4.4). 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.
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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.
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 the accounting section
(Section 7).
7.1. The Account REQUEST Packet Body
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+----------------+----------------+----------------+----------------+
| 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.
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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
the conditions on the client, and authentication context, so that
these details may be logged for accounting purposes.
See the Accounting Arguments section (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
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 (Section 3.7).
data, data_len
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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 (Section 3.7).
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.
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.
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.
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.
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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 (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 (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 a IP address so as to
avoid any ambiguities. For details of text encoding, see
(Section 3.7). IPv4 address are specified as octet numerics
separated by dots ('.'), IPv6 address text representation defined in
RFC 5952 [RFC5952].
Date Time
Absolute date/times are specified in seconds since the epoch, 12:00am
Jan 1 1970. The timezone MUST be UTC unless a timezone argument is
specified.
String
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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)
the protocol field 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 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.
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.
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acl (Numeric)
a number representing a connection access list. Applicable only to
session-based shell authorization. For details of text encoding, see
(Section 3.7).
inacl (String)
identifier (name) of an interface input access list. For details of
text encoding, see (Section 3.7).
outacl (String)
identifier (name) of an interface output access list. For details of
text encoding, see (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 user from using an escape character. Applicable only to
session-based shell authorization.
nohangup (Boolean)
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Boolean. Do not disconnect after an automatic command. Applicable
only to session-based shell authorization.
priv-lvl (Numeric)
privilege level to be assigned. Please refer to the Privilege Level
section (Section 9) below.
8.3. Accounting Arguments
The following arguments are defined for TACACS+ accounting only.
They MUST precede any argument-value pairs that are defined in the
authorization section (Section 6) above.
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 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 timezone abbreviation for all timestamps included in this packet.
A database of timezones is maintained here: TZDB [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)
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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)
string describing the status of the action. For details of text
encoding, see (Section 3.7).
9. Privilege Levels
The TACACS+ Protocol supports flexible authorization schemes through
the extensible arguments.
One scheme is built into 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. The allocation of
commands to privilege levels is highly dependent upon the deployment.
Common allocations are as follows:
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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 required to perform actions that
are mapped to a higher privilege level, then 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 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.
However, it is still common enough that it SHOULD be supported by
servers.
10. Security Considerations
The original TACACS+ Draft `The Draft' [TheDraft] from 1998 did not
address all of the key security concerns which are considered when
designing modern standards. This section addresses known limitations
and concerns which 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 original
TACACS+ Draft `The Draft' [TheDraft] 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.
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10.1. General Security of the Protocol
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 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.
Even though, to the best knowledge of 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 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.
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The following parts of this section enumerate only the session-
specific risks which 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
Best Practices section (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
original 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
Best Practices section (Section 10.5)) as it's trivial to execute a
successful man-in-the-middle attacks 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:
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, 1st octet
after the header).
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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 Best
Practices section (Section 10.5). Although Accounting sessions are
not directly involved in authentication or authorizing operations on
the device, man-in-the-middle attacker may do any of the following:
Replace accounting data with new valid or garbage which can
confuse auditors or hide information related to their
authentication and/or authorization attack attempts.
Try and poison accounting log with entries designed to make
systems behave in unintended ways (which includes TACACS+ server
and any other systems that would manage accounting entries).
In addition to these direct manipulations, different client
implementations pass different fidelity of accounting data. Some
vendors have been observed in the wild that pass sensitive data like
passwords, encryption keys and similar as part of the accounting log.
Due to lack of strong encryption with perfect forward secrecy, this
data may be revealed in 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 which ensure
privacy and integrity of the communication, and MUST be deployed over
a network which 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 original
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
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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
shared secret key, or shared key that is less than 16 characters
long.
TACACS+ server administrators SHOULD configure secret keys of minimum
16 characters 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 with
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
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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
shared key, but the packet has TAC_PLUS_UNENCRYPTED_FLAG set.
the response packet was received from the server configured not to
use obfuscation, but the packet has TAC_PLUS_UNENCRYPTED_FLAG not
set.
then the TACACS+ client MUST close 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 less weak 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 original 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.
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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 original 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 informational document describes TACACS+ protocol and its common
deployments. There is no further consideration required from IANA.
12. Acknowledgements
The authors would like to thank the following reviewers whose
comments and contributions made considerable improvements to the
document: Alan DeKok, Alexander Clouter, Chris Janicki, Tom Petch,
Robert Drake, 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.
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The authors would also like to thank the support from the OPSAWG
Chairs and advisors, especially Joe Clarke.
13. References
13.1. Normative References
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
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>.
[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,
<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>.
[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., Crocker, S., and J. Schiller,
"Randomness Requirements for Security", RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://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>.
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[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>.
[RFC8265] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 8265,
DOI 10.17487/RFC8265, October 2017,
<https://www.rfc-editor.org/info/rfc8265>.
13.2. Informative References
[TheDraft]
Carrel, D. and L. Grant, "The TACACS+ Protocol Version
1.78", June 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>.
Authors' Addresses
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: andrej@ota.si
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Douglas C. Medway Gash
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
US
EMail: dcmgash@cisco.com
David Carrel
vIPtela, Inc.
1732 North First St.
San Jose, CA 95112
US
EMail: dcarrel@viptela.com
Lol Grant
EMail: lol.grant@gmail.com
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