Network Working Group N. Mavrogiannopoulos
Internet-Draft Red Hat
Intended status: Informational September 23, 2016
Expires: March 27, 2017
The OpenConnect VPN Protocol Version 1.0
draft-mavrogiannopoulos-openconnect-00
Abstract
This document specifies version 1.0 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-
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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 March 27, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Terminology . . . . . . . . . . . . . . . . 3
1.2. Goals of This Document . . . . . . . . . . . . . . . . . 3
2. The OpenConnect Protocol . . . . . . . . . . . . . . . . . . 3
2.1. VPN Session Establishment . . . . . . . . . . . . . . . . 3
2.1.1. Server Authentication . . . . . . . . . . . . . . . . 3
2.1.2. Client Authentication . . . . . . . . . . . . . . . . 4
2.1.3. Exchange of Session Parameters . . . . . . . . . . . 9
2.1.4. Establishment of Primary TCP Channel (CSTP) . . . . . 10
2.1.5. Establishment of Secondary UDP Channel (DTLS) . . . . 11
2.2. The CSTP Channel Protocol . . . . . . . . . . . . . . . . 14
2.3. The DTLS Channel Protocol . . . . . . . . . . . . . . . . 15
2.4. The Channel Re-Key Protocol . . . . . . . . . . . . . . . 15
2.5. The Keepalive and Dead Peer Detection Protocols . . . . . 16
3. Security Considerations . . . . . . . . . . . . . . . . . . . 17
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
5. Normative References . . . . . . . . . . . . . . . . . . . . 18
Appendix A. Name for Application-Layer Protocol Negotiation . . 21
Appendix B. Compression . . . . . . . . . . . . . . . . . . . . 21
Appendix C. DTD declarations . . . . . . . . . . . . . . . . . . 21
C.1. config-auth.dtd . . . . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22
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.
While there are many competing VPN protocol solutions, none of them
was ever described in a publicly available document. Even open
source VPN solutions have their source code as the primary
description of their protocol. That allowed no easy study of each
protocol's properties and weaknesses, and that is the secondary goal
of this document, to describe a deployed TLS based [RFC5246] VPN
protocol.
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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 1.0 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 [RFC5246],
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. VPN Session 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 1.1 or later, and support the following TLS protocol
extensions.
Server Name Indication [RFC6066]: the client SHOULD provide the
DNS name of the server in the TLS handshake.
Application-Layer Protocol Negotiation [RFC7301]: the client MAY
provide this protocol name. The protocol name to be used is
defined in Appendix A.
2.1.1. Server Authentication
In the OpenConnect VPN protocol, the server is always authenticated
using its certificate. Once a client establishes a TCP connection to
the server's well known port, it initiates the TLS protocol. In the
first connection to the server, the client SHOULD verify the provided
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by the server certificate, and SHOULD store its public key for
verification of subsequent sessions. Thus, subsequent sessions
SHOULD check whether the server's key match the initial.
The server's identity in the certificate SHOULD be placed in the
certificate's SubjectAlternativeName field, and unless a special
profile is assumed, it will be of type DNSName.
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).
The server is authenticated to the client using a PKIX certificate
presented during the TLS negotiation.
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, there are legacy clients which 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.
After the TLS session is established the client irrespective of the
supported authentication methods, should send an HTTP POST request on
"/" with a config-auth XML structure of type 'init'. An example of
its contents follow.
v5.01
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The precise DTD declarations for the contents of XML messages defined
in this document are listed in Appendix C. Also the HTTP Content-
Type to be used for these XML structures MUST be 'text/xml'.
2.1.2.1. Authentication using certificates
During the initial TLS protocol handshake the server may require a
client certificate to be presented, depending on its configuration.
Because the client certificate is sent in the clear during the
handshake it 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 should send it reply.
If the certificate sent by the client was successfully validated, it
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.
0.1(1)
SSL VPN Service
In that case the client should proceed to the establishment of the
primary channel as in Section 2.1.4.
2.1.2.2. Authentication using passwords
After the TLS session is established and the the config-auth XML
structure of type 'init' is sent, the server will reply using forms
the client software should prompt the user to fill in. Its reply
utilizes a config-auth XML structure of type 'auth-request'.
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Please enter your username
The client may be asked to provide the information in separate forms
as above, or may be asked combined as below.
Please enter your username
The client software will then fill in the provided form and sent it
back to the server using an HTTP POST on the location specified by
the server (in the above examples it was "/auth"). The reply would
then be of type 'auth-reply' as in the following example.
v5.01
test
As mentioned above, the server may ask repeatedly for information
until it believes 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. In these cases the server should respond with an HTTP 200
OK status code, and proceed sending its new request.
If client authentication fails, the server MUST respond with an HTTP
401 unauthorized status code. Otherwise, on successful
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authentication the server should reply with a 200 HTTP code and use
the 'complete' config-auth XML structure as in Section 2.1.2.1.
Note, that sending the username and password in different messages
will reveal the length of them to a passive eavesdropper. For that
is is RECOMMENDED for clients to use the 'X-Pad' HTTP header, which
will contain arbitrary printable data to make the message length a
multiple of 64 bytes.
An example session is shown in figure 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 |
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| | |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
2.1.2.3. HTTP Authentication using SPNEGO
That type of authentication is performed using the HTTP SPNEGO
protocol [RFC4559], a method which is available using the Generic
Security Service API [RFC2743]. The following approach is used to
advertise the availability of the HTTP SPNEGO protocol by the client.
A client which supports the HTTP SPNEGO protocol, SHOULD indicate 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.
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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
sending the "X-Support-HTTP-Auth: true" header.
Otherwise, on successful authentication the server should reply with
a 200 HTTP code and use the 'complete' config-auth XML structure as
in Section 2.1.2.1.
2.1.3. Exchange of Session Parameters
By the receipt of a success XML structure, the client SHOULD issue an
HTTP CONNECT request. In addition it may provide the following
headers.
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.
User-Agent: A string identifying the client software.
For the options related to compression see Appendix B for more
information.
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
After a successful receipt of an HTTP CONNECT request, the server
should reply and provide the client with configuration parameters.
The available options follow.
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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 domains the provided DNS servers
respond for.
X-CSTP-Split-Include: The network address of a route which is
provided by this server.
X-CSTP-Split-Exclude: The network address of a route that is not
provided by this server.
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
will be 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 client is expected to treat the received parameters as his
networking settings. If no "X-CSTP-Split-Include" headers are
present, the client is expected to assign its default route through
the VPN.
2.1.4. Establishment of Primary TCP Channel (CSTP)
The previous HTTP message is the last HTTP message sent by the
server. After that message, the established TCP channel is used to
transport IP packets between the client and the server. The
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transferred packets encoding is discussed in Section 2.2. This
channel will be referred as CSTP in the rest of this document.
2.1.5. Establishment of Secondary UDP Channel (DTLS)
To establish the secondary UDP-based channel, which will be referred
to as the DTLS channel, the client must advertise support for it
during the issue of the HTTP CONNECT request (see Section 2.1.3).
This is done by appending the following headers to the request.
X-DTLS-Accept-Encoding: A comma separated list of accepted
compression algorithms for the DTLS channel.
X-DTLS-CipherSuite: Must contain the keyword PSK-NEGOTIATE.
The DTLS channel utilizes the PSK key exchange method. The key
material for this session is a 256-bit value generated with an
[RFC5705] exporter. The key material exporter uses the label
"EXPORTER-openconnect-psk" without the quotes, and without any
context value.
In its client hello message the client must copy the value received
in the 'X-DTLS-App-ID' header (after hex decoding it), to a TLS
application-specific ID field [I-D.mavrogiannopoulos-app-id]. That
identifier, can be used by the server to associate the client
initiated DTLS channel with the CSTP channel. The following headers
are used by the server's response to CONNECT, and are related to the
DTLS channel establishment.
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.
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 2.4. Only considered if
applicable to the negotiated DTLS protocol.
X-DTLS-Rekey-Method: The method used in DTLS rekey, see
Section 2.4. Only considered if applicable to the negotiated DTLS
protocol.
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2.1.5.1. Legacy Establishment of Secondary UDP Channel (DTLS)
Previous versions of this protocol utilized a special DTLS protocol
negotiation, based on an unpublished description of the DTLS
protocol. This section attempts to summarize this negotiation, but
may not be entirely accurate.
To establish the legacy UDP-based channel, the client must advertise
support for it during the issue of the HTTP CONNECT request (see
Section 2.1.3). This is done by appending the following headers to
the request.
X-DTLS-Accept-Encoding: A comma separated list of accepted
compression algorithms for the DTLS channel.
X-DTLS-Master-Secret: A hex encoded pre-master secret to be used
in the legacy DTLS session negotiation.
X-DTLS-CipherSuite: A colon-separated list of ciphers (e.g., the
string PSK-NEGOTIATE:AES256-SHA:AES128-SHA:DES-CBC3-SHA).
The DTLS channel utilizes session resumption as a method for
preshared-key authentication. That is the value presented in X-DTLS-
Master-Secret is set as a master secret to be resumed. The session
ID value is sent by the server on the response to CONNECT using the
'X-DTLS-Session-ID' header. That header provides a hex-encoded value
of the DTLS session ID to be used by the client. The following
headers are used by the server's response to CONNECT, and are related
to the DTLS channel establishment.
X-DTLS-Session-ID: A hex encoded value to be used as a DTLS
session ID by the client. It also serves as an identifier for the
server to associate the incoming DTLS session with the TLS
session.
X-DTLS-Port: The port number to which the client should send UDP
packets for DTLS.
X-DTLS-CipherSuite: The ciphersuite selected by the server. It
should be one of the options present in the client's X-DTLS-
CipherSuite header.
X-DTLS-Rekey-Time: The time (in seconds) after which the DTLS
session should rekey, see Section 2.4.
X-DTLS-Rekey-Method: The method used in DTLS rekey, see
Section 2.4.
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The following table lists the ciphers negotiated via the X-DTLS-
CipherSuite header, and the corresponding DTLS ciphersuite.
+--------------------+---------------------------------+------------+
| OpenConnect cipher | DTLS ciphersuite | DTLS |
| | | version |
+--------------------+---------------------------------+------------+
| DES-CBC3-SHA | TLS_RSA_WITH_3DES_EDE_CBC_SHA1 | DTLS 0.9 |
| | | (pre-draft |
| | | version) |
| | | |
| AES128-SHA | TLS_RSA_WITH_AES_128_CBC_SHA1 | DTLS 0.9 |
| | | (pre-draft |
| | | version) |
| | | |
| AES256-SHA | TLS_RSA_WITH_AES_256_CBC_SHA1 | DTLS 0.9 |
| | | (pre-draft |
| | | version) |
| | | |
| OC- | TLS_RSA_WITH_AES_128_GCM_SHA256 | DTLS 1.2 |
| DTLS1_2-AES128-GCM | | |
| | | |
| OC- | TLS_RSA_WITH_AES_256_GCM_SHA256 | DTLS 1.2 |
| DTLS1_2-AES256-GCM | | |
+--------------------+---------------------------------+------------+
Table 1
The legacy DTLS protocol negotiation described in this section, is
similar to DTLS 1.0 except for the following deviations:
The negotiated protocol version for the handshake and record
headers is 1.0 instead of 254.255.
The Hello Verify and Hello verify request messages are included in
the handshake hashes.
The handshake header is not included as part of the handshake
hashes.
The ChangeCipherSpec message is 3 byte long instead of 1, and
contains the handshake sequence number (2-bytes long) appended to
the message id.
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2.2. 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 [RFC5246]). The bytes of the header indicate
the type of data that follow, and their contents are explained in
Table 2.
+---------------------+---------------------------------------------+
| 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 3 for available types) |
| | |
| 7 | fixed to 0x00 |
+---------------------+---------------------------------------------+
Table 2
The available payload types are listed in Table 3.
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+---------------------+---------------------------------------------+
| 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. No data is |
| | associated with this request. The session |
| | will be invalidated after such 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. |
+---------------------+---------------------------------------------+
Table 3
2.3. The DTLS Channel Protocol
The format of the packets sent over the UDP channel consists of an
1-byte header followed by data. The header byte indicates the type
of data that follow as in Table 3. The header and the data are
encapsulated in a DTLS record packet (see [RFC6347]).
2.4. The Channel Re-Key Protocol
During the exchange of session parameters (Section 2.1.3), the server
advertizes 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.
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2. ssl: a TLS or DTLS rehandshake will be performed periodically.
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 determinated
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, 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 remained the same.
2.5. 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 3. 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 advertizes the suggested periods during the exchange of
session parameters (Section 2.1.3). 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.
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3. Security Considerations
This document provides a description of a protocol to establish a VPN
over a TLS channel. All security considerations of the referenced
documents in particular [RFC5246] and [RFC6347] are applicable, in
addition the following considerations.
The protocol is designed to be as compatible as possible with a
legacy VPN protocol and as such it carries cruft, such as partial
dependence on a non-standard DTLS version, and utilization of an
awkward method to establish a DTLS session which relies on session
resumption. Nevertheless, these particularities are not believed to
cause a degradation of the overall protocol security, and could be
addressed with a backwards compatible protocol upgrade.
The protocol provides a VPN channel which carries 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 don't enable compression by default, and only allow
it after notifying the users and administrators about the
consequences.
This protocol could sometimes be used because of the fact that it
ressembles the TLS protocol and thus is not detected by the available
VPN blockers. While an implementation could intentionally masquerade
its packets to ressemble a typical HTTPS session, a fully compliant
implementation will be distinct from an average HTTP session due to
the DTLS session establishment, 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.
This document defines a protocol name for Application-Layer Protocol
Negotiation. That, if used by a client would indicate to any
eavesdropping parties that the client wishes to use VPN, thus
compromising its intention privacy. On the other hand, providing
that information would help a server that re-uses the same port for
different protocols under TLS, to forward to the appropriate handler
of the connection. That is, it would allow hosting a plain HTTPS
server serving content, and a VPN server using openconnect at the
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same port. It is left to the client to decide the balance between
privacy and usability with such servers.
4. Acknowledgements
None yet.
5. Normative References
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, .
[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, June 1999,
.
[RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
Kerberos and NTLM HTTP Authentication in Microsoft
Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,
.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743,
DOI 10.17487/RFC2743, January 2000,
.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, .
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[RFC4519] Sciberras, A., Ed., "Lightweight Directory Access Protocol
(LDAP): Schema for User Applications", RFC 4519,
DOI 10.17487/RFC4519, June 2006,
.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
.
[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,
July 2014, .
[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, April 2011,
.
[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, February 2004,
.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[I-D.mavrogiannopoulos-app-id]
Mavrogiannopoulos, N. and D. Woodhouse, "A TLS
application-specific identifier", draft-mavrogiannopoulos-
app-id-00 (work in progress), September 2016.
[COMP-ISSUES]
Bhargavan, K., Fournet, C., Kohlweiss, M., Pironti, A.,
and P-Y. Strub, "TLS Compression Fingerprinting and a
Privacy-aware API for TLS", 2012.
[COMP-ISSUES2]
Kelsey, J., "Compression and information leakage of
plaintex", International Workshop on Fast Software
Encryption , 2002.
[OPENCONNECT-CLIENT]
Woodhouse, D., "http://www.infradead.org/openconnect/",
2016.
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[OPENCONNECT-SERVER]
Mavrogiannopoulos, N., "http://www.infradead.org/ocserv/",
2016.
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Appendix A. Name for Application-Layer Protocol Negotiation
Protocol: openconnect-vpn/1.0
Identification Sequence:
0x6f 0x70 0x65 0x6e 0x63 0x6f 0x6e 0x6e 0x65 0x63
0x74 0x2d 0x76 0x70 0x6e 0x2f 0x31 0x2e 0x30
Appendix B. Compression
The available compression algorithms for the CSTP and DTLS channels
are shown in Table 4. 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 implentation 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 3, or may
send uncompressed data with the 'DATA' payload. Each side MUST be
able to process both payloads.
+---------------------+---------------------------------------------+
| Algorithm | Description |
+---------------------+---------------------------------------------+
| oc-lz4 | The stateless LZ4 compression algorithm. |
| | |
| lzs | The stateless LZS (stacker) compression |
| | algorithm. |
+---------------------+---------------------------------------------+
Table 4
Appendix C. DTD declarations
C.1. config-auth.dtd
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Author's Address
Nikos Mavrogiannopoulos
Red Hat
EMail: nmav@redhat.com
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