idnits 2.17.1 draft-mavrogiannopoulos-openconnect-03.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 : ---------------------------------------------------------------------------- ** The document seems to lack an IANA Considerations section. (See Section 2.2 of https://www.ietf.org/id-info/checklist for how to handle the case when there are no actions for IANA.) ** There are 9 instances of too long lines in the document, the longest one being 6 characters in excess of 72. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (October 2, 2020) is 1300 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- == Unused Reference: 'I-D.ietf-tls-dtls13' is defined on line 715, but no explicit reference was found in the text == Outdated reference: A later version (-43) exists of draft-ietf-tls-dtls13-37 ** Obsolete normative reference: RFC 2616 (Obsoleted by RFC 7230, RFC 7231, RFC 7232, RFC 7233, RFC 7234, RFC 7235) ** Obsolete normative reference: RFC 6347 (Obsoleted by RFC 9147) Summary: 4 errors (**), 0 flaws (~~), 3 warnings (==), 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 October 2, 2020 5 Expires: April 5, 2021 7 The OpenConnect VPN Protocol Version 1.2 8 draft-mavrogiannopoulos-openconnect-03 10 Abstract 12 This document specifies version 1.2 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 https://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 April 5, 2021. 36 Copyright Notice 38 Copyright (c) 2020 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 (https://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) . . . . . 11 62 2.1.5. Establishment of Secondary UDP Channel (DTLS) . . . . 11 63 2.2. The CSTP Channel Protocol . . . . . . . . . . . . . . . . 12 64 2.3. The DTLS Channel Protocol . . . . . . . . . . . . . . . . 13 65 2.4. The Channel Re-Key Protocol . . . . . . . . . . . . . . . 13 66 2.5. The Keepalive and Dead Peer Detection Protocols . . . . . 14 67 3. Security Considerations . . . . . . . . . . . . . . . . . . . 15 68 4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16 69 5. Normative References . . . . . . . . . . . . . . . . . . . . 16 70 Appendix A. Name for Application-Layer Protocol Negotiation . . 19 71 Appendix B. Compression . . . . . . . . . . . . . . . . . . . . 19 72 Appendix C. DTD declarations . . . . . . . . . . . . . . . . . . 19 73 C.1. config-auth.dtd . . . . . . . . . . . . . . . . . . . . . 19 74 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20 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 This protocol's design follows a minimalistic modular philosophy. It 85 delegates several protocol-related elements often considered as core 86 VPN features and diversifiers, to standards protocols. That 87 delegation, allows a minimalistic core protocol which contains very 88 few security related elements and is decoupled from cryptography. 89 That in turn transfers the auditing requirements due to cryptographic 90 and negotiation protocols to dedicated for that purpose components. 91 In particular the Openconnect VPN protocol uses standard protocols 92 such as HTTP, TLS [RFC8446] and DTLS [RFC6347] to provide a VPN with 93 data security and authenticity. 95 1.1. Requirements Terminology 97 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 98 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 99 document are to be interpreted as described in [RFC2119]. 101 1.2. Goals of This Document 103 The OpenConnect protocol version 1.2 specification is intended 104 primarily for readers who will be implementing the protocol and those 105 doing cryptographic analysis of it. 107 2. The OpenConnect Protocol 109 The OpenConnect protocol combines the TLS protocol [RFC8446], 110 Datagram TLS protocol [RFC6347] and HTTP protocols [RFC2616] to 111 provide an Internet-Layer VPN channel. The channel is designed to 112 operate using UDP packets, and fallback on TCP if that's not 113 possible. 115 In brief the protocol initiates an HTTP over TLS connection on a 116 known port, where client authentication is performed. After this 117 step, the client initiates an HTTP CONNECT command to establish a VPN 118 channel over TCP. A secondary VPN channel over UDP will be 119 established using information provided by the server using HTTP 120 headers. At that point the raw IP packets flow, over the VPN 121 channels. 123 2.1. VPN Session Establishment 125 The client and server establish a TLS connection over a known port, 126 typically over 443, the port used for HTTPS. The client SHOULD 127 negotiate TLS 1.2 or later, and support the following TLS protocol 128 extensions. 130 Server Name Indication [RFC6066]: the client SHOULD provide the 131 DNS name of the server in the TLS handshake. 133 Application-Layer Protocol Negotiation [RFC7301]: the client MAY 134 provide this protocol name. The protocol name to be used is 135 defined in Appendix A. 137 2.1.1. Server Authentication 139 In the OpenConnect VPN protocol, the server is always authenticated 140 using its certificate. Once a client establishes a TCP connection to 141 the server's well known port, it initiates the TLS protocol. In the 142 first connection to the server, the client SHOULD verify the provided 143 by the server certificate, and SHOULD store its public key for 144 verification of subsequent sessions. Thus, subsequent sessions 145 SHOULD check whether the server's key match the initial. 147 The server's identity in the certificate SHOULD be placed in the 148 certificate's SubjectAlternativeName field, and unless a special 149 profile is assumed, it will be of type DNSName. 151 2.1.2. Client Authentication 153 The OpenConnect VPN protocol allows for the following types of client 154 authentication, or combinations of them. 156 1. Password: a user can authenticate itself using a password. 158 2. Certificate: a user can authenticate itself using a PKIX 159 certificate it possesses. 161 3. HTTP SPNEGO: a user can authenticate itself using a Kerberos 162 ticket, or any other mechanism supported by SPNEGO (i.e., 163 GSSAPI). 165 The server is authenticated to the client using a PKIX certificate 166 presented during the TLS negotiation. 168 It is important to note that during the password and HTTP SPNEGO 169 authentication methods, any headers allowed by the HTTP protocol can 170 be present. In fact, there are legacy clients which assume that the 171 server will keep a state using cookies, and send their username and 172 password in different TLS and HTTP connections. This practice 173 prevents the server from binding the TLS channel with the VPN session 174 [RFC5056], and is discouraged. It is RECOMMENDED for clients to 175 complete authentication in the same TLS session, and rely on TLS 176 session resumption if reconnections to the server are needed. 178 After the TLS session is established the client irrespective of the 179 supported authentication methods, should send an HTTP POST request on 180 "/" with a config-auth XML structure of type 'init'. An example of 181 its contents follow. 183 184 185 186 v5.01 187 189 The precise DTD declarations for the contents of XML messages defined 190 in this document are listed in Appendix C. Also the HTTP Content- 191 Type to be used for these XML structures MUST be 'text/xml'. 193 2.1.2.1. Authentication using certificates 195 During the initial TLS protocol handshake the server may require a 196 client certificate to be presented, depending on its configuration. 198 Because under TLS 1.2 the client certificate is sent in the clear 199 during the handshake, the certificate SHOULD NOT contain other 200 identifying information other than a username, or a pseudonymus 201 identifier. It is RECOMMENDED to place the user identifier in the DN 202 field of the certificate, using the UID object identifier 203 (0.9.2342.19200300.100.1.1) [RFC4519]. 205 After the TLS session is established and the the config-auth XML 206 structure of type 'init' is sent, the server should send it reply. 207 If the certificate sent by the client was successfully validated, it 208 should reply using the HTTP response code 200, and the contents of 209 the reply should be a config-auth XML structure of type 'complete', 210 as follows. 212 213 214 215 0.1(1) 216 217 SSL VPN Service 218 219 221 In that case the client should proceed to the establishment of the 222 primary channel as in Section 2.1.4. 224 2.1.2.2. Authentication using passwords 226 After the TLS session is established and the the config-auth XML 227 structure of type 'init' is sent, the server will reply using forms 228 the client software should prompt the user to fill in. Its reply 229 utilizes a config-auth XML structure of type 'auth-request'. 231 232 233 234 235 Please enter your username 236
237 238
239 240 242 The client may be asked to provide the information in separate forms 243 as above, or may be asked combined as below. 245 246 247 248 249 Please enter your username 250
251 252 253
254 255 257 The client software will then fill in the provided form and sent it 258 back to the server using an HTTP POST on the location specified by 259 the server (in the above examples it was "/auth"). The reply would 260 then be of type 'auth-reply' as in the following example. 262 263 264 265 v5.01 266 test 267 268 270 As mentioned above, the server may ask repeatedly for information 271 until it believes the user is authenticated. For example, the server 272 could present a second form asking for the password, after the 273 username is provided, or ask for a second password if that is 274 necessary. In these cases the server should respond with an HTTP 200 275 OK status code, and proceed sending its new request. 277 If client authentication fails, the server MUST respond with an HTTP 278 401 unauthorized status code. Otherwise, on successful 279 authentication the server should reply with a 200 HTTP code and use 280 the 'complete' config-auth XML structure as in Section 2.1.2.1. 282 Note, that including the username and password in XML messages will 283 reveal the length of them to a passive eavesdropper. For that is is 284 RECOMMENDED for clients to use an 'X-Pad' HTTP header, containing 285 arbitrary printable data to make the message length a multiple of 64 286 bytes. 288 An example session is shown in figure Figure 1. 290 ,-. 291 `-' 292 /|\ 293 | ,------. ,----------. 294 / \ |Server| |ServerDTLS| 295 Client `--+---' `----+-----' 296 | TLS handshake Client Hello | | 297 | -----------------------------------> | 298 | | | 299 | TLS handshake Finished | | 300 | <----------------------------------- | 301 | | | 302 | HTTP POST config-auth init | ,--------------------!. 303 | -----------------------------------> |This is an HTTP over|_\ 304 | | |TLS session. | 305 | | `----------------------' 306 | config-auth auth-request | | 307 | <----------------------------------- | 308 | | | 309 | HTTP POST config-auth auth-reply | | 310 | -----------------------------------> | 311 | | | 312 | config-auth complete | | 313 | <----------------------------------- | 314 | | | 315 | HTTP CONNECT | | 316 | -----------------------------------> | 317 | | | 318 | | | 319 | =================================== | 320 ====================== CSTP VPN session is established ======================= 321 | =================================== | 322 | | | 323 | | ,-------------------------!. 324 | TLS record packet with CSTP payload| |These packets show |_\ 325 | -----------------------------------> |that IP traffic can start | 326 | | |prior to the DTLS channel | 327 | | |establishment. | 328 | | `---------------------------' 329 | TLS record packet with CSTP payload| | 330 | <----------------------------------- | 331 | | | 332 | DTLS handshake Client Hello | 333 | - - - - - - - - - - - - - - - - - - - - - - - - - - - > 334 | | | 335 | DTLS handshake Finished | 336 | <- - - - - - - - - - - - - - - - - - - - - - - - - - - - 337 | | | 338 | | | 339 | =================================== | 340 ====================== DTLS VPN channel is established ======================= 341 | =================================== | 342 | | | 343 | DTLS record packet with payload | 344 | - - - - - - - - - - - - - - - - - - - - - - - - - - - > 345 | | | 346 | DTLS record packet with payload | 347 | <- - - - - - - - - - - - - - - - - - - - - - - - - - - - 348 Client ,--+---. ,----+-----. 349 ,-. |Server| |ServerDTLS| 350 `-' `------' `----------' 351 /|\ 352 | 353 / \ 355 Figure 1 357 2.1.2.3. HTTP Authentication using SPNEGO 359 That type of authentication is performed using the HTTP SPNEGO 360 protocol [RFC4559], a method which is available using the Generic 361 Security Service API [RFC2743]. The following approach is used to 362 advertise the availability of the HTTP SPNEGO protocol by the client. 363 A client which supports the HTTP SPNEGO protocol, SHOULD indicate it 364 using the following header on in its initial request to the server 365 with the config-auth 'init' XML structure. 367 X-Support-HTTP-Auth: true 369 After that the server would report a "401 Unauthorized" status code 370 and authentication would proceed as specified in the HTTP SPNEGO 371 protocol. The server may utilize the following header, to indicate 372 that alternative authentication methods are available (e.g., with 373 plain password), if authentication fails. 375 X-Support-HTTP-Auth: fallback 377 If client authentication fails, the server MUST respond with an HTTP 378 401 unauthorized status code. In that case, a client which received 379 the previous header should retry authenticating to the server without 380 sending the "X-Support-HTTP-Auth: true" header. 382 Otherwise, on successful authentication the server should reply with 383 a 200 HTTP code and use the 'complete' config-auth XML structure as 384 in Section 2.1.2.1. 386 2.1.3. Exchange of Session Parameters 388 By the receipt of a success XML structure, the client SHOULD issue an 389 HTTP CONNECT request. In addition it may provide the following 390 headers. 392 X-CSTP-Address-Type: A comma separated list of the requested 393 address types. 395 IPv4: when the client only supports IPv4 addresses. 397 IPv6: when the client only supports IPv6 addresses. 399 IPv4,IPv6: when the client supports both types of IP addresses. 401 X-CSTP-Base-MTU: The MTU of the link as estimated by the client. 403 X-CSTP-Accept-Encoding: A comma separated list of accepted 404 compression algorithms for the CSTP channel. 406 User-Agent: A string identifying the client software. 408 For the options related to compression see Appendix B for more 409 information. 411 An example CONNECT request is shown below. 413 User-Agent: Open AnyConnect VPN Agent v5.01 414 X-CSTP-Base-MTU: 1280 415 X-CSTP-Address-Type: IPv4,IPv6 416 CONNECT /CSCOSSLC/tunnel HTTP/1.1 418 After a successful receipt of an HTTP CONNECT request, the server 419 should reply and provide the client with configuration parameters. 420 The available options follow. 422 X-CSTP-Address: The IPv4 address of the client, if IPv4 has been 423 requested. 425 X-CSTP-Netmask: An IPv4 netmask to be pushed to the client, if 426 IPv4 has been requested. This should contain the mask on the 427 P-t-P link and is RECOMMENDED the server address to be the first 428 in defined network. 430 X-CSTP-Address-IP6: The IPv6 address of the client in CIDR 431 notation, if IPv6 has been requested. The prefix length is 432 RECOMMENDED to be set to 127-bits according to [RFC6164]. 434 X-CSTP-DNS: The IP address of a DNS server that can be used for 435 that session. 437 X-CSTP-Default-Domain: The DNS default search domains. Typically 438 a subset of X-CSTP-Split-DNS. If multiple, the domains are space 439 separated. 441 X-CSTP-Split-DNS: A DNS domain the provided DNS servers respond 442 for. Multiple such headers may be present for different domains. 444 X-CSTP-Split-Include: The network address of a route which is 445 provided by this server. Multiple such headers may be present. 447 X-CSTP-Split-Exclude: The network address of a route that is not 448 provided by this server. Multiple such headers may be present. 450 X-CSTP-Base-MTU: The MTU of the link as estimated by this server. 452 X-CSTP-DynDNS: Set to "true" if the server is operating with a 453 dynamic DNS address. 455 X-CSTP-Content-Encoding: if present is it set to one of the values 456 presented by the client in 'X-CSTP-Accept-Encoding' header. It 457 will be the compression algorithm used in the CSTP channel. 459 X-DTLS-Content-Encoding: if present is it set to one of the values 460 presented by the client in 'X-DTLS-Accept-Encoding' header. It 461 will be the compression algorithm used in the DTLS channel. 463 The client is expected to treat the received parameters as his 464 networking settings. If no "X-CSTP-Split-Include" headers are 465 present, the client is expected to assign its default route through 466 the VPN. 468 2.1.4. Establishment of Primary TCP Channel (CSTP) 470 The previous HTTP message is the last HTTP message sent by the 471 server. After that message, the established TCP channel is used to 472 transport IP packets between the client and the server. The 473 transferred packets encoding is discussed in Section 2.2. This 474 channel will be referred as CSTP in the rest of this document. 476 2.1.5. Establishment of Secondary UDP Channel (DTLS) 478 To establish the secondary UDP-based channel, which will be referred 479 to as the DTLS channel, the client must advertise support for it 480 during the issue of the HTTP CONNECT request (see Section 2.1.3). 481 This is done by appending the following headers to the request. 483 X-DTLS-Accept-Encoding: A comma separated list of accepted 484 compression algorithms for the DTLS channel. 486 X-DTLS-CipherSuite: Must contain the keyword PSK-NEGOTIATE. 488 The DTLS channel utilizes the DTLS 1.2 protocol (or later version) 489 with the PSK key exchange method. The key material for this session 490 is a 256-bit value generated with an [RFC5705] exporter. The key 491 material exporter uses the label "EXPORTER-openconnect-psk" without 492 the quotes, and without any context value. 494 In its client hello message the client must copy the value received 495 in the 'X-DTLS-App-ID' header (after hex decoding it), to the session 496 ID field of the DTLS client hello. That identifier, is not used for 497 session resumption, and is used by the server to associate the DTLS 498 channel with the CSTP channel. The following headers are used by the 499 server's response to CONNECT, and are related to the DTLS channel 500 establishment. 502 X-DTLS-App-ID: A hex encoded value to be used as a DTLS 503 application-specific identifier by the client. It serves as an 504 identifier for the server to associate the incoming DTLS session 505 with the TLS session. 507 X-DTLS-Port: The port number to which the client should send UDP 508 packets for DTLS. 510 X-DTLS-CipherSuite: It must contain the value "PSK-NEGOTIATE" 511 without any quotes. 513 X-DTLS-Rekey-Time: The time (in seconds) after which the DTLS 514 session should rekey, see Section 2.4. Only considered if 515 applicable to the negotiated DTLS protocol. 517 X-DTLS-Rekey-Method: The method used in DTLS rekey, see 518 Section 2.4. Only considered if applicable to the negotiated DTLS 519 protocol. 521 2.2. The CSTP Channel Protocol 523 The format of the packets sent over the primary channel consists of 524 an 8-bytes header followed by data. The whole packet in encapsulated 525 in a TLS record (see [RFC8446]). The bytes of the header indicate 526 the type of data that follow, and their contents are explained in 527 Table 1. 529 +---------------------+---------------------------------------------+ 530 | byte | value | 531 +---------------------+---------------------------------------------+ 532 | 0 | fixed to 0x53 (S) | 533 | | | 534 | 1 | fixed to 0x54 (T) | 535 | | | 536 | 2 | fixed to 0x46 (F) | 537 | | | 538 | 3 | fixed to 0x01 | 539 | | | 540 | 4-5 | The length of the packet that follows this | 541 | | header in big endian order | 542 | | | 543 | 6 | The type of the payload that follows (see | 544 | | Table 2 for available types) | 545 | | | 546 | 7 | fixed to 0x00 | 547 +---------------------+---------------------------------------------+ 549 Table 1 551 The available payload types are listed in Table 2. 553 +---------------------+---------------------------------------------+ 554 | Value | Description | 555 +---------------------+---------------------------------------------+ 556 | 0x00 | DATA: the TLS record packet contains an | 557 | | IPv4 or IPv6 packet | 558 | | | 559 | 0x03 | DPD-REQ: used for dead peer detection. Once | 560 | | sent the peer should reply with a DPD-RESP | 561 | | packet, that has the same contents as the | 562 | | original request. | 563 | | | 564 | 0x04 | DPD-RESP: used as a response to a | 565 | | previously received DPD-REQ. | 566 | | | 567 | 0x05 | DISCONNECT: sent by the client (or server) | 568 | | to terminate the session. This is followed | 569 | | by one byte indicating the disconnect | 570 | | reason. When the reason is '0xb0' the | 571 | | session should be invalidated after the | 572 | | request. | 573 | | | 574 | 0x07 | KEEPALIVE: sent by any peer. No data is | 575 | | associated with this request. | 576 | | | 577 | 0x08 | COMPRESSED DATA: a Data packet which is | 578 | | compressed prior to encryption. | 579 | | | 580 | 0x09 | TERMINATE: sent by the server to indicate | 581 | | that the server is shutting down. No data | 582 | | is associated with this request. | 583 +---------------------+---------------------------------------------+ 585 Table 2 587 2.3. The DTLS Channel Protocol 589 The format of the packets sent over the UDP channel consists of an 590 1-byte header followed by data. The header byte indicates the type 591 of data that follow as in Table 2. The header and the data are 592 encapsulated in a DTLS record packet (see [RFC6347]). 594 2.4. The Channel Re-Key Protocol 596 During the exchange of session parameters (Section 2.1.3), the server 597 advertizes the methods available for session rekey using the "X-CSTP- 598 Rekey-Method" and "X-DTLS-Rekey-Method" HTTP headers. The available 599 options for both the server and client are listed below. 601 1. none: no rekey; the session will go on until 2^48 DTLS records 602 have been exchanged, or 2^64 TLS records. 604 2. ssl: a TLS or DTLS rekey will be performed periodically. Under 605 TLS/DTLS 1.2 this is performed using a rehandshake, and in later 606 versions using a rekey. 608 3. new-tunnel: the session will tear down and the client will 609 reconnect periodically. 611 When the value is other than "none" the rekey period is determinated 612 by the "X-CSTP-Rekey-Time" and "X-DTLS-Rekey-Time" headers. These 613 headers contain the time in seconds after which a session should 614 rekey. 616 It should be noted that when the "ssl" rekey option is used under 617 TLS1.2, care must be taken by both the client and the server to 618 ensure that either safe renegotiation is used ([RFC5746]), or that 619 the identity of the peer remained the same. 621 2.5. The Keepalive and Dead Peer Detection Protocols 623 In OpenConnect there are two packet types that can be used for keep- 624 alive or dead peer detection, as shown in Table 2. These are the 625 DPD-REQ and KeepAlive packets. 627 The timings of the transmission of these packets are set by the 628 server, and they for the DPD are advisory to a client. However, any 629 peer receiving these packets MUST response with the appropriate 630 packet. For DPD-REQ packets, the response MUST be DPD-RESP, and for 631 KeepAlive packets the response must be another KeepAlive packet. The 632 main difference between these two types of packets, is that the DPD 633 packets similarly to [RFC3706] are sent when there is no traffic or 634 when the other party requests them, and allow for arbitrary data to 635 be attached, making them suitable for Path MTU detection. 637 The server advertizes the suggested periods during the exchange of 638 session parameters (Section 2.1.3). The available headers are listed 639 below. 641 X-CSTP-DPD: applicable to CSTP channel; contains a relative time 642 in seconds. 644 X-CSTP-Keepalive: applicable to CSTP channel; contains a relative 645 time in seconds. 647 X-DTLS-DPD: applicable to DTLS channel; contains a relative time 648 in seconds. 650 X-DTLS-Keepalive: applicable to DTLS channel; contains a relative 651 time in seconds. 653 3. Security Considerations 655 This document provides a description of a protocol to establish a VPN 656 over a TLS 1.2 or later channel. All security considerations of the 657 referenced documents in particular [RFC8446] and [RFC6347] are 658 applicable, in addition the following considerations. 660 The protocol is designed to be as compatible as possible with a 661 legacy VPN protocol. This compatibility is not believed to cause a 662 degradation of the overall protocol security. 664 The protocol provides a VPN channel which carries payload hidden from 665 eavesdroppers. However, the payload's length remain visible and in 666 certain scenarios that may be sufficient to determine the transferred 667 payload. Furthermore, there are scenarios where compressed payload 668 lengths may reveal more information than the uncompressed data 669 [COMP-ISSUES][COMP-ISSUES2]. For that we RECOMMEND that 670 implementations don't enable compression by default, and only allow 671 it when explicitly enabled by administrators who are aware of the 672 consequences. 674 This protocol could sometimes be used because it ressembles the TLS 675 protocol and thus is not detected by the available VPN blockers. 676 While an implementation could intentionally masquerade its packets to 677 ressemble a typical HTTPS session, a fully compliant implementation 678 will be distinct from an average HTTP session due to the DTLS session 679 establishment, and the transferred packet sizes. 681 For certificate authentication OpenConnect relies on the TLS 682 protocol. However, as mentioned in the text, TLS version 1.2 and 683 earlier do not protect the client's (or the server's) certificate 684 from eavesdroppers. For that it is RECOMMENDED that certificates to 685 be used with this protocol contain the minimum possible identifying 686 information. 688 This document defines a protocol name for Application-Layer Protocol 689 Negotiation. That, if used by a client would indicate to any 690 eavesdropping parties that the client wishes to use VPN, thus 691 compromising its intention privacy. On the other hand, providing 692 that information would help a server that re-uses the same port for 693 different protocols under TLS, to forward to the appropriate handler 694 of the connection. That is, it would allow hosting a plain HTTPS 695 server serving content, and a VPN server using openconnect at the 696 same port. It is left to the client implementation to decide the 697 balance between privacy and usability with such servers. 699 4. Acknowledgements 701 None yet. 703 5. Normative References 705 [COMP-ISSUES] 706 Bhargavan, K., Fournet, C., Kohlweiss, M., Pironti, A., 707 and P-Y. Strub, "TLS Compression Fingerprinting and a 708 Privacy-aware API for TLS", 2012. 710 [COMP-ISSUES2] 711 Kelsey, J., "Compression and information leakage of 712 plaintex", International Workshop on Fast Software 713 Encryption , 2002. 715 [I-D.ietf-tls-dtls13] 716 Rescorla, E., Tschofenig, H., and N. Modadugu, "The 717 Datagram Transport Layer Security (DTLS) Protocol Version 718 1.3", draft-ietf-tls-dtls13-37 (work in progress), March 719 2020. 721 [OPENCONNECT-CLIENT] 722 Woodhouse, D., "http://www.infradead.org/openconnect/", 723 2016. 725 [OPENCONNECT-SERVER] 726 Mavrogiannopoulos, N., "http://www.infradead.org/ocserv/", 727 2016. 729 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 730 Requirement Levels", BCP 14, RFC 2119, 731 DOI 10.17487/RFC2119, March 1997, 732 . 734 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 735 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 736 Transfer Protocol -- HTTP/1.1", RFC 2616, 737 DOI 10.17487/RFC2616, June 1999, 738 . 740 [RFC2743] Linn, J., "Generic Security Service Application Program 741 Interface Version 2, Update 1", RFC 2743, 742 DOI 10.17487/RFC2743, January 2000, 743 . 745 [RFC3706] Huang, G., Beaulieu, S., and D. Rochefort, "A Traffic- 746 Based Method of Detecting Dead Internet Key Exchange (IKE) 747 Peers", RFC 3706, DOI 10.17487/RFC3706, February 2004, 748 . 750 [RFC4519] Sciberras, A., Ed., "Lightweight Directory Access Protocol 751 (LDAP): Schema for User Applications", RFC 4519, 752 DOI 10.17487/RFC4519, June 2006, 753 . 755 [RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based 756 Kerberos and NTLM HTTP Authentication in Microsoft 757 Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006, 758 . 760 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure 761 Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007, 762 . 764 [RFC5705] Rescorla, E., "Keying Material Exporters for Transport 765 Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, 766 March 2010, . 768 [RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, 769 "Transport Layer Security (TLS) Renegotiation Indication 770 Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010, 771 . 773 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 774 Extensions: Extension Definitions", RFC 6066, 775 DOI 10.17487/RFC6066, January 2011, 776 . 778 [RFC6164] Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti, 779 L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter- 780 Router Links", RFC 6164, DOI 10.17487/RFC6164, April 2011, 781 . 783 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 784 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 785 January 2012, . 787 [RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan, 788 "Transport Layer Security (TLS) Application-Layer Protocol 789 Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, 790 July 2014, . 792 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 793 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 794 . 796 Appendix A. Name for Application-Layer Protocol Negotiation 798 Protocol: openconnect-vpn/1.2 799 Identification Sequence: 800 0x6f 0x70 0x65 0x6e 0x63 0x6f 0x6e 0x6e 0x65 0x63 801 0x74 0x2d 0x76 0x70 0x6e 0x2f 0x31 0x2e 0x32 803 Appendix B. Compression 805 The available compression algorithms for the CSTP and DTLS channels 806 are shown in Table 3. Note, that all algorithms are intentionally 807 stateless to prevent the influence of independent packets (e.g., from 808 different sources) on each others compression. That does not 809 eliminate all known attacks on compression before encryption, and for 810 that reason an implentation MUST NOT enable compression by default. 812 After compression is negotiated each side may choose to compress the 813 payload and use the 'COMPRESSED DATA' header from Table 2, or may 814 send uncompressed data with the 'DATA' payload. Each side MUST be 815 able to process both payloads. 817 +---------------------+---------------------------------------------+ 818 | Algorithm | Description | 819 +---------------------+---------------------------------------------+ 820 | oc-lz4 | The stateless LZ4 compression algorithm. | 821 | | | 822 | lzs | The stateless LZS (stacker) compression | 823 | | algorithm. | 824 +---------------------+---------------------------------------------+ 826 Table 3 828 Appendix C. DTD declarations 830 C.1. config-auth.dtd 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 854 Author's Address 856 Nikos Mavrogiannopoulos 857 Red Hat 859 EMail: nmav@redhat.com