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Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (September 23, 2016) is 2772 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 5246 (Obsoleted by RFC 8446) ** Obsolete normative reference: RFC 6347 (Obsoleted by RFC 9147) ** Obsolete normative reference: RFC 2616 (Obsoleted by RFC 7230, RFC 7231, RFC 7232, RFC 7233, RFC 7234, RFC 7235) Summary: 5 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group N. Mavrogiannopoulos 3 Internet-Draft Red Hat 4 Intended status: Informational September 23, 2016 5 Expires: March 27, 2017 7 The OpenConnect VPN Protocol Version 1.0 8 draft-mavrogiannopoulos-openconnect-00 10 Abstract 12 This document specifies version 1.0 of the OpenConnect Virtual 13 Private Network (VPN) protocol, a secure VPN protocol that provides 14 communications privacy over the Internet. That protocol is believed 15 to be compatible with CISCO's AnyConnect VPN protocol. The protocol 16 allows the establishment of VPN tunnels in a way that is designed to 17 prevent eavesdropping, tampering, or message forgery. 19 Status of This Memo 21 This Internet-Draft is submitted in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF). Note that other groups may also distribute 26 working documents as Internet-Drafts. The list of current Internet- 27 Drafts is at http://datatracker.ietf.org/drafts/current/. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 This Internet-Draft will expire on March 27, 2017. 36 Copyright Notice 38 Copyright (c) 2016 IETF Trust and the persons identified as the 39 document authors. All rights reserved. 41 This document is subject to BCP 78 and the IETF Trust's Legal 42 Provisions Relating to IETF Documents 43 (http://trustee.ietf.org/license-info) in effect on the date of 44 publication of this document. Please review these documents 45 carefully, as they describe your rights and restrictions with respect 46 to this document. Code Components extracted from this document must 47 include Simplified BSD License text as described in Section 4.e of 48 the Trust Legal Provisions and are provided without warranty as 49 described in the Simplified BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 54 1.1. Requirements Terminology . . . . . . . . . . . . . . . . 3 55 1.2. Goals of This Document . . . . . . . . . . . . . . . . . 3 56 2. The OpenConnect Protocol . . . . . . . . . . . . . . . . . . 3 57 2.1. VPN Session Establishment . . . . . . . . . . . . . . . . 3 58 2.1.1. Server Authentication . . . . . . . . . . . . . . . . 3 59 2.1.2. Client Authentication . . . . . . . . . . . . . . . . 4 60 2.1.3. Exchange of Session Parameters . . . . . . . . . . . 9 61 2.1.4. Establishment of Primary TCP Channel (CSTP) . . . . . 10 62 2.1.5. Establishment of Secondary UDP Channel (DTLS) . . . . 11 63 2.2. The CSTP Channel Protocol . . . . . . . . . . . . . . . . 14 64 2.3. The DTLS Channel Protocol . . . . . . . . . . . . . . . . 15 65 2.4. The Channel Re-Key Protocol . . . . . . . . . . . . . . . 15 66 2.5. The Keepalive and Dead Peer Detection Protocols . . . . . 16 67 3. Security Considerations . . . . . . . . . . . . . . . . . . . 17 68 4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 69 5. Normative References . . . . . . . . . . . . . . . . . . . . 18 70 Appendix A. Name for Application-Layer Protocol Negotiation . . 21 71 Appendix B. Compression . . . . . . . . . . . . . . . . . . . . 21 72 Appendix C. DTD declarations . . . . . . . . . . . . . . . . . . 21 73 C.1. config-auth.dtd . . . . . . . . . . . . . . . . . . . . . 21 74 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22 76 1. Introduction 78 The purpose of this document is to specify the OpenConnect VPN 79 protocol in a detail in order to allow for multiple interoperable 80 implementations. This is the protocol used by the OpenConnect client 81 and server [OPENCONNECT-CLIENT][OPENCONNECT-SERVER], and is believed 82 to be compatible with CISCO's AnyConnect protocol. 84 While there are many competing VPN protocol solutions, none of them 85 was ever described in a publicly available document. Even open 86 source VPN solutions have their source code as the primary 87 description of their protocol. That allowed no easy study of each 88 protocol's properties and weaknesses, and that is the secondary goal 89 of this document, to describe a deployed TLS based [RFC5246] VPN 90 protocol. 92 1.1. Requirements Terminology 94 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 95 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 96 document are to be interpreted as described in [RFC2119]. 98 1.2. Goals of This Document 100 The OpenConnect protocol version 1.0 specification is intended 101 primarily for readers who will be implementing the protocol and those 102 doing cryptographic analysis of it. 104 2. The OpenConnect Protocol 106 The OpenConnect protocol combines the TLS protocol [RFC5246], 107 Datagram TLS protocol [RFC6347] and HTTP protocols [RFC2616] to 108 provide an Internet-Layer VPN channel. The channel is designed to 109 operate using UDP packets, and fallback on TCP if that's not 110 possible. 112 In brief the protocol initiates an HTTP over TLS connection on a 113 known port, where client authentication is performed. After this 114 step, the client initiates an HTTP CONNECT command to establish a VPN 115 channel over TCP. A secondary VPN channel over UDP will be 116 established using information provided by the server using HTTP 117 headers. At that point the raw IP packets flow, over the VPN 118 channels. 120 2.1. VPN Session Establishment 122 The client and server establish a TLS connection over a known port, 123 typically over 443, the port used for HTTPS. The client SHOULD 124 negotiate TLS 1.1 or later, and support the following TLS protocol 125 extensions. 127 Server Name Indication [RFC6066]: the client SHOULD provide the 128 DNS name of the server in the TLS handshake. 130 Application-Layer Protocol Negotiation [RFC7301]: the client MAY 131 provide this protocol name. The protocol name to be used is 132 defined in Appendix A. 134 2.1.1. Server Authentication 136 In the OpenConnect VPN protocol, the server is always authenticated 137 using its certificate. Once a client establishes a TCP connection to 138 the server's well known port, it initiates the TLS protocol. In the 139 first connection to the server, the client SHOULD verify the provided 140 by the server certificate, and SHOULD store its public key for 141 verification of subsequent sessions. Thus, subsequent sessions 142 SHOULD check whether the server's key match the initial. 144 The server's identity in the certificate SHOULD be placed in the 145 certificate's SubjectAlternativeName field, and unless a special 146 profile is assumed, it will be of type DNSName. 148 2.1.2. Client Authentication 150 The OpenConnect VPN protocol allows for the following types of client 151 authentication, or combinations of them. 153 1. Password: a user can authenticate itself using a password. 155 2. Certificate: a user can authenticate itself using a PKIX 156 certificate it possesses. 158 3. HTTP SPNEGO: a user can authenticate itself using a Kerberos 159 ticket, or any other mechanism supported by SPNEGO (i.e., 160 GSSAPI). 162 The server is authenticated to the client using a PKIX certificate 163 presented during the TLS negotiation. 165 It is important to note that during the password and HTTP SPNEGO 166 authentication methods, any headers allowed by the HTTP protocol can 167 be present. In fact, there are legacy clients which assume that the 168 server will keep a state using cookies, and send their username and 169 password in different TLS and HTTP connections. This practice 170 prevents the server from binding the TLS channel with the VPN session 171 [RFC5056], and is discouraged. It is RECOMMENDED for clients to 172 complete authentication in the same TLS session, and rely on TLS 173 session resumption if reconnections to the server are needed. 175 After the TLS session is established the client irrespective of the 176 supported authentication methods, should send an HTTP POST request on 177 "/" with a config-auth XML structure of type 'init'. An example of 178 its contents follow. 180 181 182 183 v5.01 184 186 The precise DTD declarations for the contents of XML messages defined 187 in this document are listed in Appendix C. Also the HTTP Content- 188 Type to be used for these XML structures MUST be 'text/xml'. 190 2.1.2.1. Authentication using certificates 192 During the initial TLS protocol handshake the server may require a 193 client certificate to be presented, depending on its configuration. 195 Because the client certificate is sent in the clear during the 196 handshake it SHOULD NOT contain other identifying information other 197 than a username, or a pseudonymus identifier. It is RECOMMENDED to 198 place the user identifier in the DN field of the certificate, using 199 the UID object identifier (0.9.2342.19200300.100.1.1) [RFC4519]. 201 After the TLS session is established and the the config-auth XML 202 structure of type 'init' is sent, the server should send it reply. 203 If the certificate sent by the client was successfully validated, it 204 should reply using the HTTP response code 200, and the contents of 205 the reply should be a config-auth XML structure of type 'complete', 206 as follows. 208 209 210 211 0.1(1) 212 213 SSL VPN Service 214 215 217 In that case the client should proceed to the establishment of the 218 primary channel as in Section 2.1.4. 220 2.1.2.2. Authentication using passwords 222 After the TLS session is established and the the config-auth XML 223 structure of type 'init' is sent, the server will reply using forms 224 the client software should prompt the user to fill in. Its reply 225 utilizes a config-auth XML structure of type 'auth-request'. 227 228 229 230 231 Please enter your username 232
233 234
235 236 238 The client may be asked to provide the information in separate forms 239 as above, or may be asked combined as below. 241 242 243 244 245 Please enter your username 246
247 248 249
250 251 253 The client software will then fill in the provided form and sent it 254 back to the server using an HTTP POST on the location specified by 255 the server (in the above examples it was "/auth"). The reply would 256 then be of type 'auth-reply' as in the following example. 258 259 260 261 v5.01 262 test 263 264 266 As mentioned above, the server may ask repeatedly for information 267 until it believes the user is authenticated. For example, the server 268 could present a second form asking for the password, after the 269 username is provided, or ask for a second password if that is 270 necessary. In these cases the server should respond with an HTTP 200 271 OK status code, and proceed sending its new request. 273 If client authentication fails, the server MUST respond with an HTTP 274 401 unauthorized status code. Otherwise, on successful 275 authentication the server should reply with a 200 HTTP code and use 276 the 'complete' config-auth XML structure as in Section 2.1.2.1. 278 Note, that sending the username and password in different messages 279 will reveal the length of them to a passive eavesdropper. For that 280 is is RECOMMENDED for clients to use the 'X-Pad' HTTP header, which 281 will contain arbitrary printable data to make the message length a 282 multiple of 64 bytes. 284 An example session is shown in figure Figure 1. 286 ,-. 287 `-' 288 /|\ 289 | ,------. ,----------. 290 / \ |Server| |ServerDTLS| 291 Client `--+---' `----+-----' 292 | TLS handshake Client Hello | | 293 | -----------------------------------> | 294 | | | 295 | TLS handshake Finished | | 296 | <----------------------------------- | 297 | | | 298 | HTTP POST config-auth init | ,--------------------!. 299 | -----------------------------------> |This is an HTTP over|_\ 300 | | |TLS session. | 301 | | `----------------------' 302 | config-auth auth-request | | 303 | <----------------------------------- | 304 | | | 305 | HTTP POST config-auth auth-reply | | 306 | -----------------------------------> | 307 | | | 308 | config-auth complete | | 309 | <----------------------------------- | 310 | | | 311 | HTTP CONNECT | | 312 | -----------------------------------> | 313 | | | 314 | | | 315 | =================================== | 316 ====================== CSTP VPN session is established ======================= 317 | =================================== | 318 | | | 319 | | ,-------------------------!. 320 | TLS record packet with CSTP payload| |These packets show |_\ 321 | -----------------------------------> |that IP traffic can start | 322 | | |prior to the DTLS channel | 323 | | |establishment. | 324 | | `---------------------------' 325 | TLS record packet with CSTP payload| | 326 | <----------------------------------- | 327 | | | 328 | DTLS handshake Client Hello | 329 | - - - - - - - - - - - - - - - - - - - - - - - - - - - > 330 | | | 331 | DTLS handshake Finished | 332 | <- - - - - - - - - - - - - - - - - - - - - - - - - - - - 333 | | | 334 | | | 335 | =================================== | 336 ====================== DTLS VPN channel is established ======================= 337 | =================================== | 338 | | | 339 | DTLS record packet with payload | 340 | - - - - - - - - - - - - - - - - - - - - - - - - - - - > 341 | | | 342 | DTLS record packet with payload | 343 | <- - - - - - - - - - - - - - - - - - - - - - - - - - - - 344 Client ,--+---. ,----+-----. 345 ,-. |Server| |ServerDTLS| 346 `-' `------' `----------' 347 /|\ 348 | 349 / \ 351 Figure 1 353 2.1.2.3. HTTP Authentication using SPNEGO 355 That type of authentication is performed using the HTTP SPNEGO 356 protocol [RFC4559], a method which is available using the Generic 357 Security Service API [RFC2743]. The following approach is used to 358 advertise the availability of the HTTP SPNEGO protocol by the client. 359 A client which supports the HTTP SPNEGO protocol, SHOULD indicate it 360 using the following header on in its initial request to the server 361 with the config-auth 'init' XML structure. 363 X-Support-HTTP-Auth: true 365 After that the server would report a "401 Unauthorized" status code 366 and authentication would proceed as specified in the HTTP SPNEGO 367 protocol. The server may utilize the following header, to indicate 368 that alternative authentication methods are available (e.g., with 369 plain password), if authentication fails. 371 X-Support-HTTP-Auth: fallback 373 If client authentication fails, the server MUST respond with an HTTP 374 401 unauthorized status code. In that case, a client which received 375 the previous header should retry authenticating to the server without 376 sending the "X-Support-HTTP-Auth: true" header. 378 Otherwise, on successful authentication the server should reply with 379 a 200 HTTP code and use the 'complete' config-auth XML structure as 380 in Section 2.1.2.1. 382 2.1.3. Exchange of Session Parameters 384 By the receipt of a success XML structure, the client SHOULD issue an 385 HTTP CONNECT request. In addition it may provide the following 386 headers. 388 X-CSTP-Address-Type: A comma separated list of the requested 389 address types. 391 IPv4: when the client only supports IPv4 addresses. 393 IPv6: when the client only supports IPv6 addresses. 395 IPv4,IPv6: when the client supports both types of IP addresses. 397 X-CSTP-Base-MTU: The MTU of the link as estimated by the client. 399 X-CSTP-Accept-Encoding: A comma separated list of accepted 400 compression algorithms for the CSTP channel. 402 User-Agent: A string identifying the client software. 404 For the options related to compression see Appendix B for more 405 information. 407 An example CONNECT request is shown below. 409 User-Agent: Open AnyConnect VPN Agent v5.01 410 X-CSTP-Base-MTU: 1280 411 X-CSTP-Address-Type: IPv4,IPv6 412 CONNECT /CSCOSSLC/tunnel HTTP/1.1 414 After a successful receipt of an HTTP CONNECT request, the server 415 should reply and provide the client with configuration parameters. 416 The available options follow. 418 X-CSTP-Address: The IPv4 address of the client, if IPv4 has been 419 requested. 421 X-CSTP-Netmask: An IPv4 netmask to be pushed to the client, if 422 IPv4 has been requested. This should contain the mask on the 423 P-t-P link and is RECOMMENDED the server address to be the first 424 in defined network. 426 X-CSTP-Address-IP6: The IPv6 address of the client in CIDR 427 notation, if IPv6 has been requested. The prefix length is 428 RECOMMENDED to be set to 127-bits according to [RFC6164]. 430 X-CSTP-DNS: The IP address of a DNS server that can be used for 431 that session. 433 X-CSTP-Default-Domain: The DNS domains the provided DNS servers 434 respond for. 436 X-CSTP-Split-Include: The network address of a route which is 437 provided by this server. 439 X-CSTP-Split-Exclude: The network address of a route that is not 440 provided by this server. 442 X-CSTP-Base-MTU: The MTU of the link as estimated by this server. 444 X-CSTP-DynDNS: Set to "true" if the server is operating with a 445 dynamic DNS address. 447 X-CSTP-Content-Encoding: if present is it set to one of the values 448 presented by the client in 'X-CSTP-Accept-Encoding' header. It 449 will be the compression algorithm used in the CSTP channel. 451 X-DTLS-Content-Encoding: if present is it set to one of the values 452 presented by the client in 'X-DTLS-Accept-Encoding' header. It 453 will be the compression algorithm used in the DTLS channel. 455 The client is expected to treat the received parameters as his 456 networking settings. If no "X-CSTP-Split-Include" headers are 457 present, the client is expected to assign its default route through 458 the VPN. 460 2.1.4. Establishment of Primary TCP Channel (CSTP) 462 The previous HTTP message is the last HTTP message sent by the 463 server. After that message, the established TCP channel is used to 464 transport IP packets between the client and the server. The 465 transferred packets encoding is discussed in Section 2.2. This 466 channel will be referred as CSTP in the rest of this document. 468 2.1.5. Establishment of Secondary UDP Channel (DTLS) 470 To establish the secondary UDP-based channel, which will be referred 471 to as the DTLS channel, the client must advertise support for it 472 during the issue of the HTTP CONNECT request (see Section 2.1.3). 473 This is done by appending the following headers to the request. 475 X-DTLS-Accept-Encoding: A comma separated list of accepted 476 compression algorithms for the DTLS channel. 478 X-DTLS-CipherSuite: Must contain the keyword PSK-NEGOTIATE. 480 The DTLS channel utilizes the PSK key exchange method. The key 481 material for this session is a 256-bit value generated with an 482 [RFC5705] exporter. The key material exporter uses the label 483 "EXPORTER-openconnect-psk" without the quotes, and without any 484 context value. 486 In its client hello message the client must copy the value received 487 in the 'X-DTLS-App-ID' header (after hex decoding it), to a TLS 488 application-specific ID field [I-D.mavrogiannopoulos-app-id]. That 489 identifier, can be used by the server to associate the client 490 initiated DTLS channel with the CSTP channel. The following headers 491 are used by the server's response to CONNECT, and are related to the 492 DTLS channel establishment. 494 X-DTLS-App-ID: A hex encoded value to be used as a DTLS 495 application-specific identifier by the client. It serves as an 496 identifier for the server to associate the incoming DTLS session 497 with the TLS session. 499 X-DTLS-Port: The port number to which the client should send UDP 500 packets for DTLS. 502 X-DTLS-CipherSuite: It must contain the value "PSK-NEGOTIATE" 503 without any quotes. 505 X-DTLS-Rekey-Time: The time (in seconds) after which the DTLS 506 session should rekey, see Section 2.4. Only considered if 507 applicable to the negotiated DTLS protocol. 509 X-DTLS-Rekey-Method: The method used in DTLS rekey, see 510 Section 2.4. Only considered if applicable to the negotiated DTLS 511 protocol. 513 2.1.5.1. Legacy Establishment of Secondary UDP Channel (DTLS) 515 Previous versions of this protocol utilized a special DTLS protocol 516 negotiation, based on an unpublished description of the DTLS 517 protocol. This section attempts to summarize this negotiation, but 518 may not be entirely accurate. 520 To establish the legacy UDP-based channel, the client must advertise 521 support for it during the issue of the HTTP CONNECT request (see 522 Section 2.1.3). This is done by appending the following headers to 523 the request. 525 X-DTLS-Accept-Encoding: A comma separated list of accepted 526 compression algorithms for the DTLS channel. 528 X-DTLS-Master-Secret: A hex encoded pre-master secret to be used 529 in the legacy DTLS session negotiation. 531 X-DTLS-CipherSuite: A colon-separated list of ciphers (e.g., the 532 string PSK-NEGOTIATE:AES256-SHA:AES128-SHA:DES-CBC3-SHA). 534 The DTLS channel utilizes session resumption as a method for 535 preshared-key authentication. That is the value presented in X-DTLS- 536 Master-Secret is set as a master secret to be resumed. The session 537 ID value is sent by the server on the response to CONNECT using the 538 'X-DTLS-Session-ID' header. That header provides a hex-encoded value 539 of the DTLS session ID to be used by the client. The following 540 headers are used by the server's response to CONNECT, and are related 541 to the DTLS channel establishment. 543 X-DTLS-Session-ID: A hex encoded value to be used as a DTLS 544 session ID by the client. It also serves as an identifier for the 545 server to associate the incoming DTLS session with the TLS 546 session. 548 X-DTLS-Port: The port number to which the client should send UDP 549 packets for DTLS. 551 X-DTLS-CipherSuite: The ciphersuite selected by the server. It 552 should be one of the options present in the client's X-DTLS- 553 CipherSuite header. 555 X-DTLS-Rekey-Time: The time (in seconds) after which the DTLS 556 session should rekey, see Section 2.4. 558 X-DTLS-Rekey-Method: The method used in DTLS rekey, see 559 Section 2.4. 561 The following table lists the ciphers negotiated via the X-DTLS- 562 CipherSuite header, and the corresponding DTLS ciphersuite. 564 +--------------------+---------------------------------+------------+ 565 | OpenConnect cipher | DTLS ciphersuite | DTLS | 566 | | | version | 567 +--------------------+---------------------------------+------------+ 568 | DES-CBC3-SHA | TLS_RSA_WITH_3DES_EDE_CBC_SHA1 | DTLS 0.9 | 569 | | | (pre-draft | 570 | | | version) | 571 | | | | 572 | AES128-SHA | TLS_RSA_WITH_AES_128_CBC_SHA1 | DTLS 0.9 | 573 | | | (pre-draft | 574 | | | version) | 575 | | | | 576 | AES256-SHA | TLS_RSA_WITH_AES_256_CBC_SHA1 | DTLS 0.9 | 577 | | | (pre-draft | 578 | | | version) | 579 | | | | 580 | OC- | TLS_RSA_WITH_AES_128_GCM_SHA256 | DTLS 1.2 | 581 | DTLS1_2-AES128-GCM | | | 582 | | | | 583 | OC- | TLS_RSA_WITH_AES_256_GCM_SHA256 | DTLS 1.2 | 584 | DTLS1_2-AES256-GCM | | | 585 +--------------------+---------------------------------+------------+ 587 Table 1 589 The legacy DTLS protocol negotiation described in this section, is 590 similar to DTLS 1.0 except for the following deviations: 592 The negotiated protocol version for the handshake and record 593 headers is 1.0 instead of 254.255. 595 The Hello Verify and Hello verify request messages are included in 596 the handshake hashes. 598 The handshake header is not included as part of the handshake 599 hashes. 601 The ChangeCipherSpec message is 3 byte long instead of 1, and 602 contains the handshake sequence number (2-bytes long) appended to 603 the message id. 605 2.2. The CSTP Channel Protocol 607 The format of the packets sent over the primary channel consists of 608 an 8-bytes header followed by data. The whole packet in encapsulated 609 in a TLS record (see [RFC5246]). The bytes of the header indicate 610 the type of data that follow, and their contents are explained in 611 Table 2. 613 +---------------------+---------------------------------------------+ 614 | byte | value | 615 +---------------------+---------------------------------------------+ 616 | 0 | fixed to 0x53 (S) | 617 | | | 618 | 1 | fixed to 0x54 (T) | 619 | | | 620 | 2 | fixed to 0x46 (F) | 621 | | | 622 | 3 | fixed to 0x01 | 623 | | | 624 | 4-5 | The length of the packet that follows this | 625 | | header in big endian order | 626 | | | 627 | 6 | The type of the payload that follows (see | 628 | | Table 3 for available types) | 629 | | | 630 | 7 | fixed to 0x00 | 631 +---------------------+---------------------------------------------+ 633 Table 2 635 The available payload types are listed in Table 3. 637 +---------------------+---------------------------------------------+ 638 | Value | Description | 639 +---------------------+---------------------------------------------+ 640 | 0x00 | DATA: the TLS record packet contains an | 641 | | IPv4 or IPv6 packet | 642 | | | 643 | 0x03 | DPD-REQ: used for dead peer detection. Once | 644 | | sent the peer should reply with a DPD-RESP | 645 | | packet, that has the same contents as the | 646 | | original request. | 647 | | | 648 | 0x04 | DPD-RESP: used as a response to a | 649 | | previously received DPD-REQ. | 650 | | | 651 | 0x05 | DISCONNECT: sent by the client (or server) | 652 | | to terminate the session. No data is | 653 | | associated with this request. The session | 654 | | will be invalidated after such request. | 655 | | | 656 | 0x07 | KEEPALIVE: sent by any peer. No data is | 657 | | associated with this request. | 658 | | | 659 | 0x08 | COMPRESSED DATA: a Data packet which is | 660 | | compressed prior to encryption. | 661 | | | 662 | 0x09 | TERMINATE: sent by the server to indicate | 663 | | that the server is shutting down. No data | 664 | | is associated with this request. | 665 +---------------------+---------------------------------------------+ 667 Table 3 669 2.3. The DTLS Channel Protocol 671 The format of the packets sent over the UDP channel consists of an 672 1-byte header followed by data. The header byte indicates the type 673 of data that follow as in Table 3. The header and the data are 674 encapsulated in a DTLS record packet (see [RFC6347]). 676 2.4. The Channel Re-Key Protocol 678 During the exchange of session parameters (Section 2.1.3), the server 679 advertizes the methods available for session rekey using the "X-CSTP- 680 Rekey-Method" and "X-DTLS-Rekey-Method" HTTP headers. The available 681 options for both the server and client are listed below. 683 1. none: no rekey; the session will go on until 2^48 DTLS records 684 have been exchanged, or 2^64 TLS records. 686 2. ssl: a TLS or DTLS rehandshake will be performed periodically. 688 3. new-tunnel: the session will tear down and the client will 689 reconnect periodically. 691 When the value is other than "none" the rekey period is determinated 692 by the "X-CSTP-Rekey-Time" and "X-DTLS-Rekey-Time" headers. These 693 headers contain the time in seconds after which a session should 694 rekey. 696 It should be noted that when the "ssl" rekey option is used, care 697 must be taken by both the client and the server to ensure that either 698 safe renegotiation is used ([RFC5746]), or that the identity of the 699 peer remained the same. 701 2.5. The Keepalive and Dead Peer Detection Protocols 703 In OpenConnect there are two packet types that can be used for keep- 704 alive or dead peer detection, as shown in Table 3. These are the 705 DPD-REQ and KeepAlive packets. 707 The timings of the transmission of these packets are set by the 708 server, and they for the DPD are advisory to a client. However, any 709 peer receiving these packets MUST response with the appropriate 710 packet. For DPD-REQ packets, the response MUST be DPD-RESP, and for 711 KeepAlive packets the response must be another KeepAlive packet. The 712 main difference between these two types of packets, is that the DPD 713 packets similarly to [RFC3706] are sent when there is no traffic or 714 when the other party requests them, and allow for arbitrary data to 715 be attached, making them suitable for Path MTU detection. 717 The server advertizes the suggested periods during the exchange of 718 session parameters (Section 2.1.3). The available headers are listed 719 below. 721 X-CSTP-DPD: applicable to CSTP channel; contains a relative time 722 in seconds. 724 X-CSTP-Keepalive: applicable to CSTP channel; contains a relative 725 time in seconds. 727 X-DTLS-DPD: applicable to DTLS channel; contains a relative time 728 in seconds. 730 X-DTLS-Keepalive: applicable to DTLS channel; contains a relative 731 time in seconds. 733 3. Security Considerations 735 This document provides a description of a protocol to establish a VPN 736 over a TLS channel. All security considerations of the referenced 737 documents in particular [RFC5246] and [RFC6347] are applicable, in 738 addition the following considerations. 740 The protocol is designed to be as compatible as possible with a 741 legacy VPN protocol and as such it carries cruft, such as partial 742 dependence on a non-standard DTLS version, and utilization of an 743 awkward method to establish a DTLS session which relies on session 744 resumption. Nevertheless, these particularities are not believed to 745 cause a degradation of the overall protocol security, and could be 746 addressed with a backwards compatible protocol upgrade. 748 The protocol provides a VPN channel which carries payload hidden from 749 eavesdroppers. However, the payload's length remain visible and in 750 certain scenarios that may be sufficient to determine the transferred 751 payload. Furthermore, there are scenarios where compressed payload 752 lengths may reveal more information than the uncompressed data 753 [COMP-ISSUES][COMP-ISSUES2]. For that we RECOMMEND that 754 implementations don't enable compression by default, and only allow 755 it after notifying the users and administrators about the 756 consequences. 758 This protocol could sometimes be used because of the fact that it 759 ressembles the TLS protocol and thus is not detected by the available 760 VPN blockers. While an implementation could intentionally masquerade 761 its packets to ressemble a typical HTTPS session, a fully compliant 762 implementation will be distinct from an average HTTP session due to 763 the DTLS session establishment, and the transferred packet sizes. 765 For certificate authentication OpenConnect relies on the TLS 766 protocol. However, as mentioned in the text, TLS version 1.2 and 767 earlier do not protect the client's (or the server's) certificate 768 from eavesdroppers. For that it is RECOMMENDED that certificates to 769 be used with this protocol contain the minimum possible identifying 770 information. 772 This document defines a protocol name for Application-Layer Protocol 773 Negotiation. That, if used by a client would indicate to any 774 eavesdropping parties that the client wishes to use VPN, thus 775 compromising its intention privacy. On the other hand, providing 776 that information would help a server that re-uses the same port for 777 different protocols under TLS, to forward to the appropriate handler 778 of the connection. That is, it would allow hosting a plain HTTPS 779 server serving content, and a VPN server using openconnect at the 780 same port. It is left to the client to decide the balance between 781 privacy and usability with such servers. 783 4. Acknowledgements 785 None yet. 787 5. Normative References 789 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 790 (TLS) Protocol Version 1.2", RFC 5246, 791 DOI 10.17487/RFC5246, August 2008, 792 . 794 [RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, 795 "Transport Layer Security (TLS) Renegotiation Indication 796 Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010, 797 . 799 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 800 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 801 January 2012, . 803 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 804 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 805 Transfer Protocol -- HTTP/1.1", RFC 2616, 806 DOI 10.17487/RFC2616, June 1999, 807 . 809 [RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based 810 Kerberos and NTLM HTTP Authentication in Microsoft 811 Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006, 812 . 814 [RFC2743] Linn, J., "Generic Security Service Application Program 815 Interface Version 2, Update 1", RFC 2743, 816 DOI 10.17487/RFC2743, January 2000, 817 . 819 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure 820 Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007, 821 . 823 [RFC5705] Rescorla, E., "Keying Material Exporters for Transport 824 Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, 825 March 2010, . 827 [RFC4519] Sciberras, A., Ed., "Lightweight Directory Access Protocol 828 (LDAP): Schema for User Applications", RFC 4519, 829 DOI 10.17487/RFC4519, June 2006, 830 . 832 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 833 Extensions: Extension Definitions", RFC 6066, 834 DOI 10.17487/RFC6066, January 2011, 835 . 837 [RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan, 838 "Transport Layer Security (TLS) Application-Layer Protocol 839 Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, 840 July 2014, . 842 [RFC6164] Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti, 843 L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter- 844 Router Links", RFC 6164, DOI 10.17487/RFC6164, April 2011, 845 . 847 [RFC3706] Huang, G., Beaulieu, S., and D. Rochefort, "A Traffic- 848 Based Method of Detecting Dead Internet Key Exchange (IKE) 849 Peers", RFC 3706, DOI 10.17487/RFC3706, February 2004, 850 . 852 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 853 Requirement Levels", BCP 14, RFC 2119, 854 DOI 10.17487/RFC2119, March 1997, 855 . 857 [I-D.mavrogiannopoulos-app-id] 858 Mavrogiannopoulos, N. and D. Woodhouse, "A TLS 859 application-specific identifier", draft-mavrogiannopoulos- 860 app-id-00 (work in progress), September 2016. 862 [COMP-ISSUES] 863 Bhargavan, K., Fournet, C., Kohlweiss, M., Pironti, A., 864 and P-Y. Strub, "TLS Compression Fingerprinting and a 865 Privacy-aware API for TLS", 2012. 867 [COMP-ISSUES2] 868 Kelsey, J., "Compression and information leakage of 869 plaintex", International Workshop on Fast Software 870 Encryption , 2002. 872 [OPENCONNECT-CLIENT] 873 Woodhouse, D., "http://www.infradead.org/openconnect/", 874 2016. 876 [OPENCONNECT-SERVER] 877 Mavrogiannopoulos, N., "http://www.infradead.org/ocserv/", 878 2016. 880 Appendix A. Name for Application-Layer Protocol Negotiation 882 Protocol: openconnect-vpn/1.0 883 Identification Sequence: 884 0x6f 0x70 0x65 0x6e 0x63 0x6f 0x6e 0x6e 0x65 0x63 885 0x74 0x2d 0x76 0x70 0x6e 0x2f 0x31 0x2e 0x30 887 Appendix B. Compression 889 The available compression algorithms for the CSTP and DTLS channels 890 are shown in Table 4. Note, that all algorithms are intentionally 891 stateless to prevent the influence of independent packets (e.g., from 892 different sources) on each others compression. That does not 893 eliminate all known attacks on compression before encryption, and for 894 that reason an implentation MUST NOT enable compression by default. 896 After compression is negotiated each side may choose to compress the 897 payload and use the 'COMPRESSED DATA' header from Table 3, or may 898 send uncompressed data with the 'DATA' payload. Each side MUST be 899 able to process both payloads. 901 +---------------------+---------------------------------------------+ 902 | Algorithm | Description | 903 +---------------------+---------------------------------------------+ 904 | oc-lz4 | The stateless LZ4 compression algorithm. | 905 | | | 906 | lzs | The stateless LZS (stacker) compression | 907 | | algorithm. | 908 +---------------------+---------------------------------------------+ 910 Table 4 912 Appendix C. DTD declarations 914 C.1. config-auth.dtd 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 938 Author's Address 940 Nikos Mavrogiannopoulos 941 Red Hat 943 EMail: nmav@redhat.com