Internet-Draft The OpenConnect Version 1.2 July 2023
Mavrogiannopoulos Expires 24 January 2024 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-mavrogiannopoulos-openconnect-04
Published:
Intended Status:
Informational
Expires:
Author:
N. Mavrogiannopoulos
Independent

The OpenConnect VPN Protocol Version 1.2

Abstract

This document specifies version 1.2 of the OpenConnect Virtual Private Network (VPN) protocol, a secure VPN protocol that provides communications privacy over the Internet. That protocol is believed to be compatible with CISCO's AnyConnect VPN protocol. The protocol allows the establishment of VPN tunnels in a way that is designed to prevent eavesdropping, tampering, or message forgery.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 2 January 2024.

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Table of Contents

1. Introduction

The purpose of this document is to specify the OpenConnect VPN protocol in a detail in order to allow for multiple interoperable implementations. This is the protocol used by the OpenConnect client and server [OPENCONNECT-CLIENT][OPENCONNECT-SERVER], and is believed to be compatible with CISCO's AnyConnect protocol.

This protocol's design follows a minimalistic modular philosophy. It delegates several protocol-related elements often considered as core VPN features and diversifiers, to standards protocols. That delegation, allows a minimalistic core protocol which contains very few security related elements and is decoupled from cryptography. That in turn transfers the auditing requirements due to cryptographic and negotiation protocols to dedicated for that purpose components. In particular the Openconnect VPN protocol uses standard protocols such as HTTP, TLS [RFC8446] and DTLS [RFC6347] to provide a VPN with data security and authenticity.

1.1. Requirements Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

1.2. Goals of This Document

The OpenConnect protocol version 1.2 specification is intended primarily for readers who will be implementing the protocol and those doing cryptographic analysis of it.

2. The OpenConnect Protocol

The OpenConnect protocol combines the TLS protocol [RFC8446], Datagram TLS protocol [RFC6347] and HTTP protocols [RFC2616] to provide an Internet-Layer VPN channel. The channel is designed to operate using UDP packets, and fallback on TCP if that's not possible.

In brief the protocol initiates an HTTP over TLS connection on a known port, where client authentication is performed. After this step, the client initiates an HTTP CONNECT command to establish a VPN channel over TCP. A secondary VPN channel over UDP will be established using information provided by the server using HTTP headers. At that point the raw IP packets flow, over the VPN channels.

2.1. Authentication

2.1.1. Server authentication

In the OpenConnect VPN protocol, the server is always authenticated using its certificate for the HTTP over TLS session. The server's identity in the certificate SHOULD be placed in the X.509 certificate's SubjectAlternativeName field, and be of type DNSName.

This doesn't imply a particular certificate validation model. Clients also use an internet PKIX trust model, or trust on first use key validation model.

2.1.2. Client authentication

The OpenConnect VPN protocol allows for the following types of client authentication, or combinations of them.

  1. Password: a user can authenticate itself using a password.
  2. Certificate: a user can authenticate itself using a PKIX certificate it possesses.
  3. HTTP SPNEGO: a user can authenticate itself using a Kerberos ticket, or any other mechanism supported by SPNEGO (i.e., GSSAPI).

It is important to note that during the password and HTTP SPNEGO authentication methods, any headers allowed by the HTTP protocol can be present. In fact, some legacy clients assume that the server will keep a state using cookies and send their username and password in different TLS and HTTP connections. This practice prevents the server from binding the TLS channel with the VPN session [RFC5056], and is discouraged. It is RECOMMENDED for clients to complete authentication in the same TLS session, and rely on TLS session resumption if reconnections to the server are needed.

2.2. VPN tunnel establishment

The client and server establish a TLS connection over a known port, typically over 443, the port used for HTTPS. The client SHOULD negotiate TLS 1.2 or later.

2.2.1. Tunnel initiation

A client initiates the session by start a TLS connection with the server. The initial TLS Client Hello will contain a number of extensions as mandated by the TLS protocol, but the following SHOULD be included.

Server Name Indication [RFC6066]: the client SHOULD provide the DNS name of the server in the TLS handshake.

After the TLS session is established the client irrespective of the supported authentication methods, sends an HTTP POST request on "/" with a config-auth XML structure of type 'init'. The HTTP Content-Type to be used for these XML structures MUST be 'text/xml'. An example of its contents follow.

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE config-auth SYSTEM "config-auth.dtd">
    <config-auth client="vpn" type="init">
        <version who="vpn">v5.01</version>
    </config-auth>

The precise DTD declarations for the contents of XML messages are defined in this document and are listed in Appendix B.

2.2.2. Tunnel authentication using passwords

After the TLS session is established and the the config-auth XML structure of type 'init' is sent, the server requests the username and password using forms the client software prompts the user to fill in. The server's reply utilizes a config-auth XML structure of type 'auth-request'.

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE auth SYSTEM "config-auth.dtd">
    <config-auth client="vpn" type="auth-request">
        <auth id="main">
            <message>Please enter your username</message>
            <form action="/auth" method="post">
                <input label="Username:" name="username" type="text" />
            </form>
        </auth>
    </config-auth>

The client can be asked to provide the information in multiple, separate forms as the above message implies, and any number of passwords may be requested, e.g., when second factor authentication is available, a password and a second factor token may be requested. Alternatively, when the number of inputs are fixed the client may be provided a combined form as listed below.

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE auth SYSTEM "config-auth.dtd">
    <config-auth client="vpn" type="auth-request">
      <auth id="main">
        <message>Please enter your username</message>
        <form action="/auth" method="post">
            <input label="Username:" name="username" type="text"/>
            <input label="Password:" name="password" type="password"/>
        </form>
      </auth>
    </config-auth>

The client software is expected to respond to the provided form(s) and send the responses to the server using an HTTP POST on the form action location as specified in the XML message (in the above examples it was "/auth"). The reply would then be of type 'auth-reply' as in the following example.

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE config-auth SYSTEM "config-auth.dtd">
    <config-auth client="vpn" type="auth-reply">
        <version who="vpn">v5.01</version>
        <auth><username>test</username>
        </auth>
    </config-auth>

As mentioned above, the server may ask repeatedly for information until the user is authenticated. For example, the server could present a second form asking for the password after the username is provided, or ask for a second password if that is necessary, and may even use forms to prompt the user to change a password, provide additional information and so on. When multiple forms are provided the servers responds with an HTTP 200 OK status code and sends its new request.

If client authentication fails, the server MUST respond with an HTTP 401 unauthorized status code. On successful authentication the server replies with a 200 HTTP code and use the 'complete' config-auth XML structure as follows.

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE config-auth SYSTEM "config-auth.dtd">
    <config-auth client="vpn" type="complete">
      <version who="sg">0.1(1)</version>
      <auth id="success">
        <title>SSL VPN Service</title>
      </auth>
    </config-auth>

Note, that including the username and password in XML messages will reveal the length of them to a passive eavesdropper. For that is is RECOMMENDED for clients to use an 'X-Pad' HTTP header, containing arbitrary printable data to make the message length a multiple of 64 bytes.

2.2.3. Tunnel authentication using certificates

When a user is authenticated using a certificate, during the initial TLS protocol handshake the server will require a client certificate to be presented.

Because under TLS 1.2 the client certificate is sent in the clear during the handshake, the certificate SHOULD NOT contain other identifying information other than a username, or a pseudonymus identifier. It is RECOMMENDED to place the user identifier in the DN field of the certificate, using the UID object identifier (0.9.2342.19200300.100.1.1) [RFC4519].

After the TLS session is established and the the config-auth XML structure of type 'init' is sent, the server responds according to certificate validation status. If the certificate sent by the client was successfully validated, the server should reply using the HTTP response code 200, and the contents of the reply should be a config-auth XML structure of type 'complete', as follows.

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE config-auth SYSTEM "config-auth.dtd">
    <config-auth client="vpn" type="complete">
      <version who="sg">0.1(1)</version>
      <auth id="success">
        <title>SSL VPN Service</title>
      </auth>
    </config-auth>

In that case the client should proceed to the establishment of the primary CSTP channel as in Section 2.2.6.

2.2.4. Tunnel authentication using SPNEGO

The HTTP SPNEGO protocol [RFC4559] enables among others authentication using Kerberos tickets. The HTTP SPNEGO method is available using the Generic Security Service API [RFC2743]. A client which supports the HTTP SPNEGO protocol, indicates it using the following header on in its initial request to the server with the config-auth 'init' XML structure.

    X-Support-HTTP-Auth: true

After that the server would report a "401 Unauthorized" status code and authentication would proceed as specified in the HTTP SPNEGO protocol. The server may utilize the following header, to indicate that alternative authentication methods are available (e.g., with plain password), if authentication fails.

    X-Support-HTTP-Auth: fallback

If client authentication fails, the server MUST respond with an HTTP 401 unauthorized status code. In that case, a client which received the previous header should retry authenticating to the server without advertising HTTP SPNEGO, meaning the "X-Support-HTTP-Auth: true" header will not be included.

Otherwise, on successful authentication the server should reply with a 200 HTTP code and use the 'complete' config-auth XML structure as in Section 2.2.3.

Once the client is successfully validated, the server should reply using the HTTP response code 200, and the contents of the reply should be a config-auth XML structure of type 'complete', as with the certificate authentication.

2.2.5. Tunnel and channels establishment

By the receipt of a 'complete' config-auth XML structure, the client issues an HTTP CONNECT request to initiate the VPN tunnel. An example CONNECT request is shown below.

    User-Agent: Open AnyConnect VPN Agent v5.01
    X-CSTP-Base-MTU: 1280
    X-CSTP-Address-Type: IPv4,IPv6
    CONNECT /CSCOSSLC/tunnel HTTP/1.1
2.2.5.1. Client capabilities

As each client supports different capabilities, the following HTTP headers are used during the CONNECT request to advertise them.

X-CSTP-Address-Type: A comma separated list of the requested address types.

IPv4: when the client only supports IPv4 addresses.
IPv6: when the client only supports IPv6 addresses.
IPv4,IPv6: when the client supports both types of IP addresses.
X-CSTP-Base-MTU: The MTU of the link as estimated by the client.
X-CSTP-Accept-Encoding: A comma separated list of accepted compression algorithms for the CSTP channel (optional). Compression on encrypted streams introduces additional risk, see Appendix A for more information.
User-Agent: A string identifying the client software. By convention OpenConnect clients identify as "Open AnyConnect VPN Agent". This string is informative to the server and its operator.
2.2.5.2. Server response and tunnel configuration

After a successful receipt of an HTTP CONNECT request, the server responds and provides the client with configuration parameters. The available tunnel configuration options are listed below.

X-CSTP-Address: The IPv4 address of the client, if IPv4 has been requested.
X-CSTP-Netmask: An IPv4 netmask to be pushed to the client, if IPv4 has been requested. This should contain the mask on the P-t-P link and is RECOMMENDED the server address to be the first in defined network.
X-CSTP-Address-IP6: The IPv6 address of the client in CIDR notation, if IPv6 has been requested. The prefix length is RECOMMENDED to be set to 127-bits according to [RFC6164].
X-CSTP-DNS: The IP address of a DNS server that can be used for that session.
X-CSTP-Default-Domain: The DNS default search domains. Typically a subset of X-CSTP-Split-DNS. If multiple, the domains are space separated.
X-CSTP-Split-DNS: A DNS domain the provided DNS servers respond for. Multiple such headers may be present for different domains.
X-CSTP-Split-Include: The network address of a route which is provided by this server. Multiple such headers may be present.
X-CSTP-Split-Exclude: The network address of a route that is not provided by this server. Multiple such headers may be present.
X-CSTP-Base-MTU: The MTU of the link as estimated by this server.
X-CSTP-DynDNS: Set to "true" if the server is operating with a dynamic DNS address.
X-CSTP-Content-Encoding: if present is it set to one of the values presented by the client in 'X-CSTP-Accept-Encoding' header. It contains the compression algorithm used in the CSTP channel.
X-DTLS-Content-Encoding: if present is it set to one of the values presented by the client in 'X-DTLS-Accept-Encoding' header. It will be the compression algorithm used in the DTLS channel.

The received options are the client's tunnel networking configuration. If no "X-CSTP-Split-Include" headers are present, the client is expected to assign its default route through the VPN.

After the server's response to the CONNECT request, the VPN tunnel is established. This tunnel consists of two channels, the CSTP channel and the (optional) DTLS channel that are described in the next sections.

2.2.6. The primary CSTP channel - TCP

The previous HTTP message is the last HTTP message sent by the server. After that message, the established TCP connection forms a channel that is transports IP packets between the client and the server. We refer to it as the CSTP channel in the rest of this document. The encoding of the transferred packets is described further in Section 2.3.

2.2.7. The secondary DTLS channel - UDP

The secondary DTLS based channel over UDP is established optionally by clients and servers that wish to avoid the drawbacks of tunneling TCP over TCP. This channel -referred to as the DTLS channel- is established if the client advertises support for it during the HTTP CONNECT request (see Section 2.2.5). This is done by the client including the following headers in the request.

X-DTLS-CipherSuite: Must contain the keyword PSK-NEGOTIATE.
X-DTLS-Accept-Encoding: A comma separated list of accepted compression algorithms for the DTLS channel (optional). The same risks as with the primary CSTP channel apply for compression.

An example HTTP CONNECT request that advertises support for the DTLS channel is shown below.

    User-Agent: Open AnyConnect VPN Agent v5.01
    X-CSTP-Base-MTU: 1280
    X-CSTP-Address-Type: IPv4,IPv6
    X-DTLS-CipherSuite: PSK-NEGOTIATE
    CONNECT /CSCOSSLC/tunnel HTTP/1.1

The server's response to the HTTP CONNECT request, includes the following headers, if the server wishes to establish the DTLS channel.

X-DTLS-App-ID: A hex encoded value to be used as a DTLS application-specific identifier by the client. It serves as an identifier for the server to associate the incoming DTLS session with the TLS session. The identifier (before encoding) can be from 16 to 32 bytes.
X-DTLS-Port: The port number to which the client should send UDP packets for DTLS.
X-DTLS-CipherSuite: It must contain the value "PSK-NEGOTIATE" without any quotes.
X-DTLS-Rekey-Time: The time (in seconds) after which the DTLS session should rekey, see Section 2.5. Only considered if applicable to the negotiated DTLS protocol.
X-DTLS-Rekey-Method: The method used in DTLS rekey, see Section 2.5. Only considered if applicable to the negotiated DTLS protocol.
2.2.7.1. DTLS session establishment

After the DTLS channel is negotiated over the CSTP channel, it is established by the client initiating a DTLS session.

The client initiates a UDP connection to the IP address if the server and port as specified by the X-DTLS-Port value. The new UDP connection uses the DTLS 1.2 protocol (or any later version) with the PSK key exchange method. The pre-shared key material for this channel are generated by both the server and the client independently and is not exchanged. The pre-shared key is a 256-bit value generated with an [RFC5705] exporter from the TLS session of the CSTP channel. The key material exporter uses the label "EXPORTER-openconnect-psk" without the quotes, and without any context value.

In its DTLS Client Hello message the client must copy the value received in the 'X-DTLS-App-ID' header after hex decoding it, to the session ID field of the DTLS Client Hello. That identifier is not used for session resumption, and is used by the server when it receives the first UDP message to associate the new DTLS protocol connection with the corresponding CSTP channel.

2.2.8. Overview of the tunnel establishment

An overview of the established tunnel and channels is shown in Figure 1.

        ,-.
        `-'
        /|\
         |                                ,------.          ,----------.
        / \                               |Server|          |ServerDTLS|
      Client                              `--+---'          `----+-----'
        |     TLS handshake Client Hello     |                   |
        | ----------------------------------->                   |
        |                                    |                   |
        |       TLS handshake Finished       |                   |
        | <-----------------------------------                   |
        |                                    |                   |
        |     HTTP POST config-auth init     |  ,--------------------!.
        | ----------------------------------->  |This is an HTTP over|_\
        |                                    |  |TLS session.          |
        |                                    |  `----------------------'
        |      config-auth auth-request      |                   |
        | <-----------------------------------                   |
        |                                    |                   |
        |  HTTP POST config-auth auth-reply  |                   |
        | ----------------------------------->                   |
        |                                    |                   |
        |        config-auth complete        |                   |
        | <-----------------------------------                   |
        |                                    |                   |
        |            HTTP CONNECT            |                   |
        | ----------------------------------->                   |
        |                                    |                   |
        |                                    |                   |
        |            ===================================         |
====================== CSTP VPN session is established =======================
        |            ===================================         |
        |                                    |                   |
        |                                    |  ,-------------------------!.
        | TLS record packet with CSTP payload|  |These packets show       |_\
        | ----------------------------------->  |that IP traffic can start  |
        |                                    |  |prior to the DTLS channel  |
        |                                    |  |establishment.             |
        |                                    |  `---------------------------'
        | TLS record packet with CSTP payload|                   |
        | <-----------------------------------                   |
        |                                    |                   |
        |               DTLS handshake Client Hello              |
        |  - - - - - - - - - - - - - - - - - - - - - - - - - - - >
        |                                    |                   |
        |                 DTLS handshake Finished                |
        | <- - - - - - - - - - - - - - - - - - - - - - - - - - - -
        |                                    |                   |
        |                                    |                   |
        |            ===================================         |
====================== DTLS VPN channel is established =======================
        |            ===================================         |
        |                                    |                   |
        |             DTLS record packet with payload            |
        |  - - - - - - - - - - - - - - - - - - - - - - - - - - - >
        |                                    |                   |
        |             DTLS record packet with payload            |
        | <- - - - - - - - - - - - - - - - - - - - - - - - - - - -
      Client                              ,--+---.          ,----+-----.
        ,-.                               |Server|          |ServerDTLS|
        `-'                               `------'          `----------'
        /|\
         |
        / \
Figure 1

2.3. The CSTP Channel Protocol

The format of the packets sent over the primary channel consists of an 8-bytes header followed by data. The whole packet in encapsulated in a TLS record (see [RFC8446]). The bytes of the header indicate the type of data that follow, and their contents are explained in Table 1.

Table 1
byte value
0 fixed to 0x53 (S)
1 fixed to 0x54 (T)
2 fixed to 0x46 (F)
3 fixed to 0x01
4-5 The length of the packet that follows this header in big endian order
6 The type of the payload that follows (see Table 2 for available types)
7 fixed to 0x00

The available payload types are listed in Table 2.

Table 2
Value Description
0x00 DATA: the TLS record packet contains an IPv4 or IPv6 packet
0x03 DPD-REQ: used for dead peer detection. Once sent the peer should reply with a DPD-RESP packet, that has the same contents as the original request.
0x04 DPD-RESP: used as a response to a previously received DPD-REQ.
0x05 DISCONNECT: sent by the client (or server) to terminate the session. This is followed by one byte indicating the disconnect reason. When the reason is '0xb0' the session should be invalidated after the request.
0x07 KEEPALIVE: sent by any peer. No data is associated with this request.
0x08 COMPRESSED DATA: a Data packet which is compressed prior to encryption.
0x09 TERMINATE: sent by the server to indicate that the server is shutting down. No data is associated with this request.

2.4. The DTLS Channel Protocol

The format of the packets sent over the DTLS channel consists of an 1-byte header followed by data. The header byte indicates the type of data that follow as in Table 2. The header and the data are encapsulated in a DTLS record packet (see [RFC6347]).

2.5. The Channel Re-Key Protocol

During the exchange of session parameters (Section 2.2.5), the server advertises the methods available for session rekey using the "X-CSTP-Rekey-Method" and "X-DTLS-Rekey-Method" HTTP headers. The available options for both the server and client are listed below.

  1. none: no rekey; the session will go on until 2^48 DTLS records have been exchanged, or 2^64 TLS records.
  2. ssl: a TLS or DTLS rekey will be performed periodically. Under TLS/DTLS 1.2 this is performed using a rehandshake, and in later versions using a rekey.
  3. new-tunnel: the session will tear down and the client will reconnect periodically.

When the value is other than "none" the rekey period is determined by the "X-CSTP-Rekey-Time" and "X-DTLS-Rekey-Time" headers. These headers contain the time in seconds after which a session should rekey.

It should be noted that when the "ssl" rekey option is used under TLS1.2, care must be taken by both the client and the server to ensure that either safe renegotiation is used ([RFC5746]), or that the identity of the peer remains the same.

2.6. The Keepalive and Dead Peer Detection Protocols

In OpenConnect there are two packet types that can be used for keep-alive or dead peer detection, as shown in Table 2. These are the DPD-REQ and KeepAlive packets.

The timings of the transmission of these packets are set by the server, and they for the DPD are advisory to a client. However, any peer receiving these packets MUST response with the appropriate packet. For DPD-REQ packets, the response MUST be DPD-RESP, and for KeepAlive packets the response must be another KeepAlive packet. The main difference between these two types of packets, is that the DPD packets similarly to [RFC3706] are sent when there is no traffic or when the other party requests them, and allow for arbitrary data to be attached, making them suitable for Path MTU detection.

The server advertises the suggested periods during the tunnel establishment (Section 2.2.5). The available headers are listed below.

  • X-CSTP-DPD: applicable to CSTP channel; contains a relative time in seconds.
  • X-CSTP-Keepalive: applicable to CSTP channel; contains a relative time in seconds.
  • X-DTLS-DPD: applicable to DTLS channel; contains a relative time in seconds.
  • X-DTLS-Keepalive: applicable to DTLS channel; contains a relative time in seconds.

3. Security Considerations

This document provides a description of a protocol to establish a VPN tunnel over a TLS 1.2 or later channel. All security considerations of the referenced documents in particular [RFC8446] and [RFC6347] are applicable, in addition the following considerations.

The protocol is designed to be as compatible as possible with a legacy VPN protocol. This compatibility is not believed to cause a degradation of the overall protocol security.

The protocol provides a VPN tunnel split in two channels that carry payload hidden from eavesdroppers. However, the payload's length remain visible and in certain scenarios that may be sufficient to determine the transferred payload. Furthermore, there are scenarios where compressed payload lengths may reveal more information than the uncompressed data [COMP-ISSUES][COMP-ISSUES2]. For that we RECOMMEND that implementations not implement compression or not to enable it by default.

This protocol could sometimes be used because it resembles the TLS protocol and thus is not detected by the available VPN blockers. While an implementation could intentionally masquerade its packets to resemble a typical HTTPS session, a fully compliant implementation will be distinct from an average HTTP session due to the DTLS session establishment, the predictable size of the XML exchanges, and the transferred packet sizes.

For certificate authentication OpenConnect relies on the TLS protocol. However, as mentioned in the text, TLS version 1.2 and earlier do not protect the client's (or the server's) certificate from eavesdroppers. For that it is RECOMMENDED that certificates to be used with this protocol contain the minimum possible identifying information.

4. Acknowledgements

None yet.

5. Normative References

[RFC8446]
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <https://www.rfc-editor.org/info/rfc8446>.
[RFC5746]
Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, "Transport Layer Security (TLS) Renegotiation Indication Extension", RFC 5746, DOI 10.17487/RFC5746, , <https://www.rfc-editor.org/info/rfc5746>.
[RFC6347]
Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, , <https://www.rfc-editor.org/info/rfc6347>.
[RFC2616]
Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, DOI 10.17487/RFC2616, , <https://www.rfc-editor.org/info/rfc2616>.
[RFC4559]
Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows", RFC 4559, DOI 10.17487/RFC4559, , <https://www.rfc-editor.org/info/rfc4559>.
[RFC2743]
Linn, J., "Generic Security Service Application Program Interface Version 2, Update 1", RFC 2743, DOI 10.17487/RFC2743, , <https://www.rfc-editor.org/info/rfc2743>.
[RFC5056]
Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, DOI 10.17487/RFC5056, , <https://www.rfc-editor.org/info/rfc5056>.
[RFC5705]
Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, , <https://www.rfc-editor.org/info/rfc5705>.
[RFC4519]
Sciberras, A., Ed., "Lightweight Directory Access Protocol (LDAP): Schema for User Applications", RFC 4519, DOI 10.17487/RFC4519, , <https://www.rfc-editor.org/info/rfc4519>.
[RFC6066]
Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, , <https://www.rfc-editor.org/info/rfc6066>.
[RFC7301]
Friedl, S., Popov, A., Langley, A., and E. Stephan, "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, , <https://www.rfc-editor.org/info/rfc7301>.
[RFC6164]
Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti, L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-Router Links", RFC 6164, DOI 10.17487/RFC6164, , <https://www.rfc-editor.org/info/rfc6164>.
[RFC3706]
Huang, G., Beaulieu, S., and D. Rochefort, "A Traffic-Based Method of Detecting Dead Internet Key Exchange (IKE) Peers", RFC 3706, DOI 10.17487/RFC3706, , <https://www.rfc-editor.org/info/rfc3706>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[COMP-ISSUES]
Bhargavan, K., Fournet, C., Kohlweiss, M., Pironti, A., and P-Y. Strub, "TLS Compression Fingerprinting and a Privacy-aware API for TLS", .
[COMP-ISSUES2]
Kelsey, J., "Compression and information leakage of plaintex", International Workshop on Fast Software Encryption , .
[OPENCONNECT-CLIENT]
Woodhouse, D., "http://www.infradead.org/openconnect/", .
[OPENCONNECT-SERVER]
Mavrogiannopoulos, N., "http://www.infradead.org/ocserv/", .

Appendix A. Compression

The available compression algorithms for the CSTP and DTLS channels are shown in Table 3. Note, that all algorithms are intentionally stateless to prevent the influence of independent packets (e.g., from different sources) on each others compression. That does not eliminate all known attacks on compression before encryption, and for that reason an implementation MUST NOT enable compression by default.

After compression is negotiated each side may choose to compress the payload and use the 'COMPRESSED DATA' header from Table 2, or may send uncompressed data with the 'DATA' payload. Each side MUST be able to process both payloads.

Table 3
Algorithm Description
oc-lz4 The stateless LZ4 compression algorithm.
lzs The stateless LZS (stacker) compression algorithm.

Appendix B. DTD declarations

B.1. config-auth.dtd 

<!ELEMENT config-auth (version*,auth*)>
  <!ATTLIST config-auth client CDATA #FIXED "vpn">
  <!ATTLIST config-auth type (init|auth-reply|auth-request|complete) "init">
<!ELEMENT version (#PCDATA)>
  <!ATTLIST version who (sg|vpn) "sg">
<!ELEMENT auth (title*,username*,password*,message*,form*)>
  <!ATTLIST auth id (success|main|failure) "failure">
  <!ELEMENT title (#PCDATA)>
  <!ELEMENT username (#PCDATA)>
  <!ELEMENT password (#PCDATA)>
  <!ELEMENT message (#PCDATA)>
  <!ELEMENT form (input)>
    <!ATTLIST form action CDATA #FIXED "/auth">
    <!ATTLIST form method CDATA #FIXED "post">
    <!ELEMENT input (EMPTY)>
       <!ATTLIST input label CDATA "">
       <!ATTLIST input name (username|password) "username">
       <!ATTLIST input type (text|password) "text">
    <!ELEMENT select (option)>
       <!ATTLIST select label CDATA "">
       <!ATTLIST select name (group_list) "group_list">
    <!ELEMENT option (#PCDATA)>

Author's Address

Nikos Mavrogiannopoulos
Independent