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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Operations T. Dahm 3 Internet-Draft A. Ota 4 Intended status: Informational Google Inc 5 Expires: March 25, 2020 D. Medway Gash 6 Cisco Systems, Inc. 7 D. Carrel 8 vIPtela, Inc. 9 L. Grant 10 September 22, 2019 12 The TACACS+ Protocol 13 draft-ietf-opsawg-tacacs-15 15 Abstract 17 Terminal Access Controller Access-Control System Plus (TACACS+) 18 provides Device Administration for routers, network access servers 19 and other networked computing devices via one or more centralized 20 servers. This document describes the protocol that is used by 21 TACACS+. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on March 25, 2020. 40 Copyright Notice 42 Copyright (c) 2019 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (https://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 This document may contain material from IETF Documents or IETF 56 Contributions published or made publicly available before November 57 10, 2008. The person(s) controlling the copyright in some of this 58 material may not have granted the IETF Trust the right to allow 59 modifications of such material outside the IETF Standards Process. 60 Without obtaining an adequate license from the person(s) controlling 61 the copyright in such materials, this document may not be modified 62 outside the IETF Standards Process, and derivative works of it may 63 not be created outside the IETF Standards Process, except to format 64 it for publication as an RFC or to translate it into languages other 65 than English. 67 Table of Contents 69 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 70 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 71 3. Technical Definitions . . . . . . . . . . . . . . . . . . . . 4 72 3.1. Client . . . . . . . . . . . . . . . . . . . . . . . . . 4 73 3.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 4 74 3.3. Packet . . . . . . . . . . . . . . . . . . . . . . . . . 5 75 3.4. Connection . . . . . . . . . . . . . . . . . . . . . . . 5 76 3.5. Session . . . . . . . . . . . . . . . . . . . . . . . . . 5 77 3.6. Treatment of Enumerated Protocol Values . . . . . . . . . 5 78 3.7. Treatment of Text Strings . . . . . . . . . . . . . . . . 5 79 4. TACACS+ Packets and Sessions . . . . . . . . . . . . . . . . 6 80 4.1. The TACACS+ Packet Header . . . . . . . . . . . . . . . . 6 81 4.2. The TACACS+ Packet Body . . . . . . . . . . . . . . . . . 8 82 4.3. Single Connection Mode . . . . . . . . . . . . . . . . . 9 83 4.4. Session Completion . . . . . . . . . . . . . . . . . . . 9 84 4.5. Data Obfuscation . . . . . . . . . . . . . . . . . . . . 11 85 5. Authentication . . . . . . . . . . . . . . . . . . . . . . . 12 86 5.1. The Authentication START Packet Body . . . . . . . . . . 13 87 5.2. The Authentication REPLY Packet Body . . . . . . . . . . 15 88 5.3. The Authentication CONTINUE Packet Body . . . . . . . . . 16 89 5.4. Description of Authentication Process . . . . . . . . . . 17 90 5.4.1. Version Behavior . . . . . . . . . . . . . . . . . . 18 91 5.4.2. Common Authentication Flows . . . . . . . . . . . . . 19 92 5.4.3. Aborting an Authentication Session . . . . . . . . . 22 93 6. Authorization . . . . . . . . . . . . . . . . . . . . . . . . 23 94 6.1. The Authorization REQUEST Packet Body . . . . . . . . . . 23 95 6.2. The Authorization REPLY Packet Body . . . . . . . . . . . 27 96 7. Accounting . . . . . . . . . . . . . . . . . . . . . . . . . 28 97 7.1. The Account REQUEST Packet Body . . . . . . . . . . . . . 29 98 7.2. The Accounting REPLY Packet Body . . . . . . . . . . . . 30 99 8. Attribute-Value Pairs . . . . . . . . . . . . . . . . . . . . 31 100 8.1. Value Encoding . . . . . . . . . . . . . . . . . . . . . 32 101 8.2. Authorization Attributes . . . . . . . . . . . . . . . . 32 102 8.3. Accounting Attributes . . . . . . . . . . . . . . . . . . 34 103 9. Privilege Levels . . . . . . . . . . . . . . . . . . . . . . 36 104 10. Security Considerations . . . . . . . . . . . . . . . . . . . 37 105 10.1. General Security of the Protocol . . . . . . . . . . . . 37 106 10.2. Security of Authentication Sessions . . . . . . . . . . 38 107 10.3. Security of Authorization Sessions . . . . . . . . . . . 39 108 10.4. Security of Accounting Sessions . . . . . . . . . . . . 39 109 10.5. TACACS+ Best Practices . . . . . . . . . . . . . . . . . 40 110 10.5.1. Shared Secrets . . . . . . . . . . . . . . . . . . . 40 111 10.5.2. Connections and Obfuscation . . . . . . . . . . . . 41 112 10.5.3. Authentication . . . . . . . . . . . . . . . . . . . 42 113 10.5.4. Authorization . . . . . . . . . . . . . . . . . . . 42 114 10.5.5. Redirection Mechanism . . . . . . . . . . . . . . . 42 115 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 43 116 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 43 117 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 43 118 13.1. Normative References . . . . . . . . . . . . . . . . . . 43 119 13.2. Informative References . . . . . . . . . . . . . . . . . 44 120 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44 122 1. Introduction 124 Terminal Access Controller Access-Control System Plus (TACACS+) was 125 conceived initially as a general Authentication, Authorization and 126 Accounting protocol. It's use today is mainly confined to Device 127 Administration: authenticating access to network devices, providing 128 central authorization of operations, and audit of those operations. 130 A wide range of TACACS+ clients and servers are already deployed in 131 the field. The TACACS+ protocol they are based on is defined in a 132 draft document that was originally intended for IETF publication, and 133 is known as `The Draft' [TheDraft] . This did not address all of the 134 key security concerns which are considered when designing modern 135 standards. For more details please refer to security section 136 (Section 10) . 138 This is intended to document the TACACS+ protocol as it is currently 139 deployed. It is intended that all implementations which conform to 140 this document will conform to `The Draft'. However, attention is 141 drawn to the following specific adjustments of the protocol 142 specification from 'The Draft': 144 This document officially removes SENDPASS for security reasons. 146 The normative description of Legacy features such as ARAP and 147 outbound authentication has been removed. 149 The Support for forwarding to an alternative daemon 150 (TAC_PLUS_AUTHEN_STATUS_FOLLOW) has been deprecated. 152 The TACACS+ protocol separates the functions of Authentication, 153 Authorization and Accounting. It allows for arbitrary length and 154 content authentication exchanges, to support future authentication 155 mechanisms. It is extensible to provide for site customization and 156 future development features, and it uses TCP to ensure reliable 157 delivery. The protocol allows the TACACS+ client to request very 158 fine-grained access control and allows the server to respond to each 159 component of that request. 161 The separation of authentication, authorization and accounting was a 162 key element of the design of TACACS+ protocol. Essentially it makes 163 TACACS+ a suite of three protocols. This document will address each 164 one in separate sections. Although TACACS+ defines all three, an 165 implementation or configuration is not required to employ all three. 166 Separating the elements is useful for Device Administration use case, 167 specifically, for authorization of individual commands in a session. 168 Note that there is no provision made at the protocol level for 169 association of an authentication to each authorization request. 171 2. Conventions 173 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 174 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 175 "OPTIONAL" in this document are to be interpreted as described in BCP 176 14 [RFC2119] [RFC8174] when, and only when, they appear in all 177 capitals, as shown here. 179 3. Technical Definitions 181 This section provides a few basic definitions that are applicable to 182 this document 184 3.1. Client 186 The client is any device which initiates TACACS+ protocol requests to 187 mediate access, mainly for the Device Administration use case. 189 3.2. Server 191 The server receives TACACS+ protocol requests, and replies according 192 to its business model, in accordance with the flows defined in this 193 document. 195 3.3. Packet 197 All uses of the word packet in this document refer to TACACS+ 198 protocol data units unless explicitly noted otherwise. The informal 199 term "Packet" has become an established part of the definition. 201 3.4. Connection 203 TACACS+ uses TCP for its transport. TCP Server port 49 is allocated 204 by IANA for TACACS+ traffic. 206 3.5. Session 208 The concept of a session is used throughout this document. A TACACS+ 209 session is a single authentication sequence, a single authorization 210 exchange, or a single accounting exchange. 212 An accounting and authorization session will consist of a single pair 213 of packets (the request and its reply). An authentication session 214 may involve an arbitrary number of packets being exchanged. The 215 session is an operational concept that is maintained between the 216 TACACS+ client and server. It does not necessarily correspond to a 217 given user or user action. 219 3.6. Treatment of Enumerated Protocol Values 221 This document describes various enumerated values in the packet 222 header and the headers for specific packet types. For example, in 223 the Authentication start packet type, this document defines the 224 action field with three values TAC_PLUS_AUTHEN_LOGIN, 225 TAC_PLUS_AUTHEN_CHPASS and TAC_PLUS_AUTHEN_SENDAUTH. 227 If the server does not implement one of the defined options in a 228 packet that it receives, or it encounters an option that is not 229 listed in this document for a header field, then it should respond 230 with an ERROR and terminate the session. This will allow the client 231 to try a different option. 233 If an error occurs but the type of the incoming packet cannot be 234 determined, a packet with the identical cleartext header but with a 235 sequence number incremented by one and the length set to zero MUST be 236 returned to indicate an error. 238 3.7. Treatment of Text Strings 240 The TACACS+ protocol makes extensive use of text strings. The 241 original draft intended that these strings would be treated as byte 242 arrays where each byte would represent a US-ASCII character. 244 More recently, server implementations have been extended to interwork 245 with external identity services, and so a more nuanced approach is 246 needed. Text Strings in the TACACS+ protocol MUST be handled in the 247 following way: 249 Usernames 251 Usernames MUST be encoded and handled using the UsernameCasePreserved 252 Profile specified in RFC 8265 [RFC8265]. The security considerations 253 in Section 8 of that RFC apply. 255 Passwords 257 Passwords MUST be handled and processed using the OpaqueString 258 Profile specified in RFC 8265 [RFC8265]. The security considerations 259 in Section 8 of that RFC apply. 261 Binary Data Strings 263 Where specifically mentioned, data fields contain arrays of arbitrary 264 bytes as required for protocol processing. These are not intended to 265 be made visible through user interface to users. 267 Printable Text String 269 All other text fields in TACACS+ MUST be treated as printable byte 270 arrays of US-ASCII as defined by RFC 20 [RFC0020]. The term 271 "printable" used here means the fields MUST exclude the "Control 272 Characters" defined in section 5.2 of RFC 20 [RFC0020]. 274 4. TACACS+ Packets and Sessions 276 4.1. The TACACS+ Packet Header 278 All TACACS+ packets begin with the following 12-byte header. The 279 header describes the remainder of the packet: 281 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 8 282 +----------------+----------------+----------------+----------------+ 283 |major | minor | | | | 284 |version| version| type | seq_no | flags | 285 +----------------+----------------+----------------+----------------+ 286 | | 287 | session_id | 288 +----------------+----------------+----------------+----------------+ 289 | | 290 | length | 291 +----------------+----------------+----------------+----------------+ 292 The following general rules apply to all TACACS+ packet types: 294 - To signal that any variable length data fields are unused, their 295 length value is set to zero. Such fields MUST be ignored, and 296 treated as if not present. 298 - the lengths of data and message fields in a packet are specified 299 by their corresponding length fields, (and are not null 300 terminated.) 302 - All length values are unsigned and in network byte order. 304 major_version 306 This is the major TACACS+ version number. 308 TAC_PLUS_MAJOR_VER := 0xc 310 minor_version 312 The minor TACACS+ version number. 314 TAC_PLUS_MINOR_VER_DEFAULT := 0x0 316 TAC_PLUS_MINOR_VER_ONE := 0x1 318 type 320 This is the packet type. Options are: 322 TAC_PLUS_AUTHEN := 0x01 (Authentication) 324 TAC_PLUS_AUTHOR := 0x02 (Authorization) 326 TAC_PLUS_ACCT := 0x03 (Accounting) 328 seq_no 330 This is the sequence number of the current packet. The first packet 331 in a session MUST have the sequence number 1 and each subsequent 332 packet will increment the sequence number by one. Clients only send 333 packets containing odd sequence numbers, and TACACS+ servers only 334 send packets containing even sequence numbers. 336 The sequence number must never wrap i.e. if the sequence number 2^8-1 337 is ever reached, that session must terminate and be restarted with a 338 sequence number of 1. 340 flags 342 This field contains various bitmapped flags. 344 The flag bit: 346 TAC_PLUS_UNENCRYPTED_FLAG := 0x01 348 This flag indicates that the sender did not obfuscate the body of the 349 packet. The application of this flag will be covered in the security 350 section (Section 10) . 352 This flag SHOULD be clear in all deployments. Modern network traffic 353 tools support encrypted traffic when configured with the shared 354 secret (see section below), so obfuscated mode can and SHOULD be used 355 even during test. 357 The single-connection flag: 359 TAC_PLUS_SINGLE_CONNECT_FLAG := 0x04 361 This flag is used to allow a client and server to negotiate Single 362 Connection Mode. 364 All other bits MUST be ignored when reading, and SHOULD be set to 365 zero when writing. 367 session_id 369 The Id for this TACACS+ session. This field does not change for the 370 duration of the TACACS+ session. This number MUST be generated by a 371 cryptographically strong random number generation method. Failure to 372 do so will compromise security of the session. For more details 373 refer to RFC 4086 [RFC4086] 375 length 377 The total length of the packet body (not including the header). 379 4.2. The TACACS+ Packet Body 381 The TACACS+ body types are defined in the packet header. The next 382 sections of this document will address the contents of the different 383 TACACS+ bodies. 385 4.3. Single Connection Mode 387 Single Connection Mode is intended to improve performance by allowing 388 a client to multiplex multiple session on a single TCP connection. 390 The packet header contains the TAC_PLUS_SINGLE_CONNECT_FLAG used by 391 the client and server to negotiate the use of Single Connect Mode. 393 The client sets this flag, to indicate that it supports multiplexing 394 TACACS+ sessions over a single TCP connection. The client MUST NOT 395 send a second packet on a connection until single-connect status has 396 been established. 398 To indicate it will support Single Connection Mode, the server sets 399 this flag in the first reply packet in response to the first request 400 from a client. The server may set this flag even if the client does 401 not set it, but the client may ignore the flag and close the 402 connection after the session completes. 404 The flag is only relevant for the first two packets on a connection, 405 to allow the client and server to establish Single Connection Mode. 406 No provision is made for changing Single Connection Mode after the 407 first two packets: the client and server MUST ignore the flag after 408 the second packet on a connection. 410 If single Connection Mode has not been established in the first two 411 packets of a TCP connection, then both the client and the server 412 close the connection at the end of the first session. 414 The client negotiates Single Connection Mode to improve efficiency. 415 The server may refuse to allow Single Connection Mode for the client. 416 For example, it may not be appropriate to allocate a long-lasting TCP 417 connection to a specific client in some deployments. Even if the 418 server is configured to permit single Connection Mode for a specific 419 client, the server may close the connection. For example: a server 420 may be configured to time out a Single Connection Mode TCP Connection 421 after a specific period of inactivity to preserve its resources. The 422 client MUST accommodate such closures on a TCP session even after 423 Single Connection Mode has been established. 425 4.4. Session Completion 427 The REPLY packets defined for the packets types in the sections below 428 (Authentication, Authorization and Accounting) contain a status 429 field. The complete set of options for this field depend upon the 430 packet type, but all three REPLY packet types define values 431 representing PASS, ERROR and FAIL, which indicate the last packet of 432 a regular session (one which is not aborted). 434 The server responds with a PASS or a FAIL to indicate that the 435 processing of the request completed and the client can apply the 436 result (PASS or FAIL) to control the execution of the action which 437 prompted the request to be sent to the server. 439 The server responds with an ERROR to indicate that the processing of 440 the request did not complete. The client can not apply the result 441 and it MUST behave as if the server could not be connected to. For 442 example, the client tries alternative methods, if they are available, 443 such as sending the request to a backup server, or using local 444 configuration to determine whether the action which prompted the 445 request should be executed. 447 Refer to the section (Section 5.4.3) on Aborting Authentication 448 Sessions for details on handling additional status options. 450 When the session is complete, then the TCP connection should be 451 handled as follows, according to whether Single Connection Mode was 452 negotiated: 454 If Single Connection Mode was not negotiated, then the connection 455 should be closed 457 If Single Connection Mode was enabled, then the connection SHOULD be 458 left open (see section (Section 4.3) ), but may still be closed after 459 a timeout period to preserve deployment resources. 461 If Single Connection Mode was enabled, but an ERROR occurred due to 462 connection issues (such as an incorrect secret, see section 463 (Section 4.5) ), then any further new sessions MUST NOT be accepted 464 on the connection. If there are any sessions that have already been 465 established then they MAY be completed. Once all active sessions are 466 completed then the connection MUST be closed. 468 It is recommended that client implementations provide robust schemes 469 for dealing with servers which cannot be connected to. Options 470 include providing a list of servers for redundancy, and an option for 471 a local fallback configuration if no servers can be reached. Details 472 will be implementation specific. 474 The client should manage connections and handle the case of a server 475 which establishes a connection, but does not respond. The exact 476 behavior is implementation specific. It is recommended that the 477 client should close the connection after a configurable timeout. 479 4.5. Data Obfuscation 481 The body of packets may be obfuscated. The following sections 482 describe the obfuscation method that is supported in the protocol. 483 In 'The Draft' this process was actually referred to as Encryption, 484 but the algorithm would not meet modern standards, and so will not be 485 termed as encryption in this document. 487 The obfuscation mechanism relies on a secret key, a shared secret 488 value that is known to both the client and the server. The secret 489 keys MUST remain secret. 491 Server implementations MUST allow a unique secret key to be 492 associated with each client. It is a site-dependent decision as to 493 whether the use of separate keys is appropriate. 495 The flag field may be set as follows: 497 TAC_PLUS_UNENCRYPTED_FLAG = 0x0 499 In this case, the packet body is obfuscated by XOR-ing it byte-wise 500 with a pseudo-random pad. 502 ENCRYPTED {data} = data ^ pseudo_pad 504 The packet body can then be de-obfuscated by XOR-ing it byte-wise 505 with a pseudo random pad. 507 data = ENCRYPTED {data} ^ pseudo_pad 509 The pad is generated by concatenating a series of MD5 hashes (each 16 510 bytes long) and truncating it to the length of the input data. 512 Whenever used in this document, MD5 refers to the "RSA Data Security, 513 Inc. MD5 Message-Digest Algorithm" as specified in RFC 1321 [RFC1321] 514 . 516 pseudo_pad = {MD5_1 [,MD5_2 [ ... ,MD5_n]]} truncated to len(data) 518 The first MD5 hash is generated by concatenating the session_id, the 519 secret key, the version number and the sequence number and then 520 running MD5 over that stream. All of those input values are 521 available in the packet header, except for the secret key which is a 522 shared secret between the TACACS+ client and server. 524 The version number and session_id are extracted from the header 525 Subsequent hashes are generated by using the same input stream, but 526 concatenating the previous hash value at the end of the input stream. 528 MD5_1 = MD5{session_id, key, version, seq_no} MD5_2 = MD5{session_id, 529 key, version, seq_no, MD5_1} .... MD5_n = MD5{session_id, key, 530 version, seq_no, MD5_n-1} 532 When a server detects that the secret(s) it has configured for the 533 device mismatch, it MUST return ERROR. For details of TCP connection 534 handling on ERROR, refer to section (Section 4.4) . 536 TAC_PLUS_UNENCRYPTED_FLAG == 0x1 538 In this case, the entire packet body is in cleartext. Obfuscation 539 and de-obfuscation are null operations. This method should be 540 avoided unless absolutely required for debug purposes, when tooling 541 does not permit de-obfuscation. 543 If deployment is configured for obfuscating a connection then the 544 request MUST be dropped if TAC_PLUS_UNENCRYPTED_FLAG is set to true. 546 After a packet body is de-obfuscated, the lengths of the component 547 values in the packet are summed. If the sum is not identical to the 548 cleartext datalength value from the header, the packet MUST be 549 discarded, and an ERROR signaled. For details of TCP connection 550 handling on ERROR, refer to section (Section 4.4) . 552 Commonly such failures are seen when the keys are mismatched between 553 the client and the TACACS+ server. 555 5. Authentication 557 Authentication is the action of determining who a user (or entity) 558 is. Authentication can take many forms. Traditional authentication 559 employs a name and a fixed password. However, fixed passwords are 560 vulnerable security, so many modern authentication mechanisms utilize 561 "one-time" passwords or a challenge-response query. TACACS+ is 562 designed to support all of these, and be flexible enough to handle 563 any future mechanisms. Authentication generally takes place when the 564 user first logs in to a machine or requests a service of it. 566 Authentication is not mandatory; it is a site-configured option. 567 Some sites do not require it. Others require it only for certain 568 services (see authorization below). Authentication may also take 569 place when a user attempts to gain extra privileges, and must 570 identify himself or herself as someone who possesses the required 571 information (passwords, etc.) for those privileges. 573 5.1. The Authentication START Packet Body 575 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 8 576 +----------------+----------------+----------------+----------------+ 577 | action | priv_lvl | authen_type | authen_service | 578 +----------------+----------------+----------------+----------------+ 579 | user_len | port_len | rem_addr_len | data_len | 580 +----------------+----------------+----------------+----------------+ 581 | user ... 582 +----------------+----------------+----------------+----------------+ 583 | port ... 584 +----------------+----------------+----------------+----------------+ 585 | rem_addr ... 586 +----------------+----------------+----------------+----------------+ 587 | data... 588 +----------------+----------------+----------------+----------------+ 590 Packet fields are as follows: 592 action 594 This indicates the authentication action. Valid values are listed 595 below. 597 TAC_PLUS_AUTHEN_LOGIN := 0x01 599 TAC_PLUS_AUTHEN_CHPASS := 0x02 601 TAC_PLUS_AUTHEN_SENDAUTH := 0x04 603 priv_lvl 605 This indicates the privilege level that the user is authenticating 606 as. Please refer to the Privilege Level section (Section 9) below. 608 authen_type 610 The type of authentication. Please see section Common Authentication 611 Flows (Section 5.4.2) . Valid values are: 613 TAC_PLUS_AUTHEN_TYPE_ASCII := 0x01 615 TAC_PLUS_AUTHEN_TYPE_PAP := 0x02 617 TAC_PLUS_AUTHEN_TYPE_CHAP := 0x03 619 TAC_PLUS_AUTHEN_TYPE_MSCHAP := 0x05 620 TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 := 0x06 622 authen_service 624 This is the service that is requesting the authentication. Valid 625 values are: 627 TAC_PLUS_AUTHEN_SVC_NONE := 0x00 629 TAC_PLUS_AUTHEN_SVC_LOGIN := 0x01 631 TAC_PLUS_AUTHEN_SVC_ENABLE := 0x02 633 TAC_PLUS_AUTHEN_SVC_PPP := 0x03 635 TAC_PLUS_AUTHEN_SVC_PT := 0x05 637 TAC_PLUS_AUTHEN_SVC_RCMD := 0x06 639 TAC_PLUS_AUTHEN_SVC_X25 := 0x07 641 TAC_PLUS_AUTHEN_SVC_NASI := 0x08 643 TAC_PLUS_AUTHEN_SVC_FWPROXY := 0x09 645 The TAC_PLUS_AUTHEN_SVC_NONE option is intended for the authorization 646 application of this field that indicates that no authentication was 647 performed by the device. 649 The TAC_PLUS_AUTHEN_SVC_LOGIN option indicates regular login (as 650 opposed to ENABLE) to a client device. 652 The TAC_PLUS_AUTHEN_SVC_ENABLE option identifies the ENABLE 653 authen_service, which refers to a service requesting authentication 654 in order to grant the user different privileges. This is comparable 655 to the Unix "su(1)" command, which substitutes the current user's 656 identity with another. An authen_service value of NONE is only to be 657 used when none of the other authen_service values are appropriate. 658 ENABLE may be requested independently, no requirements for previous 659 authentications or authorizations are imposed by the protocol. 661 Other options are included for legacy/backwards compatibility. 663 user, user_len 665 The username is optional in this packet, depending upon the class of 666 authentication. If it is absent, the client MUST set user_len to 0. 668 If included, the user_len indicates the length of the user field, in 669 bytes. 671 port, port_len 673 The name of the client port on which the authentication is taking 674 place, and its length in bytes. The value of this field is client 675 specific. The port_len indicates the length of the port field, in 676 bytes. For details of text encoding, see (Section 3.7) . 678 rem_addr, rem_addr_len 680 A string indicating the remote location from which the user has 681 connected to the client. For details of text encoding, see 682 (Section 3.7) . 684 When TACACS+ was used for dial-up services, this value contained the 685 caller ID 687 When TACACS+ is used for Device Administration, the user is normally 688 connected via a network, and in this case the value is intended to 689 hold a network address, IPv4 or IPv6. For IPv6 address text 690 representation defined please see RFC 5952 [RFC5952] . 692 This field is optional (since the information may not be available). 693 The rem_addr_len indicates the length of the user field, in bytes. 695 data, data_len 697 This field is used to send data appropriate for the action and 698 authen_type. It is described in more detail in the section Common 699 Authentication flows (Section 5.4.2) . The data_len indicates the 700 length of the data field, in bytes. 702 5.2. The Authentication REPLY Packet Body 704 The TACACS+ server sends only one type of authentication packet (a 705 REPLY packet) to the client. 707 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 8 708 +----------------+----------------+----------------+----------------+ 709 | status | flags | server_msg_len | 710 +----------------+----------------+----------------+----------------+ 711 | data_len | server_msg ... 712 +----------------+----------------+----------------+----------------+ 713 | data ... 714 +----------------+----------------+ 715 status 717 The current status of the authentication. Valid values are: 719 TAC_PLUS_AUTHEN_STATUS_PASS := 0x01 721 TAC_PLUS_AUTHEN_STATUS_FAIL := 0x02 723 TAC_PLUS_AUTHEN_STATUS_GETDATA := 0x03 725 TAC_PLUS_AUTHEN_STATUS_GETUSER := 0x04 727 TAC_PLUS_AUTHEN_STATUS_GETPASS := 0x05 729 TAC_PLUS_AUTHEN_STATUS_RESTART := 0x06 731 TAC_PLUS_AUTHEN_STATUS_ERROR := 0x07 733 TAC_PLUS_AUTHEN_STATUS_FOLLOW := 0x21 735 flags 737 Bitmapped flags that modify the action to be taken. The following 738 values are defined: 740 TAC_PLUS_REPLY_FLAG_NOECHO := 0x01 742 server_msg, server_msg_len 744 A message to be displayed to the user. This field is optional. The 745 server_msg_len indicates the length of the server_msg field, in 746 bytes. For details of text encoding, see (Section 3.7) . 748 data, data_len 750 This field holds data that is a part of the authentication exchange 751 and is intended for client processing, not the user. It is not a 752 printable text encoding. Examples of its use are shown in the 753 section Common Authentication flows (Section 5.4.2) . The data_len 754 indicates the length of the data field, in bytes. 756 5.3. The Authentication CONTINUE Packet Body 758 This packet is sent from the client to the server following the 759 receipt of a REPLY packet. 761 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 8 762 +----------------+----------------+----------------+----------------+ 763 | user_msg len | data_len | 764 +----------------+----------------+----------------+----------------+ 765 | flags | user_msg ... 766 +----------------+----------------+----------------+----------------+ 767 | data ... 768 +----------------+ 770 user_msg, user_msg_len 772 This field is the string that the user entered, or the client 773 provided on behalf of the user, in response to the server_msg from a 774 REPLY packet. The user_len indicates the length of the user field, 775 in bytes. 777 data, data_len 779 This field carries information that is specific to the action and the 780 authen_type for this session. Valid uses of this field are described 781 below. It is not a printable text encoding. The data_len indicates 782 the length of the data field, in bytes. 784 flags 786 This holds the bitmapped flags that modify the action to be taken. 787 The following values are defined: 789 TAC_PLUS_CONTINUE_FLAG_ABORT := 0x01 791 5.4. Description of Authentication Process 793 The action, authen_type and authen_service fields (described above) 794 combine to indicate what kind of authentication is to be performed. 795 Every authentication START, REPLY and CONTINUE packet includes a data 796 field. The use of this field is dependent upon the kind of the 797 Authentication. 799 This document defines a core set of authentication flows to be 800 supported by TACACS+. Each authentication flow consists of a START 801 packet. The server responds either with a request for more 802 information (GETDATA, GETUSER or GETPASS) or a termination PASS, 803 FAIL, ERROR or RESTART. The actions and meanings when the server 804 sends a RESTART or ERROR are common and are described further below. 806 When the REPLY status equals TAC_PLUS_AUTHEN_STATUS_GETDATA, 807 TAC_PLUS_AUTHEN_STATUS_GETUSER or TAC_PLUS_AUTHEN_STATUS_GETPASS, 808 then authentication continues and the server SHOULD provide 809 server_msg content for the client to prompt the user for more 810 information. The client MUST then return a CONTINUE packet 811 containing the requested information in the user_msg field. 813 The client should interpret TAC_PLUS_AUTHEN_STATUS_GETUSER as a 814 request for username and TAC_PLUS_AUTHEN_STATUS_GETPASS as a request 815 for password. The TAC_PLUS_AUTHEN_STATUS_GETDATA is the generic 816 request for more information to flexibly support future requirements. 818 If the information being requested by the server form the client is 819 sensitive, then the server should set the TAC_PLUS_REPLY_FLAG_NOECHO 820 flag. When the client queries the user for the information, the 821 response MUST NOT be reflected in the user interface as it is 822 entered. 824 The data field is only used in the REPLY where explicitly defined 825 below. 827 5.4.1. Version Behavior 829 The TACACS+ protocol is versioned to allow revisions while 830 maintaining backwards compatibility. The version number is in every 831 packet header. The changes between minor_version 0 and 1 apply only 832 to the authentication process, and all deal with the way that CHAP 833 and PAP authentications are handled. minor_version 1 may only be used 834 for authentication kinds that explicitly call for it in the table 835 below: 837 LOGIN CHPASS SENDAUTH 838 ASCII v0 v0 - 839 PAP v1 - v1 840 CHAP v1 - v1 841 MS-CHAPv1/2 v1 - v1 843 The '-' symbol represents that the option is not valid. 845 All authorization and accounting and ASCII authentication use 846 minor_version number of 0. 848 PAP, CHAP and MS-CHAP login use minor_version 1. The normal exchange 849 is a single START packet from the client and a single REPLY from the 850 server. 852 The removal of SENDPASS was prompted by security concerns, and is no 853 longer considered part of the TACACS+ protocol. 855 5.4.2. Common Authentication Flows 857 This section describes common authentication flows. If the server 858 does not implement an option, it MUST respond with 859 TAC_PLUS_AUTHEN_STATUS_FAIL. 861 5.4.2.1. ASCII Login 863 action = TAC_PLUS_AUTHEN_LOGIN 864 authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII 865 minor_version = 0x0 867 This is a standard ASCII authentication. The START packet MAY 868 contain the username. If the user does not include the username then 869 the server MUST obtain it from the client with a CONTINUE 870 TAC_PLUS_AUTHEN_STATUS_GETUSER. If the user does not provide a 871 username then the server can send another 872 TAC_PLUS_AUTHEN_STATUS_GETUSER request, but the server MUST limit the 873 number of retries that are permitted, recommended limit is three 874 attempts. When the server has the username, it will obtain the 875 password using a continue with TAC_PLUS_AUTHEN_STATUS_GETPASS. ASCII 876 login uses the user_msg field for both the username and password. 877 The data fields in both the START and CONTINUE packets are not used 878 for ASCII logins, any content MUST be ignored. The session is 879 composed of a single START followed by zero or more pairs of REPLYs 880 and CONTINUEs, followed by a final REPLY indicating PASS, FAIL or 881 ERROR. 883 5.4.2.2. PAP Login 885 action = TAC_PLUS_AUTHEN_LOGIN 886 authen_type = TAC_PLUS_AUTHEN_TYPE_PAP 887 minor_version = 0x1 889 The entire exchange MUST consist of a single START packet and a 890 single REPLY. The START packet MUST contain a username and the data 891 field MUST contain the PAP ASCII password. A PAP authentication only 892 consists of a username and password RFC 1334 [RFC1334] (Obsolete). 893 The REPLY from the server MUST be either a PASS, FAIL or ERROR. 895 5.4.2.3. CHAP login 897 action = TAC_PLUS_AUTHEN_LOGIN 898 authen_type = TAC_PLUS_AUTHEN_TYPE_CHAP 899 minor_version = 0x1 901 The entire exchange MUST consist of a single START packet and a 902 single REPLY. The START packet MUST contain the username in the user 903 field and the data field is a concatenation of the PPP id, the 904 challenge and the response. 906 The length of the challenge value can be determined from the length 907 of the data field minus the length of the id (always 1 octet) and the 908 length of the response field (always 16 octets). 910 To perform the authentication, the server calculates the PPP hash as 911 defined in the PPP Authentication RFC RFC 1334 [RFC1334] and then 912 compares that value with the response. The MD5 algorithm option is 913 always used. The REPLY from the server MUST be a PASS, FAIL or 914 ERROR. 916 The selection of the challenge and its length are not an aspect of 917 the TACACS+ protocol. However, it is strongly recommended that the 918 client/endstation interaction is configured with a secure challenge. 919 The TACACS+ server can help by rejecting authentications where the 920 challenge is below a minimum length (Minimum recommended is 8 bytes). 922 5.4.2.4. MS-CHAP v1 login 924 action = TAC_PLUS_AUTHEN_LOGIN 925 authen_type = TAC_PLUS_AUTHEN_TYPE_MSCHAP 926 minor_version = 0x1 928 The entire exchange MUST consist of a single START packet and a 929 single REPLY. The START packet MUST contain the username in the user 930 field and the data field will be a concatenation of the PPP id, the 931 MS-CHAP challenge and the MS-CHAP response. 933 The length of the challenge value can be determined from the length 934 of the data field minus the length of the id (always 1 octet) and the 935 length of the response field (always 49 octets). 937 To perform the authentication, the server will use a combination of 938 MD4 and DES on the user's secret and the challenge, as defined in RFC 939 2433 [RFC2433] and then compare the resulting value with the 940 response. The REPLY from the server MUST be a PASS or FAIL. 942 For best practices, please refer to RFC 2433 [RFC2433] . The TACACS+ 943 server MUST reject authentications where the challenge deviates from 944 8 bytes as defined in the RFC. 946 5.4.2.5. MS-CHAP v2 login 948 action = TAC_PLUS_AUTHEN_LOGIN 949 authen_type = TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 950 minor_version = 0x1 952 The entire exchange MUST consist of a single START packet and a 953 single REPLY. The START packet MUST contain the username in the user 954 field and the data field will be a concatenation of the PPP id, the 955 MS-CHAP challenge and the MS-CHAP response. 957 The length of the challenge value can be determined from the length 958 of the data field minus the length of the id (always 1 octet) and the 959 length of the response field (always 49 octets). 961 To perform the authentication, the server will use the algorithm 962 specified RFC 2759 [RFC2759] on the user's secret and challenge and 963 then compare the resulting value with the response. The REPLY from 964 the server MUST be a PASS or FAIL. 966 For best practices for MS-CHAP v2, please refer to RFC2759 [RFC2759] 967 . The TACACS+ server MUST reject authentications where the challenge 968 deviates from 16 bytes as defined in the RFC. 970 5.4.2.6. Enable Requests 972 action = TAC_PLUS_AUTHEN_LOGIN 973 priv_lvl = implementation dependent 974 authen_type = not used 975 service = TAC_PLUS_AUTHEN_SVC_ENABLE 977 This is an ENABLE request, used to change the current running 978 privilege level of a user. The exchange MAY consist of multiple 979 messages while the server collects the information it requires in 980 order to allow changing the principal's privilege level. This 981 exchange is very similar to an ASCII login (Section 5.4.2.1) . 983 In order to readily distinguish enable requests from other types of 984 request, the value of the authen_service field MUST be set to 985 TAC_PLUS_AUTHEN_SVC_ENABLE when requesting an ENABLE. It MUST NOT be 986 set to this value when requesting any other operation. 988 5.4.2.7. ASCII change password request 990 action = TAC_PLUS_AUTHEN_CHPASS 991 authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII 993 This exchange consists of multiple messages while the server collects 994 the information it requires in order to change the user's password. 995 It is very similar to an ASCII login. The status value 996 TAC_PLUS_AUTHEN_STATUS_GETPASS MUST only be used when requesting the 997 "new" password. It MAY be sent multiple times. When requesting the 998 "old" password, the status value MUST be set to 999 TAC_PLUS_AUTHEN_STATUS_GETDATA. 1001 5.4.3. Aborting an Authentication Session 1003 The client may prematurely terminate a session by setting the 1004 TAC_PLUS_CONTINUE_FLAG_ABORT flag in the CONTINUE message. If this 1005 flag is set, the data portion of the message may contain an message 1006 explaining the reason for the abort. For details of text encoding, 1007 see (Section 3.7) . This information will be handled by the server 1008 according to the requirements of the deployment. The session is 1009 terminated, for more details about session termination, refer to 1010 section (Section 4.4) . 1012 In cases of PASS, FAIL or ERROR, the server can insert a message into 1013 server_msg to be displayed to the user. 1015 The Draft `The Draft' [TheDraft] defined a mechanism to direct 1016 authentication requests to an alternative server. This mechanism is 1017 regarded as insecure, is deprecated, and not covered here. The 1018 client should treat TAC_PLUS_AUTHEN_STATUS_FOLLOW as 1019 TAC_PLUS_AUTHEN_STATUS_FAIL 1021 If the status equals TAC_PLUS_AUTHEN_STATUS_ERROR, then the host is 1022 indicating that it is experiencing an unrecoverable error and the 1023 authentication will proceed as if that host could not be contacted. 1024 The data field may contain a message to be printed on an 1025 administrative console or log. 1027 If the status equals TAC_PLUS_AUTHEN_STATUS_RESTART, then the 1028 authentication sequence is restarted with a new START packet from the 1029 client, with new session Id, and seq_no set to 1. This REPLY packet 1030 indicates that the current authen_type value (as specified in the 1031 START packet) is not acceptable for this session. The client may try 1032 an alternative authen_type. 1034 If a client does not implement TAC_PLUS_AUTHEN_STATUS_RESTART option, 1035 then it MUST process the response as if the status was 1036 TAC_PLUS_AUTHEN_STATUS_FAIL. 1038 6. Authorization 1040 In the TACACS+ Protocol, authorization is the action of determining 1041 what a user is allowed to do. Generally, authentication precedes 1042 authorization, though it is not mandatory that a client use the same 1043 service for authentication that it will use for authorization. An 1044 authorization request may indicate that the user is not authenticated 1045 (we don't know who they are). In this case it is up to the server to 1046 determine, according to its configuration, if an unauthenticated user 1047 is allowed the services in question. 1049 Authorization does not merely provide yes or no answers, but it may 1050 also customize the service for the particular user. A common use of 1051 authorization is to provision a shell session when a user first logs 1052 into a device to administer it. The TACACS+ server might respond to 1053 the request by allowing the service, but placing a time restriction 1054 on the login shell. For a list of common attributes used in 1055 authorization, see the Authorization Attributes section (Section 8.2) 1056 . 1058 In the TACACS+ protocol an authorization is always a single pair of 1059 messages: a REQUEST from the client followed by a REPLY from the 1060 server. 1062 The authorization REQUEST message contains a fixed set of fields that 1063 indicate how the user was authenticated and a variable set of 1064 arguments that describe the services and options for which 1065 authorization is requested. 1067 The REPLY contains a variable set of response arguments (attribute- 1068 value pairs) that can restrict or modify the client's actions. 1070 6.1. The Authorization REQUEST Packet Body 1071 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 1072 +----------------+----------------+----------------+----------------+ 1073 | authen_method | priv_lvl | authen_type | authen_service | 1074 +----------------+----------------+----------------+----------------+ 1075 | user_len | port_len | rem_addr_len | arg_cnt | 1076 +----------------+----------------+----------------+----------------+ 1077 | arg_1_len | arg_2_len | ... | arg_N_len | 1078 +----------------+----------------+----------------+----------------+ 1079 | user ... 1080 +----------------+----------------+----------------+----------------+ 1081 | port ... 1082 +----------------+----------------+----------------+----------------+ 1083 | rem_addr ... 1084 +----------------+----------------+----------------+----------------+ 1085 | arg_1 ... 1086 +----------------+----------------+----------------+----------------+ 1087 | arg_2 ... 1088 +----------------+----------------+----------------+----------------+ 1089 | ... 1090 +----------------+----------------+----------------+----------------+ 1091 | arg_N ... 1092 +----------------+----------------+----------------+----------------+ 1094 authen_method 1096 This indicates the authentication method used by the client to 1097 acquire the user information. As this information is not always 1098 subject to verification, it is recommended that this field is 1099 ignored. 1101 TAC_PLUS_AUTHEN_METH_NOT_SET := 0x00 1103 TAC_PLUS_AUTHEN_METH_NONE := 0x01 1105 TAC_PLUS_AUTHEN_METH_KRB5 := 0x02 1107 TAC_PLUS_AUTHEN_METH_LINE := 0x03 1109 TAC_PLUS_AUTHEN_METH_ENABLE := 0x04 1111 TAC_PLUS_AUTHEN_METH_LOCAL := 0x05 1113 TAC_PLUS_AUTHEN_METH_TACACSPLUS := 0x06 1115 TAC_PLUS_AUTHEN_METH_GUEST := 0x08 1117 TAC_PLUS_AUTHEN_METH_RADIUS := 0x10 1118 TAC_PLUS_AUTHEN_METH_KRB4 := 0x11 1120 TAC_PLUS_AUTHEN_METH_RCMD := 0x20 1122 KRB5 and KRB4 are Kerberos version 5 and 4. LINE refers to a fixed 1123 password associated with the terminal line used to gain access. 1124 LOCAL is a client local user database. ENABLE is a command that 1125 authenticates in order to grant new privileges. TACACSPLUS is, of 1126 course, TACACS+. GUEST is an unqualified guest authentication. 1127 RADIUS is the Radius authentication protocol. RCMD refers to 1128 authentication provided via the R-command protocols from Berkeley 1129 Unix. 1131 priv_lvl 1133 This field is used in the same way as the priv_lvl field in 1134 authentication request and is described in the Privilege Level 1135 section (Section 9) below. It indicates the users current privilege 1136 level. 1138 authen_type 1140 This field corresponds to the authen_type field in the authentication 1141 section (Section 5) above. It indicates the type of authentication 1142 that was performed. If this information is not available, then the 1143 client will set authen_type to: TAC_PLUS_AUTHEN_TYPE_NOT_SET := 0x00. 1144 This value is valid only in authorization and accounting requests. 1146 authen_service 1148 This field is the same as the authen_service field in the 1149 authentication section (Section 5) above. It indicates the service 1150 through which the user authenticated. 1152 user, user_len 1154 This field contains the user's account name. The user_len MUST 1155 indicate the length of the user field, in bytes. 1157 port, port_len 1159 This field matches the port field in the authentication section 1160 (Section 5) above. The port_len indicates the length of the port 1161 field, in bytes. 1163 rem_addr, rem_addr_len 1164 This field matches the rem_addr field in the authentication section 1165 (Section 5) above. The rem_addr_len indicates the length of the port 1166 field, in bytes. 1168 arg_cnt 1170 The number of authorization arguments to follow 1172 arg_1 ... arg_N, arg_1_len .... arg_N_len 1174 The arguments are the primary elements of the authorization 1175 interaction. In the request packet, they describe the specifics of 1176 the authorization that is being requested. Each argument is encoded 1177 in the packet as a single arg field (arg_1... arg_N) with a 1178 corresponding length fields (which indicates the length of each 1179 argument in bytes). 1181 The authorization arguments in both the REQUEST and the REPLY are 1182 attribute-value pairs. The attribute and the value are in a single 1183 string and are separated by either a "=" (0X3D) or a "*" (0X2A). The 1184 equals sign indicates a mandatory argument. The asterisk indicates 1185 an optional one. For details of text encoding, see (Section 3.7) . 1187 An attribute name MUST NOT contain either of the separators. An 1188 attribute value MAY contain the separators. This means that the 1189 arguments must be parsed until the first separator is encountered, 1190 all characters in the argument, after this separator, are interpreted 1191 as the argument value. 1193 Optional arguments are ones that may be disregarded by either client 1194 or server. Mandatory arguments require that the receiving side can 1195 handle the attribute, that is: its implementation and configuration 1196 includes the details of how to act on it. If the client receives a 1197 mandatory argument that it cannot handle, it MUST consider the 1198 authorization to have failed. The value part of an attribute-value 1199 pair may be empty, that is: the length of the value may be zero. 1201 Attribute-value strings are not NULL terminated, rather their length 1202 value indicates their end. The maximum length of an attribute-value 1203 string is 255 characters. The minimum is two characters (one name- 1204 value character and the separator) 1206 Though the attributes allow extensibility, a common core set of 1207 authorization attributes SHOULD be supported by clients and servers, 1208 these are listed in the Authorization Attributes (Section 8.2) 1209 section below. 1211 6.2. The Authorization REPLY Packet Body 1213 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 1214 +----------------+----------------+----------------+----------------+ 1215 | status | arg_cnt | server_msg len | 1216 +----------------+----------------+----------------+----------------+ 1217 + data_len | arg_1_len | arg_2_len | 1218 +----------------+----------------+----------------+----------------+ 1219 | ... | arg_N_len | server_msg ... 1220 +----------------+----------------+----------------+----------------+ 1221 | data ... 1222 +----------------+----------------+----------------+----------------+ 1223 | arg_1 ... 1224 +----------------+----------------+----------------+----------------+ 1225 | arg_2 ... 1226 +----------------+----------------+----------------+----------------+ 1227 | ... 1228 +----------------+----------------+----------------+----------------+ 1229 | arg_N ... 1230 +----------------+----------------+----------------+----------------+ 1232 status This field indicates the authorization status 1234 TAC_PLUS_AUTHOR_STATUS_PASS_ADD := 0x01 1236 TAC_PLUS_AUTHOR_STATUS_PASS_REPL := 0x02 1238 TAC_PLUS_AUTHOR_STATUS_FAIL := 0x10 1240 TAC_PLUS_AUTHOR_STATUS_ERROR := 0x11 1242 TAC_PLUS_AUTHOR_STATUS_FOLLOW := 0x21 1244 server_msg, server_msg_len 1246 This is a string that may be presented to the user. The 1247 server_msg_len indicates the length of the server_msg field, in 1248 bytes. For details of text encoding, see (Section 3.7) . 1250 data, data_len 1252 This is a string that may be presented on an administrative display, 1253 console or log. The decision to present this message is client 1254 specific. The data_len indicates the length of the data field, in 1255 bytes. For details of text encoding, see (Section 3.7) . 1257 arg_cnt 1258 The number of authorization arguments to follow. 1260 arg_1 ... arg_N, arg_1_len .... arg_N_len 1262 The arguments describe the specifics of the authorization that is 1263 being requested. For details of the content of the args, refer to: 1264 Authorization Attributes (Section 8.2) section below. Each argument 1265 is encoded in the packet as a single arg field (arg_1... arg_N) with 1266 a corresponding length fields (which indicates the length of each 1267 argument in bytes). 1269 If the status equals TAC_PLUS_AUTHOR_STATUS_FAIL, then the requested 1270 authorization MUST be denied. 1272 If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_ADD, then the 1273 arguments specified in the request are authorized and the arguments 1274 in the response MUST be applied according to the rules described 1275 above. 1277 If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_REPL then the client 1278 MUST use the authorization attribute-value pairs (if any) in the 1279 response, instead of the authorization attribute-value pairs from the 1280 request. 1282 To approve the authorization with no modifications, the server sets 1283 the status to TAC_PLUS_AUTHOR_STATUS_PASS_ADD and the arg_cnt to 0. 1285 A status of TAC_PLUS_AUTHOR_STATUS_ERROR indicates an error occurred 1286 on the server. For the differences between ERROR and FAIL, refer to 1287 section Session Completion (Section 4.4) . None of the arg values 1288 have any relevance if an ERROR is set, and must be ignored. 1290 When the status equals TAC_PLUS_AUTHOR_STATUS_FOLLOW, then the 1291 arg_cnt MUST be 0. In that case, the actions to be taken and the 1292 contents of the data field are identical to the 1293 TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication. 1295 7. Accounting 1297 Accounting is typically the third action after authentication and 1298 authorization. But again, neither authentication nor authorization 1299 is required. Accounting is the action of recording what a user is 1300 doing, and/or has done. Accounting in TACACS+ can serve two 1301 purposes: It may be used as an auditing tool for security services. 1302 It may also be used to account for services used, such as in a 1303 billing environment. To this end, TACACS+ supports three types of 1304 accounting records. Start records indicate that a service is about 1305 to begin. Stop records indicate that a service has just terminated, 1306 and Update records are intermediate notices that indicate that a 1307 service is still being performed. TACACS+ accounting records contain 1308 all the information used in the authorization records, and also 1309 contain accounting specific information such as start and stop times 1310 (when appropriate) and resource usage information. A list of 1311 accounting attributes is defined in the accounting section 1312 (Section 7) . 1314 7.1. The Account REQUEST Packet Body 1316 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 1317 +----------------+----------------+----------------+----------------+ 1318 | flags | authen_method | priv_lvl | authen_type | 1319 +----------------+----------------+----------------+----------------+ 1320 | authen_service | user_len | port_len | rem_addr_len | 1321 +----------------+----------------+----------------+----------------+ 1322 | arg_cnt | arg_1_len | arg_2_len | ... | 1323 +----------------+----------------+----------------+----------------+ 1324 | arg_N_len | user ... 1325 +----------------+----------------+----------------+----------------+ 1326 | port ... 1327 +----------------+----------------+----------------+----------------+ 1328 | rem_addr ... 1329 +----------------+----------------+----------------+----------------+ 1330 | arg_1 ... 1331 +----------------+----------------+----------------+----------------+ 1332 | arg_2 ... 1333 +----------------+----------------+----------------+----------------+ 1334 | ... 1335 +----------------+----------------+----------------+----------------+ 1336 | arg_N ... 1337 +----------------+----------------+----------------+----------------+ 1339 flags 1341 This holds bitmapped flags. 1343 TAC_PLUS_ACCT_FLAG_START := 0x02 1345 TAC_PLUS_ACCT_FLAG_STOP := 0x04 1347 TAC_PLUS_ACCT_FLAG_WATCHDOG := 0x08 1349 All other fields are defined in the authorization and authentication 1350 sections above and have the same semantics. They provide details for 1351 the conditions on the client, and authentication context, so that 1352 these details may be logged for accounting purposes. 1354 See section 12 Accounting Attribute-value Pairs for the dictionary of 1355 attributes relevant to accounting. 1357 7.2. The Accounting REPLY Packet Body 1359 The purpose of accounting is to record the action that has occurred 1360 on the client. The server MUST reply with success only when the 1361 accounting request has been recorded. If the server did not record 1362 the accounting request then it MUST reply with ERROR. 1364 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 1365 +----------------+----------------+----------------+----------------+ 1366 | server_msg len | data_len | 1367 +----------------+----------------+----------------+----------------+ 1368 | status | server_msg ... 1369 +----------------+----------------+----------------+----------------+ 1370 | data ... 1371 +----------------+ 1373 status 1375 This is the return status. Values are: 1377 TAC_PLUS_ACCT_STATUS_SUCCESS := 0x01 1379 TAC_PLUS_ACCT_STATUS_ERROR := 0x02 1381 TAC_PLUS_ACCT_STATUS_FOLLOW := 0x21 1383 server_msg, server_msg_len 1385 This is a string that may be presented to the user. The 1386 server_msg_len indicates the length of the server_msg field, in 1387 bytes. For details of text encoding, see (Section 3.7) . 1389 data, data_len 1391 This is a string that may be presented on an administrative display, 1392 console or log. The decision to present this message is client 1393 specific. The data_len indicates the length of the data field, in 1394 bytes. For details of text encoding, see (Section 3.7) . 1396 When the status equals TAC_PLUS_ACCT_STATUS_FOLLOW, then the actions 1397 to be taken and the contents of the data field are identical to the 1398 TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication. 1400 TACACS+ accounting is intended to record various types of events on 1401 clients, for example: login sessions, command entry, and others as 1402 required by the client implementation. These events are collectively 1403 referred to in `The Draft' [TheDraft] as "tasks". 1405 The TAC_PLUS_ACCT_FLAG_START flag indicates that this is a start 1406 accounting message. Start messages will only be sent once when a 1407 task is started. The TAC_PLUS_ACCT_FLAG_STOP indicates that this is 1408 a stop record and that the task has terminated. The 1409 TAC_PLUS_ACCT_FLAG_WATCHDOG flag means that this is an update record. 1411 Summary of Accounting Packets 1413 +----------+-------+-------+-------------+-------------------------+ 1414 | Watchdog | Stop | Start | Flags & 0xE | Meaning | 1415 +----------+-------+-------+-------------+-------------------------+ 1416 | 0 | 0 | 0 | 0 | INVALID | 1417 | 0 | 0 | 1 | 2 | Start Accounting Record | 1418 | 0 | 1 | 0 | 4 | Stop Accounting Record | 1419 | 0 | 1 | 1 | 6 | INVALID | 1420 | 1 | 0 | 0 | 8 | Watchdog, no update | 1421 | 1 | 0 | 1 | A | Watchdog, with update | 1422 | 1 | 1 | 0 | C | INVALID | 1423 | 1 | 1 | 1 | E | INVALID | 1424 +----------+-------+-------+-------------+-------------------------+ 1426 The START and STOP flags are mutually exclusive. 1428 The WATCHDOG flag is used by the client to communicate ongoing status 1429 of a long-running task. Update records are sent at the client's 1430 discretion. The frequency of the update depends upon the intended 1431 application: A watchdog to provide progress indication will require 1432 higher frequency than a daily keep-alive. When the WATCHDOG flag is 1433 set along with the START flag, it indicates that the update record 1434 provides additional or updated arguments from the original START 1435 record. If the START flag is not set, then this indicates only that 1436 task is still running, and no new information is provided (servers 1437 MUST ignore any arguments). The STOP flag MUST NOT be set in 1438 conjunction with the WATCHDOG flag. 1440 The Server MUST respond with TAC_PLUS_ACCT_STATUS_ERROR if the client 1441 requests an INVALID option. 1443 8. Attribute-Value Pairs 1445 TACACS+ is intended to be an extensible protocol. The attributes 1446 used in Authorization and Accounting are not limited by this 1447 document. Some attributes are defined below for common use cases, 1448 clients MUST use these attributes when supporting the corresponding 1449 use cases. 1451 8.1. Value Encoding 1453 All attribute values are encoded as strings. For details of text 1454 encoding, see (Section 3.7) . The following type representations 1455 SHOULD be followed 1457 Numeric 1459 All numeric values in an attribute-value string are provided as 1460 decimal numbers, unless otherwise stated. 1462 Boolean 1464 All Boolean attributes are encoded with values "true" or "false". 1466 IP-Address 1468 It is recommended that hosts be specified as a IP address so as to 1469 avoid any ambiguities. For details of text encoding, see 1470 (Section 3.7) . IPv4 address are specified as octet numerics 1471 separated by dots ('.'), IPv6 address text representation defined in 1472 RFC 5952 [RFC5952] . 1474 Date Time 1476 Absolute date/times are specified in seconds since the epoch, 12:00am 1477 Jan 1 1970. The timezone MUST be UTC unless a timezone attribute is 1478 specified. 1480 String 1482 Many values have no specific type representation and so are 1483 interpreted as plain strings. 1485 Empty Values 1487 Attributes may be submitted with no value, in which case they consist 1488 of the name and the mandatory or optional separator. For example, 1489 the attribute "cmd" which has no value is transmitted as a string of 1490 four characters "cmd=" 1492 8.2. Authorization Attributes 1494 service (String) 1496 The primary service. Specifying a service attribute indicates that 1497 this is a request for authorization or accounting of that service. 1498 For example: "shell", "tty-server", "connection", "system" and 1499 "firewall", others may be chosen for the required application. This 1500 attribute MUST always be included. 1502 protocol (String) 1504 the protocol field may be used to indicate a subset of a service. 1506 cmd (String) 1508 a shell (exec) command. This indicates the command name of the 1509 command that is to be run. The "cmd" attribute MUST be specified if 1510 service equals "shell". 1512 Authorization of shell commands is a common use-case for the TACACS+ 1513 protocol. Command Authorization generally takes one of two forms: 1514 session-based and command-based. 1516 For session-based shell authorization, the "cmd" argument will have 1517 an empty value. The client determines which commands are allowed in 1518 a session according to the arguments present in the authorization. 1520 In command-based authorization, the client requests that the server 1521 determine whether a command is allowed by making an authorization 1522 request for each command. The "cmd" argument will have the command 1523 name as its value. 1525 cmd-arg (String) 1527 an argument to a shell (exec) command. This indicates an argument 1528 for the shell command that is to be run. Multiple cmd-arg attributes 1529 may be specified, and they are order dependent. 1531 acl (Numeric) 1533 a number representing a connection access list. Applicable only to 1534 session-based shell authorization. For details of text encoding, see 1535 (Section 3.7) . 1537 inacl (String) 1539 identifier (name) of an interface input access list. For details of 1540 text encoding, see (Section 3.7) . 1542 outacl (String) 1544 identifier (name) of an interface output access list. For details of 1545 text encoding, see (Section 3.7) . 1547 addr (IP-Address) 1549 a network address 1551 addr-pool (String) 1553 The identifier of an address pool from which the client can assign an 1554 address. 1556 timeout (Numeric) 1558 an absolute timer for the connection (in minutes). A value of zero 1559 indicates no timeout. 1561 idletime (Numeric) 1563 an idle-timeout for the connection (in minutes). A value of zero 1564 indicates no timeout. 1566 autocmd (String) 1568 an auto-command to run. Applicable only to session-based shell 1569 authorization. 1571 noescape (Boolean) 1573 Prevents user from using an escape character. Applicable only to 1574 session-based shell authorization. 1576 nohangup (Boolean) 1578 Boolean. Do not disconnect after an automatic command. Applicable 1579 only to session-based shell authorization. 1581 priv-lvl (Numeric) 1583 privilege level to be assigned. Please refer to the Privilege Level 1584 section (Section 9) below. 1586 8.3. Accounting Attributes 1588 The following attributes are defined for TACACS+ accounting only. 1589 They MUST precede any attribute-value pairs that are defined in the 1590 authorization section (Section 6) above. 1592 task_id (String) 1593 Start and stop records for the same event MUST have matching task_id 1594 attribute values. The client MUST ensure that active task_ids are 1595 not duplicated: a client MUST NOT reuse a task_id a start record 1596 until it has sent a stop record for that task_id. Servers MUST NOT 1597 make assumptions about the format of a task_id. 1599 start_time (Date Time) 1601 The time the action started (in seconds since the epoch.). 1603 stop_time (Date Time) 1605 The time the action stopped (in seconds since the epoch.) 1607 elapsed_time (Numeric) 1609 The elapsed time in seconds for the action. 1611 timezone (String) 1613 The timezone abbreviation for all timestamps included in this packet. 1614 A database of timezones is maintained here: TZDB [TZDB] . 1616 event (String) 1618 Used only when "service=system". Current values are "net_acct", 1619 "cmd_acct", "conn_acct", "shell_acct" "sys_acct" and "clock_change". 1620 These indicate system-level changes. The flags field SHOULD indicate 1621 whether the service started or stopped. 1623 reason (String) 1625 Accompanies an event attribute. It describes why the event occurred. 1627 bytes (Numeric) 1629 The number of bytes transferred by this action 1631 bytes_in (Numeric) 1633 The number of bytes transferred by this action from the endstation to 1634 the client port 1636 bytes_out (Numeric) 1638 The number of bytes transferred by this action from the client to the 1639 endstation port 1640 paks (Numeric) 1642 The number of packets transferred by this action. 1644 paks_in (Numeric) 1646 The number of input packets transferred by this action from the 1647 endstation to the client port. 1649 paks_out (Numeric) 1651 The number of output packets transferred by this action from the 1652 client port to the endstation. 1654 err_msg (String) 1656 string describing the status of the action. For details of text 1657 encoding, see (Section 3.7) . 1659 9. Privilege Levels 1661 The TACACS+ Protocol supports flexible authorization schemes through 1662 the extensible attributes. 1664 One scheme is built into the protocol and has been extensively used 1665 for Session-based shell authorization: Privilege Levels. Privilege 1666 Levels are ordered values from 0 to 15 with each level being a 1667 superset of the next lower value. Configuration and implementation 1668 of the client will map actions (such as the permission to execute of 1669 specific commands) to different privilege levels. The allocation of 1670 commands to privilege levels is highly dependent upon the deployment. 1671 Common allocations are as follows: 1673 TAC_PLUS_PRIV_LVL_MIN := 0x00. The level normally allocated to an 1674 unauthenticated session. 1676 TAC_PLUS_PRIV_LVL_USER := 0x01. The level normally allocated to a 1677 regular authenticated session 1679 TAC_PLUS_PRIV_LVL_ROOT := 0x0f. The level normally allocated to a 1680 session authenticated by a highly privileged user to allow 1681 commands with significant system impact. 1683 TAC_PLUS_PRIV_LVL_MAX := 0x0f. The highest privilege level. 1685 A Privilege level can be assigned to a shell (EXEC) session when it 1686 starts. The client will permit the actions associated with this 1687 level to be executed. This privilege level is returned by the Server 1688 in a session-based shell authorization (when "service" equals "shell" 1689 and "cmd" is empty). When a user required to perform actions that 1690 are mapped to a higher privilege level, then an ENABLE type 1691 reauthentication can be initiated by the client. The client will 1692 insert the required privilege level into the authentication header 1693 for enable authentication request. 1695 The use of Privilege levels to determine session-based access to 1696 commands and resources is not mandatory for clients. Although the 1697 privilege level scheme is widely supported, its lack of flexibility 1698 in requiring a single monotonic hierarchy of permissions means that 1699 other session-based command authorization schemes have evolved, and 1700 so it is no longer mandatory for clients to use it. However, it is 1701 still common enough that it SHOULD be supported by servers. 1703 10. Security Considerations 1705 The original TACACS+ Draft `The Draft' [TheDraft] from 1998 did not 1706 address all of the key security concerns which are considered when 1707 designing modern standards. This section addresses known limitations 1708 and concerns which will impact overall security of the protocol and 1709 systems where this protocol is deployed to manage central 1710 authentication, authorization or accounting for network device 1711 administration. 1713 Multiple implementations of the protocol described in the original 1714 TACACS+ Draft `The Draft' [TheDraft] have been deployed. As the 1715 protocol was never standardized, current implementations may be 1716 incompatible in non-obvious ways, giving rise to additional security 1717 risks. This section does not claim to enumerate all possible 1718 security vulnerabilities. 1720 10.1. General Security of the Protocol 1722 TACACS+ protocol does not include a security mechanism that would 1723 meet modern-day requirements. These security mechanisms would be 1724 best referred to as "obfuscation" and not "encryption" since they 1725 provide no meaningful integrity, privacy or replay protection. An 1726 attacker with access to the data stream should be assumed to be able 1727 to read and modify all TACACS+ packets. Without mitigation, a range 1728 of risks such as the following are possible: 1730 Accounting information may be modified by the man-in-the-middle 1731 attacker, making such logs unsuitable and not trustable for 1732 auditing purposes. 1734 Invalid or misleading values may be inserted by the man-in-the- 1735 middle attacker in various fields at known offsets to try and 1736 circumvent the authentication or authorization checks even inside 1737 the obfuscated body. 1739 While the protocol provides some measure of transport privacy, it is 1740 vulnerable to at least the following attacks: 1742 Brute force attacks exploiting increased efficiency of MD5 digest 1743 computation. 1745 Known plaintext attacks which may decrease the cost of brute force 1746 attack. 1748 Chosen plaintext attacks which may decrease the cost of a brute 1749 force attack. 1751 No forward secrecy. 1753 Even though, to the best knowledge of authors, this method of 1754 encryption wasn't rigorously tested, enough information is available 1755 that it is best referred to as "obfuscation" and not "encryption". 1757 For these reasons, users deploying TACACS+ protocol in their 1758 environments MUST limit access to known clients and MUST control the 1759 security of the entire transmission path. Attackers who can guess 1760 the key or otherwise break the obfuscation will gain unrestricted and 1761 undetected access to all TACACS+ traffic. Ensuring that a 1762 centralized AAA system like TACACS+ is deployed on a secured 1763 transport is essential to managing the security risk of such an 1764 attack. 1766 The following parts of this section enumerate only the session- 1767 specific risks which are in addition to general risk associated with 1768 bare obfuscation and lack of integrity checking. 1770 10.2. Security of Authentication Sessions 1772 Authentication sessions SHOULD be used via a secure transport (see 1773 Best Practices section (Section 10.5) ) as the man-in-the-middle 1774 attack may completely subvert them. Even CHAP, which may be 1775 considered resistant to password interception, is unsafe as it does 1776 not protect the username from a trivial man-in-the-middle attack. 1778 This document deprecates the redirection mechanism using the 1779 TAC_PLUS_AUTHEN_STATUS_FOLLOW option which was included in the 1780 original draft. As part of this process, the secret key for a new 1781 server was sent to the client. This public exchange of secret keys 1782 means that once one session is broken, it may be possible to leverage 1783 that key to attacking connections to other servers. This mechanism 1784 MUST NOT be used in modern deployments. It MUST NOT be used outside 1785 a secured deployment. 1787 10.3. Security of Authorization Sessions 1789 Authorization sessions SHOULD be used via a secure transport (see 1790 Best Practices section (Section 10.5) ) as it's trivial to execute a 1791 successful man-in-the-middle attacks that changes well-known 1792 plaintext in either requests or responses. 1794 As an example, take the field "authen_method". It's not unusual in 1795 actual deployments to authorize all commands received via the device 1796 local serial port (a console port) as that one is usually considered 1797 secure by virtue of the device located in a physically secure 1798 location. If an administrator would configure the authorization 1799 system to allow all commands entered by the user on a local console 1800 to aid in troubleshooting, that would give all access to all commands 1801 to any attacker that would be able to change the "authen_method" from 1802 TAC_PLUS_AUTHEN_METH_TACACSPLUS to TAC_PLUS_AUTHEN_METH_LINE. In 1803 this regard, the obfuscation provided by the protocol itself wouldn't 1804 help much, because: 1806 Lack of integrity means that any byte in the payload may be 1807 changed without either side detecting the change. 1809 Known plaintext means that an attacker would know with certainty 1810 which octet is the target of the attack (in this case, 1st octet 1811 after the header). 1813 In combination with known plaintext, the attacker can determine 1814 with certainty the value of the crypto-pad octet used to obfuscate 1815 the original octet. 1817 10.4. Security of Accounting Sessions 1819 Accounting sessions SHOULD be used via a secure transport (see Best 1820 Practices section (Section 10.5). Although Accounting sessions are 1821 not directly involved in authentication or authorizing operations on 1822 the device, man-in-the-middle attacker may do any of the following: 1824 Replace accounting data with new valid or garbage which can 1825 confuse auditors or hide information related to their 1826 authentication and/or authorization attack attempts. 1828 Try and poison accounting log with entries designed to make 1829 systems behave in unintended ways (which includes TACACS+ server 1830 and any other systems that would manage accounting entries). 1832 In addition to these direct manipulations, different client 1833 implementations pass different fidelity of accounting data. Some 1834 vendors have been observed in the wild that pass sensitive data like 1835 passwords, encryption keys and similar as part of the accounting log. 1836 Due to lack of strong encryption with perfect forward secrecy, this 1837 data may be revealed in future, leading to a security incident. 1839 10.5. TACACS+ Best Practices 1841 With respect to the observations about the security issues described 1842 above, a network administrator MUST NOT rely on the obfuscation of 1843 the TACACS+ protocol and TACACS+ MUST be deployed over networks which 1844 ensure privacy and integrity of the communication. TACACS+ MUST be 1845 used within a secure deployment. Failure to do so will impact 1846 overall network security. 1848 TACACS+ SHOULD be deployed over a network which is separated from 1849 other traffic. 1851 The following recommendations impose restrictions on how the protocol 1852 is applied. These restrictions were not imposed in the original 1853 draft. New implementations, and upgrades of current implementations, 1854 MUST implement these recommendations. Vendors SHOULD provide 1855 mechanisms to assist the administrator to achieve these best 1856 practices. 1858 10.5.1. Shared Secrets 1860 TACACS+ servers and clients MUST treat shared secrets as sensitive 1861 data to be managed securely, as would be expected for other sensitive 1862 data such as identity credential information. TACACS+ servers MUST 1863 NOT leak sensitive data. For example, TACACS+ servers MUST NOT 1864 expose shared secrets in logs. 1866 TACACS+ servers MUST allow a dedicated secret key to be defined for 1867 each client. 1869 TACACS+ servers SHOULD warn administrators if secret keys are not 1870 unique per client. 1872 TACACS+ server administrators SHOULD always define a secret for each 1873 client. 1875 TACACS+ servers and clients MUST support shared keys that are at 1876 least 32 characters long. 1878 TACACS+ servers MUST support policy to define minimum complexity for 1879 shared keys. 1881 TACACS+ clients SHOULD NOT allow servers to be configured without 1882 shared secret key, or shared key that is less than 16 characters 1883 long. 1885 TACACS+ server administrators SHOULD configure secret keys of minimum 1886 16 characters length. 1888 TACACS+ server administrators SHOULD change secret keys at regular 1889 intervals. 1891 10.5.2. Connections and Obfuscation 1893 TACACS+ servers MUST allow the definition of individual clients. The 1894 servers MUST only accept network connection attempts from these 1895 defined, known clients. 1897 TACACS+ servers MUST reject connections with 1898 TAC_PLUS_UNENCRYPTED_FLAG set, when there is a shared secret set on 1899 the server for the client requesting the connection. 1901 If an invalid shared secret is detected when processing packets for a 1902 client, TACACS+ servers MUST NOT accept any new sessions on that 1903 connection. TACACS+ servers MUST terminate the connection on 1904 completion of any sessions that were previously established with a 1905 valid shared secret on that connection. 1907 TACACS+ clients MUST NOT set TAC_PLUS_UNENCRYPTED_FLAG when a secret 1908 is defined. Clients MUST be implemented in a way that requires 1909 explicit configuration to enable the use of 1910 TAC_PLUS_UNENCRYPTED_FLAG. 1912 When a TACACS+ client receives responses from servers where: 1914 the response packet was received from the server configured with 1915 shared key, but the packet has TAC_PLUS_UNENCRYPTED_FLAG set. 1917 the response packet was received from the server configured not to 1918 use obfuscation, but the packet has TAC_PLUS_UNENCRYPTED_FLAG not 1919 set. 1921 then the TACACS+ client MUST close TCP session, and process the 1922 response in the same way that a TAC_PLUS_AUTHEN_STATUS_FAIL 1923 (authentication sessions) or TAC_PLUS_AUTHOR_STATUS_FAIL 1924 (authorization sessions) was received. 1926 10.5.3. Authentication 1928 To help TACACS+ administrators select less weak authentication 1929 options, TACACS+ servers MUST allow the administrator to configure 1930 the server to only accept challenge/response options for 1931 authentication (TAC_PLUS_AUTHEN_TYPE_CHAP or 1932 TAC_PLUS_AUTHEN_TYPE_MSCHAP or TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 for 1933 authen_type). 1935 TACACS+ server administrators SHOULD enable the option mentioned in 1936 the previous paragraph. TACACS+ Server deployments SHOULD ONLY 1937 enable other options (such as TAC_PLUS_AUTHEN_TYPE_ASCII or 1938 TAC_PLUS_AUTHEN_TYPE_PAP) when unavoidable due to requirements of 1939 identity/password systems. 1941 TACACS+ server administrators SHOULD NOT allow the same credentials 1942 to be applied in challenge-based (TAC_PLUS_AUTHEN_TYPE_CHAP or 1943 TAC_PLUS_AUTHEN_TYPE_MSCHAP or TAC_PLUS_AUTHEN_TYPE_MSCHAPV2) and non 1944 challenge-based authen_type options as the insecurity of the latter 1945 will compromise the security of the former. 1947 TAC_PLUS_AUTHEN_SENDAUTH and TAC_PLUS_AUTHEN_SENDPASS options 1948 mentioned in the original draft SHOULD NOT be used, due to their 1949 security implications. TACACS+ servers SHOULD NOT implement them. 1950 If they must be implemented, the servers MUST default to the options 1951 being disabled and MUST warn the administrator that these options are 1952 not secure. 1954 10.5.4. Authorization 1956 The authorization and accounting features are intended to provide 1957 extensibility and flexibility. There is a base dictionary defined in 1958 this document, but it may be extended in deployments by using new 1959 attribute names. The cost of the flexibility is that administrators 1960 and implementors MUST ensure that the attribute and value pairs 1961 shared between the clients and servers have consistent 1962 interpretation. 1964 TACACS+ clients that receive an unrecognized mandatory attribute MUST 1965 evaluate server response as if they received 1966 TAC_PLUS_AUTHOR_STATUS_FAIL. 1968 10.5.5. Redirection Mechanism 1970 The original draft described a redirection mechanism 1971 (TAC_PLUS_AUTHEN_STATUS_FOLLOW). This feature is difficult to 1972 secure. The option to send secret keys in the server list is 1973 particularly insecure, as it can reveal client shared secrets. 1975 TACACS+ servers MUST deprecate the redirection mechanism. 1977 If the redirection mechanism is implemented then TACACS+ servers MUST 1978 disable it by default, and MUST warn TACACS+ server administrators 1979 that it must only be enabled within a secure deployment due to the 1980 risks of revealing shared secrets. 1982 TACACS+ clients SHOULD deprecate this feature by treating 1983 TAC_PLUS_AUTHEN_STATUS_FOLLOW as TAC_PLUS_AUTHEN_STATUS_FAIL. 1985 11. IANA Considerations 1987 This informational document describes TACACS+ protocol and its common 1988 deployments. There is no further consideration required from IANA. 1990 12. Acknowledgements 1992 The authors would like to thank the following reviewers whose 1993 comments and contributions made considerable improvements to the 1994 document: Alan DeKok, Alexander Clouter, Chris Janicki, Tom Petch, 1995 Robert Drake, among many others. 1997 The authors would particularly like to thank Alan DeKok, who provided 1998 significant insights and recommendations on all aspects of the 1999 document and the protocol. Alan DeKok has dedicated considerable 2000 time and effort to help improve the document, identifying weaknesses 2001 and providing remediation. 2003 The authors would also like to thank the support from the OPSAWG 2004 Chairs and advisors. 2006 13. References 2008 13.1. Normative References 2010 [RFC0020] Cerf, V., "ASCII format for network interchange", STD 80, 2011 RFC 20, DOI 10.17487/RFC0020, October 1969, 2012 . 2014 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 2015 April 1992. 2017 [RFC1334] Lloyd, B. and W. Simpson, "PPP Authentication Protocols", 2018 RFC 1334, DOI 10.17487/RFC1334, October 1992, 2019 . 2021 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2022 Requirement Levels", BCP 14, RFC 2119, 2023 DOI 10.17487/RFC2119, March 1997, 2024 . 2026 [RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions", 2027 RFC 2433, DOI 10.17487/RFC2433, October 1998, 2028 . 2030 [RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", 2031 RFC 2759, DOI 10.17487/RFC2759, January 2000, 2032 . 2034 [RFC4086] Eastlake 3rd, D., Crocker, S., and J. Schiller, 2035 "Randomness Requirements for Security", RFC 4086, 2036 DOI 10.17487/RFC4086, June 2005, 2037 . 2039 [RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6 2040 Address Text Representation", RFC 5952, 2041 DOI 10.17487/RFC5952, August 2010, 2042 . 2044 [RFC8265] Saint-Andre, P. and A. Melnikov, "Preparation, 2045 Enforcement, and Comparison of Internationalized Strings 2046 Representing Usernames and Passwords", RFC 8265, 2047 DOI 10.17487/RFC8265, October 2017, 2048 . 2050 13.2. Informative References 2052 [TheDraft] 2053 Carrel, D. and L. Grant, "The TACACS+ Protocol Version 2054 1.78", June 1997, 2055 . 2057 [TZDB] Eggert, P. and A. Olson, "Sources for Time Zone and 2058 Daylight Saving Time Data", 1987, 2059 . 2061 Authors' Addresses 2062 Thorsten Dahm 2063 Google Inc 2064 1600 Amphitheatre Parkway 2065 Mountain View, CA 94043 2066 US 2068 EMail: thorstendlux@google.com 2070 Andrej Ota 2071 Google Inc 2072 1600 Amphitheatre Parkway 2073 Mountain View, CA 94043 2074 US 2076 EMail: andrej@ota.si 2078 Douglas C. Medway Gash 2079 Cisco Systems, Inc. 2080 170 West Tasman Dr. 2081 San Jose, CA 95134 2082 US 2084 EMail: dcmgash@cisco.com 2086 David Carrel 2087 vIPtela, Inc. 2088 1732 North First St. 2089 San Jose, CA 95112 2090 US 2092 EMail: dcarrel@viptela.com 2094 Lol Grant 2096 EMail: lol.grant@gmail.com