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RFC 7869

Informational
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The "vnc" URI Scheme

 


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Independent Submission                                         D. Warden
Request for Comments: 7869                              Dell Products LP
Category: Informational                                      I. Iordanov
ISSN: 2070-1721                                                 Undatech
                                                                May 2016


                          The "vnc" URI Scheme

Abstract

   Virtual Network Computing (VNC) software provides remote desktop
   functionality.  This document describes a Uniform Resource Identifier
   (URI) scheme enabling the launch of VNC clients from other
   applications.  The scheme specifies parameters useful in securely
   connecting clients with remote hosts.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7869.

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.

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Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Language ......................................3
   2. The "vnc" URI Scheme ............................................3
      2.1. URI Scheme Syntax ..........................................3
           2.1.1. URI Parameters ......................................4
           2.1.2. Data Types ..........................................9
      2.2. Processing URIs ...........................................11
           2.2.1. Error Handling .....................................12
           2.2.2. Connection Profile Matching ........................12
      2.3. Connection Channel Types ..................................12
           2.3.1. The "Integrated SSH" Channel Type ..................12
           2.3.2. The "Secure Tunnel" Channel Type ...................14
   3. Security Considerations ........................................15
      3.1. Application Trust .........................................16
      3.2. URI Handling ..............................................16
      3.3. Host Identification .......................................17
      3.4. Connection Database Integrity .............................18
   4. IANA Considerations ............................................18
      4.1. "vnc" Scheme ..............................................18
      4.2. Remote Framebuffer Security Types .........................18
      4.3. VNC URI Group .............................................19
      4.4. VNC URI Connection Channel Types ..........................19
      4.5. VNC URI ID Hash Algorithms ................................19
      4.6. VNC URI Parameters ........................................21
   5. References .....................................................22
      5.1. Normative References ......................................22
      5.2. Informative References ....................................23
   Appendix A. "vnc" URI Template ....................................24
   Acknowledgments ...................................................25
   Authors' Addresses ................................................25

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1.  Introduction

   Virtual Network Computing (VNC) clients are used to support remote
   desktop connectivity based on the Remote Framebuffer (RFB) Protocol
   [RFC6143].  It is often desirable to integrate such functionality
   with other software.  However, the lack of a standard method for
   specifying VNC client parameters has limited such integration.

   The "vnc" Uniform Resource Identifier (URI) scheme specified in this
   document facilitates the launch of VNC clients from applications in
   browser-based, desktop, and mobile environments.  Using this scheme,
   users and application vendors will be able to integrate remote
   desktop capabilities without being tied to a particular client.

   Remote desktop clients often store connection profiles in a local
   connection database.  By associating connections specified in a URI
   with those stored in a database, client-specific options can be
   automatically applied to a connection launched from another
   application, even when that application is unaware of those options.

   Connections to VNC servers are often secured using mechanisms
   including Transport Layer Security / Secure Sockets Layer (TLS/SSL)
   tunneling [RFC5246] and Secure Shell (SSH) [RFC4251] tunneling, which
   are outside the scope of the RFB protocol.  Defining the behavior of
   these client-integrated security options enables their use with "vnc"
   URIs.

1.1.  Requirements Language

   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 RFC 2119 [RFC2119].

   In this document, these words will appear with that interpretation
   only when in ALL CAPS.  Lowercase uses of these words are not to be
   interpreted as carrying the significance described in RFC 2119.

2.  The "vnc" URI Scheme

2.1.  URI Scheme Syntax

   The normative syntax of the "vnc" URI is defined in the <vnc-uri>
   rule in the following syntax specification.  This specification uses
   the Augmented Backus-Naur Form (ABNF) as described in [RFC5234].  The
   "vnc" URI conforms to the generic URI syntax specified in [RFC3986].
   The <userinfo>, <host>, <port>, <unreserved>, and <pct-encoded> rules
   are defined in [RFC3986].

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   vnc-uri = "vnc://" [ userinfo "@" ] [ host [ ":" port ] ]
             [ "?" [ vnc-params ] ]

   vnc-params = param "=" value *("&" param "=" value) ["&"]

   param = 1*( param-char )

   value = *( param-char )

   param-char = unreserved / pct-encoded / unreserved-symbols

   unreserved-symbols = ":" / "/" / "@" / "!" / "$" / "'"
                        / "(" / ")" / "*" / "," / ";"

   The "?", "=", and "&" characters are used to delimit VNC parameters
   and must be percent-encoded when representing a data octet as
   specified in [RFC3986].  Within the <vnc-params> portion of a "vnc"
   URI, the <unreserved-symbols> do not have special meaning and need
   not be percent-encoded when representing a data octet.

   A "vnc" URI has the general form:

      vnc://host:port?param1=value1&param2=value2...

   The host information and each parameter value specify information
   used in establishing or operating the remote desktop session as
   specified in Section 2.1.1.

   For example:

      vnc://10.0.0.1:5901?VncPassword=secret&SecurityType=2

   This example indicates a VNC connection to the host at IP "10.0.0.1"
   on port "5901" with VNC password "secret" using the VNC
   Authentication security type.

2.1.1.  URI Parameters

   A description of host information and URI parameters is provided in
   this section.  Information on the constraints of various data types
   is provided in Section 2.1.2.  All parameters are considered
   optional; however, a client will not be able to connect without
   sufficient information.

   A parameter without a specified default value indicates that no
   default value is implied by this URI scheme; however, VNC clients can
   apply implementation-dependent default behaviors otherwise consistent
   with this document.

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   The <userinfo> value is deprecated and processed only in an
   implementation-specific manner.  The <userinfo> component MUST NOT be
   generated in an environment where a client supporting an updated URI
   format is expected to be available.  When processing a URI value from
   an untrusted source, VNC clients SHOULD alert the user in order to
   mitigate the risk that the URI is constructed to obscure the identity
   of the remote host unless the URI can be validated or backwards-
   compatibility considerations make an alert impractical.

   The <host> and <port> values in the "vnc" URI specify the address of
   the VNC server on the remote host:

   +------------+------------+-----------------------------+----------+
   | Name       | Type       | Description                 | Default  |
   +------------+------------+-----------------------------+----------+
   | host       | string     | VNC server hostname or IP   | none     |
   +------------+------------+-----------------------------+----------+
   | port       | ushort     | VNC server port             | 5900     |
   +------------+------------+-----------------------------+----------+

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   The "vnc" URI parameter values specify remote desktop connection or
   session properties, including aspects of client operation, usability,
   and security as specified in the table below:

   +---------------+---------+-----------------------------+----------+
   | Name          | Type    | Description                 | Default  |
   +---------------+---------+-----------------------------+----------+
   |ConnectionName | string  | Name of connection profile  | none     |
   +---------------+---------+-----------------------------+----------+
   |VncUsername    | string  | VNC server username         | none     |
   +---------------+---------+-----------------------------+----------+
   |VncPassword    | string  | VNC server password         | none     |
   +---------------+---------+-----------------------------+----------+
   |SecurityType   | enum    | RFB security type used      | none     |
   |               | <rfbsec>|                             |          |
   +---------------+---------+-----------------------------+----------+
   |ChannelType    | enum    | Connection channel type     | none     |
   |               | <chan>  |                             |          |
   +---------------+---------+-----------------------------+----------+
   |SshHost        | string  | SSH server hostname or IP   | <host>   |
   +---------------+---------+-----------------------------+----------+
   |SshPort        | ushort  | SSH server port             | 22       |
   +---------------+---------+-----------------------------+----------+
   |SshUsername    | string  | SSH username                | none     |
   +---------------+---------+-----------------------------+----------+
   |SshPassword    | string  | SSH password                | none     |
   +---------------+---------+-----------------------------+----------+
   |IdHashAlgorithm| enum    | Hash algorithm used with    | none     |
   |               | <idhash>| "IdHash" parameter          |          |
   +---------------+---------+-----------------------------+----------+
   |IdHash         | string  | Expected hash of remote     | none     |
   |               | <hex>   | public key or certificate   |          |
   +---------------+---------+-----------------------------+----------+
   |ColorLevel     | enum    | Client color depth/mode     | none     |
   |               | <clevel>|                             |          |
   +---------------+---------+-----------------------------+----------+
   |ViewOnly       | boolean | Client is view only         | false    |
   +---------------+---------+-----------------------------+----------+
   |SaveConnection | boolean | Store connection info       | none     |
   +---------------+---------+-----------------------------+----------+

   o  ConnectionName, SaveConnection

      "ConnectionName" is used to identify a connection profile in both
      the launching application and VNC client.  Profiles are applied as
      described in Section 2.2.2.  If omitted, the client MAY generate a
      name based on the host, port, and/or other parameters.  The VNC
      client MAY normalize the name as required.

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      If true, "SaveConnection" indicates a connection profile should be
      created or updated and stored in the client connection database.
      If false, no profile should be updated or persisted.

   o  VncUsername, VncPassword, SecurityType

      The "SecurityType" parameter indicates which RFB security type
      applies to the connection.  RFB security types are recorded in the
      IANA "Remote Framebuffer Security Types" registry created by
      [RFC6143].  The VNC client will use this information to determine
      which parameters are required and establish the connection.

      VNC clients can sometimes automatically negotiate a security type
      with a server.  Specifying the security type controls the security
      negotiation.  Specifying the security type also allows a client to
      prompt for necessary security parameters prior to establishing a
      connection.  Parameters may take time to enter on mobile clients
      and could otherwise result in timeouts and/or security lockouts.
      If the specified type is not supported by the server, an error
      SHOULD be indicated as described in Section 2.2.1.

      "VncUsername" and "VncPassword" are used when applicable to
      authenticate to the VNC server using the specified "SecurityType".
      Since passwords often contain arbitrary characters, they will
      often require percent encoding.

   o  ChannelType

      "ChannelType" specifies the transport stream used to carry
      connection data.  This allows a client to initiate a connection
      using a secure transport protocol such as SSH prior to connecting
      to the VNC server socket.  Use of this value in the context of the
      "Integrated SSH" and "Secure Tunnel" channel types is provided in
      Section 2.3.

   o  SshHost, SshPort, SshUsername, SshPassword

      The SSH parameters are intended for use with the "Integrated SSH"
      channel type described in Section 2.3.1.  These parameters can
      also be used with any future SSH-based channel types.  Since
      passwords often contain arbitrary characters, they will often
      require percent encoding.

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   o  IdHashAlgorithm, IdHash

      The "IdHashAlgorithm" and "IdHash" values are used to verify the
      expected identity of the remote system based on its public key or
      certificate.  Use of these values in the context of the
      "Integrated SSH" and "Secure Tunnel" channel types is provided in
      Section 2.3.

   o  ColorLevel

      The "ColorLevel" parameter specifies the color model to use for
      data transfer and display as specified in Section 2.1.2.  If the
      requested color model is unsupported, the behavior is
      implementation dependent.

   o  ViewOnly

      If "ViewOnly" is true, the VNC client SHOULD operate in a display-
      only mode and refrain from sending input data including KeyEvent,
      PointerEvent, and ClientCutText messages specified in Section 7.5
      of [RFC6143] unless this mode is unsupported by the client.

   Parameter names SHOULD be provided in the case specified in this
   document; however, for compatibility, clients SHOULD accept
   parameters in a case-insensitive manner.  Values SHALL be interpreted
   in a case-sensitive manner, unless otherwise noted.

   Additional parameters likely to be useful with multiple VNC clients
   can be added to the "VNC URI Parameters" registry as specified in
   Section 4.6 of this document.  Individual clients MAY support
   parameters specific to that client.  VNC clients supporting
   application-specific parameters SHOULD include a distinguishing
   prefix within the parameter name, such as the name of the application
   package specified in source code except when precluded by
   compatibility constraints.  For example:

      vnc://?com.dell.vncclient.ScreenMode=2&

   It can also be expected that clients will maintain backward
   compatibility with legacy URI formats and parameters.

   Legacy software applications respond to "vnc" URIs in different ways
   and may fail to behave as expected.  It is advisable to test "vnc"
   URIs with specific applications or consult application-specific
   documentation.

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2.1.2.  Data Types

   "vnc" URIs can be percent-encoded as specified in [RFC3986] and MUST
   be decoded.  After decoding, the following type constraints and
   semantics apply:

   o  string

      Values of "string" type are UTF-encoded strings as specified in
      [RFC3629].

      The "string<hex>" subtype used in the "IdHash" consists of colon-
      delimited ":" octets displayed in hexadecimal.  For example:

         5D:D2:39:57

      Comparison of "string<hex>" values SHALL be case insensitive;
      however, the uppercase notation is preferred for readability.

   o  enum

      The "enum" types consist of specific enumerated subtypes and are
      represented by their decimal value.

      The "enum<rfbsec>" values represent an RFB security type included
      in the IANA "Remote Framebuffer Security Types" registry created
      by [RFC6143].

      "enum<chan>" values represent connection channel types listed in
      the "VNC URI Connection Channel Types" registry created by
      Section 4.4 of this document.  Initial values are:

         Value     Description
         --------  --------------
         1         Standard TCP
         23        Secure Tunnel
         24        Integrated SSH

      The "Standard TCP" channel type represents a generic TCP
      connection.  The "Secure Tunnel" and "Integrated SSH" [RFC4252]
      channel types are described in Section 2.3.

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      Values of the "enum<idhash>" parameter represent secure hash
      algorithms in the "VNC URI Hash Algorithms" registry created by
      Section 4.5 of this document.  The initial values include:

         Value     Description
         --------  ------------
         1         MD5
         2         SHA1
         4         SHA256

      The MD5 algorithm is described in [RFC1321].  The SHA-1 and
      SHA-256 algorithms are described in [SHS].

      Values of the "enum<clevel>" subtype represent a color level.  In
      the table below, the columns have the meaning specified in
      Section 7.4 of [RFC6143]:

         BPP = bits-per-pixel
         TC = true-color-flag
         RM = red-max
         GM = green-max
         BM = blue-max
         RS = red-shift
         GS = green-shift
         BS = blue-shift

      The values are:

         Value  Description      BPP Depth TC RM   GM   BM   RS GS BS
         -----  ---------------  --- ----- -- ---- ---- ---- -- -- --
         1      Black and White  8   3     t  1    1    1    2  1  0
         2      Grayscale        8   6     t  3    3    3    4  2  0
         3      8 Colors         8   3     t  1    1    1    2  1  0
         4      64 Colors        8   6     t  3    3    3    4  2  0
         5      256 Colors       8   8     t  7    7    3    0  3  6
         6      16-bit Color     16  16    t  31   63   31   11 5  0
         7      24-bit Color     32  24    t  255  255  255  16 8  0
         8      30-bit Color     32  30    t  1023 1023 1023 0  10 20

      A value of "t" indicates the true-color-flag should be set.  The
      big-endian-flag (see Section 7.4 of [RFC6143]) should be set as
      required for the system.

   o  ushort

      The "ushort" values represent unsigned 16-bit integers expressed
      in decimal digits with value between 0-65535 inclusive.

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   o  boolean

      "boolean" values represent conditions that are true or false and
      are represented as either "true" or "false" respectively.  For
      maximum compatibility, clients SHOULD accept the value 1 as
      representing true values and 0 as representing false values.
      Clients SHOULD perform parsing of "boolean" values in a case-
      insensitive manner.

      An example "vnc" URI including several of these data types is:

         vnc://localhost:5900?ConnectionName=Server&SecurityType=2&
            IdHash=0D:3A:72:08:57:EA:4D:30&SaveConnection=false&

      Note the above example should be considered to be a contiguous
      string without line breaks or whitespace and is broken into
      multiple lines in this document for readability.

2.2.  Processing URIs

   Conceptually, a "vnc" URI supports only a "VIEW" operation,
   indicating the user wishes to view the remote desktop accessible via
   the URI reference.

   In general, when a VNC client receives a "vnc" URI, it will initiate
   a remote desktop connection with the RFB protocol using the specified
   host information and parameter values.  Initiating the connection
   using a connection channel mechanism such as those specified in
   Section 2.3 might require processing prior to establishing the RFB
   connection.  A client MAY attempt to automatically discover or
   negotiate appropriate connection channel, security, or other
   parameter values.

   The process for negotiating security types is specified in [RFC6143].
   Supported connection channels could be discovered by testing channel
   types to detect when a channel is successfully established.  To best
   integrate with other applications, the VNC client SHOULD initiate the
   connection with minimal or no user intervention, whenever sufficient
   information is available and adequate security is preserved.

   Host information and parameter values may be provided through
   connection profiles.  When a parameter value is not available from
   either a URI or a connection profile described in Section 2.2.2, the
   default value specified in Section 2.1.1 SHOULD be applied.  If
   available parameters are not sufficient to establish a connection,
   the VNC client SHOULD present a session initiation data-entry screen.

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2.2.1.  Error Handling

   In a typical interactive environment, if an error prevents a session
   from being established, the VNC client presents an error message to
   the user.  When the message is acknowledged, the console application
   can show a session initiation data-entry screen populated with
   available session parameters, or it can terminate.  If an error
   occurs after a session is successfully established that terminates
   the connection, the VNC client presents a termination notification to
   the user.  When the termination notification is acknowledged, the
   client can present a reconnection prompt or terminate.

   When an error occurs in a dedicated environment (such as a kiosk
   system), the system can transmit an alert to the remote operator,
   record a log entry, and execute appropriate fallback behavior such as
   automatically attempting to reestablish a session or displaying a
   generic message requesting servicing.

2.2.2.  Connection Profile Matching

   VNC clients MAY store remote desktop session settings in connection
   profiles.  If the client is able to uniquely identify and associate a
   connection request with a connection profile based on the
   "ConnectionName" parameter value, remote host IP address, or hostname
   / fully qualified domain name, the VNC client SHOULD apply profile
   values for those settings that do not have values supplied in the
   "vnc" URI.  When profile data is unavailable, the VNC client MAY
   apply global application defaults for settings not supplied in the
   URI and for which the scheme does not specify a default value.  The
   VNC client MUST NOT override supplied parameters with profile values
   or global defaults.

   When the "SaveConnection" parameter value is true, within the VNC
   client, a connection profile SHOULD be created or updated with the
   values supplied in the "vnc" URI.  Profile updates and storage should
   be consistent with the recommendations in Section 3.4.

2.3.  Connection Channel Types

2.3.1.  The "Integrated SSH" Channel Type

   The "Integrated SSH" channel type establishes an SSH connection to a
   host, authenticates with SSH password authentication, establishes a
   secure tunnel to the VNC host/port, and then connects to the VNC
   server using a supported "SecurityType".  The secure tunnel will
   provide encryption and data integrity, while verifying the public key
   authenticates the server.  The SSH architecture is specified in
   [RFC4251].  The steps are detailed below:

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   1. The VNC client initiates a transport-level connection to the
      "SshHost" on the "SshPort" specified in the parameter values with
      a key exchange as described in [RFC4253].

   2. When the VNC client receives the server key (or certificate), the
      hash of the key (or certificate) is computed using the algorithm
      corresponding to the "IdHashAlgorithm" parameter value and
      compared with the expected "IdHash" value (if available).  If the
      certificate hash cannot be verified, the client alerts the user or
      operator.  In a typical interactive environment, the alert
      provides the remote system's identifying information including the
      hash value and allows the user to terminate the connection.  The
      alert could allow the user to accept the key and continue
      establishing the connection.  In a dedicated environment (such as
      a kiosk system), the system can transmit an alert to the remote
      operator, record a log entry, and execute appropriate fallback
      behavior such as displaying a generic message requesting
      servicing.

   3. The SSH client authenticates the user using the "SshUsername" and
      "SshPassword" parameter values according to the "password"
      authentication mechanism described in [RFC4252].

   4. The SSH client opens a TCP/IP channel as specified in [RFC4254]
      from the local system to the system indicated by the <host> and
      <port> information values.

   5. The VNC client establishes an RFB connection to the VNC server
      over the channel and authenticates using the "SecurityType" as
      described in [RFC6143] or other reference.

   The VNC client MAY establish the connection described in this section
   using an external SSH client, by launching the client and then
   connecting to a secure tunnel created between a local port and the
   VNC server.

   If the VNC client is supplied with additional parameters outside the
   scope of this document, it MAY perform a variation of these steps
   consistent with the underlying protocols, for example, by using
   "publickey" SSH client authentication [RFC4252] or providing another
   form of authentication to the VNC server.  The specific negotiation
   of SSH parameters such as cipher suite configuration is outside the
   scope of this document.

   Many SSH clients present key hashes using MD5, and it can be expected
   that launching applications will specify the hash be displayed in the
   manner its users are familiar with.

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   For compatibility, when the "SecurityType" parameter value is
   "Integrated SSH" (24), a VNC client MUST treat the value as a request
   to use "Integrated SSH" as the "ChannelType".  However, this value
   SHOULD NOT be supplied for the "SecurityType" parameter unless
   required for backward compatibility as the channel is established
   prior to connecting to the server and is not consistent with the
   negotiation of other security types.

2.3.2.  The "Secure Tunnel" Channel Type

   The "Secure Tunnel" channel type establishes a TLS connection with a
   remote server using certificate authentication, over which a
   connection to the VNC server is established using a supported
   "SecurityType".  The secure tunnel will provide encryption and data
   integrity, while verifying the certificate authenticates the server.
   The TLS protocol is specified in [RFC5246].  The steps are detailed
   below:

   1. The VNC client initiates the TLS Handshake Protocol with a system
      indicated by the <host> and <port> information values.

   2. When the server certificate is received, the hash of the key
      certificate is computed using the algorithm corresponding to the
      "IdHashAlgorithm" parameter value and compared with the expected
      "IdHash" value (if available).  If the certificate hash cannot be
      verified, the client alerts the user or operator.  In a typical
      interactive environment, the alert provides the remote system's
      identifying information and allows the user to terminate the
      connection.  The alert could allow the user to accept the key and
      continue establishing the connection.  In a dedicated environment
      (such as a kiosk system), the system can transmit an alert to the
      remote operator, record a log entry, and execute appropriate
      fallback behavior such as displaying a generic message requesting
      servicing.

      When providing identifying information of a host identified by an
      X.509 certificate [RFC5280] [X.509], the certificate subject,
      issuer, validity period, and certificate hash is typically
      included.  The VNC client MAY verify the validity of the
      certificate.  If the validity of a certificate is not confirmed,
      the alert includes a statement indicating such information has not
      been verified.

   3. The client finishes establishing the TLS tunnel.

   4. The VNC client establishes an RFB connection to the VNC server
      over the channel and authenticates using the "SecurityType" as
      described in [RFC6143] or other reference.

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   If the VNC client is supplied with additional parameters, it MAY
   perform a variation of these steps consistent with the underlying
   protocols, for example, by providing another form of authentication
   to the VNC server.  The negotiation of specific TLS parameters such
   as cipher suite configuration is outside the scope of this document.

   The TLS protocol provides backwards compatibility with SSLv3;
   however, due to known security flaws, it SHOULD NOT be used.

   For compatibility, when the "SecurityType" parameter value is "Secure
   Tunnel" (23), a VNC client MUST treat the value as a request to use
   "Secure Tunnel" as the "ChannelType".  However, this value SHOULD NOT
   be supplied for the "SecurityType" parameter unless required for
   backward compatibility as the channel must be established prior to
   connecting to the server and is not consistent with the negotiation
   of other security types.

3.  Security Considerations

   General security concerns involving URI schemes are discussed in
   [RFC3986].  In implementing support for the "vnc" URI scheme, areas
   for particular consideration include application trust, URI handling,
   host identification, and connection database security.

   Remote desktop connectivity requires the transmission of security
   credentials, which could be included in a URI.  If those credentials
   are not kept secure, an attacker can gain access to any systems using
   those credentials.  Host addresses and connection parameters might
   also be considered sensitive, as such information can be used in
   planning an attack.

   URIs can also contain host identification information.  It is
   important to securely identify the remote host system to which a
   connection is established.  If a user connects to an attacker's
   system, user data, including credentials, can be exposed.

   Note that the RFB protocol itself may not encrypt data.  To protect
   data in transit, RFB should be tunneled over TLS [RFC5246], SSH
   [RFC4251], or another secure protocol.

   Some VNC systems can be used without authentication.  To protect the
   remote host, strong passwords or other authentication mechanisms need
   to be used.

Top      ToC       Page 16 
3.1.  Application Trust

   A malicious application receiving VNC credentials via URI or other
   means can obviously misuse those credentials.  To protect against
   this, users should only install applications from trusted sources.
   The integrity of application packages can be verified through digital
   signatures.

   Applications launching VNC clients can elect to launch only
   particular trusted clients and can specify those clients through
   platform-specific mechanisms.  Package integrity can be verified
   programmatically by querying the package manager for digital
   signatures or other platform-specific means.

   The risk to a VNC client from a launching application is generally
   much lower, since the launching application will not receive
   credentials or data from the client.  A VNC client can verify its
   caller thorough platform-specific means.

   VNC clients ought not to accept potentially destructive parameters
   from untrusted launching applications without explicit user
   confirmation.  For example, a client-specific parameter that runs an
   arbitrary command upon establishing an SSH connection used for VNC
   tunneling is potentially destructive and high risk.

3.2.  URI Handling

   Within a mobile or desktop environment, application launch will
   typically involve in-memory URI data transmission facilitated and
   secured by the operating system.

   When "vnc" URIs are exchanged or used within a system, their contents
   might be exposed by process listings or other instrumentation.  Users
   need to avoid including sensitive information in "vnc" URIs that
   could be exposed to unauthorized observation.

   If sensitive URI information is exchanged across a network, for
   example, by providing a list of connection URIs in a web page, the
   data needs to be encrypted in transit and only be accessible to
   authorized users.

   When an application detects potentially sensitive information in a
   "vnc" URI, it needs to be handled securely or discarded.  In
   particular, URI data on persistent storage needs to be encrypted as
   described in Section 3.4.

Top      ToC       Page 17 
   Since "vnc" URIs may contain sensitive information, applications
   should avoid logging the URIs even when errors occur.  Users need to
   avoid including sensitive information in "vnc" URIs that are used
   with applications where logging is unavoidable.

   Applications that process URIs in a generic way, such as web
   browsers, might not detect that sensitive information is contained in
   a URI and could cache or store that information insecurely.  It is
   advisable to avoid including credentials and other sensitive
   information in URIs that are likely to be processed in a generic way
   unless such caching and storage is disabled or otherwise secured.

3.3.  Host Identification

   In the absence of verifiable host identification, a VNC client
   application is vulnerable to spoofing and man-in-the-middle attacks
   that capture VNC or host OS credentials and user data.  To prevent
   such attacks, administrators SHOULD secure their VNC communications
   with TLS [RFC5246] or SSH [RFC4251] tunnels or other connection
   mechanisms identifying remote hosts via certificate or public key.
   VNC clients MUST verify the respective certificates or public keys to
   confirm the remote host's identity.

   An application launching a VNC client via URI MAY provide a
   certificate hash or public key hash identifying the remote host.  VNC
   clients maintaining a connection database can also store certificate
   or public key data suitable for validating a host's identity.

   If connecting to a system identified by certificate or public key and
   a remote system ID hash cannot be matched to available identifying
   data, the VNC client needs to alert the user or operator.  In a
   typical interactive environment, the alert will provide the remote
   system's identifying information and allow the user to terminate the
   connection.  The alert can allow the user to accept the information
   and continue establishing the connection.  In a dedicated environment
   (such as a kiosk system), the system can transmit an alert to the
   remote operator, record a log entry, and execute appropriate fallback
   behavior such as displaying a generic message requesting servicing.

   When providing identifying information of a host identified by an
   X.509 certificate [RFC5280] [X.509], the certificate subject, issuer,
   validity period, and certificate hash need to be included.  The VNC
   client can verify the certificate validity.  If the validity of a
   certificate is not determined, the alert needs to include a statement
   indicating such information has not been verified.

Top      ToC       Page 18 
   Identifying information of a host identified by public key, such as
   the endpoint of an SSH connection using a raw key, needs to include a
   hash of the key.

3.4.  Connection Database Integrity

   A VNC client application and/or launching application can maintain a
   connection database containing remote host information, credentials,
   and/or connection parameters.  Applications storing credentials need
   to ensure they are stored in an encrypted format with a decryption
   process requiring user-supplied or device-specific data.  If
   supported, it is advisable for applications to have a setting
   disabling storage of credentials.

   If available, the VNC client connection database can store
   certificate or public key data used to verify host identification.
   To prevent a malicious URI from overriding the database, if
   identification information in the URI conflicts with information in
   the database, the user or operator needs to be alerted.  In a typical
   interactive environment, the user can be prompted to accept the new
   information prior to updating the database.

4.  IANA Considerations

   The "vnc" scheme has been registered in the "Uniform Resource
   Identifier (URI) Schemes" registry.

   The "Remote Framebuffer Security Types", "VNC URI Connection Channel
   Types", "VNC URI ID Hash Algorithms", and "VNC URI Parameters"
   registries support elements of the scheme.

4.1.  "vnc" Scheme

   IANA has added the "vnc" scheme to the "Uniform Resource Identifier
   (URI) Schemes" registry with description "Remote Framebuffer
   Protocol" and reference to this document.  A registration template is
   provided in Appendix A.

   The IANA schemes registry is currently located at
   <http://www.iana.org/assignments/uri-schemes>.

4.2.  Remote Framebuffer Security Types

   This document references the existing IANA "Remote Framebuffer
   Security Types" registry in specifying security type options.  RFB
   security types are supported in "vnc" URIs.

Top      ToC       Page 19 
   Security mechanisms integrated with VNC clients might need to alter
   the process by which a connection is established prior to the
   security handshake described in Section 7.1.2 of [RFC6143].  Such
   mechanisms should be reflected in the "VNC URI Connection Channel
   Types" registry described in Section 4.4 of this document rather than
   the "Remote Framebuffer Security Types" registry, as their use cannot
   be negotiated by the mechanism specified in [RFC6143].

   Exceptions can be made for backwards compatibility.  IANA has updated
   the "Secure Tunnel" and "Integrated SSH" security types to refer to
   this document.

4.3.  VNC URI Group

   IANA has created a "Virtual Network Computing (VNC) Uniform Resource
   Identifier (URI)" group.  This group contains application-level, URI-
   related registries distinct from those used by the RFB protocol
   itself.

4.4.  VNC URI Connection Channel Types

   IANA has created a "VNC URI Connection Channel Types" registry within
   the "Virtual Network Computing (VNC) Uniform Resource Identifier
   (URI)" group.  The registry includes Value, Description, and
   Reference columns.  The initial contents of the registry are
   described in this document.  The values of the "Secure Tunnel" and
   "Integrated SSH" types are copied from the RFB Security Types
   registry.  They are:

   Value     Description      Reference
   --------  ---------------  --------------
   0         Reserved         this document
   1         Standard TCP     this document
   23        Secure Tunnel    this document
   24        Integrated SSH   this document

   The maximum acceptable value is 2,147,483,647.

   Future assignments to this registry should be made through the "First
   Come First Served" process described in [RFC5226].

4.5.  VNC URI ID Hash Algorithms

   IANA has created a "VNC URI ID Hash Algorithms" registry within the
   "Virtual Network Computing (VNC) Uniform Resource Identifier (URI)"
   group.  The registry includes Value, Description, and Reference
   columns.

Top      ToC       Page 20 
   The initial hash algorithms specified are a subset of the algorithms
   contained in the "TLS HashAlgorithm Registry".  The initial contents
   of the registry are:

   Value     Description   Reference
   --------  ------------  --------------
   0         Reserved      this document
   1         MD5           this document
   2         SHA1          this document
   4         SHA256        this document

   The maximum acceptable value is 2,147,483,647.

   Future assignments to this registry should be made through the "First
   Come First Served" process described in [RFC5226].

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4.6.  VNC URI Parameters

   IANA has created a "VNC URI Parameters" registry within the "VNC URI"
   group.

   The initial contents are described in this document.  They are:

   +-----------------+-----------------------------+-----------------+
   | Name            | Description                 | Reference       |
   +-----------------+-----------------------------+-----------------+
   | ConnectionName  | Name of connection profile  | this document   |
   +-----------------+-----------------------------+-----------------+
   | VncUsername     | VNC server username         | this document   |
   +-----------------+-----------------------------+-----------------+
   | VncPassword     | VNC server password         | this document   |
   +-----------------+-----------------------------+-----------------+
   | SecurityType    | RFB security type used      | this document   |
   +-----------------+-----------------------------+-----------------+
   | ChannelType     | Connection channel type     | this document   |
   +-----------------+-----------------------------+-----------------+
   | SshHost         | SSH server hostname or IP   | this document   |
   +-----------------+-----------------------------+-----------------+
   | SshPort         | SSH server port             | this document   |
   +-----------------+-----------------------------+-----------------+
   | SshUsername     | SSH username                | this document   |
   +-----------------+-----------------------------+-----------------+
   | SshPassword     | SSH password                | this document   |
   +-----------------+-----------------------------+-----------------+
   | IdHashAlgorithm | Hash algorithm used with    | this document   |
   |                 | "IdHash" parameter          |                 |
   +-----------------+-----------------------------+-----------------+
   | IdHash          | Expected hash of remote     | this document   |
   |                 | public key or certificate   |                 |
   +-----------------+-----------------------------+-----------------+
   | ColorLevel      | Client color depth/mode     | this document   |
   +-----------------+-----------------------------+-----------------+
   | ViewOnly        | Client is view only         | this document   |
   +-----------------+-----------------------------+-----------------+
   | SaveConnection  | Store connection info       | this document   |
   +-----------------+-----------------------------+-----------------+

   Future assignments to this registry should be made through the "First
   Come First Served" process described in [RFC5226].

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5.  References

5.1.  Normative References

   [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
             DOI 10.17487/RFC1321, April 1992,
             <http://www.rfc-editor.org/info/rfc1321>.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119,
             DOI 10.17487/RFC2119, March 1997,
             <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
             STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003,
             <http://www.rfc-editor.org/info/rfc3629>.

   [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
             Resource Identifier (URI): Generic Syntax", STD 66, RFC
             3986, DOI 10.17487/RFC3986, January 2005,
             <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
             Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
             January 2006, <http://www.rfc-editor.org/info/rfc4251>.

   [RFC4252] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
             Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
             January 2006, <http://www.rfc-editor.org/info/rfc4252>.

   [RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
             Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
             January 2006, <http://www.rfc-editor.org/info/rfc4253>.

   [RFC4254] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
             Connection Protocol", RFC 4254, DOI 10.17487/RFC4254,
             January 2006, <http://www.rfc-editor.org/info/rfc4254>.

   [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax
             Specifications: ABNF", STD 68, RFC 5234,
             DOI 10.17487/RFC5234, January 2008,
             <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246,
             DOI 10.17487/RFC5246, August 2008,
             <http://www.rfc-editor.org/info/rfc5246>.

Top      ToC       Page 23 
   [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
             Housley, R., and W. Polk, "Internet X.509 Public Key
             Infrastructure Certificate and Certificate Revocation List
             (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
             <http://www.rfc-editor.org/info/rfc5280>.

   [RFC6143] Richardson, T. and J. Levine, "The Remote Framebuffer
             Protocol", RFC 6143, DOI 10.17487/RFC6143, March 2011,
             <http://www.rfc-editor.org/info/rfc6143>.

   [SHS]     National Institute of Standards and Technology, "Secure
             Hash Standard", NIST FIPS PUB 180-4,
             DOI 10.6028/NIST.FIPS.180-4, August 2015.

5.2.  Informative References

   [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 5226,
             DOI 10.17487/RFC5226, May 2008,
             <http://www.rfc-editor.org/info/rfc5226>.

   [RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines and
             Registration Procedures for URI Schemes", BCP 35, RFC 7595,
             DOI 10.17487/RFC7595, June 2015,
             <http://www.rfc-editor.org/info/rfc7595>.

   [X.509]   ITU-T, "Information technology - Open Systems
             Interconnection - The Directory: Public-key and attribute
             certificate frameworks", ITU-T Recommendation X.509,
             ISO/IEC 9594-8, 2005.

Top      ToC       Page 24 
Appendix A.  "vnc" URI Template

   This template is provided for registration of the "vnc" URI in the
   IANA "Uniform Resource Identifier (URI) Schemes" registry as
   specified in [RFC7595].

   Scheme name: vnc

   Status: Permanent

   Applications/protocols that use this scheme name: Virtual Network
   Computing (VNC) remote desktop applications use vnc URIs.  VNC
   applications use the Remote Framebuffer (RFB) protocol.

   Contact: IESG <iesg@ietf.org>.

   Change Controller: See the authors of this document.  Change control
   is through the IESG on behalf of the IETF <iesg@ietf.org>.

   References: This document.

Top      ToC       Page 25 
Acknowledgments

   Dominic Parkes and the staff of RealVNC Ltd. graciously reviewed this
   document and provided constructive comments.

   RFB and VNC are registered trademarks of RealVNC Ltd. in the United
   States and in other countries.

Authors' Addresses

   David Warden
   Dell Products LP
   200 Dell Way
   Round Rock, TX 78682
   United States

   Phone: 512-728-0380
   Email: David_Warden@dell.com
   URI: http://www.dell.com


   Iordan Iordanov
   Undatech
   260 Scarlet Road, Apt. 503
   Toronto, ON M6N 4X6
   Canada

   Email: iiordanov@gmail.com