--- 1/draft-ietf-opsawg-tacacs-07.txt 2018-02-19 08:13:36.020578797 -0800 +++ 2/draft-ietf-opsawg-tacacs-08.txt 2018-02-19 08:13:36.096581023 -0800 @@ -1,56 +1,56 @@ Operations T. Dahm Internet-Draft A. Ota Intended status: Informational Google Inc -Expires: February 22, 2018 D. Medway Gash +Expires: August 23, 2018 D. Medway Gash Cisco Systems, Inc. D. Carrel vIPtela, Inc. L. Grant - August 21, 2017 + February 19, 2018 The TACACS+ Protocol - draft-ietf-opsawg-tacacs-07 + draft-ietf-opsawg-tacacs-08 Abstract TACACS+ provides Device Administration for routers, network access servers and other networked computing devices via one or more centralized servers. This document describes the protocol that is used by TACACS+. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- - Drafts is at http://datatracker.ietf.org/drafts/current/. + Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on February 22, 2018. + This Internet-Draft will expire on August 23, 2018. Copyright Notice - Copyright (c) 2017 IETF Trust and the persons identified as the + Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents - (http://trustee.ietf.org/license-info) in effect on the date of + (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this @@ -62,58 +62,59 @@ not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Technical Definitions . . . . . . . . . . . . . . . . . . . . 4 3. TACACS+ Connections and Sessions . . . . . . . . . . . . . . 4 3.1. Connection . . . . . . . . . . . . . . . . . . . . . . . 4 - 3.2. Session . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 3.3. Single Connect Mode . . . . . . . . . . . . . . . . . . . 5 - 3.4. Session Completion . . . . . . . . . . . . . . . . . . . 5 - 3.5. Treatment of Enumerated Protocol Values . . . . . . . . . 6 + 3.2. Session . . . . . . . . . . . . . . . . . . . . . . . . . 5 + 3.3. Single Connection Mode . . . . . . . . . . . . . . . . . 5 + 3.4. Session Completion . . . . . . . . . . . . . . . . . . . 6 + 3.5. Treatment of Enumerated Protocol Values . . . . . . . . . 7 3.6. Text Encoding . . . . . . . . . . . . . . . . . . . . . . 7 3.7. Data Obfuscation . . . . . . . . . . . . . . . . . . . . 7 3.8. The TACACS+ Packet Header . . . . . . . . . . . . . . . . 9 3.9. The TACACS+ Packet Body . . . . . . . . . . . . . . . . . 11 4. Authentication . . . . . . . . . . . . . . . . . . . . . . . 11 - 4.1. The Authentication START Packet Body . . . . . . . . . . 11 + 4.1. The Authentication START Packet Body . . . . . . . . . . 12 4.2. The Authentication REPLY Packet Body . . . . . . . . . . 14 - 4.3. The Authentication CONTINUE Packet Body . . . . . . . . . 15 + 4.3. The Authentication CONTINUE Packet Body . . . . . . . . . 16 4.4. Description of Authentication Process . . . . . . . . . . 16 4.4.1. Version Behaviour . . . . . . . . . . . . . . . . . . 17 - 4.4.2. Common Authentication Flows . . . . . . . . . . . . . 17 + 4.4.2. Common Authentication Flows . . . . . . . . . . . . . 18 4.4.3. Aborting an Authentication Session . . . . . . . . . 21 5. Authorization . . . . . . . . . . . . . . . . . . . . . . . . 22 - 5.1. The Authorization REQUEST Packet Body . . . . . . . . . . 23 + 5.1. The Authorization REQUEST Packet Body . . . . . . . . . . 22 5.2. The Authorization REPLY Packet Body . . . . . . . . . . . 26 6. Accounting . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.1. The Account REQUEST Packet Body . . . . . . . . . . . . . 28 6.2. The Accounting REPLY Packet Body . . . . . . . . . . . . 29 7. Attribute-Value Pairs . . . . . . . . . . . . . . . . . . . . 30 - 7.1. Authorization Attributes . . . . . . . . . . . . . . . . 31 - 7.2. Accounting Attributes . . . . . . . . . . . . . . . . . . 34 - + 7.1. Value Encoding . . . . . . . . . . . . . . . . . . . . . 31 + 7.2. Authorization Attributes . . . . . . . . . . . . . . . . 31 + 7.3. Accounting Attributes . . . . . . . . . . . . . . . . . . 34 8. Privilege Levels . . . . . . . . . . . . . . . . . . . . . . 35 9. TACACS+ Security Considerations . . . . . . . . . . . . . . . 36 - 9.1. Overall Security of The Protocol . . . . . . . . . . . . 36 + 9.1. General Security of The Protocol . . . . . . . . . . . . 36 9.2. Security of Authentication Sessions . . . . . . . . . . . 38 9.3. Security of Authorization Sessions . . . . . . . . . . . 38 - 9.4. Security of Accounting Sessions . . . . . . . . . . . . . 38 + 9.4. Security of Accounting Sessions . . . . . . . . . . . . . 39 9.5. TACACS+ Deployment Recommendations . . . . . . . . . . . 39 - 9.6. TACACS+ Client Implementation Recommendations . . . . . . 39 - 9.7. TACACS+ Server Implementation Recommendations . . . . . . 40 - 9.8. TACACS+ Security and Operational Concerns . . . . . . . . 40 - 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 41 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 + 9.6. TACACS+ Client Implementation Recommendations . . . . . . 40 + 9.7. TACACS+ Server Implementation Recommendations . . . . . . 41 + 9.8. TACACS+ Security and Operational Concerns . . . . . . . . 41 + 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 42 + 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 42 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42 1. Introduction Terminal Access Controller Access-Control System Plus (TACACS+) was originally conceived as a general Authentication, Authorization and Accounting protocol. It is primarily used today for Device Administration: authenticating access to network devices, providing central authorization of operations, and audit of those operations. A wide range of TACACS+ clients and servers are already deployed in @@ -125,51 +126,56 @@ document will conform to `The Draft'. However, attention is drawn to the following specific adjustments of the protocol specification from 'The Draft': This document officially removes SENDPASS for security reasons. The normative description of Legacy features such as ARAP and outbound authentication have been removed, however the required enumerations are kept. + The Support for forwarding to an alternative daemon + (TAC_PLUS_AUTHEN_STATUS_FOLLOW) has been deprecated. + 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. + content authentication exchanges, to support future authentication + mechanisms. It is extensible to provide for site customization and + future development features, and it uses TCP to ensure reliable + delivery. The protocol allows the TACACS+ client to request 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. + The separation of authentication, authorization and accounting was a + key element of the design of TACACS+ protocol. Essentially it makes + TACACS+ a suite of three protocols. This document will address each + one in separate sections. Although TACACS+ defines all three, but an + implementation or configuration is not required to employ all three. + Separating the elements is useful for Device Administration use case, + specifically, for authorization of individual commands in a session. + Note that there is no provision made at the protocol level for + association of an authentication to each authroization request. This document restricts itself to a description of the protocol that is used by TACACS+. It does not cover deployment or best practices. 2. Technical Definitions This section provides a few basic definitions that are applicable to this document Client The client is any device, (often a Network Access Server) that provides access services. The clients usually provide a character mode front end and then allow the user to telnet or rlogin to another - host. A client may also support protocol based access services. + host. 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+ @@ -188,58 +194,59 @@ session is a single authentication sequence, a single authorization exchange, or a single accounting exchange. An accounting and authorization session will consist of a single pair of packets (the request and its reply). An authentication session may involve an arbitrary number of packets being exchanged. The session is an operational concept that is maintained between the TACACS+ client and server. It does not necessarily correspond to a given user or user action. -3.3. Single Connect Mode +3.3. Single Connection Mode Single Connection Mode is intended to improve performance by allowing a client to multiplex multiple session on a single TCP connection. The packet header contains the TAC_PLUS_SINGLE_CONNECT_FLAG used by the client and server to negotiate the use of Single Connect Mode. The client sets this flag, to indicate that it supports multiplexing TACACS+ sessions over a single TCP connection. The client MUST NOT send a second packet on a connection until single-connect status has been established. - To indicate it will support Single Connect Mode, the server sets this - flag in the first reply packet in response to the first request from - a client. The server may set this flag even if the client does not - set it, but the client may ignore the flag and close the connection - after the session completes. + To indicate it will support Single Connection Mode, the server sets + this flag in the first reply packet in response to the first request + from a client. The server may set this flag even if the client does + not set it, but the client may ignore the flag and close the + connection after the session completes. The flag is only relevant for the first two packets on a connection, - to allow the client and server to establish Single Connect Mode. - This protocol does not define a procedure for changing Single Connect - Mode after the first two packets. + to allow the client and server to establish Single Connection Mode. + No provision is made for changing Single Connection Mode after the + first two packets: the client and server MUST ignore the flag after + the second packet on a connection. - If single Connect Mode has not been established in the first two + If single Connection Mode has not been established in the first two packets of a TCP connection, then both the client and the server close the connection at the end of the first session. The client negotiates single Connection Mode to improve efficiency. The server may refuse to allow Single connection Mode for the client. - For example it may not fit the specific deployment to allocate a long - lasting TCP connection to a specific client. Even if the server is - configured to permit single Connection Mode for a specific client, - the server may close the connection. For example: a server may be - configured to time out a Single Connection Mode TCP Connection after - a specific period of inactivity to preserve its resources. The + For example, it may not be appropriate to allocate a long lasting TCP + connection to a specific client in some deployments. Even if the + server is configured to permit single Connection Mode for a specific + client, the server may close the connection. For example: a server + may be configured to time out a Single Connection Mode TCP Connection + after a specific period of inactivity to preserve its resources. The client MUST accommodate such closures on a TCP session even after - Single Conenction Mode has been established. + Single Connection Mode has been established. 3.4. Session Completion The REPLY packets defined for the packets types in the sections below (Authentication, Authorization and Accounting) contain a status field. The complete set of options for this field depend upon the packet type, but all three REPLY packet types define values representing PASS, ERROR and FAIL, which indicate the last packet of a regular session (one which is not aborted). @@ -253,37 +260,48 @@ and it MUST behave as if the server could not be connected to. For example, the client try alternative methods, if they are available, such as sending the request to a backup server, or using local configuration to determine whether the action which prompted the request should be executed. Refer to the section (Section 4.4.3) on Aborting Authentication Sessions for details on handling additional status options . When the session is complete, then the TCP connection should be - handled as follows, according to whether Single Connect Mode was + handled as follows, according to whether Single Connection Mode was negotiated: If Single Connection Mode was not negotiated, then the connection should be closed If Single Connection Mode was enabled, then the connection SHOULD be left open (see section (Section 3.3) ), but may still be closed after a timeout period to preserve deployment resources If Single Connection Mode was enabled, but an ERROR occurred due to connection issues (such as an incorrect secret, see section (Section 3.7) ), then any further new sessions MUST NOT be accepted on the connection. If there are any sessions that have already been established then they MAY be completed. Once all active sessions are completed then the connection MUST be closed. + It is recommended that client implementations provide robust schemes + for dealing with servers which cannot be connected to. Options + include providing a list of servers for redundancy, and an option for + a local fallback configuration if no servers can be reached. Details + will be implmentation specific. + + The client should manage connections and handle the case of a server + which establishes a connection, but does not respond. The exact + behavior is implementation specific. It is recommended that the + client should close the connection after a configurable timeout. + 3.5. Treatment of Enumerated Protocol Values This document describes various enumerated values in the packet header and the headers for specific packet types. for example in the Authentication start packet type, this document defines the action field with three values TAC_PLUS_AUTHEN_LOGIN, TAC_PLUS_AUTHEN_CHPASS and TAC_PLUS_AUTHEN_SENDAUTH. If the server does not implement one of the defined options in a packet that it receives, or it encounters an option that is not @@ -291,114 +309,110 @@ with a ERROR and terminate the session. This will allow the client to try a different option. If an error occurs but the type of the incoming packet cannot be determined, a packet with the identical cleartext header but with a sequence number incremented by one and the length set to zero MUST be returned to indicate an error. 3.6. Text Encoding - All text fields in TACACS+ MUST be US-ASCII, excepting special - consideration given to user field and data fields used for passwords. + All text fields in TACACS+ MUST be printable US-ASCII, excepting + special consideration given to user field and data fields used for + passwords. To ensure interoperability of current deployments, the TACACS+ client and server MUST handle user fields and those data fields used for passwords as 8 bit octet strings. The deployment operator MUST - ensure that consistent character encoding is applied. The encoding - SHOULD be UTF-8, and other encodings outside US-ASCII SHOULD be - deprecated. + ensure that consistent character encoding is applied from the end + client to the server. The encoding SHOULD be UTF-8, and other + encodings outside printable US-ASCII SHOULD be deprecated. 3.7. Data Obfuscation The body of packets may be obfuscated. The following sections - describe the obfuscation mechanism that is supported in the protocol. + describe the obfuscation method that is supported in the protocol. In 'The Draft' this process was actually referred to as Encryption, - but by modern day standards the mechanims would not meet the - requirements of an encryption mechanism. + but the algorithm would not meet modern standards, and so will not be + termed as encryption in this document. - The obfuscation mechanism relies on a secret key, it is referring to - a shared secret value that is known to both the client and the - server. This document does not discuss the management and storage of - those keys. It is an implementation detail of the server and client, - as to whether they will maintain only one key, or a different key for - each client or server with which they communicate. For security - reasons, the latter options MUST be available, but it is a site - dependent decision as to whether the use of separate keys is - appropriate. + The obfuscation mechanism relies on a secret key, 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, other than + to require that the secret keys MUST remain secret. + + Server implementations MUST allow a unique secret key to be + associated with every client. It is a site dependent decision as to + whether the use of separate keys is appropriate. The flag field may be set as follows: - TAC_PLUS_UNENCRYPTED_FLAG == 0x0 + TAC_PLUS_UNENCRYPTED_FLAG = 0x0 In this case, the packet body is obfuscated by XOR-ing it byte-wise with a pseudo random pad. - ENCRYPTED {data} == data ^ pseudo_pad + ENCRYPTED {data} = data ^ pseudo_pad + + The packet body can then be de-obfuscated by XOR-ing it byte-wise + with a pseudo random pad. + + data = ENCRYPTED {data} ^ pseudo_pad + The pad is generated by concatenating a series of MD5 hashes (each 16 bytes long) and truncating it to the length of the input data. Whenever used in this document, MD5 refers to the "RSA Data Security, Inc. MD5 Message-Digest Algorithm" as specified in RFC 1321 [RFC1321] . pseudo_pad = {MD5_1 [,MD5_2 [ ... ,MD5_n]]} truncated to len(data) The first MD5 hash is generated by concatenating the session_id, the secret key, the version number and the sequence number and then running MD5 over that stream. All of those input values are available in the packet header, except for the secret key which is a shared secret between the TACACS+ client and server. - The version number is the one byte concatenation of the major and - minor version numbers. - - The session id is used in network byte order. + The version number and session_id are used as extracted from the + header Subsequent hashes are generated by using the same input stream, but concatenating the previous hash value at the end of the input stream. MD5_1 = MD5{session_id, key, version, seq_no} MD5_2 = MD5{session_id, key, version, seq_no, MD5_1} .... MD5_n = MD5{session_id, key, version, seq_no, MD5_n-1} When a server detects that the 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. + device mismatch, it MUST return ERROR. For details of TCP connection + handling on ERROR, refer to section section (Section 3.4) TAC_PLUS_UNENCRYPTED_FLAG == 0x1 In this case, the entire packet body is in cleartext. Obfuscation and de-obfuscation are null operations. This method should be avoided unless absolutely required for debug purposes, when tooling does not permit de-obfuscation. - If deployment is configured for obfuscating a connection then do no - skip de-obfuscation simply because an incoming packet indicates that - it is not obfuscated. If the flag is not set when expected, then it - must be dropped. + If deployment is configured for obfuscating a connection then the + request MUST be dropped if TAC_PLUS_UNENCRYPTED_FLAG is set to true. After a packet body is de-obfuscated, the lengths of the component values in the packet are summed. If the sum is not identical to the cleartext datalength value from the header, the packet MUST be - discarded, and an error signalled. The underlying TCP connection MAY - also be closed, if it is not being used for other sessions in single- - connect mode. + discarded, and an ERROR signaled. For details of TCP connection + handling on ERROR, refer to section (Section 3.4) Commonly such failures are seen when the keys are mismatched between the client and the TACACS+ server. - If an error must be declared but the type of the incoming packet - cannot be determined, a packet with the identical cleartext header - but with a sequence number incremented by one and the length set to - zero MUST be returned to indicate an error. - 3.8. The TACACS+ Packet Header All TACACS+ packets begin with the following 12 byte header. The header describes the remainder of the packet: 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 +----------------+----------------+----------------+----------------+ |major | minor | | | | |version| version| type | seq_no | flags | +----------------+----------------+----------------+----------------+ @@ -445,28 +460,28 @@ sequence number of 1. flags This field contains various bitmapped flags. The flag bit: TAC_PLUS_UNENCRYPTED_FLAG := 0x01 - This flag indicates that the sender did not obfuscate the bode of the + This flag indicates that the sender did not obfuscate the body of the packet. The application of this flag will be covered in the security - section (Section 9) . section. + section (Section 9) . This flag SHOULD be clear in all deployments. Modern network traffic - tools easily support encryted traffic when configured with the shared - secret (see section below), so even in test scenarios, the obfuscated - mode SHOULD be used. + tools support encrypted traffic when configured with the shared + secret (see section below), so obfuscated mode can and SHOULD be used + even during test. The single-connection flag: TAC_PLUS_SINGLE_CONNECT_FLAG := 0x04 This flag is used to allow a client and server to negotiate Single Connection Mode. session_id @@ -480,48 +496,49 @@ The total length of the packet body (not including the header). 3.9. The TACACS+ Packet Body The TACACS+ body types are defined in the packet header. The next sections of this document will address the contents of the different TACACS+ bodies. The following general rules apply to all TACACS+ body types: - To signal that any variable length data fields are unused, their - length value is set to zero. + length value is set to zero. Such fields MUST be ignored, and + treated as if not present. - the lengths of data and message fields in a packet are specified by their corresponding length fields, (and are not null terminated.) - All length values are unsigned and in network byte order. 4. Authentication Authentication is the action of determining who a user (or entity) is. Authentication can take many forms. Traditional authentication - utilizes a name and a fixed password. However, fixed passwords have - limitations, mainly in the area of security. Many modern - authentication mechanisms utilize "one-time" passwords or a - challenge-response query. TACACS+ is designed to support all of - these, and be powerful enough to handle any future mechanisms. - Authentication generally takes place when the user first logs in to a - machine or requests a service of it. + employs a name and a fixed password. However, fixed passwords are + vulnerable security, so many modern authentication mechanisms utilize + "one-time" passwords or a challenge-response query. TACACS+ is + designed to support all of these, and be flexible enough to handle + any future mechanisms. Authentication generally takes place when the + user first logs in to a machine or requests a service of it. Authentication is not mandatory; it is a site-configured option. Some sites do not require it. Others require it only for certain services (see authorization below). Authentication may also take place when a user attempts to gain extra privileges, and must identify himself or herself as someone who possesses the required information (passwords, etc.) for those privileges. 4.1. The Authentication START Packet Body + 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 +----------------+----------------+----------------+----------------+ | action | priv_lvl | authen_type | authen_service | +----------------+----------------+----------------+----------------+ | user_len | port_len | rem_addr_len | data_len | +----------------+----------------+----------------+----------------+ | user ... +----------------+----------------+----------------+----------------+ | port ... +----------------+----------------+----------------+----------------+ @@ -586,21 +602,21 @@ 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 + The TAC_PLUS_AUTHEN_SVC_LOGIN option indicates regular login (as opposed to ENABLE) to a client device. The TAC_PLUS_AUTHEN_SVC_ENABLE option identifies the ENABLE authen_service, which refers to a service requesting authentication in order to grant the user different privileges. This is comparable to the Unix "su(1)" command. An authen_service value of NONE is only to be used when none of the other authen_service values are appropriate. ENABLE may be requested independently, no requirements for previous authentications or authorizations are imposed by the protocol. @@ -604,41 +620,41 @@ appropriate. ENABLE may be requested independently, no requirements for previous authentications or authorizations are imposed by the protocol. Other options are included for legacy/backwards compatibility. user, user_len The username is optional in this packet, depending upon the class of authentication. If it is absent, the client MUST set user_len to 0. - If included, the user_len indicates the length of the user field, in bytes. port, port_len - The US-ASCII name of the client port on which the authentication is - taking place, and its length in bytes. The value of this field is - client specific. (For example, Cisco uses "tty10" to denote the - tenth tty line and "Async10" to denote the tenth async interface). - The port_len indicates the length of the port field, in bytes. + The printable US-ASCII name of the client port on which the + authentication is taking place, and its length in bytes. The value + of this field is client specific. (For example, Cisco uses "tty10" + to denote the tenth tty line and "Async10" to denote the tenth async + interface). The port_len indicates the length of the port field, in + bytes. rem_addr, rem_addr_len - An US-ASCII string indicating the remote location from which the user - has connected to the client. It is intended to hold a network - address if the user is connected via a network, a caller ID is the - user is connected via ISDN or a POTS, or any other remote location - information that is available. This field is optional (since the - information may not be available). The rem_addr_len indicates the - length of the user field, in bytes. + A printable US-ASCII string indicating the remote location from which + the user has connected to the client. It is intended to hold a + network address if the user is connected via a network, a caller ID + is the user is connected via ISDN or a POTS, or any other remote + location information that is available. This field is optional + (since the information may not be available). The rem_addr_len + indicates the length of the user field, in bytes. data, data_len This field is used to send data appropriate for the action and authen_type. It is described in more detail in the section Common Authentication flows (Section 4.4.2) . The data_len indicates the length of the data field, in bytes. 4.2. The Authentication REPLY Packet Body @@ -675,24 +692,23 @@ flags Bitmapped flags that modify the action to be taken. The following values are defined: TAC_PLUS_REPLY_FLAG_NOECHO := 0x01 server_msg, server_msg_len - A message to be displayed to the user. This field is optional. If - it exists, it is intended to be presented to the user. US-ASCII - charset MUST be used. The server_msg_len indicates the length of the - server_msg field, in bytes. + A message to be displayed to the user. This field is optional. The + printable US-ASCII charset MUST be used. The server_msg_len + indicates the length of the server_msg field, in bytes. data, data_len This field holds data that is a part of the authentication exchange and is intended for the client, not the user. Examples of its use are shown in the section Common Authentication flows (Section 4.4.2) . The data_len indicates the length of the data field, in bytes. 4.3. The Authentication CONTINUE Packet Body @@ -729,27 +746,26 @@ TAC_PLUS_CONTINUE_FLAG_ABORT := 0x01 4.4. Description of Authentication Process The action, authen_type and authen_service fields (described above) combine to indicate what kind of authentication is to be performed. Every authentication START, REPLY and CONTINUE packet includes a data field. The use of this field is dependent upon the kind of the Authentication. - This document defines a standard set of the kinds of authentication + This document defines a core set of authentication flows to be supported by TACACS+. Each authentication flow consists of a START packet. The server responds either with a request for more information (GETDATA, GETUSER or GETPASS) or a termination PASS, - FAIL, ERROR, RESTART or FOLLOW. The actions and meanings when the - server sends a RESTART, ERROR or FOLLOW are common and are described - further below. + FAIL, ERROR or RESTART. The actions and meanings when the server + sends a RESTART or ERROR are common and are described further below. When the REPLY status equals TAC_PLUS_AUTHEN_STATUS_GETDATA, TAC_PLUS_AUTHEN_STATUS_GETUSER or TAC_PLUS_AUTHEN_STATUS_GETPASS, then authentication continues and the server SHOULD provide server_msg content for the client to prompt the user for more information. The client MUST then return a CONTINUE packet containing the requested information in the user_msg field. The client should interpret TAC_PLUS_AUTHEN_STATUS_GETUSER as a request for username and TAC_PLUS_AUTHEN_STATUS_GETPASS as a request @@ -782,152 +798,163 @@ The '-' symbol represents that the option is not valid. All authorisation and accounting and ASCII authentication use minor_version number of 0. PAP, CHAP and MS-CHAP login use minor_version 1. The normal exchange is a single START packet from the client and a single REPLY from the server. - SENDAUTH is only used for PPP when performing outbound - authentication. - The removal of SENDPASS was prompted by security concerns, and is no longer considered part of the TACACS+ protocol. 4.4.2. Common Authentication Flows This section describes common authentication flows. If the server does not implement an option, it MUST respond with TAC_PLUS_AUTHEN_STATUS_FAIL. - Inbound ASCII Login +4.4.2.1. ASCII Login + action = TAC_PLUS_AUTHEN_LOGIN authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII minor_version = 0x0 This is a standard ASCII authentication. The START packet MAY contain the username. If the user does not include the username then the server MUST obtain it from the client with a CONTINUE - TAC_PLUS_AUTHEN_STATUS_GETUSER. When the server has the username, it - will obtain the password using a continue with - TAC_PLUS_AUTHEN_STATUS_GETPASS. ASCII login uses the user_msg field - for both the username and password. The data fields in both the - START and CONTINUE packets are not used for ASCII logins, any content - MUST be ignored. The session is composed of a single START followed - by zero or more pairs of REPLYs and CONTINUEs, followed by a final - REPLY indicating PASS, FAIL or ERROR. + TAC_PLUS_AUTHEN_STATUS_GETUSER. If the user does not provide a + username then the server can send another + TAC_PLUS_AUTHEN_STATUS_GETUSER request, but the server MUST limit the + number of retries that are permitted, recommended limit is three + attempts. When the server has the username, it will obtain the + password using a continue with TAC_PLUS_AUTHEN_STATUS_GETPASS. ASCII + login uses the user_msg field for both the username and password. + The data fields in both the START and CONTINUE packets are not used + for ASCII logins, any content MUST be ignored. The session is + composed of a single START followed by zero or more pairs of REPLYs + and CONTINUEs, followed by a final REPLY indicating PASS, FAIL or + ERROR. - Inbound PAP Login +4.4.2.2. PAP Login action = TAC_PLUS_AUTHEN_LOGIN authen_type = TAC_PLUS_AUTHEN_TYPE_PAP minor_version = 0x1 The entire exchange MUST consist of a single START packet and a single REPLY. The START packet MUST contain a username and the data field MUST contain the PAP ASCII password. A PAP authentication only consists of a username and password RFC 1334 [RFC1334] . The REPLY from the server MUST be either a PASS, FAIL or ERROR. - Inbound CHAP login +4.4.2.3. CHAP login action = TAC_PLUS_AUTHEN_LOGIN authen_type = TAC_PLUS_AUTHEN_TYPE_CHAP minor_version = 0x1 The entire exchange MUST consist of a single START packet and a single REPLY. The START packet MUST contain the username in the user field and the data field is a concatenation of the PPP id, the challenge and the response. The length of the challenge value can be determined from the length of the data field minus the length of the id (always 1 octet) and the length of the response field (always 16 octets). - To perform the authentication, the server calculates the PAP hash as + To perform the authentication, the server calculates the PPP hash as defined in the PPP Authentication RFC RFC 1334 [RFC1334] and then - compare that value with the response. The REPLY from the server MUST - be a PASS, FAIL or ERROR. + compare that value with the response. The MD5 algorithm option is + alays used. The REPLY from the server MUST be a PASS, FAIL or ERROR. - The client condcuts the exchange with the endstation and then sends - the resulting materials (challenge and responsee) to the server. So - although the selection of the challenge and its length are not an - aspect of the TACACS+ protocol, it is strongly recommended that the - client/endstation interaction is configured with a secure challenge - in mind, and the TACACS+ server can help by rejecting authentications - where the challenge is below a minimum length (for example, 8 bytes). + In cases where the client conducts the exchange with the endstation + and then sends the resulting materials (challenge and responsee) to + the server, the selection of the challenge and its length are not an + aspect of the TACACS+ protocol. However, it is strongly recommended + that the client/endstation interaction is configured with a secure + challenge. The TACACS+ server can help by rejecting authentications + where the challenge is below a minimum length (Minimum recommended is + 8 bytes). - Inbound MS-CHAP v1 login + In cases where the TACACS+ Server generates the challenge then it + MUST change for every request and MUST be derived from a strong + cryptographic source. + +4.4.2.4. MS-CHAP v1 login action = TAC_PLUS_AUTHEN_LOGIN authen_type = TAC_PLUS_AUTHEN_TYPE_MSCHAP minor_version = 0x1 The entire exchange MUST consist of a single START packet and a single REPLY. The START packet MUST contain the username in the user field and the data field will be a concatenation of the PPP id, the MS-CHAP challenge and the MS-CHAP response. The length of the challenge value can be determined from the length of the data field minus the length of the id (always 1 octet) and the length of the response field (always 49 octets). To perform the authentication, the server will use a combination of MD4 and DES on the user's secret and the challenge, as defined in RFC 2433 [RFC2433] and then compare the resulting value with the response. The REPLY from the server MUST be a PASS or FAIL. - For best practices, please refer to RFC 2433 [RFC2433] + For best practices, please refer to RFC 2433 [RFC2433] . The TACACS+ + server MUST rejects authentications where the challenge deviates from + 8 bytes as defined in the RFC. - Inbound MS-CHAP v2 login +4.4.2.5. MS-CHAP v2 login action = TAC_PLUS_AUTHEN_LOGIN authen_type = TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 minor_version = 0x1 The entire exchange MUST consist of a single START packet and a single REPLY. The START packet MUST contain the username in the user field and the data field will be a concatenation of the PPP id, the MS-CHAP challenge and the MS-CHAP response. The length of the challenge value can be determined from the length of the data field minus the length of the id (always 1 octet) and the length of the response field (always 49 octets). To perform the authentication, the server will use the algorithm specified RFC 2759 [RFC2759] on the user's secret and challenge and then compare the resulting value with the response. The REPLY from the server MUST be a PASS or FAIL. For best practices for MS-CHAP v2, please refer to RFC2759 [RFC2759] + . The TACACS+ server MUST rejects authentications where the challenge + deviates from 16 bytes as defined in the RFC. - Enable Requests +4.4.2.6. Enable Requests action = TAC_PLUS_AUTHEN_LOGIN priv_lvl = implementation dependent authen_type = not used service = TAC_PLUS_AUTHEN_SVC_ENABLE This is an ENABLE request, used to change the current running privilege level of a user. The exchange MAY consist of multiple messages while the server collects the information it requires in order to allow changing the principal's privilege level. This - exchange is very similar to an Inbound ASCII login. + exchange is very similar to an ASCII login (Section 4.4.2.1) . In order to readily distinguish enable requests from other types of request, the value of the authen_service field MUST be set to TAC_PLUS_AUTHEN_SVC_ENABLE when requesting an ENABLE. It MUST NOT be set to this value when requesting any other operation. - ASCII change password request +4.4.2.7. ASCII change password request action = TAC_PLUS_AUTHEN_CHPASS authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII This exchange consists of multiple messages while the server collects the information it requires in order to change the user's password. It is very similar to an ASCII login. The status value TAC_PLUS_AUTHEN_STATUS_GETPASS MUST only be used when requesting the "new" password. It MAY be sent multiple times. When requesting the "old" password, the status value MUST be set to @@ -941,56 +968,23 @@ message explaining the reason for the abort. This information will be handled by the server according to the requirements of the deployment. The session is terminated, for more details about session temrination, oplease refer to section (Section 3.4) In the case of PALL, FAIL or ERROR, the server can insert a message into server_msg to be displayed to the user. The Draft `The Draft' [TheDraft] defined a mechanism to direct authentication requests to an alternative server. This mechanism is - regarded as legacy and its implementation is optional. - - If this feature is not implemented, then the client should treat - TAC_PLUS_AUTHEN_STATUS_FOLLOW as TAC_PLUS_AUTHEN_STATUS_FAIL - - When the status equals TAC_PLUS_AUTHEN_STATUS_FOLLOW the packet - indicates that the TACACS+ server requests that authentication is - performed with an alternate server. The data field MUST contain - ASCII text describing one or more servers. A server description - appears like this: - - [@@]>[@] - - If more than one host is specified, they MUST be separated into rows - by an ASCII Carriage Return (0x0D). - - The protocol and key are optional, and apply only to host in the same - row. The protocol can describe an alternate way of performing the - authentication, other than TACACS+. If the protocol is not present, - then TACACS+ is assumed. - - Protocols are ASCII numbers corresponding to the methods listed in - the authen_method field of authorization packets (defined below). - The host is specified as either a fully qualified domain name, or an - ASCII numeric IPV4 address specified as octets separated by dots - ('.'), or IPV6 address text representation defined in RFC 4291. - - If a key is supplied, the client MAY use the key in order to - authenticate to that host. The client may use a preconfigured key - for the host if it has one. - - Use of the hosts in a TAC_PLUS_AUTHEN_STATUS_FOLLOW packet is at the - discretion of the TACACS+ client. It may choose to use any one, all - 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. + regarded as insecure, is deprecated, and not covered here. The + client should treat TAC_PLUS_AUTHEN_STATUS_FOLLOW as + TAC_PLUS_AUTHEN_STATUS_FAIL If the status equals TAC_PLUS_AUTHEN_STATUS_ERROR, then the host is indicating that it is experiencing an unrecoverable error and the authentication will proceed as if that host could not be contacted. The data field may contain a message to be printed on an administrative console or log. If the status equals TAC_PLUS_AUTHEN_STATUS_RESTART, then the authentication sequence is restarted with a new START packet from the client, with new session Id, and seq_no set to 1. This REPLY packet @@ -1012,21 +1006,21 @@ (we don't know who they are). In this case it is up to the server to determine, according to its configuration, if an unauthenticated user is allowed the services in question. Authorization does not merely provide yes or no answers, but it may also customize the service for the particular user. A common use of authorization is to provision a shell session when a user first logs in to a device to administer it. The TACACS+ server might respond to the request by allowing the service, but placing a time restriction on the login shell. For a list of common attributes used in - authorization, see the Authorization Attributes section (Section 7.1) + authorization, see the Authorization Attributes section (Section 7.2) . In the TACACS+ protocol an authorization is always a single pair of messages: a REQUEST from the client followed by a REPLY from the server. The authorization REQUEST message contains a fixed set of fields that indicate how the user was authenticated and a variable set of arguments that describe the services and options for which authorization is requested. @@ -1055,21 +1048,23 @@ | arg_2 ... +----------------+----------------+----------------+----------------+ | ... +----------------+----------------+----------------+----------------+ | arg_N ... +----------------+----------------+----------------+----------------+ authen_method This indicates the authentication method used by the client to - acquire the user information. + acquire the user information. As this information is not always + subject to verification, it is recommended that this field is + ignored. 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 @@ -1104,36 +1099,36 @@ authen_type This field corrsponds to the authen_type field in the authentication section (Section 4) above. It indicates the type of authentication that was performed. If this information is not available, then the client will set authen_type to: TAC_PLUS_AUTHEN_TYPE_NOT_SET := 0x00. This value is valid only in authorization and accounting requests. authen_service - This field matches the authen_service field in the authentication - section (Section 4) above. It indicates the service through which - the user authenticated. + This field is the same as the authen_service field in the + authentication section (Section 4) above. It indicates the service + through which the user authenticated. user, user_len This field contains the user's account name. The user_len MUST indicate the length of the user field, in bytes. port, port_len + This field matches the port field in the authentication section (Section 4) above. The port_len indicates the length of the port field, in bytes. rem_addr, rem_addr_len - This field matches the rem_addr field in the authentication section (Section 4) above. The rem_addr_len indicates the length of the port field, in bytes. arg_cnt The number of authorization arguments to follow arg_1 ... arg_N, arg_1_len .... arg_N_len @@ -1139,44 +1134,46 @@ The arguments are the primary elements of the authorization interaction. In the request packet they describe the specifics of the authorization that is being requested. Each argument is encoded in the packet as a single arg filed (arg_1... arg_N) with a corresponding length fields (which indicates the length of each argument in bytes). The authorization arguments in both the REQUEST and the REPLY are attribute-value pairs. The attribute and the value are in a single - US-ASCII string and are separated by either a "=" (0X3D) or a "*" - (0X2A). The equals sign indicates a mandatory argument. The + printable 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. + separators. This means that the arguments must be parsed until the + first separator is encountered, all characters in the argument, after + this separator, are interpreted as the argument value. Optional arguments are ones that may be disregarded by either client or server. Mandatory arguments require that the receiving side can handle the attribute, that is: its implementation and configuration includes the details of how to act on it. If the client receives a mandatory argument that it cannot handle, it MUST consider the authorization to have failed. It is legal to send an attribute-value pair with a zero length value. Attribute-value strings are not NULL terminated, rather their length value indicates their end. The maximum length of an attribute-value string is 255 characters. The minimum is two characters (one name value character and the separator) Though the attributes allow extensibility, a common core set of authorization attributes SHOULD be supported by clients and servers, - these are listed in the Authorization Attributes (Section 7.1) + these are listed in the Authorization Attributes (Section 7.2) section below. 5.2. The Authorization REPLY Packet Body 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 +----------------+----------------+----------------+----------------+ | status | arg_cnt | server_msg len | +----------------+----------------+----------------+----------------+ + data_len | arg_1_len | arg_2_len | +----------------+----------------+----------------+----------------+ @@ -1200,40 +1197,39 @@ TAC_PLUS_AUTHOR_STATUS_PASS_REPL := 0x02 TAC_PLUS_AUTHOR_STATUS_FAIL := 0x10 TAC_PLUS_AUTHOR_STATUS_ERROR := 0x11 TAC_PLUS_AUTHOR_STATUS_FOLLOW := 0x21 server_msg, server_msg_len - This is an US-ASCII string that may be presented to the user. The - server_msg_len indicates the length of the server_msg field, in - bytes. + This is a printable US-ASCII string that may be presented to the + user. The server_msg_len indicates the length of the server_msg + field, in bytes. data, data_len - This is an US-ASCII string that may be presented on an administrative - display, console or log. The decision to present this message is - client specific. The data_len indicates the length of the data - field, in bytes. + This is a printable US-ASCII string that may be presented on an + administrative display, console or log. The decision to present this + message is client specific. The data_len indicates the length of the + data field, in bytes. arg_cnt - The number of authorization arguments to follow. arg_1 ... arg_N, arg_1_len .... arg_N_len The arguments describe the specifics of the authorization that is being requested. For details of the content of the args, refer to: - Authorization Attributes (Section 7.1) section below. Each argument + Authorization Attributes (Section 7.2) section below. Each argument is encoded in the packet as a single arg field (arg_1... arg_N) with a corresponding length fields (which indicates the length of each argument in bytes). If the status equals TAC_PLUS_AUTHOR_STATUS_FAIL, then the requested authorization MUST be denied. If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_ADD, then the arguments specified in the request are authorized and the arguments in the response MUST be applied according to the rules described @@ -1340,307 +1336,314 @@ This is the return status. Values are: TAC_PLUS_ACCT_STATUS_SUCCESS := 0x01 TAC_PLUS_ACCT_STATUS_ERROR := 0x02 TAC_PLUS_ACCT_STATUS_FOLLOW := 0x21 server_msg, server_msg_len - This is a US-ASCII string that may be presented to the user. The - server_msg_len indicates the length of the server_msg field, in - bytes. + This is a printable US-ASCII string that may be presented to the + user. The server_msg_len indicates the length of the server_msg + field, in bytes. data, data_len - This is a US-ASCII string that may be presented on an administrative - display, console or log. The decision to present this message is - client specific. The data_len indicates the length of the data - field, in bytes. + This is a printable US-ASCII string that may be presented on an + administrative display, console or log. The decision to present this + message is client specific. The data_len indicates the length of the + data field, in bytes. When the status equals TAC_PLUS_ACCT_STATUS_FOLLOW, then the actions to be taken and the contents of the data field are identical to the TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication. TACACS+ accounting is intended to record various types of events on clients, for example: login sessions, command entry, and others as required by the client implementation. These events are collectively referred to in `The Draft' [TheDraft] as "tasks". The TAC_PLUS_ACCT_FLAG_START flag indicates that this is a start accounting message. Start messages will only be sent once when a task is started. The TAC_PLUS_ACCT_FLAG_STOP indicates that this is a stop record and that the task has terminated. The TAC_PLUS_ACCT_FLAG_WATCHDOG flag means that this is an update record. - Update records are sent at the client's discretion if the task has - not finished. Summary of Accounting Packets +----------+-------+-------+-------------+-------------------------+ | Watchdog | Stop | Start | Flags & 0xE | Meaning | +----------+-------+-------+-------------+-------------------------+ | 0 | 0 | 0 | 0 | INVALID | | 0 | 0 | 1 | 2 | Start Accounting Record | | 0 | 1 | 0 | 4 | Stop Accounting Record | | 0 | 1 | 1 | 6 | INVALID | | 1 | 0 | 0 | 8 | Watchdog, no update | | 1 | 0 | 1 | A | Watchdog, with update | | 1 | 1 | 0 | C | INVALID | | 1 | 1 | 1 | E | INVALID | +----------+-------+-------+-------------+-------------------------+ - The START and STOP flags are mutually exclusive. When the WATCHDOG - flag is set along with the START flag, it indicates that the update - record is a duplicate of the original START record. If the START - flag is not set, then this indicates only that task is still running. - The STOP flag MUST NOT be set in conjunction with the WATCHDOG flag. + The START and STOP flags are mutually exclusive. + + The WATCHDOG flag is used by the client to communicate ongoing status + of a long running task. Update records are sent at the client's + discretion. The frequency of the update depends upon the intended + application: A watchdog to provide progress indication will require + higher frequency than a daily keep-alive. When the WATCHDOG flag is + set along with the START flag, it indicates that the update record + provides additional or updated arguments from the original START + record. If the START flag is not set, then this indicates only that + task is still running, and no new information is provided (servers + MUST ignore any arguments). The STOP flag MUST NOT be set in + conjunction with the WATCHDOG flag. The Server MUST respond with TAC_PLUS_ACCT_STATUS_ERROR if the client requests an INVALID option. 7. Attribute-Value Pairs TACACS+ is intended to be an extensible protocol. The attributes - used in Authorization and Accounting are not limited by thsi + used in Authorization and Accounting are not limited by this document. Some attributes are defined below for common use cases, clients MUST use these attributes when supporting the corresponding use cases. +7.1. Value Encoding + + All attribute values are encoded as printable US-ASCII strings. The + following type representations SHOULD be followed + + Numeric + All numeric values in an attribute-value string are provided as - decimal US-ASCII numbers, unless otherwise stated. + decimal printable US-ASCII numbers, unless otherwise stated. - All boolean attributes are encoded with values "true" or "false". + Boolean + + All boolean attributes are encoded as printable US-ASCII with values + "true" or "false". + + IP-Address It is recommended that hosts be specified as a IP address so as to - avoid any ambiguities. ASCII numeric IPV4 address are specified as - octets separated by dots ('.'), IPV6 address text representation + avoid any ambiguities. IPV4 address are specified as US-ASCII octet + numerics separated by dots ('.'), IPV6 address text representation defined in RFC 4291. - Absolute times are specified in seconds since the epoch, 12:00am Jan - 1 1970. The timezone MUST be UTC unless a timezone attribute is - specified. + Date Time + + Absolute date/times are specified in seconds since the epoch, 12:00am + Jan 1 1970. The timezone MUST be UTC unless a timezone attribute is + specified. Stardate is canonically inconsistent and so SHOULD NOT be + used. + + String + + Many values have no specific type representation and so are + interpreted as plain strings. + + Empty Values Attributes may be submitted with no value, in which case they consist of the name and the mandatory or optional separator. For example, the attribute "cmd" which has no value is transmitted as a string of four characters "cmd=" -7.1. Authorization Attributes +7.2. Authorization Attributes - service + service (String) The primary service. Specifying a service attribute indicates that this is a request for authorization or accounting of that service. + For example: "shell", "tty-server", "connection", "system" and "firewall". This attribute MUST always be included. - protocol + protocol (String) - the ptotocol field may be used to indicate a subset of a setvice. + the protocol field may be used to indicate a subset of a service. - cmd + cmd (String) a shell (exec) command. This indicates the command name of the command that is to be run. The "cmd" attribute MUST be specified if service equals "shell". Authorization of shell commands is a common use-case for the TACACS+ protocol. Command Authorization generally takes one of two forms: session-based and command-based. For session-based shell authorization, the "cmd" argument will have an empty value. The client determines which commands are allowed in a session according to the arguments present in the authorization. In command-based authorization, the client requests that the server determine whether a command is allowed by making an authorization request for each command. The "cmd" argument will have the command name as its value. - cmd-arg + cmd-arg (String) an argument to a shell (exec) command. This indicates an argument for the shell command that is to be run. Multiple cmd-arg attributes may be specified, and they are order dependent. - acl + acl (Numeric) - US-ASCII number representing a connection access list. Applicable - only to session-based shell authorization. + printable US-ASCII number representing a connection access list. + Applicable only to session-based shell authorization. - inacl + inacl (String) - US-ASCII identifier for an interface input access list. + printable US-ASCII identifier for an interface input access list. - outacl + outacl (String) - US-ASCII identifier for an interface output access list. + printable US-ASCII identifier for an interface output access list. - addr + addr (IP-Address) a network address - - addr-pool + addr-pool (String) The identifier of an address pool from which the client can assign an address. - routing + routing (Boolean) - Boolean. Specifies whether routing information is to be propagated - to, and accepted from this interface. + Specifies whether routing information is to be propagated to, and + accepted from this interface. - route + route (String) Indicates a route that is to be applied to this interface. Values MUST be of the form " []". If a is not specified, the resulting route is via the requesting peer. - timeout + timeout (Numeric) an absolute timer for the connection (in minutes). A value of zero indicates no timeout. - idletime + idletime (Numeric) + an idle-timeout for the connection (in minutes). A value of zero indicates no timeout. - autocmd + autocmd (String) an auto-command to run. Applicable only to session-based shell authorization. - noescape + noescape (Boolean) - Boolean. Prevents user from using an escape character. Applicable - only to session-based shell authorization. + Prevents user from using an escape character. Applicable only to + session-based shell authorization. - nohangup + nohangup (Boolean) Boolean. Do not disconnect after an automatic command. Applicable only to session-based shell authorization.y. - priv-lvl + priv-lvl (Numeric) privilege level to be assigned. Please refer to the Privilege Level section (Section 8) below. - remote_user - + remote_user (String) 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 (String) remote host (authen_method must have the value TAC_PLUS_AUTHEN_METH_RCMD) - callback-dialstring - - Indicates that callback is to be done. Value is a dialstring, or - empty. Empty value indicates that the service MAY choose to get the - dialstring through other means. - - callback-line - - The line number to use for a callback. - - callback-rotary - The rotary number to use for a callback. - - nocallback-verify - - Do not require authentication after callback. - -7.2. Accounting Attributes +7.3. Accounting Attributes The following attributes are defined for TACACS+ accounting only. They MUST precede any attribute-value pairs that are defined in the authorization section (Section 5) above. - task_id + task_id (String) Start and stop records for the same event MUST have matching task_id attribute values. The client MUST ensure that active task_ids are not duplicated: a client MUST NOT reuse a task_id a start record until it has sent a stop record for that task_id. Servers MUST not make assumptions about the format of a task_id. - start_time + start_time (Date Time) The time the action started (in seconds since the epoch.). - stop_time + stop_time (Date Time) The time the action stopped (in seconds since the epoch.) - elapsed_time + elapsed_time (Numeric) The elapsed time in seconds for the action. - timezone + timezone (String) The timezone abbreviation for all timestamps included in this packet. - event + event (String) Used only when "service=system". Current values are "net_acct", "cmd_acct", "conn_acct", "shell_acct" "sys_acct" and "clock_change". These indicate system level changes. The flags field SHOULD indicate whether the service started or stopped. - reason + reason (String) Accompanies an event attribute. It describes why the event occurred. - bytes + bytes (Numeric) + The number of bytes transferred by this action - bytes_in + bytes_in (Numeric) - The number of input bytes transferred by this action to the port + The number of bytes transferred by this action from the endstation to + the client port - bytes_out + bytes_out (Numeric) - The number of output bytes transferred by this action from the port + The number of bytes transferred by this action from the client to the + endstation port - paks + paks (Numeric) The number of packets transferred by this action. - paks_in + paks_in (Numeric) - The number of input packets transferred by this action to the port. + The number of input packets transferred by this action from the + endstation to the client port. - paks_out + paks_out (Numeric) The number of output packets transferred by this action from the - port. - - status - - The numeric status value associated with the action. This is a - signed four (4) byte word in network byte order. 0 is defined as - success. Negative numbers indicate errors. Positive numbers - indicate non-error failures. The exact status values may be defined - by the client. + client port to the endstation. - err_msg + err_msg (String) - An US-ASCII string describing the status of the action. + A printable US-ASCII string describing the status of the action. 8. Privilege Levels The TACACS+ Protocol supports flexible authorization schemes through the extensible attributes. One scheme is built in to the protocol and has been extensively used for Session-based shell authorization: Privilege Levels. Privilege Levels are ordered values from 0 to 15 with each level being a superset of the next lower value. Configuration and implementation @@ -1654,50 +1657,51 @@ TAC_PLUS_PRIV_LVL_USER := 0x01 TAC_PLUS_PRIV_LVL_MIN := 0x00 A Privilege level can be assigned to a shell (EXEC) session when it starts starts (for example, TAC_PLUS_PRIV_LVL_USER). The client will permit the actions associated with this level to be executed. This privilege level is returned by the Server in a session-based shell authorization (when "service" equals "shell" and "cmd" is empty). - When a user required to perfrom actions that are mapped to a higher + When a user required to perform actions that are mapped to a higher privilege level, then an ENABLE type reuthentication can be initiated by the client, in a way similar to su in unix. The client will insert the required privilege level into the authentication header for enable authentication request. The use of Privilege levels to determine session-based access to commands and resources is not mandatory for clients. Although the privilege level scheme is widely supported, its lack of flexibility in requiring a single monotonic hierarchy of permissions means that other session-based command authorization schemes have evolved, and so it is no longer mandatory for clients to use it. However, it is still common enough that it SHOULD be supported by servers. 9. TACACS+ Security Considerations - Although in widespread use, the TACACS+ protocol (as defined in "the - Draft") does not meet modern security standards on its own. For this - reason, the authors intend to follow up this document with a more - secure version of the protocol. + The original TACACS+ Draft[1] from 1998 did not address all of the + key security concerns which are considered when designing modern + standards. This section addresses known limitations and concerns + which will impact overall security of the protocol and systems where + this protocol is deployed to manage central authentication, + authorization or accounting for network device administration. - TACACS+ was originally specified in "The Draft" (1998) is incomplete, - and leaves key points unspecified. As a result, software authors - have had to make implementation choices about what should, or should - not, be done in certain situations. These implementation choices are - somewhat constrained by ad hoc interoperability tests. That is, all - TACACS+ clients and servers interoperate, so there is a rough - consensus on how the protocol works. + Multiple implementations of the protocol described in the draft[1] + have been deployed. As the protocol was never standardised, current + implementations may be incompatible in non-obvious ways, giving rise + to additional security risks about which authors might not be aware + of. This section does not claim to enumerate all possible security + vulnerabilities. -9.1. Overall Security of The Protocol +9.1. General Security of The Protocol TACACS+ protocol does not include a security mechanism that would meet modern day requirements. Support for MD5-based crypto pad encryption fails to provide any kind of transport integrity, which presents at least the following risks: Accounting information may be modified by the man-in-the-middle attacker, making such logs unsuitable and untrustable for auditing purposes. @@ -1715,119 +1719,153 @@ Brute force attacks exploiting increased efficiency of MD5 digest computation. Known plaintext attacks which may decrease the cost of brute force attack. Chosen plaintext attacks which may decrease the cost of a brute force attack. - No forward secrecy means that original data may be revealed at the - later time and still provide valuable information to the attacker. + No forward secrecy. Even though, to the best knowledge of authors, this method of encryption wasn't rigorously tested, authors feel that enough information is available that it is best referred to as "obfuscation" - and not "encryption" and as such it MUST NOT BE relied upon to - provide privacy. + and not "encryption". For example, a "session_id" can be replaced by an alternate one, which could allow an unprivileged administrator to "steal" the authorization from a session for a privileged administrator. An attacker could also update the "flags" field to indicate that one or the other end of a connection requires TAC_PLUS_UNENCRYPTED_FLAG, which would subvert the obfuscation mechanism. For this reasons, users deploying TACACS+ protocol in their environments MUST limit access to known clients and MUST control the security of the entire transmission path. Attacks who can guess the - key or otherwise break the obfuscation WILL gain unrestricted and + key or otherwise break the obfuscation will gain unrestricted and undetected access to all TACACS+ traffic. The security risk of such attack succeeding against a centralised AAA system like TACACS+ cannot be overstated. + The following parts of this section enumerate only the session- + specific risks which are in addition to general risk associated with + bare obfuscation and lack of integrity checking. + 9.2. Security of Authentication Sessions - The authentication options include options which MUST NOT be used - outside a secured deployment. Specifically, options which permit the - exchange of clear-text passwords or MSCHAPv1 and MS-CHAPv2. As of - the publication of this document, there has been no similar attacks - on the CHAP protocol. + Authentication sessions SHOULD NOT be used via unsecure transport as + the man-in-the-middle attack may completely subvert them. Even CHAP, + which may me considered resistant to password interception, is unsafe + as it does not protect the username from a trivial man-in-the-middle + attack. Section 4.4.3 permits the redirection of a session to another server via the TAC_PLUS_AUTHEN_STATUS_FOLLOW mechanism. As part of this process, the secret key for a new server can be sent to the client. This public exchange of secret keys means that once one session is - broken, it may be possible to leverage that attack to attacking + broken, it may be possible to leverage that key to attacking connections to other servers. This option MUST NOT be used outside a - secured deployment. + secured deployment of protocol clients or outside of secure + transport. 9.3. Security of Authorization Sessions - TACACS+ authorization is specifically separate from authentication. - Careful consideration must be given to whether this mode is - appropriate for the target deployment. Authorization sessions are - not cryptographically linked to any authentication sessions. - Instead, sessions are tied together implicitly by the contents of the - other fields, such as "use", "port", "rem_addr", etc. + Authorization sessions MUST be used via secure transport only as it's + trivial to execute a successful man-in-the-middle attacks that + changes well-known plaintext in either requests or responses. - The specification allows for the exchange of attribute-value pairs. - While a few such attributes are defined here, the protocol is - extensible, and vendors can define their own attributes. There is no - registry for such attributes, and in the absence of a published - specification, no way for a client or server to know the meaning of a - new attribute. + As an example, take the field "authen_method". It's not unusual in + actual deployments to authorize all commands received via the device + local serial port (a console port) as that one is usually considered + secure by virtue of the device located in a physically secure + location. If an administrator would configure the authorization + system to allow all commands entered by the user on a local console + to aid in troubleshooting, that would give all access to all commands + to any attacker that would be able to change the "authen_method" from + TAC_PLUS_AUTHEN_METH_TACACSPLUS to TAC_PLUS_AUTHEN_METH_LINE. In + this regard, the obfuscation provided by the protocol itself wouldn't + help much, because: - As a result, implemetors MUST ensure that new attribute-value pairs - are used consistently to communicate between client and server - implementations. + Lack of integrity means that any byte in the payload may be + changed w/o either side detecting the change. + + Known plaintext means that an attacker would know with certainty + which octet is the target of the attack (in this case, 1st octet + after the header). + + In combination with known plaintext, the attacker can determine + with certainty the value of the crypto-pad octet used to obfuscate + the original octet. 9.4. Security of Accounting Sessions - The security considerations for accounting sessions are largely the - same as for authorization sessions. This section describes - additional issues specific to accounting sessions. + Accounting sessions are not directly involved in authentication or + authorizing operations on the device. However, man-in-the-middle + attacker may do any of the following: - There is no way in TACACS+ to signal that accounting is required. - There is no way for a server to signal a client how often accounting - is required. The accounting packets are received solely at the - clients discretion. Adding such functionality would assist with - auditing of user actions. + Replace accounting data with new valid or garbage which prevents + to provide distraction or hide information related to their + authentication and/or authorization attack attempts. - The "task_id" field is defined only for accounting packets, and not - for authentication or authorization packets. As such, it is - difficult to correlate accounting data with a previous authentication - or authorization request. + Try and poison accounting log with entries designed to make + systems behave in unintended ways (which includes TACACS+ server + and any other systems that would manage accounting entries). + + In addition to these direct manipulations, different client + implementations pass different fidelity of accounting data. Some + vendors have been observed in the wild that pass sensitive data like + passwords, encryption keys and similar as part of the accounting log. + Due to lack of strong encryption with perfect forward secrecy, this + data may be revealed in future, leading to a security incident. 9.5. TACACS+ Deployment Recommendations - Due to the above concerns with the protocol, it is critical that it - be deployed in a secure manner. The following recommendations are + Due to above observations about the TACACS+ protocol, it is critical + to only deploy it using secure transport. A secure transport for + TACACS+ is defined as any means that ensure privacy and integrity of + all data passed between clients and servers. There are multiple + means of achieving this, all of them beyond the scope of this + document. + + Symmetric encryption key represents a possible attack vector at the + protocol itself. For this reason, servers SHOULD accept only those + network connection attempts that arrive from known clients. This + limits the exposure and prevents remote brute force attacks from + unknown clients. + + Due to the security deficiencies of the protocol, it is critical that + it be deployed in a secure manner. The following recommendations are made for those deploying and configuring TACACS+ as a solution for - Device Administration: + device administration: - Secure the Deployment: TACACS+ does not provide modern security so - TACACS+ MUST BE employed over networks which ensure privacy and - integrity of the communication. The way this is ensured will - depend upon the organisational means: a dedicated and secure - management network where available in enterprise deployments, or - IPsec where dedicated networks are not available. + Secure the Deployment: TACACS+ MUST BE deployed over networks + which ensure an appropriate privacy and integrity of the + communication. Definition of an appropriate level of privacy and + integrity is organisation-dependent What is apropriate level of + The way this is ensured will depend upon the organisational means: + a dedicated and secure management network where available in + enterprise deployments, or IPsec where dedicated networks are not + available. Always set a secret key (recommended minimum 14 characters) on the client and server when configuring the connection between them. Servers MUST be configured with a list of known clients. Servers MUST be configured to reject requests from clients not on the list. A unique secret key SHOULD be configured for every individual client. + Implementors should consider shared secrets to be sensitive data, + and managed securely. + Restrict to TAC_PLUS_AUTHEN_TYPE_CHAP for authen_type where possible. Use other options only when unavoidable due to requirements of identity/password systems. Servers SHOULD be restricted to requiring TACACS+ authentication for authorization requests (i.e. TAC_PLUS_AUTHEN_METH_TACACSPLUS is used). Avoid the use of the redirection mechanism. TAC_PLUS_AUTHEN_STATUS_FOLLOW, specifically avoid the option to @@ -1897,21 +1935,32 @@ Potential confusion between clients and servers from different vendors of the meaning of specific argument attributes. Potential confusion between clients and servers from different vendors of the meaning of specific commands. In summary: It is strongly advised that TACACS+ MUST be used within a secure deployment. Failure to do so may impact overall network security. -10. References +10. Acknowledgements + + The Authors would like to thank the following reviewers whose + comments and contributions made considerable improvements to the + document: Alan DeKok (who provided significant insights and + recommendations on all aspects of documenting the protocol), + Alexander Clouter, Chris Janicki, Tom Petch, Robert Drake + + The Authors would also like to thanks the support from the OPSAWG + Chairs and advisors. + +11. References [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. [RFC1334] Lloyd, B. and W. Simpson, "PPP Authentication Protocols", RFC 1334, DOI 10.17487/RFC1334, October 1992, . [RFC1750] Eastlake 3rd, D., Crocker, S., and J. Schiller, "Randomness Recommendations for Security", RFC 1750, @@ -1921,22 +1970,22 @@ [RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions", RFC 2433, DOI 10.17487/RFC2433, October 1998, . [RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", RFC 2759, DOI 10.17487/RFC2759, January 2000, . [TheDraft] Carrel, D. and L. Grant, "The TACACS+ Protocol Version - 1.78", June 1997, . + 1.78", June 1997, + . Authors' Addresses Thorsten Dahm Google Inc 1600 Amphitheatre Parkway Mountain View, CA 94043 US EMail: thorstendlux@google.com @@ -1959,10 +2008,12 @@ David Carrel vIPtela, Inc. 1732 North First St. San Jose, CA 95112 US EMail: dcarrel@viptela.com Lol Grant + + EMail: lol.grant@gmail.com