IETF protocol: http://tools.ietf.org/html/draft-grant-tacacs-02
A free tacacs+ server: http://tacacs.net/
The TACACS+ Protocol
Version 1.78
Status of This Memo
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Abstract
TACACS+ provides access control for routers, network access servers
and other networked computing devices via one or more centralized
servers. TACACS+ provides separate authentication, authorization and
accounting services. This document describes the protocol that is
used by TACACS+.
1. Introduction
The TACACS+ protocol is the latest generation of TACACS. TACACS is a
simple UDP based access control protocol originally developed by BBN
for the MILNET. Cisco has enhanced (extended) TACACS several times,
and Cisco's implementation, based on the original TACACS, is referred
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to as XTACACS. The TACACS protocol is described in [2].
TACACS+ improves on TACACS and XTACACS by separating the functions of
Authentication, Authorization and Accounting and by encrypting all
traffic between the NAS and the daemon. It allows for arbitrary
length and content authentication exchanges which will allow any
authentication mechanism to be utilized with TACACS+ clients. It is
extensible to provide for site customization and future development
features, and it uses TCP to ensure reliable delivery. The protocol
allows the TACACS+ client to request very fine grained access control
and allows the daemon to respond to each component of that request.
The separation of authentication, authorization and accounting is a
fundamental component of the design of TACACS+. The distinction
between them is very important so this document will address each one
separately. It is important to note that TACACS+ provides for all
three, but an implementation or configuration is not required to
employ all three. Each one serves a unique purpose that alone is use-
ful, and together can be quite powerful. A very important benefit to
separating authentication from authorization is that authorization
(and per-user profiles) can be a dynamic process. Instead of a one-
shot user profile, TACACS+ can be integrated with other negotiations,
such as a PPP negotiation, for far greater flexibility. The account-
ing portion can serve to provide security auditing or accounting/
billing services.
TACACS+ uses TCP for its transport. The daemon should listen at port
49 which is the "LOGIN" port assigned for the TACACS protocol. This
port is reserved in the assigned numbers RFC for both UDP and TCP.
Current TACACS and extended TACACS implementations use port 49.
2. Technical Definitions
This section provides a few basic definitions that are applicable to
this document.
Authentication
Authentication is the action of determining who a user (or entity)
is. Authentication can take many forms. Traditional authentication
utilizes a name and a fixed password. Most computers work this way,
and TACACS+ can also work this way. However, fixed passwords have
limitations, mainly in the area of security. Many modern authentica-
tion mechanisms utilize "one-time" passwords or a challenge-response
query. TACACS+ is designed to support all of these, and should be
powerful enough to handle any future mechanisms. Authentication gen-
erally takes place when the user first logs in to a machine or
requests a service of it.
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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 themselves
as someone who possesses the required information (passwords, etc.)
for those privileges.
Authorization
It is important to distinguish Authorization from Authentication.
Authorization is the action of determining what a user is allowed to
do. Generally authentication precedes authorization, but again, this
is not required. 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 authorization agent to determine if an unauthenticated user
is allowed the services in question.
In TACACS+, authorization does not merely provide yes or no answers,
but it may also customize the service for the particular user. Exam-
ples of when authorization would be performed are: When a user first
logs in and wants to start a shell, or when a user starts PPP and
wants to use IP over PPP with a particular IP address. The TACACS+
daemon might respond to these requests by allowing the service, but
placing a time restriction on the login shell, or by requiring IP
access lists on the PPP connection. For a list of authorization
attributes, see the authorization section below.
Accounting
Accounting is typically the third action after authentication and
authorization. But again, neither authentication nor authorization
are 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 to account for services used, such as in a billing
environment. It may also be used as an auditing tool for security
services. 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
attributes is defined in the accounting section.
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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.
The session concept is important because a session identifier is used
as a part of the encryption, and it is used by both ends to distin-
guish between packets belonging to multiple sessions.
Multiple sessions may be supported simultaneously and/or consecu-
tively on a single TCP connection if both the daemon and client sup-
port this. If multiple sessions are not being multiplexed over a
single tcp connection, a new connection should be opened for each
TACACS+ session and closed at the end of that session. For accounting
and authorization, this implies just a single pair of packets
exchanged over the connection (the request and its reply). For
authentication, a single session may involve an arbitrary number of
packets being exchanged.
The session is an operational concept that is maintained between the
TACACS+ client and daemon. It does not necessarily correspond to a
given user or user action.
NAS
A NAS is a Network Access Server. This is any device that provides
access services. Nowadays, a NAS is typically more than just a termi-
nal server. Terminal servers usually provide a character mode front
end and then allow the user to telnet or rlogin to another host. A
NAS may also support protocol based access services and may support
PPP, ARAP, LAT, XREMOTE, and others.
MUST
This word means that the definition is an absolute requirement of the
specification.
MUST NOT
This phrase means that the definition is an absolute prohibition of
the specification.
SHOULD
This word, or the adjective "recommended", means that there may exist
valid reasons in particular circumstances to ignore this item, but
the full implications should be understood and carefully weighed
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before choosing a different course.
MAY
This word, or the adjective "optional", means that this item is one
of an allowed set of alternatives. An implementation which does not
include this option MUST be prepared to interoperate with another
implementation which does include the option.
3. The TACACS+ packet header
All TACACS+ packets always begin with the following 12 byte header.
The header is always cleartext and describes the remainder of the
packet:
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+
|major | minor | | | |
|version| version| type | seq_no | flags |
+----------------+----------------+----------------+----------------+
| |
| session_id |
+----------------+----------------+----------------+----------------+
| |
| length |
+----------------+----------------+----------------+----------------+
major_version
This is the major TACACS+ version number.
TAC_PLUS_MAJOR_VER := 0xc
minor_version
The minor TACACS+ version number. This is intended to allow revisions
to the TACACS+ protocol while maintaining backwards compatibility.
Minor version 1 is currently defined for some commands. It may only
be used for commands that explicitly call for it in this document.
All other requests must use the default value.
TAC_PLUS_MINOR_VER_DEFAULT := 0x0
TAC_PLUS_MINOR_VER_ONE := 0x1
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See the compatibility section at the end of the document.
When a daemon receives a packet with a minor_version that it does not
support, it should return an ERROR status with the minor_version set
to the closest supported value.
type
This is the packet type. Legal values 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 for the current
session. The first TACACS+ packet in a session MUST have the sequence
number 1 and each subsequent packet will increment the sequence
number by one. Thus clients only send packets containing odd sequence
numbers, and TACACS+ daemons 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 unencrypted flag bit says whether encryption is being used on the
body of the TACACS+ packet (the entire portion after the header).
TAC_PLUS_UNENCRYPTED_FLAG := 0x01
If this flag is set, the packet is not encrypted. If this flag is
cleared, the packet is encrypted.
Unencrypted packets are intended for testing, and are not recommended
for normal use.
The single-connection flag:
TAC_PLUS_SINGLE_CONNECT_FLAG := 0x04
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If a NAS sets this flag, this indicates that it supports multiplexing
TACACS+ sessions over a single tcp connection. The flag need only be
examined on the first two packets for any given connection since the
single-connect status of a connection, once established, should not
be changed. The connection must instead be closed and a new connec-
tion opened, if required.
If the daemon sets this flag in the first reply packet in response to
the first packet from a NAS, this indicates its willingness to sup-
port single-connection over the current connection. The daemon may
set this flag even if the NAS does not set it, but the NAS is under
no obligation to honor it.
session_id
The Id for this TACACS+ session. The session id should be randomly
chosen. This field does not change for the duration of the TACACS+
session. (If this value is not a cryptographically strong random
number, it will compromise the protocol's security. [6])
length
The total length of the TACACS+ packet body (not including the
header). This value is in network byte order. Packets are never pad-
ded beyond this length.
4. The TACACS+ packet body
The TACACS+ body types are defined in the packet header. The
remainder of this document will address the contents of the different
TACACS+ bodies. The following general rules apply to all TACACS+ body
types:
- The entire body is protected by the encryption mechanism indicated
in the header.
- Any variable length data fields which are unused MUST have a length
value equal to zero.
- Unused fixed length fields SHOULD have values of zero.
- All data and message fields in a TACACS+ packet MUST NOT be null
terminated.
- All length values are unsigned and in network byte order.
- There should be no padding in any of the fields or at the end of a
packet.
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5. Body Encryption
The body of TACACS+ packets may be encrypted. The following sections
describe the encryption mechanisms that are supported. Only one
encryption mechanism SHOULD be used within a single session.
When the encryption mechanism relies on a secret key, it is referring
to a shared secret value that is known to both the client and the
daemon. This document does not discuss the management and storage of
those keys. It is an implementation detail of the daemon and client,
as to whether they will maintain only one key, or a different key for
each client or daemon with which they communicate. For security rea-
sons, the latter options should be available, but it is a site depen-
dent decision as to whether the use of separate keys is appropriate.
The encrypted flag field may be set as follows:
TAC_PLUS_UNENCRYPTED_FLAG == 0x0
In this case, the packet body is encrypted by XOR-ing it byte-wise
with a pseudo random pad.
ENCRYPTED {data} == 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 [3].
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 run-
ning 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 daemon.
The version number is the one byte combination of the major and minor
version numbers.
The session id is used in the byte order in which it appears in the
TACACS+ header. (i.e. in network byte order, not host byte order).
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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}
TAC_PLUS_UNENCRYPTED_FLAG == 0x1
In this case, the entire packet body is in cleartext. Encryption and
decryption are null operations. This method should only be used for
debugging. It does not provide data protection or authentication and
is highly susceptible to packet spoofing. Implementing this encryp-
tion method is optional.
NOTE: implementations should take care not to skip decryption simply
because an incoming packet indicates that it is not encrypted.
After a packet body is decrypted, the lengths of the component values
in the packet should be summed and checked against the cleartext
datalength value from the header. Any packets which fail this check
should be discarded and an error signalled. Commonly such failures
may be expected to be seen when there are mismatched keys between the
NAS and the TACACS+ server.
If an error must be declared but the type of the incoming packet can-
not be determined, a packet with the identical cleartext header but
with a sequence number incremented by one and the length set to zero
may be returned to indicate an error.
6. Body types
All further discussions of TACACS+ packet bodies assumes that any
encryption/decryption has already been performed. From here on, we
are only concerned with the cleartext data.
There are several constant fields in many of the following bodies. A
few merit mention here as they apply to most packet bodies.
The user is the username or user id that is authenticated or being
authenticated.
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The port is an ascii description of the port on which the user is
connected.
The rem_addr is a "best effort" description of the remote location
from which the user has connected to the client. In many cases, the
remote address will not be available or will be unreliable at best,
but it may be useful when included.
The user_msg is always the ASCII input from the user.
The server_msg is always used to hold a message that is intended to
be presented to the user. In some contexts it may be optional as to
whether to actually present it.
The data field has several uses but is not used in all packets.
6.1. Authentication
TACACS+ authentication has three packet types: START, CONTINUE and
REPLY. START and CONTINUE are always sent by the client and REPLY is
always sent by the daemon.
Authentication begins with the client sending a START message to the
daemon. The START message describes the type of authentication to be
performed, and may contain the username and some authentication data.
The START packet is only ever sent as the first message in a TACACS+
authentication session, or as the packet immediately following a res-
tart. (A restart may be requested by the daemon in a REPLY packet). A
START packet always has seq_no equal to 1.
In response to a START packet, the daemon sends a REPLY. The REPLY
message indicates whether the authentication is finished, or whether
it should continue. If the REPLY indicates that authentication should
continue, then it will also indicate what new information is
requested. The client will get that information and return it in a
CONTINUE message.
The daemon MUST always send a REPLY to both the START and the CON-
TINUE messages, the only exception being if the client indicates an
abort in the CONTINUE, in which case the session is immediately
aborted.
Thus, there are zero or more pairs of packets where the client sends
a CONTINUE and the daemon sends a REPLY.
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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 | service |
+----------------+----------------+----------------+----------------+
| user len | port len | rem_addr len | data len |
+----------------+----------------+----------------+----------------+
| user ...
+----------------+----------------+----------------+----------------+
| port ...
+----------------+----------------+----------------+----------------+
| rem_addr ...
+----------------+----------------+----------------+----------------+
| data...
+----------------+----------------+----------------+----------------+
Packet fields are as follows:
action
This describes the authentication action to be performed. Legal
values are:
TAC_PLUS_AUTHEN_LOGIN := 0x01
TAC_PLUS_AUTHEN_CHPASS := 0x02
TAC_PLUS_AUTHEN_SENDPASS := 0x03 (deprecated)
TAC_PLUS_AUTHEN_SENDAUTH := 0x04
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priv_lvl
This indicates the privilege level that the user is authenticating
as. Privilege levels are ordered values from 0 to 15 with each level
representing a privilege level that is a superset of the next lower
value. If a NAS client uses a different privilege level scheme, then
mapping must be provided. Pre-defined values are:
TAC_PLUS_PRIV_LVL_MAX := 0x0f
TAC_PLUS_PRIV_LVL_ROOT := 0x0f
TAC_PLUS_PRIV_LVL_USER := 0x01
TAC_PLUS_PRIV_LVL_MIN := 0x00
authen_type
The type of authentication that is being performed. Legal values are:
TAC_PLUS_AUTHEN_TYPE_ASCII := 0x01
TAC_PLUS_AUTHEN_TYPE_PAP := 0x02
TAC_PLUS_AUTHEN_TYPE_CHAP := 0x03
TAC_PLUS_AUTHEN_TYPE_ARAP := 0x04
TAC_PLUS_AUTHEN_TYPE_MSCHAP := 0x05
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service
This is the service that is requesting the authentication. Legal
values are:
TAC_PLUS_AUTHEN_SVC_NONE := 0x00
TAC_PLUS_AUTHEN_SVC_LOGIN := 0x01
TAC_PLUS_AUTHEN_SVC_ENABLE := 0x02
TAC_PLUS_AUTHEN_SVC_PPP := 0x03
TAC_PLUS_AUTHEN_SVC_ARAP := 0x04
TAC_PLUS_AUTHEN_SVC_PT := 0x05
TAC_PLUS_AUTHEN_SVC_RCMD := 0x06
TAC_PLUS_AUTHEN_SVC_X25 := 0x07
TAC_PLUS_AUTHEN_SVC_NASI := 0x08
TAC_PLUS_AUTHEN_SVC_FWPROXY := 0x09
The ENABLE service refers to a service requesting authentication in order
to grant the user different privileges. This is comparable to the Unix
"su(1)" command. A service value of NONE should only be used when none of
the other service values are appropriate.
user
The username. It is optional in this packet.
port
The ASCII name of the client port on which the authentication is tak-
ing place. The value of this field is client specific. (For example,
Cisco uses "tty10" to denote the tenth tty line and "Async10" to
denote the tenth async interface).
rem_addr
An ASCII string that describes the user's remote location. This field
is optional (since the information may not be available). It is
intended to hold a network address if the user is connected via a
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network, a caller ID is the user is connected via ISDN or a POTS, or
any other remote location information that is available. This field
value is client specified.
data
This field is used to send data appropriate for the action and
authen_type. It is described in more detail below.
7. The authentication REPLY packet body
The TACACS+ daemon sends only one type of authentication packet (a
REPLY packet) to the client. The REPLY packet body looks as follows:
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+
| status | flags | server_msg len |
+----------------+----------------+----------------+----------------+
| data len | server_msg ...
+----------------+----------------+----------------+----------------+
| data ...
+----------------+----------------+
status
The current status of the authentication. Legal values are:
TAC_PLUS_AUTHEN_STATUS_PASS := 0x01
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
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flags
Bitmapped flags that modify the action to be taken. The following
values are defined:
TAC_PLUS_REPLY_FLAG_NOECHO := 0x01
server_msg
A message to be displayed to the user. This field is optional. If it
exists, it is intended to be presented to the user.
data
This field holds data that is a part of the authentication exchange
and is intended for the NAS, not the user. Valid uses of this field
are described below.
8. The authentication CONTINUE packet body
This packet is sent from the NAS to the daemon 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
This field is the string that the user entered, or the NAS provided
on behalf of the user, in response to the server_msg from a REPLY
packet.
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data
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.
flags
This holds the bitmapped flags that modify the action to be taken.
The following values are defined:
TAC_PLUS_CONTINUE_FLAG_ABORT := 0x01
9. The authentication process
The flavor of the authentication is determined by the action and the
authen_type fields in the START packet. First we should discuss some
general fields that apply to all flavors of authentication exchanges.
The user and data fields in the START packet are defined below for
each flavor.
The priv_lvl, service, port and rem_addr in the START packet are all
provided to help identify the conditions on the NAS. In the CONTINUE
packet, the user_msg and data fields are defined below for each fla-
vor. For all REPLY packets, the server_msg may contain a message to
be displayed to the user, but the data field usage varies and is
described below.
The descriptions below first describe "normal" authentication where,
in response to a START packet, the daemon either sends a request for
more information (GETDATA, GETUSER or GETPASS) or a termination (PASS
or FAIL). The actions and meanings when the daemon sends a RESTART,
ERROR or FOLLOW are common and are described further below.
When the REPLY status equals TAC_PLUS_AUTHEN_STATUS_GETDATA,
TAC_PLUS_AUTHEN_STATUS_GETUSER or TAC_PLUS_AUTHEN_STATUS_GETPASS,
then authentication continues and the server_msg may be used by 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.
TAC_PLUS_AUTHEN_STATUS_GETDATA is the generic request for more infor-
mation. All three cause the same action to be performed, but in the
case of TAC_PLUS_AUTHEN_STATUS_GETUSER, the client can know that the
information that the user responds with is a username, and for
TAC_PLUS_AUTHEN_STATUS_GETPASS, that the user response represents a
password.
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If the TAC_PLUS_REPLY_FLAG_NOECHO flag is set in the REPLY, then the
user response should not be echoed as it is entered. The data field
is only used in the REPLY where explicitly defined below.
9.0.1. 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 principal. The exchange MAY consist of multiple
messages while the daemon collects the information it requires in
order to allow changing the principal's privilege level. This
exchange is very similar to an Inbound ASCII login (which see).
In order to readily distinguish enable requests from other types of
request, the value of the 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.
9.0.2. Inbound ASCII Login
action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII
This is a standard ASCII authentication. The START packet may contain
the username, but need not do so. The data fields in both the START
and CONTINUE packets are not used for ASCII logins. There is a single
START followed by zero or more pairs of REPLYs and CONTINUEs, fol-
lowed by a terminating REPLY (PASS or FAIL).
9.0.3. Inbound PAP Login
action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_PAP
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minor_version = 0x1
The entire exchange MUST consist of a single START packet and a sin-
gle 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 [4]. The REPLY from the daemon
MUST be either a PASS or FAIL.
9.0.4. Inbound CHAP login
action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_CHAP
minor_version = 0x1
The entire exchange MUST consist of a single START packet and a sin-
gle 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
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 daemon will run MD5 over the id,
the user's secret and the challenge, as defined in the PPP Authenti-
cation RFC [4] and then compare that value with the response. The
REPLY from the daemon MUST be a PASS or FAIL.
9.0.5. Inbound MS-CHAP 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 sin-
gle 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.
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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 daemon will use a combination of
MD4 and DES on the user's secret and the challenge, as defined in [7]
and then compare the resulting value with the response. The REPLY
from the daemon MUST be a PASS or FAIL.
9.0.6. Outbound MS-CHAP request
action = TAC_PLUS_AUTHEN_SENDAUTH
authen_type = TAC_PLUS_AUTHEN_TYPE_MSCHAP
minor_version = 0x1
This is used when the NAS needs to provide MS-CHAP authentication
credentials to the remote PPP peer. 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 and the challenge.
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). The
daemon will use MD4 and DES to process the user's secret and the
challenge, as defined in [7].
The REPLY from the daemon MUST be a PASS or FAIL. If the status is
PASS, then the data field MUST contain the 49-octet output, in which
24 octets are MD4 output for the Microsoft LAN Manager compatible
challenge response, 24 octets are MD4 output for the Microsoft Win-
dows NT compatible challenge response and 1 octet is the flag to
determine which part of the response packet should be utilized.
9.0.7. Inbound ARAP login
action = TAC_PLUS_AUTHEN_LOGIN
authen_type = TAC_PLUS_AUTHEN_TYPE_ARAP
minor_version = 0x1
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The entire exchange MUST consist of a single START packet and a sin-
gle REPLY. The START packet MUST contain the username in the user
field and the data field will be a concatenation of the NAS's chal-
lenge to the remote peer (8 octets) the remote peer's challenge to
the NAS (8 octets) and the remote peer's response to the NAS's chal-
lenge (8 octets).
The daemon must run DES encryption over both the challenges using the
user's secret as the DES key, as described in the ARAP specification
[5]. For a successful authentication, the encrypted NAS challenge
MUST be identical to the peer's response. The REPLY from the daemon
MUST be a PASS or FAIL. The encrypted peer challenge (8 octets) is
returned in the data field of a REPLY packet if the status is set to
PASS.
9.0.8. Outbound PAP request
action = TAC_PLUS_AUTHEN_SENDAUTH
authen_type = TAC_PLUS_AUTHEN_TYPE_PAP
minor_version = 0x1
This is used when the NAS needs to provide PAP authentication creden-
tials to the remote PPP peer. The entire exchange MUST consist of a
single START packet and a single REPLY. The START packet contains a
username in the user field. A REPLY with status set to PASS MUST con-
tain a cleartext password in the data field. Caution is urged when
using this. By sending a cleartext password to the NAS, that password
will then be passed to the remote PPP peer. It should be ensured that
the provided password can never be used to authenticate back to the
NAS. Use of this is discouraged, but supported for complete intero-
perability with the PPP protocol.
9.0.9. Outbound CHAP request
action = TAC_PLUS_AUTHEN_SENDAUTH
authen_type = TAC_PLUS_AUTHEN_TYPE_CHAP
minor_version = 0x1
This is used when the NAS needs to provide CHAP authentication
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credentials to the remote PPP peer. 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 and the challenge.
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). The
daemon will run MD5 over the id, the user's secret and the challenge,
as defined in the PPP Authentication RFC [4].
The REPLY from the daemon MUST be a PASS or FAIL. If the status is
PASS, then the data field MUST contain the 16 octet MD5 output
9.0.10. Outbound ASCII and ARAP request
action = TAC_PLUS_AUTHEN_SENDAUTH
authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII
authen_type = TAC_PLUS_AUTHEN_TYPE_ARAP
This is an error. This action is not supported for ASCII logins and
in not needed for ARAP since ARAP authentication is already a two way
protocol.
9.0.11. ASCII change password request
action = TAC_PLUS_AUTHEN_CHPASS
authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII
This exchange consists of multiple messages while the daemon 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.
9.0.12. PPP change password request
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action = TAC_PLUS_AUTHEN_CHPASS
authen_type = TAC_PLUS_AUTHEN_TYPE_PAP
authen_type = TAC_PLUS_AUTHEN_TYPE_CHAP
This is never valid. The PPP protocol does not support password
changing.
9.0.13. ARAP change password request
action = TAC_PLUS_AUTHEN_CHPASS
authen_type = TAC_PLUS_AUTHEN_TYPE_ARAP
This exchange consists of a single START packet and a single REPLY.
The START packet MUST contain the username and the data field con-
tains both the old and the new passwords encrypted (**FORMAT NOT
KNOWN AT THIS TIME **). The reply is a PASS or FAIL and the data
field is unused.
10. Aborting a session
The client may prematurely terminate a session by setting the
TAC_PLUS_CONTINUE_FLAG_ABORT flag in the CONTINUE message. If this
flag is set, the data portion of the message may contain an ASCII
message explaining the reason for the abort. The session is ter-
minated and no REPLY message is sent.
There are three other possible return status values that can be used
in a REPLY packet. These can be sent regardless of the action or
authen_type. Each of these indicates that the TACACS+ authentication
session should be terminated. In each case, the server_msg may con-
tain a message to be displayed to the user.
When the status equals TAC_PLUS_AUTHEN_STATUS_FOLLOW the packet indi-
cates that the TACACS+ daemon requests that authentication should be
performed with an alternate daemon. The data field MUST contain ASCII
text describing one or more daemons. A daemon description appears
like this:
[@<protocol>@]<host>[@<key>]
The protocol and key are optional. The protocol can describe an
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alternate way of performing the authentication, other than TACACS+.
If the protocol is not present, then TACACS+ is assumed.
Protocols are ASCII numbers corresponding to the methods listed in
the authen_method field of authorization packets (defined below). The
host is specified as either a fully qualified domain name, or an
ASCII numeric IP address specified as octets separated by dots (`.').
If a key is supplied, the client MAY use the key in order to authen-
ticate to that host. If more than one host is specified, they MUST be
separated by an ASCII <CR> (0x0D).
Use of the hosts in a TAC_PLUS_AUTHEN_STATUS_FOLLOW packet is at the
discretion of the TACACS+ client. It may choose to use any one, all
or none of these hosts. If it chooses to use none, then it MUST treat
the authentication as if the return status was
TAC_PLUS_AUTHEN_STATUS_FAIL.
While the order of hosts in this packet indicates a preference, but
the client is not obliged to use that ordering.
If the status equals TAC_PLUS_AUTHEN_STATUS_ERROR, then the host is
indicating that it is experiencing an unrecoverable error and the
authentication should proceed as if that host could not be contacted.
The data field may contain a message to be printed on an administra-
tive console or log.
If the status equals TAC_PLUS_AUTHEN_STATUS_RESTART, then the authen-
tication sequence should be restarted with a new START packet from
the client. This REPLY packet indicates that the current authen_type
value (as specified in the START packet) is not acceptable for this
session, but that others may be.
The TAC_PLUS_AUTHEN_STATUS_RESTART REPLY packet may contain a list of
valid authen_type values in the data portion of the packet. The
authen_type values are a single byte in length so the data_len value
indicates the number of authen_type values included. This packet is
only currently intended for PPP authentication when multiple authen-
tication mechanisms are available and can be negotiated between the
client and the remote peer. This also requires future PPP authentica-
tion extensions which have not yet been passed through the IETF. If a
client chooses not to accept the TAC_PLUS_AUTHEN_STATUS_RESTART
packet, then it should be TREATED as if the status was
TAC_PLUS_AUTHEN_STATUS_FAIL.
11. Authorization
TACACS+ authorization is an extensible way of providing remote
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authorization services. An authorization session is defined as a
single pair of messages, a REQUEST followed by a RESPONSE.
The authorization REQUEST message contains a fixed set of fields that
describe the authenticity of the user or process, and a variable set
of arguments that describes the services and options for which
authorization is requested.
The RESPONSE contains a variable set of response arguments
(attribute-value pairs) which can restrict or modify the clients
actions.
The arguments in both a REQUEST and a RESPONSE can be specified as
either mandatory or optional. An optional argument is one that may or
may not be used, modified or even understood by the recipient.
A mandatory argument MUST be both understood and used. This allows
for extending the attribute list while providing secure backwards
compatibility.
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11.1. The authorization REQUEST packet body
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+
| authen_method | priv_lvl | authen_type | authen_service |
+----------------+----------------+----------------+----------------+
| user len | port len | rem_addr len | arg_cnt |
+----------------+----------------+----------------+----------------+
| arg 1 len | arg 2 len | ... | arg N len |
+----------------+----------------+----------------+----------------+
| user ...
+----------------+----------------+----------------+----------------+
| port ...
+----------------+----------------+----------------+----------------+
| rem_addr ...
+----------------+----------------+----------------+----------------+
| arg 1 ...
+----------------+----------------+----------------+----------------+
| arg 2 ...
+----------------+----------------+----------------+----------------+
| ...
+----------------+----------------+----------------+----------------+
| arg N ...
+----------------+----------------+----------------+----------------+
authen_method
This indicates the authentication method used by the client to
acquire the user information.
TAC_PLUS_AUTHEN_METH_NOT_SET := 0x00
TAC_PLUS_AUTHEN_METH_NONE := 0x01
TAC_PLUS_AUTHEN_METH_KRB5 := 0x02
TAC_PLUS_AUTHEN_METH_LINE := 0x03
TAC_PLUS_AUTHEN_METH_ENABLE := 0x04
TAC_PLUS_AUTHEN_METH_LOCAL := 0x05
TAC_PLUS_AUTHEN_METH_TACACSPLUS := 0x06
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TAC_PLUS_AUTHEN_METH_GUEST := 0x08
TAC_PLUS_AUTHEN_METH_RADIUS := 0x10
TAC_PLUS_AUTHEN_METH_KRB4 := 0x11
TAC_PLUS_AUTHEN_METH_RCMD := 0x20
KRB5 and KRB4 are kerberos version 5 and 4. LINE refers to a fixed
password associated with the line used to gain access. LOCAL is a NAS
local user database. ENABLE is a command that authenticates in order
to grant new privileges. TACACSPLUS is, of course, TACACS+. GUEST is
an unqualified guest authentication, such as an ARAP guest login.
RADIUS is the Radius authentication protocol. RCMD refers to authen-
tication provided via the R-command protocols from Berkeley Unix.
(One should be aware of the security limitations to R-command authen-
tication.)
priv_lvl
This field matches the priv_lvl field in the authentication section
above. It indicates the users current privilege level.
authen_type
This field matches the authen_type field in the authentication sec-
tion above. It indicates the type of authentication that was per-
formed.
authen_service
This field matches the service field in the authentication section
above. It indicates the service through which the user authenticated.
user
This field contains the user's account name.
port
This field matches the port field in the authentication section
above.
rem_addr
This field matches the rem_addr field in the authentication section
above.
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arg_cnt
The number of authorization arguments to follow
arg
An attribute-value pair that describes the command to be performed.
(see below)
The authorization arguments in both the REQUEST and the RESPONSE are
attribute-value pairs. The attribute and the value are in a single
ascii string and are separated by either a "=" (0X3D) or a "*"
(0X2A). The equals sign indicates a mandatory argument. The asterisk
indicates an optional one.
Optional arguments are ones that may be disregarded by either client
or daemon. Mandatory arguments require that the receiving side under-
stands the attribute and will act on it. If the client receives a
mandatory argument that it cannot oblige or does not understand, it
MUST consider the authorization to have failed. It is legal to send
an attribute-value pair with a NULL (zero length) value.
Attribute-value strings are not NULL terminated, rather their length
value indicates their end. The maximum length of an attribute-value
string is 255 characters. The following attributes are defined:
12. Table 1: Attribute-value Pairs
service
The primary service. Specifying a service attribute indicates that
this is a request for authorization or accounting of that service.
Current values are "slip", "ppp", "arap", "shell", "tty-daemon",
"connection", "system" and "firewall". This attribute MUST always be
included.
protocol
a protocol that is a subset of a service. An example would be any PPP
NCP. Currently known values are "lcp", "ip", "ipx", "atalk", "vines",
"lat", "xremote", "tn3270", "telnet", "rlogin", "pad", "vpdn", "ftp",
"http", "deccp", "osicp" and "unknown".
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cmd
a shell (exec) command. This indicates the command name for a shell
command that is to be run. This attribute MUST be specified if ser-
vice equals "shell". A NULL value indicates that the shell itself is
being referred to.
cmd-arg
an argument to a shell (exec) command. This indicates an argument for
the shell command that is to be run. Multiple cmd-arg attributes may
be specified, and they are order dependent.
acl
ASCII number representing a connection access list. Used only when
service=shell and cmd=NULL
inacl
ASCII identifier for an interface input access list.
outacl
ASCII identifier for an interface output access list.
zonelist
A numeric zonelist value. (Applicable to AppleTalk only).
addr
a network address
addr-pool
The identifier of an address pool from which the NAS should assign an
address.
routing
A boolean. Specifies whether routing information is to be propagated
to, and accepted from this interface.
route
Indicates a route that is to be applied to this interface. Values
MUST be of the form "<dst_address> <mask> [<routing_addr>]". If a
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<routing_addr> is not specified, the resulting route should be via
the requesting peer.
timeout
an absolute timer for the connection (in minutes). A value of zero
indicates no timeout.
idletime
an idle-timeout for the connection (in minutes). A value of zero
indicates no timeout.
autocmd
an auto-command to run. Used only when service=shell and cmd=NULL
noescape
Boolean. Prevents user from using an escape character. Used only when
service=shell and cmd=NULL
nohangup
Boolean. Do no disconnect after an automatic command. Used only when
service=shell and cmd=NULL
priv_lvl
privilege level to be assigned.
remote_user
remote userid (authen_method must have the value
TAC_PLUS_AUTHEN_METH_RCMD)
remote_host
remote host (authen_method must have the value
TAC_PLUS_AUTHEN_METH_RCMD)
callback-dialstring
Indicates that callback should be done. Value is NULL, or a dial-
string. A NULL value indicates that the service MAY choose to get the
dialstring through other means.
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callback-line
The line number to use for a callback.
callback-rotary
The rotary number to use for a callback.
nocallback-verify
Do not require authentication after callback.
For all boolean attributes, valid values are "true" or "false". A
value of NULL means an attribute with a zero length string for its value
i.e. cmd=NULL is actually transmitted as the string of 4 characters
"cmd=".
If a host is specified in a cmd-arg or addr, it is recommended that it
be specified as a numeric address so as to avoid any ambiguities.
In the case of rcmd authorizations, the authen_method will be set to
TAC_PLUS_AUTHEN_METH_RCMD and the remote_user and remote_host attributes
will provide the remote user and host information to enable rhost style
authorization. The response may request that a privilege level be set
for the user.
The protocol attribute is intended for use with PPP. When service equals
"ppp" and protocol equals "lcp", the message describes the PPP link
layer service. For other values of protocol, this describes a PPP NCP
(network layer service). A single PPP session can support multiple NCPs.
The attributes addr, inacl, outacl, route and routing may be used for
all network protocol types that are supported. Their format and meaning
is determined by the values of the service or protocol attributes. Not
all are necessarily implemented for any given network protocol.
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12.1. The authorization RESPONSE packet body
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+
| status | arg_cnt | server_msg len |
+----------------+----------------+----------------+----------------+
+ data len | arg 1 len | arg 2 len |
+----------------+----------------+----------------+----------------+
| ... | arg N len | server_msg ...
+----------------+----------------+----------------+----------------+
| data ...
+----------------+----------------+----------------+----------------+
| arg 1 ...
+----------------+----------------+----------------+----------------+
| arg 2 ...
+----------------+----------------+----------------+----------------+
| ...
+----------------+----------------+----------------+----------------+
| arg N ...
+----------------+----------------+----------------+----------------+
status
This field indicates the authorization status
TAC_PLUS_AUTHOR_STATUS_PASS_ADD := 0x01
TAC_PLUS_AUTHOR_STATUS_PASS_REPL := 0x02
TAC_PLUS_AUTHOR_STATUS_FAIL := 0x10
TAC_PLUS_AUTHOR_STATUS_ERROR := 0x11
TAC_PLUS_AUTHOR_STATUS_FOLLOW := 0x21
server_msg
This is an ASCII string that may be presented to the user. The decision
to present this message is client specific.
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data
This is an ASCII string that may be presented on an administrative
display, console or log. The decision to present this message is client
specific.
arg_cnt
The number of authorization arguments to follow.
arg
An attribute-value pair that describes the command to be performed. (see
below)
If the status equals TAC_PLUS_AUTHOR_STATUS_FAIL, then the appropriate
action is to deny the user action.
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 are to be used IN ADDITION to those arguments.
If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_REPL then the
arguments in the request are to be completely replaced by the
arguments in the response.
If the intended action is to approve the authorization with no
modifications, then the status should be set to
TAC_PLUS_AUTHOR_STATUS_PASS_ADD and the arg_cnt should be set to
0.
A status of TAC_PLUS_AUTHOR_STATUS_ERROR indicates an error occurred
on the daemon.
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.
None of the arg values have any relevance if an ERROR is set.
13. Accounting
TACACS+ accounting is very similar to authorization. The packet for-
mat is also similar. There is a fixed portion and an extensible por-
tion. The extensible portion uses all the same attribute-value pairs
that authorization uses, and adds several more.
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13.1. The account REQUEST packet body
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+
| flags | authen_method | priv_lvl | authen_type |
+----------------+----------------+----------------+----------------+
| authen_service | user len | port len | rem_addr len |
+----------------+----------------+----------------+----------------+
| arg_cnt | arg 1 len | arg 2 len | ... |
+----------------+----------------+----------------+----------------+
| arg N len | user ...
+----------------+----------------+----------------+----------------+
| port ...
+----------------+----------------+----------------+----------------+
| rem_addr ...
+----------------+----------------+----------------+----------------+
| arg 1 ...
+----------------+----------------+----------------+----------------+
| arg 2 ...
+----------------+----------------+----------------+----------------+
| ...
+----------------+----------------+----------------+----------------+
| arg N ...
+----------------+----------------+----------------+----------------+
flags
This holds bitmapped flags.
TAC_PLUS_ACCT_FLAG_MORE := 0x01 (deprecated)
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.
The following new attributes are defined for TACACS+ accounting only.
When these attribute-value pairs are included in the argument list,
they should precede any attribute-value pairs that are defined in the
authorization section above.
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Table 2: Accounting Attribute-value Pairs
task_id
Start and stop records for the same event MUST have matching (unique)
task_id's
start_time
The time the action started (in seconds since the epoch, 12:00am Jan
1 1970).
stop_time
The time the action stopped (in seconds since the epoch.)
elapsed_time
The elapsed time in seconds for the action. Useful when the device
does not keep real time.
timezone
The timezone abbreviation for all timestamps included in this packet.
event
Used only when "service=system". Current values are "net_acct",
"cmd_acct", "conn_acct", "shell_acct" "sys_acct" and "clock_change".
These indicate system level changes. The flags field SHOULD indicate
whether the service started or stopped.
reason
Accompanies an event attribute. It describes why the event occurred.
bytes
The number of bytes transferred by this action
bytes_in
The number of input bytes transferred by this action
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bytes_out
The number of output bytes transferred by this action
paks
The number of packets transferred by this action.
paks_in
The number of input packets transferred by this action.
paks_out
The number of output packets transferred by this action.
status
The numeric status value associated with the action. This is a signed
four (4) byte word in network byte order. 0 is defined as success.
Negative numbers indicate errors. Positive numbers indicate non-error
failures. The exact status values may be defined by the client.
err_msg
An ascii string describing the status of the action.
NOTE: All numeric values in an attribute-value string are provided as
decimal ASCII numbers.
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13.2. The accounting REPLY packet body
The response to an accounting message is used to indicate that the
accounting function on the daemon has completed and securely
committed the record. This provides the client the best possible
guarantee that the data is indeed logged.
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
This is an ASCII string that may be presented to the user. The deci-
sion to present this message is client specific.
data
This is an ASCII string that may be presented on an administrative
display, console or log. The decision to present this message is
client specific.
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
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TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication.
The daemon MUST terminate the session after sending a REPLY.
The TAC_PLUS_ACCT_FLAG_START flag indicates that this is a start
accounting message. Start messages should only be sent once when a
task is started. The TAC_PLUS_ACCT_FLAG_STOP indicates that this is a
stop record and that the task has terminated. The
TAC_PLUS_ACCT_FLAG_WATCHDOG flag means that this is an update record.
Update records are sent at the client's discretion when the task is
still running.
The START and STOP flags are mutually exclusive. When the WATCHDOG
flag is set along with the START flag, it indicates that the update
record is a duplicate of the original START record. If the START flag
is not set, then this indicates a minimal record indicating only that
task is still running. The STOP flag MUST NOT be set in conjunction
with the WATCHDOG flag.
14. Compatibility between Minor Versions 0 and 1
Whenever a TACACS+ daemon receives a packet with a minor_version that
it does not support, it should return an ERROR status with the
minor_version set to the supported value closest to the requested
value.
The changes between minor_version 0 and 1 all deal with the way that
CHAP, ARAP and PAP authentications are handled.
In minor_version 0, CHAP, ARAP and outbound PAP authentications were
performed by the NAS sending a SENDPASS packet to the daemon. The
SENDPASS requested a copy of the user's plaintext password so that
the NAS could complete the authentication. The CHAP hashing and ARAP
encryption were all performed on the NAS. Inbound PAP performed a
normal LOGIN, sending the username in the START packet and then wait-
ing for a GETPASS and sending the password in a CONTINUE packet.
In minor_version 1, CHAP, ARAP and inbound PAP use LOGIN to perform
inbound authentication and the exchanges use the data field so that
the NAS only sends a single START packet and expects to receive a
PASS or FAIL. SENDPASS has been deprecated and SENDAUTH introduced,
so that the NAS can request authentication credentials for authenti-
cating to a remote peer. SENDAUTH is only used for PPP when perform-
ing outbound authentication.
NOTE: Only those requests which have changed from their minor_version
0 implementation (i.e. ARAP, CHAP and PAP) should use the new
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minor_version number of 1. All other requests (whose implementation
has not changed) MUST continue to use the same minor_version number
of 0 that they have always used.
If a daemon or NAS implementation desires to provide support for
minor_number 0 TACACS+ hosts, it MUST pay attention to the
minor_version in the TACACS+ header (as it should anyway) and be
prepared to support the SENDPASS operation.
The removal of SENDPASS was prompted by security concerns, and imple-
mentors should think very carefully about how they wish to provide
this service. On a NAS, the minor_version 0 compatibility can be lay-
ered such that higher layers only need to understand the
minor_version 1 methodology, with the compatibility layer translating
requests appropriately when contacting an older daemon.
On a TACACS+ server, when detecting minor_number 0, the daemon should
allow for PAP authentications that do not send the password in the
data field, but instead expect to read the PAP password from a subse-
quent CONTINUE packet.
If the daemon supports SENDPASS, then it should be prepared to handle
such requests for CHAP and ARAP and even PAP, when outbound authenti-
cation takes place.
15. Notes to Implementors
For those interested in integrating one-time password support into
TACACS+ daemons, there are some subtleties to consider. TACACS+ is
designed to make this straightforward, but two cases require some
extra care.
One-time password support with ARAP and PPP's CHAP authentication
protocols is NOT straightforward, but there are work arounds. The
problem lies in the nature of ARAP and CHAP authentication. Both
employ a challenge-response protocol that requires a copy of the
cleartext password to be stored at both ends. Unfortunately, due to
their cryptographic nature, one-time password systems can rarely pro-
vide the cleartext version of the next password.
A simple workaround is to have the user enter their username as a
combination of the username and the one-time password, separated by a
special character, and a fixed password can be used in the password
field. The fixed password can be assigned on a per user basis or as a
single site-wide password.
For the separator character, Cisco Systems has been using the `*'
Carrel/Grant [Page 38]
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(asterisk) character. After some deliberation, it was decided that it
was the least likely character to be found in a username.
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16. References
[1] D. Carrel, L. Grant, "The TACACS+ API Definition"
[2] C. Finseth, RFC 1492, "An Access Control Protocol, Sometimes
Called TACACS", July 1993.
[3] R. Rivest, RFC 1321, "The MD5 Message-Digest Algorithm", April
1992.
[4] B. Lloyd, W. Simpson, RFC 1334, "PPP Authentication Protocols",
October 1992.
[5] Apple Computer Corp. AppleTalk Remote Access Protocol (ARAP)
Version 2.0 External Reference Specification. Preliminary docu-
ment (no date available).
[6] D. Eastlake, S. Crocker, J. Schiller, RFC 1750, "Randomness
Recommendations for Security", December 1994.
[7] S. Cobb, Microsoft PPP CHAP Extensions, Network working group
Informational memo, Revision 1.2, March 1995.
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17. Revision History
v1.75: first IETF submission
v1.76: fix encrypted flag text
v1.77: Add service 5 for Protocol translation
v1.78: Add ms chap description
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Table of Contents
Introduction ................................................... 1
Technical Definitions .......................................... 2
The TACACS+ packet header ...................................... 5
The TACACS+ packet body ........................................ 7
Body Encryption ................................................ 8
Body types ..................................................... 9
Authentication ................................................. 10
Enable Requests ................................................ 17
Inbound ASCII Login ............................................ 17
Inbound PAP Login .............................................. 17
Inbound CHAP login ............................................. 18
Inbound MS-CHAP login .......................................... 18
Outbound MS-CHAP request ....................................... 19
Inbound ARAP login ............................................. 19
Outbound PAP request ........................................... 20
Outbound CHAP request .......................................... 20
Outbound ASCII and ARAP request ................................ 21
ASCII change password request .................................. 21
PPP change password request .................................... 21
ARAP change password request ................................... 22
Authorization .................................................. 23
The authorization REQUEST packet body .......................... 24
Table 1: Attribute-value Pairs ................................. 27
The authorization RESPONSE packet body ......................... 30
Accounting ..................................................... 32
The account REQUEST packet body ................................ 32
The accounting REPLY packet body ............................... 35
Compatibility between Minor Versions 0 and 1 ................... 37
Notes to Implementors .......................................... 38
References ..................................................... 40
Revision History ............................................... 41
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