The TACACS+ Protocol

Terminal Access Controller Access-Control System Plus (TACACS+) was originally conceived as a general Authentication, Authorization and Accounting protocol. It is primarily used today for Device Administration: authenticating access to network devices, providing central authorization of operations, and audit of those operations. A wide range of TACACS+ clients and servers are already deployed in the field. The TACACS+ protocol they are based on is defined in a draft document that was originally intended for IETF publication. This document is known as `The Draft' . It is intended that all implementations which conform to this document will conform to `The Draft'. However, the following specific adjustments of the protocol specification from 'The Draft' should be noted: This document officially removes SENDPASS for security reasons. The normative description of Legacy features such as ARAP and outbound authentication have been removed, however the required enumerations are kept. The TACACS+ protocol separates the functions of Authentication, Authorization and Accounting. It allows for arbitrary length and content authentication exchanges, which will support any authentication mechanism to be utilized with TACACS+ clients. It is extensible to provide for site customization and future development features, and it uses TCP to ensure reliable delivery. The protocol allows the TACACS+ client to request very fine-grained access control and allows the server to respond to each component of that request. The separation of authentication, authorization and accounting is a fundamental component of the design of TACACS+. The distinction between them is very important so this document will address each one separately. It is important to note that TACACS+ provides for all three, but an implementation or configuration is not required to employ all three. Each one serves a unique purpose that alone is useful, and together can be quite powerful. This document restricts itself to a description of the protocol that is used by TACACS+. It does not cover deployment or best practices.

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. However, fixed passwords have limitations, mainly in the area of security. Many modern authentication mechanisms utilize "one-time" passwords or a challenge-response query. TACACS+ is designed to support all of these, and should be powerful enough to handle any future mechanisms. Authentication generally takes place when the user first logs in to a machine or requests a service of it. Authentication is not mandatory; it is a site-configured option. Some sites do not require it. Others require it only for certain services (see authorization below). Authentication may also take place when a user attempts to gain extra privileges, and must identify himself or herself as someone who possesses the required information (passwords, etc.) for those privileges. 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. Examples 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+ server 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 . 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 attributes is defined in the accounting section . Client The client is any device, (often a Network Access Server) that provides access services. The clients usually provide a character mode front end and then allow the user to telnet or rlogin to another host. A client may also support protocol based access services. Server The server receives TACACS+ protocol requests, and replies according to its business model, in accordance with the flows defined in this document. Packet All uses of the word packet in this document refer to TACACS+ protocol packets unless explicitly noted otherwise.

TACACS+ uses TCP for its transport. Server port 49 is allocated for TACACS+ traffic.

The protocol includes an obfuscation mechanism referred to in the original draft as Body Encryption. It is intended to follow up this document with enhancements to TACACS+ security. It is recommended to separate the management traffic from the regular network access traffic, and to use Body Encryption in production environments.

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.

The packet header (see below) contains a flag to allow sessions to be multiplexed on a connection. If a client sets this flag, this indicates 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. If the server sets this flag in the first reply packet in response to the first packet from a client, this indicates its willingness to support single-connection over the current connection. The server may set this flag even if the client does not set it, but the client is under no obligation to honor it. The flag is only relevant for the first two packets on a connection, to allow the client and server to establish single connection mode. The flag MUST be ignored after these two packets since the single-connect status of a connection, once established, must not be changed. The connection must instead be closed and a new connection opened, if required. When single-connect status is established, multiple sessions MUST be allowed simultaneously and/or consecutively on a single TCP connection. If single-connect status has not been established in the first two packets of a TCP connection, then the connection must be closed at the end of the first session.

All TACACS+ packets begin with the following 12 byte header. The header describes the remainder of the packet:

major_version This is the major TACACS+ version number. TAC_PLUS_MAJOR_VER := 0xc minor_version The minor TACACS+ version number. TAC_PLUS_MINOR_VER_DEFAULT := 0x0 TAC_PLUS_MINOR_VER_ONE := 0x1 type This is the packet type. Legal values are: TAC_PLUS_AUTHEN := 0x01 (Authentication) 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 packet in a session MUST have the sequence number 1 and each subsequent packet will increment the sequence number by one. Thus clients only send packets containing odd sequence numbers, and TACACS+ servers only send packets containing even sequence numbers. The sequence number must never wrap i.e. if the sequence number 2^8-1 is ever reached, that session must terminate and be restarted with a sequence number of 1. flags This field contains various bitmapped flags. The unencrypted flag bit says whether encryption is being used on the body of the packet (the entire portion after the header). TAC_PLUS_UNENCRYPTED_FLAG := 0x01 If this flag is set, then body encryption is not used. 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 This flag is used to allow a client and server to agree whether multiple sessions may be multiplexed onto a single connection. 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, see RFC 1750 ) length The total length of the packet body (not including the header). This value is in network byte order. Packets are never padded beyond this length.

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: - 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 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.

The body of packets may be encrypted. The following sections describe the encryption mechanism that is supported to enable backwards compatibility with 'The Draft'. 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 server. This document does not discuss the management and storage of those keys. It is an implementation detail of the server and client, as to whether they will maintain only one key, or a different key for each client or server with which they communicate. For security reasons, the latter options should be available, but it is a site dependent 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 RFC 1321 . pseudo_pad = {MD5_1 [,MD5_2 [ ... ,MD5_n]]} truncated to len(data) The first MD5 hash is generated by concatenating the session_id, the secret key, the version number and the sequence number and then running MD5 over that stream. All of those input values are available in the packet header, except for the secret key which is a shared secret between the TACACS+ client and server. The version number is the one byte concatenation of the major and minor version numbers. The session id is used in network byte order. Subsequent hashes are generated by using the same input stream, but concatenating the previous hash value at the end of the input stream. MD5_1 = MD5{session_id, key, version, seq_no} MD5_2 = MD5{session_id, key, version, seq_no, MD5_1} .... MD5_n = MD5{session_id, key, version, seq_no, MD5_n-1} When a server detects that the secret(s) it has configured for the device mismatch, it MUST return ERROR. The handling of the TCP connection by the server is implementation independent. TAC_PLUS_UNENCRYPTED_FLAG == 0x1 In this case, the entire packet body is in cleartext. 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 encryption method is optional. NOTE: Implementations should take care not to skip decryption simply because an incoming packet indicates that it is not encrypted. If the unencrypted flag is not set, and the packet is not encrypted, it must be dropped. 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 client and the TACACS+ server. If an error must be declared but the type of the incoming packet cannot be determined, a packet with the identical cleartext header but with a sequence number incremented by one and the length set to zero MUST be returned to indicate an error.

All text fields in TACACS+ MUST be US-ASCII, excepting special consideration given to user field and data fields used for passwords. To ensure interoperability of current deployments, the TACACS+ client and server MUST handle user field and data fields used for passwords as 8 bit octet strings. The deployment operator MUST ensure that consistent character encoding is applied. The encoding SHOULD be UTF-8, and other encodings outside US-ASCII SHOULD be deprecated.

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_SENDAUTH := 0x04 priv_lvl This indicates the privilege level that the user is authenticating as. Please refer to the Privilege Level section below. 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 (deprecated) TAC_PLUS_AUTHEN_TYPE_MSCHAP := 0x05 TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 := 0x06 authen_service This is the service that is requesting the authentication. Legal values are: TAC_PLUS_AUTHEN_SVC_NONE := 0x00 TAC_PLUS_AUTHEN_SVC_LOGIN := 0x01 TAC_PLUS_AUTHEN_SVC_ENABLE := 0x02 TAC_PLUS_AUTHEN_SVC_PPP := 0x03 TAC_PLUS_AUTHEN_SVC_ARAP := 0x04 TAC_PLUS_AUTHEN_SVC_PT := 0x05 TAC_PLUS_AUTHEN_SVC_RCMD := 0x06 TAC_PLUS_AUTHEN_SVC_X25 := 0x07 TAC_PLUS_AUTHEN_SVC_NASI := 0x08 TAC_PLUS_AUTHEN_SVC_FWPROXY := 0x09 The TAC_PLUS_AUTHEN_SVC_NONE option is intended for the authorization application of this field that indicates that no authentication was performed by the device. The TAC_PLUS_AUTHEN_SVC_LOGIN option is identifies regular login (as opposed to ENABLE) to a client device. The TAC_PLUS_AUTHEN_SVC_ENABLE option identifies the ENABLE 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. A service value of NONE should only be used when none of the other 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 The username. It is optional in this packet, depending upon the class of authentication. port The US-ASCII name of the client port on which the authentication is taking 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 US-ASCII string this is a "best effort" description of the remote location from which the user has connected to the client. It is intended to hold a network address if the user is connected via a network, a caller ID is the user is connected via ISDN or a POTS, or any other remote location information that is available. This field is optional (since the information may not be available). data 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 .

The TACACS+ server sends only one type of authentication packet (a REPLY packet) to the client. The REPLY packet body looks as follows:

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 flags Bitmapped flags that modify the action to be taken. The following values are defined: TAC_PLUS_REPLY_FLAG_NOECHO := 0x01 server_msg c A message to be displayed to the user. This field is optional. If it exists, it is intended to be presented to the user. US-ASCII charset must be used. data This field holds data that is a part of the authentication exchange and is intended for the client, not the user. It is described in more detail in the section Common Authentication flows .

This packet is sent from the client to the server following the receipt of a REPLY packet.

user_msg 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. 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

The action, authen_type and service fields (described above) combine to determine 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. A set of standard kinds of authentication is defined in this document. Each authentication flow consists of a START packet. The server responds either with a request for more information (GETDATA, GETUSER or GETPASS) or a termination (PASS or FAIL). The actions and meanings when the server sends a RESTART, ERROR or FOLLOW are common and are described further below. When the REPLY status equals TAC_PLUS_AUTHEN_STATUS_GETDATA, TAC_PLUS_AUTHEN_STATUS_GETUSER or TAC_PLUS_AUTHEN_STATUS_GETPASS, then authentication continues and the 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. All three cause the same action to be performed, but the use of TAC_PLUS_AUTHEN_STATUS_GETUSER, indicates to the client that the user response will be interpreted as a username, and for TAC_PLUS_AUTHEN_STATUS_GETPASS, that the user response represents will be interpreted as a password. TAC_PLUS_AUTHEN_STATUS_GETDATA is the generic request for more information to flexibly support future requirements. If the TAC_PLUS_REPLY_FLAG_NOECHO flag is set in the REPLY, then the user response must not be echoed as it is entered. The data field is only used in the REPLY where explicitly defined below.

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:

The '-' symbol represents that the option is not valid. When a server 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. In minor_version 0, Inbound PAP performed a normal LOGIN, sending the username in the START packet and then waiting for a GETPASS and sending the password in a CONTINUE packet. In minor_version 1, CHAP and inbound PAP use LOGIN to perform inbound authentication and the exchanges use the data field so that the client only sends a single START packet and expects to receive a PASS or FAIL. SENDAUTH is only used for PPP when performing outbound authentication. NOTE: Only those requests which have changed from their minor_version 0 implementation (i.e. CHAP, MS-CHAP and PAP authentications) should use the new minor_version number of 1. All other requests (i.e. all authorisation and accounting and ASCII authentication) MUST continue to use the same minor_version number of 0. The removal of SENDPASS was prompted by security concerns, and is no longer considered part of the TACACS+ protocol.

This section describes common authentication flows. If the options are implemented, they MUST follow the description. If the server does not implement an option, it should respond with TAC_PLUS_AUTHEN_STATUS_ERROR. Inbound ASCII Login

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, followed by a terminating REPLY (PASS or FAIL). Inbound PAP Login

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 . The REPLY from the server MUST be either a PASS or FAIL. Inbound CHAP login

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 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 will run MD5 over the id, the user's secret and the challenge, as defined in the PPP Authentication RFC RFC 1334 and then compare that value with the response. The REPLY from the server MUST be a PASS or FAIL. Inbound MS-CHAP v1 login

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 and then compare the resulting value with the response. The REPLY from the server MUST be a PASS or FAIL. For best practices, please refer to RFC 2433 Inbound MS-CHAP v2 login

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 the algorithm specified RFC2759 on the user's secret and challenge and then compare the resulting value with the response. The REPLY from the server MUST be a PASS or FAIL. For best practices for MS-CHAP v2, please refer to RFC2759 Enable Requests

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 server collects the information it requires in order to allow changing the principal's privilege level. This exchange is very similar to an Inbound ASCII login. In order to readily distinguish enable requests from other types of request, the value of the service field MUST be set to TAC_PLUS_AUTHEN_SVC_ENABLE when requesting an ENABLE. It MUST NOT be set to this value when requesting any other operation. ASCII change password request

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.

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 terminated 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 contain a message to be displayed to the user. When the status equals TAC_PLUS_AUTHEN_STATUS_FOLLOW the packet indicates that the TACACS+ server requests that authentication should be performed with an alternate server. The data field MUST contain ASCII text describing one or more servers. A server description appears like this: [@<protocol>@]<host>>[@<key>] If more than one host is specified, they MUST be separated into rows by an ASCII Carriage Return (0x0D). The protocol and key are optional, and apply only to host in the same row. The protocol can describe an alternate way of performing the authentication, other than TACACS+. If the protocol is not present, then TACACS+ is assumed. Protocols are ASCII numbers corresponding to the methods listed in the authen_method field of authorization packets (defined below). The host is specified as either a fully qualified domain name, or an ASCII numeric IPV4 address specified as octets separated by dots ('.'), or IPV6 adress test representation defined in RFC 4291. If a key is supplied, the client MAY use the key in order to authenticate to that host. The client may use a preconfigured key for the host if it has one. If not then the client may communicate with the host using unencrypted option. 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 administrative console or log. If the status equals TAC_PLUS_AUTHEN_STATUS_RESTART, then the authentication 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. 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.

This part of the TACACS+ protocol provides an extensible way of providing remote 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 indicate how the user was authenticated or processed and a variable set of arguments that describe the services and options for which authorization is requested. The RESPONSE contains a variable set of response arguments (attribute-value pairs) that 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.

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 TAC_PLUS_AUTHEN_METH_GUEST := 0x08 TAC_PLUS_AUTHEN_METH_RADIUS := 0x10 TAC_PLUS_AUTHEN_METH_KRB4 := 0x11 TAC_PLUS_AUTHEN_METH_RCMD := 0x20 KRB5 and KRB4 are Kerberos version 5 and 4. LINE refers to a fixed password associated with the terminal line used to gain access. LOCAL is a client local user database. ENABLE is a command that authenticates in order to grant new privileges. TACACSPLUS is, of course, TACACS+. GUEST is an unqualified guest authentication, such as an ARAP guest login. RADIUS is the Radius authentication protocol. RCMD refers to authentication provided via the R-command protocols from Berkeley Unix. (One should be aware of the security limitations to R-command authentication.) 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 below. It indicates the users current privilege level. authen_type This field matches the authen_type field in the authentication section above. It indicates the type of authentication that was performed. If this information is not available, then the client should set authen_type to: TAC_PLUS_AUTHEN_TYPE_NOT_SET := 0x00. This value is valid only in authorization and accounting requests. authen_service This field matches the 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. 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 US-ASCII string and are separated by either a "=" (0X3D) or a "*" (0X2A). The equals sign indicates a mandatory argument. The asterisk indicates an optional one. It is not legal for an attribute name to contain either of the separators. It is legal for attribute values to contain the separators. Optional arguments are ones that may be disregarded by either client or server. Mandatory arguments require that the receiving side understands 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 minimum is two characters (one name value and the separator)

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 US-ASCII string that may be presented to the user. The decision to present this message is client specific. data This is an US-ASCII string that may be presented on an administrative display, console or log. The decision to present this message is client specific. 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 server. 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, and must be ignored.

TACACS+ accounting is very similar to authorization. The packet format is also similar. There is a fixed portion and an extensible portion. The extensible portion uses all the same attribute-value pairs that authorization uses, and adds several more.

flags This holds bitmapped flags. TAC_PLUS_ACCT_FLAG_START := 0x02 TAC_PLUS_ACCT_FLAG_STOP := 0x04 TAC_PLUS_ACCT_FLAG_WATCHDOG := 0x08 All other fields are defined in the authorization and authentication sections above and have the same semantics. See section 12 Accounting Attribute-value Pairs for the dictionary of attributes relevant to accounting.

The response to an accounting message is used to indicate that the accounting function on the server has completed. The server should reply with success only when the record has been committed to the required level of security, relieving the burden on the client from ensuring any better form of accounting is required.

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 a US-ASCII string that may be presented to the user. The decision to present this message is client specific. data This is a US-ASCII string that may be presented on an administrative display, console or log. The decision to present this message is client specific. 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. The server 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. Summary of Accounting Packets

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. The Server MUST respond with TAC_PLUS_ACCT_STATUS_ERROR if the client requests an INVALID option.

TACACS+ is intended to be an extensible protocol. The attributes used in Authorization and Accounting are not fixed. Some attributes are defined below for common use cases, clients MUST use these attributes when supporting the corresponding use cases. All numeric values in an attribute-value string are provided as decimal US-ASCII numbers, unless otherwise stated. All boolean attributes are encoded with values "true" or "false". It is recommended that hosts be specified as a numeric address so as to avoid any ambiguities. Absolute times should be specified in seconds since the epoch, 12:00am Jan 1 1970. The timezone MUST be UTC unless a timezone attribute is specified. Attributes may be submitted with no value, in which case they consist of the name and the mandatory or optional separator. For example, the attribute "cmd" which has no value is transmitted as a string of 4 characters "cmd="

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", "shell", "tty-server", "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". 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 service equals "shell". If no value is present then 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 US-ASCII number representing a connection access list. Used only when value of service is "shell"" and cmd has no value. inacl US-ASCII identifier for an interface input access list. outacl US-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 client 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 <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 not disconnect after an automatic command. Used only when service=shell and cmd=NULL priv-lvl privilege level to be assigned. Please refer to the Privilege Level section below. remote_user remote userid (authen_method must have the value TAC_PLUS_AUTHEN_METH_RCMD). In the case of rcmd authorizations, the authen_method will be set to TAC_PLUS_AUTHEN_METH_RCMD and the remote_user and remote_host attributes will provide the remote user and host information to enable rhost style authorization. The response may request that a privilege level be set for the user. remote_host remote host (authen_method must have the value TAC_PLUS_AUTHEN_METH_RCMD) callback-dialstring Indicates that callback should be done. Value is NULL, or a dialstring. A NULL value indicates that the service MAY choose to get the dialstring through other means. callback-line The line number to use for a callback. callback-rotary The rotary number to use for a callback. nocallback-verify Do not require authentication after callback.

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. task_id Start and stop records for the same event MUST have matching task_id attribute values. The client must not reuse a specific task_id in a start record until it has sent a stop record for that task_id. start_time The time the action started (). 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 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 US-ASCII string describing the status of the action.

The TACACS+ Protocol supports flexible authorization schemes through the extensible attributes. One scheme is built in to the protocol: Privilege Levels. 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. 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 If a client uses a different privilege level scheme, then it must map the privilege level to scheme above. Privilege Levels are applied in two ways in the TACACS+ protocol: - As an argument in authorization EXEC phase (when service=shell and cmd=NULL), where it is primarily used to set the initial privilege level for the EXEC session. - In the packet headers for Authentication, Authorization and Accounting. The privilege level in the header is primarily significant in the Authentication phase for enable authentication where a different privilege level is required. The use of Privilege levels to determine session-based access to commands and resources is not mandatory for clients, but it is in common use so SHOULD be supported by servers.