idnits 2.17.1 draft-ietf-opsawg-tacacs-17.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == The document seems to contain a disclaimer for pre-RFC5378 work, but was first submitted on or after 10 November 2008. The disclaimer is usually necessary only for documents that revise or obsolete older RFCs, and that take significant amounts of text from those RFCs. If you can contact all authors of the source material and they are willing to grant the BCP78 rights to the IETF Trust, you can and should remove the disclaimer. Otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (January 27, 2020) is 1551 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- == Missing Reference: 'RFC8174' is mentioned on line 184, but not defined ** Obsolete normative reference: RFC 1334 (Obsoleted by RFC 1994) Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). 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: July 30, 2020 D. Medway Gash 6 Cisco Systems, Inc. 7 D. Carrel 8 vIPtela, Inc. 9 L. Grant 10 January 27, 2020 12 The TACACS+ Protocol 13 draft-ietf-opsawg-tacacs-17 15 Abstract 17 This document describes the Terminal Access Controller Access-Control 18 System Plus (TACACS+) protocol which is widely deployed today to 19 provide Device Administration for routers, network access servers and 20 other networked computing devices via one or more centralized 21 servers. 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 July 30, 2020. 40 Copyright Notice 42 Copyright (c) 2020 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 . . . . . . . . . . . . . . . . . . . . . . . . . 5 73 3.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 5 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 . . . . . . . . . . . . . . . . 6 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 . . . . . . . . . . . . . . . . . 8 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 . . . . . . . . . 17 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. Argument-Value Pairs . . . . . . . . . . . . . . . . . . . . 31 100 8.1. Value Encoding . . . . . . . . . . . . . . . . . . . . . 32 101 8.2. Authorization Arguments . . . . . . . . . . . . . . . . . 33 102 8.3. Accounting Arguments . . . . . . . . . . . . . . . . . . 35 103 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 . . . . . . . . . . . . . . . 43 115 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 . . . . . . . . . . . . . . . . . . . . . . . 45 122 1. Introduction 124 This document describes the Terminal Access Controller Access-Control 125 System Plus (TACACS+) protocol. It was conceived initially as a 126 general Authentication, Authorization and Accounting (AAA) protocol. 127 It is widely deployed today but is mainly confined for a specific 128 subset of AAA: Device Administration, that is: authenticating access 129 to network devices, providing central authorization of operations, 130 and audit of those operations. 132 A wide range of TACACS+ clients and servers are already deployed in 133 the field. The TACACS+ protocol they are based on is defined in a 134 draft document that was originally intended for IETF publication, but 135 was never standardized. The draft document is known as `The Draft' 136 [TheDraft]. 138 This Draft was a product of its time, and did not address all of the 139 key security concerns which are considered when designing modern 140 standards. Deployment must therefore be executed with care. These 141 concerns are addressed in the security section (Section 10). 143 The primary intent of this informational document is to clarify the 144 subset of `The Draft' which is common to implementations supporting 145 Device Administration. It is intended that all implementations which 146 conform to this document will conform to `The Draft'. However, it is 147 not intended that all implementations which conform to 'The Draft' 148 will conform to this document. The following features from `The 149 Draft' have been removed: 151 This document officially removes SENDPASS for security reasons. 153 The normative description of Legacy features such as ARAP and 154 outbound authentication has been removed. 156 The Support for forwarding to an alternative daemon 157 (TAC_PLUS_AUTHEN_STATUS_FOLLOW) has been deprecated. 159 The TACACS+ protocol allows for arbitrary length and content 160 authentication exchanges, to support alternative authentication 161 mechanisms. It is extensible to provide for site customization and 162 future development features, and it uses TCP to ensure reliable 163 delivery. The protocol allows the TACACS+ client to request fine- 164 grained access control and allows the server to respond to each 165 component of that request. 167 The separation of authentication, authorization and accounting is a 168 key element of the design of TACACS+ protocol. Essentially it makes 169 TACACS+ a suite of three protocols. This document will address each 170 one in separate sections. Although TACACS+ defines all three, an 171 implementation or deployment is not required to employ all three. 172 Separating the elements is useful for Device Administration use case, 173 specifically, for authorization of individual commands in a session. 174 Note that there is no provision made at the protocol level for 175 association of an authentication to authorization requests. 177 2. Conventions 179 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 180 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 181 "OPTIONAL" in this document are to be interpreted as described in BCP 182 14 [RFC2119] [RFC8174] when, and only when, they appear in all 183 capitals, as shown here. 185 3. Technical Definitions 187 This section provides a few basic definitions that are applicable to 188 this document 190 3.1. Client 192 The client is any device which initiates TACACS+ protocol requests to 193 mediate access, mainly for the Device Administration use case. 195 3.2. Server 197 The server receives TACACS+ protocol requests, and replies according 198 to its business model, in accordance with the flows defined in this 199 document. 201 3.3. Packet 203 All uses of the word packet in this document refer to TACACS+ 204 protocol data units unless explicitly noted otherwise. The informal 205 term "Packet" has become an established part of the definition. 207 3.4. Connection 209 TACACS+ uses TCP for its transport. TCP Server port 49 is allocated 210 by IANA for TACACS+ traffic. 212 3.5. Session 214 The concept of a session is used throughout this document. A TACACS+ 215 session is a single authentication sequence, a single authorization 216 exchange, or a single accounting exchange. 218 An accounting and authorization session will consist of a single pair 219 of packets (the request and its reply). An authentication session 220 may involve an arbitrary number of packets being exchanged. The 221 session is an operational concept that is maintained between the 222 TACACS+ client and server. It does not necessarily correspond to a 223 given user or user action. 225 3.6. Treatment of Enumerated Protocol Values 227 This document describes various enumerated values in the packet 228 header and the headers for specific packet types. For example, in 229 the Authentication start packet type, this document defines the 230 action field with three values TAC_PLUS_AUTHEN_LOGIN, 231 TAC_PLUS_AUTHEN_CHPASS and TAC_PLUS_AUTHEN_SENDAUTH. 233 If the server does not implement one of the defined options in a 234 packet that it receives, or it encounters an option that is not 235 listed in this document for a header field, then it should respond 236 with an ERROR and terminate the session. This will allow the client 237 to try a different option. 239 If an error occurs but the type of the incoming packet cannot be 240 determined, a packet with the identical cleartext header but with a 241 sequence number incremented by one and the length set to zero MUST be 242 returned to indicate an error. 244 3.7. Treatment of Text Strings 246 The TACACS+ protocol makes extensive use of text strings. The 247 original draft intended that these strings would be treated as byte 248 arrays where each byte would represent a US-ASCII character. 250 More recently, server implementations have been extended to interwork 251 with external identity services, and so a more nuanced approach is 252 needed. Usernames MUST be encoded and handled using the 253 UsernameCasePreserved Profile specified in RFC 8265 [RFC8265]. The 254 security considerations in Section 8 of that RFC apply. 256 Where specifically mentioned, data fields contain arrays of arbitrary 257 bytes as required for protocol processing. These are not intended to 258 be made visible through user interface to users. 260 All other text fields in TACACS+ MUST be treated as printable byte 261 arrays of US-ASCII as defined by RFC 20 [RFC0020]. The term 262 "printable" used here means the fields MUST exclude the "Control 263 Characters" defined in section 5.2 of RFC 20 [RFC0020]. 265 4. TACACS+ Packets and Sessions 267 4.1. The TACACS+ Packet Header 269 All TACACS+ packets begin with the following 12-byte header. The 270 header describes the remainder of the packet: 272 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 8 273 +----------------+----------------+----------------+----------------+ 274 |major | minor | | | | 275 |version| version| type | seq_no | flags | 276 +----------------+----------------+----------------+----------------+ 277 | | 278 | session_id | 279 +----------------+----------------+----------------+----------------+ 280 | | 281 | length | 282 +----------------+----------------+----------------+----------------+ 284 The following general rules apply to all TACACS+ packet types: 286 - To signal that any variable length data fields are unused, the 287 corresponding length values are set to zero. Such fields MUST be 288 ignored, and treated as if not present. 290 - the lengths of data and message fields in a packet are specified 291 by their corresponding length fields, (and are not null 292 terminated.) 294 - All length values are unsigned and in network byte order. 296 major_version 298 This is the major TACACS+ version number. 300 TAC_PLUS_MAJOR_VER := 0xc 302 minor_version 304 The minor TACACS+ version number. 306 TAC_PLUS_MINOR_VER_DEFAULT := 0x0 308 TAC_PLUS_MINOR_VER_ONE := 0x1 310 type 312 This is the packet type. Options are: 314 TAC_PLUS_AUTHEN := 0x01 (Authentication) 316 TAC_PLUS_AUTHOR := 0x02 (Authorization) 318 TAC_PLUS_ACCT := 0x03 (Accounting) 320 seq_no 322 This is the sequence number of the current packet. The first packet 323 in a session MUST have the sequence number 1 and each subsequent 324 packet will increment the sequence number by one. TACACS+ Clients 325 only send packets containing odd sequence numbers, and TACACS+ 326 servers only send packets containing even sequence numbers. 328 The sequence number must never wrap i.e. if the sequence number 2^8-1 329 is ever reached, that session must terminate and be restarted with a 330 sequence number of 1. 332 flags 333 This field contains various bitmapped flags. 335 The flag bit: 337 TAC_PLUS_UNENCRYPTED_FLAG := 0x01 339 This flag indicates that the sender did not obfuscate the body of the 340 packet. This option MUST NOT be used in production. The application 341 of this flag will be covered in the security section (Section 10). 343 This flag SHOULD be clear in all deployments. Modern network traffic 344 tools support encrypted traffic when configured with the shared 345 secret (see section below), so obfuscated mode can and SHOULD be used 346 even during test. 348 The single-connection flag: 350 TAC_PLUS_SINGLE_CONNECT_FLAG := 0x04 352 This flag is used to allow a client and server to negotiate Single 353 Connection Mode (Section 4.3). 355 All other bits MUST be ignored when reading, and SHOULD be set to 356 zero when writing. 358 session_id 360 The Id for this TACACS+ session. This field does not change for the 361 duration of the TACACS+ session. This number MUST be generated by a 362 cryptographically strong random number generation method. Failure to 363 do so will compromise security of the session. For more details 364 refer to RFC 4086 [RFC4086]. 366 length 368 The total length of the packet body (not including the header). 370 4.2. The TACACS+ Packet Body 372 The TACACS+ body types are defined in the packet header. The next 373 sections of this document will address the contents of the different 374 TACACS+ bodies. 376 4.3. Single Connection Mode 378 Single Connection Mode is intended to improve performance where there 379 is a lot of traffic between a client and a server by allowing the 380 client to multiplex multiple session on a single TCP connection. 382 The packet header contains the TAC_PLUS_SINGLE_CONNECT_FLAG used by 383 the client and server to negotiate the use of Single Connect Mode. 385 The client sets this flag, to indicate that it supports multiplexing 386 TACACS+ sessions over a single TCP connection. The client MUST NOT 387 send a second packet on a connection until single-connect status has 388 been established. 390 To indicate it will support Single Connection Mode, the server sets 391 this flag in the first reply packet in response to the first request 392 from a client. The server may set this flag even if the client does 393 not set it, but the client may ignore the flag and close the 394 connection after the session completes. 396 The flag is only relevant for the first two packets on a connection, 397 to allow the client and server to establish Single Connection Mode. 398 No provision is made for changing Single Connection Mode after the 399 first two packets: the client and server MUST ignore the flag after 400 the second packet on a connection. 402 If single Connection Mode has not been established in the first two 403 packets of a TCP connection, then both the client and the server 404 close the connection at the end of the first session. 406 The client negotiates Single Connection Mode to improve efficiency. 407 The server may refuse to allow Single Connection Mode for the client. 408 For example, it may not be appropriate to allocate a long-lasting TCP 409 connection to a specific client in some deployments. Even if the 410 server is configured to permit single Connection Mode for a specific 411 client, the server may close the connection. For example: a server 412 MUST be configured to time out a Single Connection Mode TCP 413 Connection after a specific period of inactivity to preserve its 414 resources. The client MUST accommodate such closures on a TCP 415 session even after Single Connection Mode has been established. 417 The TCP connection underlying the Single Connection Mode will close 418 eventually, either because of the timeout from the server or from an 419 intermediate link. If a session is in progress when the client 420 detects disconnect then the client should handle it as described in 421 Section 4.4. If a session is not in progress, then the client will 422 need to detect this, and restart the single connection mode when the 423 it initiates the next session. 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 cannot apply the result and 441 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 Section 5.4.3 on Aborting Authentication Sessions for 448 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 4.3), but may still be closed after a timeout 459 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 4.5), 463 then any further new sessions MUST NOT be accepted on the connection. 464 If there are any sessions that have already been established then 465 they MAY be completed. Once all active sessions are completed then 466 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 MUST be configured with the following bit as follows: 497 TAC_PLUS_UNENCRYPTED_FLAG = 0x0 499 So that the packet body is obfuscated by XOR-ing it byte-wise with a 500 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 514 [RFC1321]. 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 4.4. 536 TAC_PLUS_UNENCRYPTED_FLAG == 0x1 538 This option is deprecated and MUST NOT be used in production. In 539 this case, the entire packet body is in cleartext. A request MUST be 540 dropped if TAC_PLUS_UNENCRYPTED_FLAG is set to true. 542 After a packet body is de-obfuscated, the lengths of the component 543 values in the packet are summed. If the sum is not identical to the 544 cleartext datalength value from the header, the packet MUST be 545 discarded, and an ERROR signaled. For details of TCP connection 546 handling on ERROR, refer to Section 4.4. 548 Commonly such failures are seen when the keys are mismatched between 549 the client and the TACACS+ server. 551 5. Authentication 553 Authentication is the action of determining who a user (or entity) 554 is. Authentication can take many forms. Traditional authentication 555 employs a name and a fixed password. However, fixed passwords are 556 vulnerable security, so many modern authentication mechanisms utilize 557 "one-time" passwords or a challenge-response query. TACACS+ is 558 designed to support all of these, and be flexible enough to handle 559 any future mechanisms. Authentication generally takes place when the 560 user first logs in to a machine or requests a service of it. 562 Authentication is not mandatory; it is a site-configured option. 563 Some sites do not require it. Others require it only for certain 564 services (see authorization below). Authentication may also take 565 place when a user attempts to gain extra privileges, and must 566 identify himself or herself as someone who possesses the required 567 information (passwords, etc.) for those privileges. 569 5.1. The Authentication START Packet Body 571 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 8 572 +----------------+----------------+----------------+----------------+ 573 | action | priv_lvl | authen_type | authen_service | 574 +----------------+----------------+----------------+----------------+ 575 | user_len | port_len | rem_addr_len | data_len | 576 +----------------+----------------+----------------+----------------+ 577 | user ... 578 +----------------+----------------+----------------+----------------+ 579 | port ... 580 +----------------+----------------+----------------+----------------+ 581 | rem_addr ... 582 +----------------+----------------+----------------+----------------+ 583 | data... 584 +----------------+----------------+----------------+----------------+ 586 Packet fields are as follows: 588 action 590 This indicates the authentication action. Valid values are listed 591 below. 593 TAC_PLUS_AUTHEN_LOGIN := 0x01 595 TAC_PLUS_AUTHEN_CHPASS := 0x02 597 TAC_PLUS_AUTHEN_SENDAUTH := 0x04 599 priv_lvl 601 This indicates the privilege level that the user is authenticating 602 as. Please refer to the Privilege Level section (Section 9) below. 604 authen_type 606 The type of authentication. Please see section Common Authentication 607 Flows (Section 5.4.2). Valid values are: 609 TAC_PLUS_AUTHEN_TYPE_ASCII := 0x01 611 TAC_PLUS_AUTHEN_TYPE_PAP := 0x02 613 TAC_PLUS_AUTHEN_TYPE_CHAP := 0x03 615 TAC_PLUS_AUTHEN_TYPE_MSCHAP := 0x05 616 TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 := 0x06 618 authen_service 620 This is the service that is requesting the authentication. Valid 621 values are: 623 TAC_PLUS_AUTHEN_SVC_NONE := 0x00 625 TAC_PLUS_AUTHEN_SVC_LOGIN := 0x01 627 TAC_PLUS_AUTHEN_SVC_ENABLE := 0x02 629 TAC_PLUS_AUTHEN_SVC_PPP := 0x03 631 TAC_PLUS_AUTHEN_SVC_PT := 0x05 633 TAC_PLUS_AUTHEN_SVC_RCMD := 0x06 635 TAC_PLUS_AUTHEN_SVC_X25 := 0x07 637 TAC_PLUS_AUTHEN_SVC_NASI := 0x08 639 TAC_PLUS_AUTHEN_SVC_FWPROXY := 0x09 641 The TAC_PLUS_AUTHEN_SVC_NONE option is intended for the authorization 642 application of this field that indicates that no authentication was 643 performed by the device. 645 The TAC_PLUS_AUTHEN_SVC_LOGIN option indicates regular login (as 646 opposed to ENABLE) to a client device. 648 The TAC_PLUS_AUTHEN_SVC_ENABLE option identifies the ENABLE 649 authen_service, which refers to a service requesting authentication 650 in order to grant the user different privileges. This is comparable 651 to the Unix "su(1)" command, which substitutes the current user's 652 identity with another. An authen_service value of NONE is only to be 653 used when none of the other authen_service values are appropriate. 654 ENABLE may be requested independently, no requirements for previous 655 authentications or authorizations are imposed by the protocol. 657 Other options are included for legacy/backwards compatibility. 659 user, user_len 661 The username is optional in this packet, depending upon the class of 662 authentication. If it is absent, the client MUST set user_len to 0. 664 If included, the user_len indicates the length of the user field, in 665 bytes. 667 port, port_len 669 The name of the client port on which the authentication is taking 670 place. The value of this field is free format text and is client 671 specific. Examples of this this argument include "tty10" to denote 672 the tenth tty line and "async10" to denote the tenth async interface. 673 The client documentation SHOULD define the values and their meanings 674 for this field. For details of text encoding, see (Section 3.7). 675 port_len indicates the length of the port field, in bytes. 677 rem_addr, rem_addr_len 679 A string indicating the remote location from which the user has 680 connected to the client. For details of text encoding, see 681 (Section 3.7). 683 When TACACS+ was used for dial-up services, this value contained the 684 caller ID 686 When TACACS+ is used for Device Administration, the user is normally 687 connected via a network, and in this case the value is intended to 688 hold a network address, IPv4 or IPv6. For IPv6 address text 689 representation defined please see RFC 5952 [RFC5952]. 691 This field is optional (since the information may not be available). 692 The rem_addr_len indicates the length of the user field, in bytes. 694 data, data_len 696 This field is used to send data appropriate for the action and 697 authen_type. It is described in more detail in the section Common 698 Authentication flows (Section 5.4.2). The data_len indicates the 699 length of the data field, in bytes. 701 5.2. The Authentication REPLY Packet Body 703 The TACACS+ server sends only one type of authentication packet (a 704 REPLY packet) to the client. 706 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 8 707 +----------------+----------------+----------------+----------------+ 708 | status | flags | server_msg_len | 709 +----------------+----------------+----------------+----------------+ 710 | data_len | server_msg ... 711 +----------------+----------------+----------------+----------------+ 712 | data ... 713 +----------------+----------------+ 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 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 a 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 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 arguments used in 1055 authorization, see the Authorization Arguments section (Section 8.2). 1057 In the TACACS+ protocol an authorization is always a single pair of 1058 messages: a REQUEST from the client followed by a REPLY from the 1059 server. 1061 The authorization REQUEST message contains a fixed set of fields that 1062 indicate how the user was authenticated and a variable set of 1063 arguments that describe the services and options for which 1064 authorization is requested. 1066 The REPLY contains a variable set of response arguments (argument- 1067 value pairs) that can restrict or modify the client's actions. 1069 6.1. The Authorization REQUEST Packet Body 1070 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 1071 +----------------+----------------+----------------+----------------+ 1072 | authen_method | priv_lvl | authen_type | authen_service | 1073 +----------------+----------------+----------------+----------------+ 1074 | user_len | port_len | rem_addr_len | arg_cnt | 1075 +----------------+----------------+----------------+----------------+ 1076 | arg_1_len | arg_2_len | ... | arg_N_len | 1077 +----------------+----------------+----------------+----------------+ 1078 | user ... 1079 +----------------+----------------+----------------+----------------+ 1080 | port ... 1081 +----------------+----------------+----------------+----------------+ 1082 | rem_addr ... 1083 +----------------+----------------+----------------+----------------+ 1084 | arg_1 ... 1085 +----------------+----------------+----------------+----------------+ 1086 | arg_2 ... 1087 +----------------+----------------+----------------+----------------+ 1088 | ... 1089 +----------------+----------------+----------------+----------------+ 1090 | arg_N ... 1091 +----------------+----------------+----------------+----------------+ 1093 authen_method 1095 This indicates the authentication method used by the client to 1096 acquire the user information. As this information is not always 1097 subject to verification, it is recommended that this field is 1098 ignored. 1100 TAC_PLUS_AUTHEN_METH_NOT_SET := 0x00 1102 TAC_PLUS_AUTHEN_METH_NONE := 0x01 1104 TAC_PLUS_AUTHEN_METH_KRB5 := 0x02 1106 TAC_PLUS_AUTHEN_METH_LINE := 0x03 1108 TAC_PLUS_AUTHEN_METH_ENABLE := 0x04 1110 TAC_PLUS_AUTHEN_METH_LOCAL := 0x05 1112 TAC_PLUS_AUTHEN_METH_TACACSPLUS := 0x06 1114 TAC_PLUS_AUTHEN_METH_GUEST := 0x08 1116 TAC_PLUS_AUTHEN_METH_RADIUS := 0x10 1117 TAC_PLUS_AUTHEN_METH_KRB4 := 0x11 1119 TAC_PLUS_AUTHEN_METH_RCMD := 0x20 1121 KRB5 and KRB4 are Kerberos version 5 and 4. This document does not 1122 cover how the client performed the authentication, so normative 1123 references will not be given . LINE refers to a fixed password 1124 associated with the terminal line used to gain access. LOCAL is a 1125 client local user database. ENABLE is a command that authenticates 1126 in order to grant new privileges. TACACSPLUS is, of course, TACACS+. 1127 GUEST is an unqualified guest authentication. RADIUS is the Radius 1128 authentication protocol. RCMD refers to authentication provided via 1129 the R-command protocols from Berkeley 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 argument-value pairs. The argument 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 argument name MUST NOT contain either of the separators. An 1188 argument 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 argument, 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 argument-value 1199 pair may be empty, that is: the length of the value may be zero. 1201 Argument-value strings are not NULL terminated, rather their length 1202 value indicates their end. The maximum length of an argument-value 1203 string is 255 characters. The minimum is two characters (one name- 1204 value character and the separator) 1206 Though the arguments allow extensibility, a common core set of 1207 authorization arguments SHOULD be supported by clients and servers, 1208 these are listed in the Authorization Arguments (Section 8.2) section 1209 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 Arguments (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 argument-value pairs (if any) in the 1279 response, instead of the authorization argument-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 Session Completion (Section 4.4). None of the arg values have any 1288 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 arguments 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 the Accounting Arguments section (Section 8.3) for the dictionary 1355 of arguments 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. Argument-Value Pairs 1445 TACACS+ is intended to be an extensible protocol. The arguments used 1446 in Authorization and Accounting are not limited by this document. 1447 Some arguments are defined below for common use cases, clients MUST 1448 use these arguments when supporting the corresponding use cases. 1450 8.1. Value Encoding 1452 All argument values are encoded as strings. For details of text 1453 encoding, see (Section 3.7). The following type representations 1454 SHOULD be followed 1456 Numeric 1458 All numeric values in an argument-value string are provided as 1459 decimal numbers, unless otherwise stated. All arguments include a 1460 length field, and TACACS+ implementations MUST verify that they can 1461 accommodate the lengths of numeric arguments before attempting to 1462 process them. If the length cannot be accommodated then the argument 1463 MUST be regarded as not handled and the logic in authorization 1464 section (Section 6.1) regarding the processing of arguments MUST be 1465 applied. 1467 Boolean 1469 All Boolean arguments are encoded with values "true" or "false". 1471 IP-Address 1473 It is recommended that hosts be specified as a IP address so as to 1474 avoid any ambiguities. For details of text encoding, see 1475 (Section 3.7). IPv4 address are specified as octet numerics 1476 separated by dots ('.'), IPv6 address text representation defined in 1477 RFC 5952 [RFC5952]. 1479 Date Time 1481 Absolute date/times are specified in seconds since the epoch, 12:00am 1482 Jan 1 1970. The timezone MUST be UTC unless a timezone argument is 1483 specified. 1485 String 1487 Many values have no specific type representation and are interpreted 1488 as plain strings. 1490 Empty Values 1492 Arguments may be submitted with no value, in which case they consist 1493 of the name and the mandatory or optional separator. For example, 1494 the argument "cmd" which has no value is transmitted as a string of 1495 four characters "cmd=" 1497 8.2. Authorization Arguments 1499 service (String) 1501 The primary service. Specifying a service argument indicates that 1502 this is a request for authorization or accounting of that service. 1503 For example: "shell", "tty-server", "connection", "system" and 1504 "firewall", others may be chosen for the required application. This 1505 argument MUST always be included. 1507 protocol (String) 1509 the protocol field may be used to indicate a subset of a service. 1511 cmd (String) 1513 a shell (exec) command. This indicates the command name of the 1514 command that is to be run. The "cmd" argument MUST be specified if 1515 service equals "shell". 1517 Authorization of shell commands is a common use-case for the TACACS+ 1518 protocol. Command Authorization generally takes one of two forms: 1519 session-based and command-based. 1521 For session-based shell authorization, the "cmd" argument will have 1522 an empty value. The client determines which commands are allowed in 1523 a session according to the arguments present in the authorization. 1525 In command-based authorization, the client requests that the server 1526 determine whether a command is allowed by making an authorization 1527 request for each command. The "cmd" argument will have the command 1528 name as its value. 1530 cmd-arg (String) 1532 an argument to a shell (exec) command. This indicates an argument 1533 for the shell command that is to be run. Multiple cmd-arg arguments 1534 may be specified, and they are order dependent. 1536 acl (Numeric) 1538 a number representing a connection access list. Applicable only to 1539 session-based shell authorization. For details of text encoding, see 1540 (Section 3.7). 1542 inacl (String) 1543 identifier (name) of an interface input access list. For details of 1544 text encoding, see (Section 3.7). 1546 outacl (String) 1548 identifier (name) of an interface output access list. For details of 1549 text encoding, see (Section 3.7). 1551 addr (IP-Address) 1553 a network address 1555 addr-pool (String) 1557 The identifier of an address pool from which the client can assign an 1558 address. 1560 timeout (Numeric) 1562 an absolute timer for the connection (in minutes). A value of zero 1563 indicates no timeout. 1565 idletime (Numeric) 1567 an idle-timeout for the connection (in minutes). A value of zero 1568 indicates no timeout. 1570 autocmd (String) 1572 an auto-command to run. Applicable only to session-based shell 1573 authorization. 1575 noescape (Boolean) 1577 Prevents user from using an escape character. Applicable only to 1578 session-based shell authorization. 1580 nohangup (Boolean) 1582 Boolean. Do not disconnect after an automatic command. Applicable 1583 only to session-based shell authorization. 1585 priv-lvl (Numeric) 1587 privilege level to be assigned. Please refer to the Privilege Level 1588 section (Section 9) below. 1590 8.3. Accounting Arguments 1592 The following arguments are defined for TACACS+ accounting only. 1593 They MUST precede any argument-value pairs that are defined in the 1594 authorization section (Section 6) above. 1596 task_id (String) 1598 Start and stop records for the same event MUST have matching task_id 1599 argument values. The client MUST ensure that active task_ids are not 1600 duplicated: a client MUST NOT reuse a task_id a start record until it 1601 has sent a stop record for that task_id. Servers MUST NOT make 1602 assumptions about the format of a task_id. 1604 start_time (Date Time) 1606 The time the action started (in seconds since the epoch.). 1608 stop_time (Date Time) 1610 The time the action stopped (in seconds since the epoch.) 1612 elapsed_time (Numeric) 1614 The elapsed time in seconds for the action. 1616 timezone (String) 1618 The timezone abbreviation for all timestamps included in this packet. 1619 A database of timezones is maintained here: TZDB [TZDB]. 1621 event (String) 1623 Used only when "service=system". Current values are "net_acct", 1624 "cmd_acct", "conn_acct", "shell_acct" "sys_acct" and "clock_change". 1625 These indicate system-level changes. The flags field SHOULD indicate 1626 whether the service started or stopped. 1628 reason (String) 1630 Accompanies an event argument. It describes why the event occurred. 1632 bytes (Numeric) 1634 The number of bytes transferred by this action 1636 bytes_in (Numeric) 1637 The number of bytes transferred by this action from the endstation to 1638 the client port 1640 bytes_out (Numeric) 1642 The number of bytes transferred by this action from the client to the 1643 endstation port 1645 paks (Numeric) 1647 The number of packets transferred by this action. 1649 paks_in (Numeric) 1651 The number of input packets transferred by this action from the 1652 endstation to the client port. 1654 paks_out (Numeric) 1656 The number of output packets transferred by this action from the 1657 client port to the endstation. 1659 err_msg (String) 1661 string describing the status of the action. For details of text 1662 encoding, see (Section 3.7). 1664 9. Privilege Levels 1666 The TACACS+ Protocol supports flexible authorization schemes through 1667 the extensible arguments. 1669 One scheme is built into the protocol and has been extensively used 1670 for Session-based shell authorization: Privilege Levels. Privilege 1671 Levels are ordered values from 0 to 15 with each level being a 1672 superset of the next lower value. Configuration and implementation 1673 of the client will map actions (such as the permission to execute of 1674 specific commands) to different privilege levels. The allocation of 1675 commands to privilege levels is highly dependent upon the deployment. 1676 Common allocations are as follows: 1678 TAC_PLUS_PRIV_LVL_MIN := 0x00. The level normally allocated to an 1679 unauthenticated session. 1681 TAC_PLUS_PRIV_LVL_USER := 0x01. The level normally allocated to a 1682 regular authenticated session 1683 TAC_PLUS_PRIV_LVL_ROOT := 0x0f. The level normally allocated to a 1684 session authenticated by a highly privileged user to allow 1685 commands with significant system impact. 1687 TAC_PLUS_PRIV_LVL_MAX := 0x0f. The highest privilege level. 1689 A Privilege level can be assigned to a shell (EXEC) session when it 1690 starts. The client will permit the actions associated with this 1691 level to be executed. This privilege level is returned by the Server 1692 in a session-based shell authorization (when "service" equals "shell" 1693 and "cmd" is empty). When a user required to perform actions that 1694 are mapped to a higher privilege level, then an ENABLE type 1695 reauthentication can be initiated by the client. The client will 1696 insert the required privilege level into the authentication header 1697 for enable authentication request. 1699 The use of Privilege levels to determine session-based access to 1700 commands and resources is not mandatory for clients. Although the 1701 privilege level scheme is widely supported, its lack of flexibility 1702 in requiring a single monotonic hierarchy of permissions means that 1703 other session-based command authorization schemes have evolved. 1704 However, it is still common enough that it SHOULD be supported by 1705 servers. 1707 10. Security Considerations 1709 The original TACACS+ Draft `The Draft' [TheDraft] from 1998 did not 1710 address all of the key security concerns which are considered when 1711 designing modern standards. This section addresses known limitations 1712 and concerns which will impact overall security of the protocol and 1713 systems where this protocol is deployed to manage central 1714 authentication, authorization or accounting for network device 1715 administration. 1717 Multiple implementations of the protocol described in the original 1718 TACACS+ Draft `The Draft' [TheDraft] have been deployed. As the 1719 protocol was never standardized, current implementations may be 1720 incompatible in non-obvious ways, giving rise to additional security 1721 risks. This section does not claim to enumerate all possible 1722 security vulnerabilities. 1724 10.1. General Security of the Protocol 1726 TACACS+ protocol does not include a security mechanism that would 1727 meet modern-day requirements. These security mechanisms would be 1728 best referred to as "obfuscation" and not "encryption" since they 1729 provide no meaningful integrity, privacy or replay protection. An 1730 attacker with access to the data stream should be assumed to be able 1731 to read and modify all TACACS+ packets. Without mitigation, a range 1732 of risks such as the following are possible: 1734 Accounting information may be modified by the man-in-the-middle 1735 attacker, making such logs unsuitable and not trustable for 1736 auditing purposes. 1738 Invalid or misleading values may be inserted by the man-in-the- 1739 middle attacker in various fields at known offsets to try and 1740 circumvent the authentication or authorization checks even inside 1741 the obfuscated body. 1743 While the protocol provides some measure of transport privacy, it is 1744 vulnerable to at least the following attacks: 1746 Brute force attacks exploiting increased efficiency of MD5 digest 1747 computation. 1749 Known plaintext attacks which may decrease the cost of brute force 1750 attack. 1752 Chosen plaintext attacks which may decrease the cost of a brute 1753 force attack. 1755 No forward secrecy. 1757 Even though, to the best knowledge of authors, this method of 1758 encryption wasn't rigorously tested, enough information is available 1759 that it is best referred to as "obfuscation" and not "encryption". 1761 For these reasons, users deploying TACACS+ protocol in their 1762 environments MUST limit access to known clients and MUST control the 1763 security of the entire transmission path. Attackers who can guess 1764 the key or otherwise break the obfuscation will gain unrestricted and 1765 undetected access to all TACACS+ traffic. Ensuring that a 1766 centralized AAA system like TACACS+ is deployed on a secured 1767 transport is essential to managing the security risk of such an 1768 attack. 1770 The following parts of this section enumerate only the session- 1771 specific risks which are in addition to general risk associated with 1772 bare obfuscation and lack of integrity checking. 1774 10.2. Security of Authentication Sessions 1776 Authentication sessions SHOULD be used via a secure transport (see 1777 Best Practices section (Section 10.5)) as the man-in-the-middle 1778 attack may completely subvert them. Even CHAP, which may be 1779 considered resistant to password interception, is unsafe as it does 1780 not protect the username from a trivial man-in-the-middle attack. 1782 This document deprecates the redirection mechanism using the 1783 TAC_PLUS_AUTHEN_STATUS_FOLLOW option which was included in the 1784 original draft. As part of this process, the secret key for a new 1785 server was sent to the client. This public exchange of secret keys 1786 means that once one session is broken, it may be possible to leverage 1787 that key to attacking connections to other servers. This mechanism 1788 MUST NOT be used in modern deployments. It MUST NOT be used outside 1789 a secured deployment. 1791 10.3. Security of Authorization Sessions 1793 Authorization sessions SHOULD be used via a secure transport (see 1794 Best Practices section (Section 10.5)) as it's trivial to execute a 1795 successful man-in-the-middle attacks that changes well-known 1796 plaintext in either requests or responses. 1798 As an example, take the field "authen_method". It's not unusual in 1799 actual deployments to authorize all commands received via the device 1800 local serial port (a console port) as that one is usually considered 1801 secure by virtue of the device located in a physically secure 1802 location. If an administrator would configure the authorization 1803 system to allow all commands entered by the user on a local console 1804 to aid in troubleshooting, that would give all access to all commands 1805 to any attacker that would be able to change the "authen_method" from 1806 TAC_PLUS_AUTHEN_METH_TACACSPLUS to TAC_PLUS_AUTHEN_METH_LINE. In 1807 this regard, the obfuscation provided by the protocol itself wouldn't 1808 help much, because: 1810 Lack of integrity means that any byte in the payload may be 1811 changed without either side detecting the change. 1813 Known plaintext means that an attacker would know with certainty 1814 which octet is the target of the attack (in this case, 1st octet 1815 after the header). 1817 In combination with known plaintext, the attacker can determine 1818 with certainty the value of the crypto-pad octet used to obfuscate 1819 the original octet. 1821 10.4. Security of Accounting Sessions 1823 Accounting sessions SHOULD be used via a secure transport (see Best 1824 Practices section (Section 10.5). Although Accounting sessions are 1825 not directly involved in authentication or authorizing operations on 1826 the device, man-in-the-middle attacker may do any of the following: 1828 Replace accounting data with new valid or garbage which can 1829 confuse auditors or hide information related to their 1830 authentication and/or authorization attack attempts. 1832 Try and poison accounting log with entries designed to make 1833 systems behave in unintended ways (which includes TACACS+ server 1834 and any other systems that would manage accounting entries). 1836 In addition to these direct manipulations, different client 1837 implementations pass different fidelity of accounting data. Some 1838 vendors have been observed in the wild that pass sensitive data like 1839 passwords, encryption keys and similar as part of the accounting log. 1840 Due to lack of strong encryption with perfect forward secrecy, this 1841 data may be revealed in future, leading to a security incident. 1843 10.5. TACACS+ Best Practices 1845 With respect to the observations about the security issues described 1846 above, a network administrator MUST NOT rely on the obfuscation of 1847 the TACACS+ protocol. TACACS+ MUST be used within a secure 1848 deployment: TACACS+ MUST be deployed over networks which ensure 1849 privacy and integrity of the communication, and MUST be deployed over 1850 a network which is separated from other traffic. Failure to do so 1851 will impact overall network security. 1853 The following recommendations impose restrictions on how the protocol 1854 is applied. These restrictions were not imposed in the original 1855 draft. New implementations, and upgrades of current implementations, 1856 MUST implement these recommendations. Vendors SHOULD provide 1857 mechanisms to assist the administrator to achieve these best 1858 practices. 1860 10.5.1. Shared Secrets 1862 TACACS+ servers and clients MUST treat shared secrets as sensitive 1863 data to be managed securely, as would be expected for other sensitive 1864 data such as identity credential information. TACACS+ servers MUST 1865 NOT leak sensitive data. For example, TACACS+ servers MUST NOT 1866 expose shared secrets in logs. 1868 TACACS+ servers MUST allow a dedicated secret key to be defined for 1869 each client. 1871 TACACS+ server management systems MUST provide a mechanism to track 1872 secret key lifetimes and notify administrators to update them 1873 periodically. TACACS+ server administrators SHOULD change secret 1874 keys at regular intervals. 1876 TACACS+ servers SHOULD warn administrators if secret keys are not 1877 unique per client. 1879 TACACS+ server administrators SHOULD always define a secret for each 1880 client. 1882 TACACS+ servers and clients MUST support shared keys that are at 1883 least 32 characters long. 1885 TACACS+ servers MUST support policy to define minimum complexity for 1886 shared keys. 1888 TACACS+ clients SHOULD NOT allow servers to be configured without 1889 shared secret key, or shared key that is less than 16 characters 1890 long. 1892 TACACS+ server administrators SHOULD configure secret keys of minimum 1893 16 characters length. 1895 10.5.2. Connections and Obfuscation 1897 TACACS+ servers MUST allow the definition of individual clients. The 1898 servers MUST only accept network connection attempts from these 1899 defined, known clients. 1901 TACACS+ servers MUST reject connections with 1902 TAC_PLUS_UNENCRYPTED_FLAG set. There MUST always be a shared secret 1903 set on the server for the client requesting the connection. 1905 If an invalid shared secret is detected when processing packets for a 1906 client, TACACS+ servers MUST NOT accept any new sessions on that 1907 connection. TACACS+ servers MUST terminate the connection on 1908 completion of any sessions that were previously established with a 1909 valid shared secret on that connection. 1911 TACACS+ clients MUST NOT set TAC_PLUS_UNENCRYPTED_FLAG. Clients MUST 1912 be implemented in a way that requires explicit configuration to 1913 enable the use of TAC_PLUS_UNENCRYPTED_FLAG, this option MUST NOT be 1914 used when the client is in production 1916 When a TACACS+ client receives responses from servers where: 1918 the response packet was received from the server configured with 1919 shared key, but the packet has TAC_PLUS_UNENCRYPTED_FLAG set. 1921 the response packet was received from the server configured not to 1922 use obfuscation, but the packet has TAC_PLUS_UNENCRYPTED_FLAG not 1923 set. 1925 then the TACACS+ client MUST close TCP session, and process the 1926 response in the same way that a TAC_PLUS_AUTHEN_STATUS_FAIL 1927 (authentication sessions) or TAC_PLUS_AUTHOR_STATUS_FAIL 1928 (authorization sessions) was received. 1930 10.5.3. Authentication 1932 To help TACACS+ administrators select less weak authentication 1933 options, TACACS+ servers MUST allow the administrator to configure 1934 the server to only accept challenge/response options for 1935 authentication (TAC_PLUS_AUTHEN_TYPE_CHAP or 1936 TAC_PLUS_AUTHEN_TYPE_MSCHAP or TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 for 1937 authen_type). 1939 TACACS+ server administrators SHOULD enable the option mentioned in 1940 the previous paragraph. TACACS+ Server deployments SHOULD ONLY 1941 enable other options (such as TAC_PLUS_AUTHEN_TYPE_ASCII or 1942 TAC_PLUS_AUTHEN_TYPE_PAP) when unavoidable due to requirements of 1943 identity/password systems. 1945 TACACS+ server administrators SHOULD NOT allow the same credentials 1946 to be applied in challenge-based (TAC_PLUS_AUTHEN_TYPE_CHAP or 1947 TAC_PLUS_AUTHEN_TYPE_MSCHAP or TAC_PLUS_AUTHEN_TYPE_MSCHAPV2) and non 1948 challenge-based authen_type options as the insecurity of the latter 1949 will compromise the security of the former. 1951 TAC_PLUS_AUTHEN_SENDAUTH and TAC_PLUS_AUTHEN_SENDPASS options 1952 mentioned in the original draft SHOULD NOT be used, due to their 1953 security implications. TACACS+ servers SHOULD NOT implement them. 1954 If they must be implemented, the servers MUST default to the options 1955 being disabled and MUST warn the administrator that these options are 1956 not secure. 1958 10.5.4. Authorization 1960 The authorization and accounting features are intended to provide 1961 extensibility and flexibility. There is a base dictionary defined in 1962 this document, but it may be extended in deployments by using new 1963 argument names. The cost of the flexibility is that administrators 1964 and implementers MUST ensure that the argument and value pairs shared 1965 between the clients and servers have consistent interpretation. 1967 TACACS+ clients that receive an unrecognized mandatory argument MUST 1968 evaluate server response as if they received 1969 TAC_PLUS_AUTHOR_STATUS_FAIL. 1971 10.5.5. Redirection Mechanism 1973 The original draft described a redirection mechanism 1974 (TAC_PLUS_AUTHEN_STATUS_FOLLOW). This feature is difficult to 1975 secure. The option to send secret keys in the server list is 1976 particularly insecure, as it can reveal client shared secrets. 1978 TACACS+ servers MUST deprecate the redirection mechanism. 1980 If the redirection mechanism is implemented then TACACS+ servers MUST 1981 disable it by default, and MUST warn TACACS+ server administrators 1982 that it must only be enabled within a secure deployment due to the 1983 risks of revealing shared secrets. 1985 TACACS+ clients SHOULD deprecate this feature by treating 1986 TAC_PLUS_AUTHEN_STATUS_FOLLOW as TAC_PLUS_AUTHEN_STATUS_FAIL. 1988 11. IANA Considerations 1990 This informational document describes TACACS+ protocol and its common 1991 deployments. There is no further consideration required from IANA. 1993 12. Acknowledgements 1995 The authors would like to thank the following reviewers whose 1996 comments and contributions made considerable improvements to the 1997 document: Alan DeKok, Alexander Clouter, Chris Janicki, Tom Petch, 1998 Robert Drake, John Heasley, among many others. 2000 The authors would particularly like to thank Alan DeKok, who provided 2001 significant insights and recommendations on all aspects of the 2002 document and the protocol. Alan DeKok has dedicated considerable 2003 time and effort to help improve the document, identifying weaknesses 2004 and providing remediation. 2006 The authors would also like to thank the support from the OPSAWG 2007 Chairs and advisors, especially Joe Clarke. 2009 13. References 2011 13.1. Normative References 2013 [RFC0020] Cerf, V., "ASCII format for network interchange", STD 80, 2014 RFC 20, DOI 10.17487/RFC0020, October 1969, 2015 . 2017 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 2018 April 1992. 2020 [RFC1334] Lloyd, B. and W. Simpson, "PPP Authentication Protocols", 2021 RFC 1334, DOI 10.17487/RFC1334, October 1992, 2022 . 2024 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2025 Requirement Levels", BCP 14, RFC 2119, 2026 DOI 10.17487/RFC2119, March 1997, 2027 . 2029 [RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions", 2030 RFC 2433, DOI 10.17487/RFC2433, October 1998, 2031 . 2033 [RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", 2034 RFC 2759, DOI 10.17487/RFC2759, January 2000, 2035 . 2037 [RFC4086] Eastlake 3rd, D., Crocker, S., and J. Schiller, 2038 "Randomness Requirements for Security", RFC 4086, 2039 DOI 10.17487/RFC4086, June 2005, 2040 . 2042 [RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6 2043 Address Text Representation", RFC 5952, 2044 DOI 10.17487/RFC5952, August 2010, 2045 . 2047 [RFC8265] Saint-Andre, P. and A. Melnikov, "Preparation, 2048 Enforcement, and Comparison of Internationalized Strings 2049 Representing Usernames and Passwords", RFC 8265, 2050 DOI 10.17487/RFC8265, October 2017, 2051 . 2053 13.2. Informative References 2055 [TheDraft] 2056 Carrel, D. and L. Grant, "The TACACS+ Protocol Version 2057 1.78", June 1997, 2058 . 2060 [TZDB] Eggert, P. and A. Olson, "Sources for Time Zone and 2061 Daylight Saving Time Data", 1987, 2062 . 2064 Authors' Addresses 2066 Thorsten Dahm 2067 Google Inc 2068 1600 Amphitheatre Parkway 2069 Mountain View, CA 94043 2070 US 2072 EMail: thorstendlux@google.com 2074 Andrej Ota 2075 Google Inc 2076 1600 Amphitheatre Parkway 2077 Mountain View, CA 94043 2078 US 2080 EMail: andrej@ota.si 2082 Douglas C. Medway Gash 2083 Cisco Systems, Inc. 2084 170 West Tasman Dr. 2085 San Jose, CA 95134 2086 US 2088 EMail: dcmgash@cisco.com 2090 David Carrel 2091 vIPtela, Inc. 2092 1732 North First St. 2093 San Jose, CA 95112 2094 US 2096 EMail: dcarrel@viptela.com 2098 Lol Grant 2100 EMail: lol.grant@gmail.com