Internet DRAFT - draft-warden-appsawg-vnc-scheme
draft-warden-appsawg-vnc-scheme
Applications Area Working Group D. Warden
Internet Draft Dell Products LP
Intended status: Informational I. Iordanov
Expires: August 2016 Undatech
February 25, 2016
The "vnc" URI Scheme
draft-warden-appsawg-vnc-scheme-10.txt
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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 6, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction...................................................2
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...........................................8
2.2. Processing URIs..........................................10
2.2.1. Error Handling......................................11
2.2.2. Connection Profile Matching.........................11
2.3. Connection Channel Types.................................12
2.3.1. The "Integrated SSH" Channel Type...................12
2.3.2. The "Secure Tunnel" Channel Type....................13
3. Security Considerations.......................................15
3.1. Application Trust........................................15
3.2. URI Handling.............................................16
3.3. Host Identification......................................16
3.4. Connection Database Integrity............................17
4. IANA Considerations...........................................18
4.1. "vnc" Scheme.............................................18
4.2. Remote Framebuffer Security Types........................18
4.3. VNC URI Group............................................18
4.4. VNC URI Connection Channel Types.........................19
4.5. VNC URI ID Hash Algorithms...............................19
4.6. VNC URI Parameters.......................................19
5. References....................................................21
5.1. Normative References.....................................21
5.2. Informative References...................................22
6. Acknowledgments...............................................22
Appendix A. "vnc" URI Template...................................23
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
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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. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
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 (BNF) 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].
vnc-uri = "vnc://" [ userinfo "@" ] [ host [ ":" port ] ]
[ "?" [ vnc-params ] ]
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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¶m2=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
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 |
+------------+------------+-----------------------------+----------+
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 |
+---------------+---------+-----------------------------+----------+
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+---------------+---------+-----------------------------+----------+
|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
The "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.
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. However, in addition to controlling the security
negotiation, specifying the security type also allows for a
client to prompt in advance for necessary security parameters.
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.
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The "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
The channel type 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.
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 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
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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.
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].
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"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 Secure Shell (SSH)
[RFC4252] channel types are described in Section 2.3.
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 SHA1 and SHA256
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 Greyscale 8 6 t 3 3 3 4 2 0
3 8 Colors 8 3 t 1 1 1 2 1 0
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Value Description BPP Depth TC RM GM BM RS GS BS
----- --------------- --- ----- -- ---- ---- ---- -- -- --
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 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.
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
an RFB protocol remote desktop connection 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.
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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.
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 which 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.
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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:
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 a 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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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.
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.
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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
X509 certificate [RFC5280], 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 a RFB connection to the VNC server
over the channel and authenticates using the "SecurityType" as
described in [RFC6143] or other reference.
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.
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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.
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.
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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 a 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, the URI data that is persisted needs to be encrypted as
described in Section 3.4.
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.
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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
X509 certificate [RFC5280], the certificate subject, issuer,
validity period, and certificate hash needs 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.
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
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typical interactive environment, the user can be prompted to accept
the new information prior to updating the database.
4. IANA Considerations
The "vnc" scheme should be registered in the URI schemes registry.
IANA "Remote Framebuffer Security Types", "VNC URI Connection
Channel Types", "VNC URI ID Hash Algorithms", and "VNC URI
Parameters" registries will support elements of the scheme.
4.1. "vnc" Scheme
IANA is asked to add 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/uri-schemes.xhtml
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.
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 Chanel
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 is asked
to update the "Secure Tunnel" and "Integrated SSH" security types
listed for compatibility to refer to this document.
4.3. VNC URI Group
IANA is asked to create a "VNC URI" Group. This group will contain
application-level URI related registries distinct from those used by
the RFB protocol itself.
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4.4. VNC URI Connection Channel Types
IANA is asked to create a "VNC URI Connection Channel Types"
registry within the "VNC URI" group. The registry should include
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 should be:
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 is asked to create a "VNC URI ID Hash Algorithms" registry
within the "VNC URI" group. The registry should include Value,
Description, and Reference columns.
The initial hash algorithms specified are a subset of the algorithms
contained in the "TLS HashAlgorithm Registry". The initial contents
of the registry should be:
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].
4.6. VNC URI Parameters
IANA is asked to create a "VNC URI Parameters" registry within the
"VNC URI" group.
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The initial contents are described in this document. They should be:
+-----------------+-----------------------------+-----------------+
| 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 these registries 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,
April 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of
ISO 10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
[RFC4254] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Connection Protocol", RFC 4254, January 2006.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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, May 2008.
[RFC6143] Richardson, T., and J. Levine, "The Remote Framebuffer
Protocol", RFC 6143, March 2011.
[SHS] NIST FIPS PUB 180-2, "Secure Hash Standard", National
Institute of Standards and Technology, U.S. Department of
Commerce, August 2002.
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5.2. Informative References
[RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35, RFC
7595, June 2015.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[X.509] ITU-T Recommendation X.509 (2005) | ISO/IEC 9594-8:2005,
Information technology - Open Systems Interconnection -
The Directory: Public-key and attribute certificate
frameworks.
6. 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 U.S.
and in other countries.
This document was prepared using 2-Word-v2.0.template.dot.
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Appendix A. "vnc" URI Template
This template is provided for registration of the VNC URI in the
IANA URI schemes registry as specified in [RFC7595].
URI Scheme name: vnc
Status: Permanent
URI scheme syntax: See Section 2 of this document.
Scheme semantics: See Section 2 of this document.
Encoding considerations: See Section 2 of this document.
Applications/protocols that use this URI scheme name: Virtual
Network Computing (VNC) remote desktop applications use vnc URIs.
VNC applications use the Remote Framebuffer (RFB) protocol.
Interoperability considerations: 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.
Security considerations: See Section 3 of this document.
Contact: IESG <iesg@ietf.org>.
Author/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.
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Authors' Addresses
David Warden
Dell Products LP
200 Dell Way
Round Rock, Texas 78682
U.S.A.
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
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