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======================
(Work In Progress)
Mark McLoughlin (mark@skynet.ie)
December 1st, 2003
1. Problem Description
======================
In enterprise installations system administrators typically
have to deal with a large number of pretty basic problems on users'
machines. Remotely taking control of a user's desktop to fix the
problem while at the same time training the user as to how to resolve
the problem for themselves is an effective and simple way to handle
these types of support scenarios.
Currently there is no way to do this with GNOME.
2. Overview
===========
The basic requirement for such a tool is some method of
sharing a desktop session between multiple users. The sysadmin sees
what the user sees and the user sees what the sysadmin sees.
However, the technology behind this is obviously useful in
other ways. Here in Sun, for example, we make widespread use of VNC
for some basic collaboration. Targetting this project purely at the
Remote Assistance use case will leave some users wondering "why
... why on earth did you make it impossible for us to use this like
VNC?".
This project, therefore, also encompasses the use case of
a simple form of collaboration by sharing access to a desktop
session.
There are various existing technologies in this area which all
work in very similar ways. This project will follow those same basic
architectural principals.
The core part of such a system is a protocol by which
information about what is happening on the screen of the "host"
machine (in this case, the user's machine) is sent to the "client"
machine (the sysadmin's machine). The client also needs to be able to
relay back key presses and pointer manipulation information to the
host.
There are several existing such protocols available - the RFB
(Remote FrameBuffer) protocol used by VNC, RDP (Remote Desktop
Protocol) used by Window's Remote Desktop and the Sun Ray protocol.
On the host machine a mechanism is needed by which all drawing
primitives are proxied via the protocol to the client machine as well
as a mechanism by which key/pointer events from the client are passed
to the windowing system. Also needed is an authentication mechanism by
which access to the host system can be restricted.
On the client machine an application which allows the user to
connect to the host machine, authenticate, display the contents of the
host display and forward input events to the host is required.
3. Task List, by Commonality and Frequency
==========================================
Below, each of the user tasks the project aims to facilitate
is listed, grouped by the proportion of users who will perform the
task and how often it will be performed. In the absence of personas to
encapsulate our target user base, the groupings are judgement based.
Each task also has (C), (I) or (NTH) depicting whether support
for that task is core, important or nice to have functionality.
First lets take a look at the "Remote Assistance" use case on
the host side and then on the client side.
=> Host
* Frequent by Many:
1. Verify/approve a connection before it can be
established. (C)
* Frequent by Few:
1. Request assistance from a colleague or system
administrator. (I)
=> Client
* Frequent by Many:
1. View a desktop remotely. (C)
2. Interact with a remote desktop. (C)
3. Browse the network for a machine to connect to. (I)
4. Make the remote desktop occupy the entire local screen
i.e. a fullscreen option. (NTH)
* Occasional by Many:
1. Open a connection to a user's desktop given only the
hostname of the user's machine (C)
2. Open a connection to a user's desktop without having the
user be aware of your connection. (I)
3. Receive a remote assitance request and open a connection to
that user's desktop. (I)
Now, lets take a look at the additional tasks with a simple
VNC-like collaboration tool.
=> Host
* Frequent by Many:
1. Inform others how to connect to your desktop. (I)
* Frequent by Few:
1. View the number of and the details of established
connections. (I)
2. Close established connections. (I)
3. Toggle established connections between interactive and
non-interactive. (NTH)
* Occasional by Many:
1. Allow/dis-allow remote connections to the desktop. (C)
2. Allow/dis-allow interactive connections. (C)
2. Restrict remote access to your desktop in some way
(e.g. set a password, specify specific users ...) (C)
=> Client
* Frequent by Many:
1. Open a connection to a remote desktop given some details
from the host user (C)
4. Functional Requirements
==========================
=> Host
1. Ability to approve or reject a request to remotely connect to
the desktop. (C)
2. Ability to request assistance from a pre-defined source. (I)
3. Ability to request assistance from a colleague. (I)
4. Ability to allow/dis-allow remote connections to the desktop. (C)
5. Ability to allow/dis-allow interactive connections. (C)
6. Ability to restrict access to the desktop in some way
(e.g. assign a password) (C)
7. Ability to give, to another user, enough details for that user
to connect to your desktop. (I)
8. A list of currently open connections, including details of
the other endpoint of the connection and whether or not the
connection is interactive. (I)
9. Ability to close a connection. (I)
10. Ability to toggle a connection between interactive and
non-interactive. (I)
=> Client
1. Ability to view a desktop remotely. (C)
2. Ability to interact with a desktop remotely. (C)
3. Ability (for a user with sufficient priviledges) to connect to
a given user's desktop on a given host. (C)
4. Ability to browse the network for hosts which you can then connect
to. (I)
5. Ability (for a system administrator) to connect to a given user's
desktop without the user being aware of the connection. (I)
6. Ability to view the remote desktop in fullscreen mode. (NTH)
7. Ability to connect to a desktop given a remote assitance
request. (I)
8. Ability to connect to a desktop given some details from the
host user. (I)
5. Notes/Design Constraints/Caveats
===================================
* I don't think the "Request Assistance" task is a very important
one. In most cases where a user has support personnel available I
think picking up the phone will be a much more common way of
requesting assistance rather than clicking a button and twidling
your thumbs waiting for someone to respond.
6. Review of Existing Technologies
==================================
6.1. Existing Products - VNC/RFB
================================
RFB[1] (Remote FrameBuffer) is the protocol used by VNC. The
emphasis in the design of the protocol was to make very few
requirements of the client. The client has no need to maintain
explicit state and clients are able to disconnect and re-connect to
the server while preserving the state of the user interface.
The dislay part of the protocol is based around a single
simple graphics primitive "put a rectangle of pixel data at a given
position". Each rectangle may be encoded in any one of a number of
encodings allowing for compression or usage of parts of the client's
existing copy of the framebuffer. Updates are requested by the client
rather than pushed out by the server allowing the protcol to adapt to
slower networks and/or clients - i.e. with a slow network or client
the rate of updates are greatly reduced and the client ignores the
transient state of the framebuffer.
The protocol is quite extensible. Extra encodings can be
advertised by the server and used if the client supports the
encoding. Use of encodings are not only limited to how frame buffer
updates are encoded on the wire, but also extra psuedo-encodings may
be added which can do anything from inform the client of a change in
cursor shape, a change in the size of the screen or even things like
extra in-band communication between the server and client.
There seems to be many different implementations of VNC
available. Available RFB server implementations include:
* libvncserver[2]: a "library" for implementing VNC
servers. It basically just seems to be a hacked up version of an
existing VNC server in an archive with some header files. Not a very
nice API, but quite a few projects are using it successfully.
* xf4vnc[3]: a controlled fork of XFree86 which allows you to
run an Xserver which doubles as a VNC server or export your local
framebuffer through a loadable module.
* realVNC[4]: seems to be a continuation of the original VNC
project which has an Xserver which doubles as a VNC server. Thus,
there is no method by which you can remote the local framebuffer.
* x0rfbserver: an X11 client which acts as an RFB server and
exports the local display by polling the display for updates using X
and pushing the updates out to clients using RFB.
* krfb[5]: a pretty nice looking VNC server for KDE which is
based on the same concept (and indeed uses some of the code of)
x0rfbserver. It also uses libvncserver.
I won't list the VNC client's available, there seem to be
many, but suffice to say there are X11, Windows and OS X clients
available along with, interestingly, several implementations of a Java
client which can be run embedded in the browser as an applet.
Tim Waugh has written a nice article[6] on VNC and the many
projects around the technology.
In summary, the RFB protocol has a number of advantages:
1) Simple and open protocol.
2) Rate-limited by the client, pretty low bandwidth/latency
requirements.
3) Extensible.
4) Several open source implementations available.
5) Many existing clients available for different platforms.
6.2. Existing Protocols - Remote Desktop/RDP
============================================
"Remote Desktop"[7,8,9] is Microsoft's technology in this
area. The RDP protocol itself is essentially an extension of the
ITU-T T.128 (aka T.SHARE) application sharing protocol[10].
The protocol is a good deal more complex than the RFB protocol
and the protocol supports a very much larger set of functionality than
the RFB protocol e.g.
* Printer re-direction - you can print something on the host
machine to a printer local to the remote desktop client.
* Sound re-direction - sounds from the host can be heard at
the client.
* Multi-point sharing - the remote desktop session does not
neccessarily consist of only windows/applications on a single
host. Multiple hosts can share applications to the session.
None of these features are needed here given the functional
requirements above.
Also, the protocol has been further extended by Microsoft to
such an extent that it hardly be considered an "open" protocol.
Another problem with Window's Remote Desktop compared to VNC
is the limited client availability. On Linux, the rdesktop[11] project
provides a Remote Desktop client (tsclient[12] is a GNOME-like frontend
for it) but on Windows, the only client I know of is the Window's
Remote Desktop client.
6.3. Existing Protocols - Sun Ray
=================================
There's not a lot of information out there on SLIM, the
protocol behind Sun Ray. About the only details available are from a
paper[13] investigating the performace of the protocol.
SLIM, like RDP, is designed to immediately push all frame
buffer updates to the client. Therefore, on low bandwidth connections
the updates would just pile up. One would assume this is the reason
Sun Ray requires a dedicated network.
Also, Sun Ray has no client implementation available apart
from the Sun Ray Enterprise Appliance itself.
7. System Design
================
7.1. Overview
=============
The host side is to be implemented as a VNC server using the
libvncserver library. The VNC server will act as an X client and poll
the local X display for the contents of the framebuffer and notify the
VNC clients if there have been any changes. Input events coming from
the clients will be injected into the X display using the XTEST[14,15]
extension.
The VNC client we will most likely be a modified version of
an existing Java client. The advantage of having a Java client is that
it may be used to connect to the host from any platform.
7.2. Monitoring The Local Display
=================================
To implement a VNC server you need to know the contents of the
local framebuffer in order to pass this information onto the VNC
clients.
Currently, as an X client, there is only one way to do this
and that is by doing a GetImage on the root window which basically
copies the entire framebuffer from the X server to the X client. The
main problem with this approach is that without knowing what parts of
the framebuffer has actually changed since the last time you updated,
you are wasting an enormous amount of resources copying the entire
framebuffer each time.
There are a number of possiblities to lessen the inefficiency
here. The first is to limit the amount of polling you do per update of
the framebuffer. For example, every update you could just check a
certain number of scanlines against your local copy of the frame
buffer and if parts of the scanline differ, then do a GetImage on a
number of tiles which capture those changes to the scanline. This is
the approach taken by krfb and x0rfbserver.
Another possibility is an X extension to notify the X client
of changes to the framebuffer, thereby negating the need for
continually polling the X server. When the client receives the
notification it can do a GetImage to update its copy of the
framebuffer with the latest changes. Keith Packard is currently
working on an extension to do exactly this called XDAMAGE[16].
Initially we will use the x0rfbserver approach of polling the
screen, but will later implement support for the XDAMAGE extension
when it becomes available.
7.3. Input Event Handling
=========================
The VNC server will need to handle two types of input events
coming from VNC clients - keyboard and pointer events. These events
will be injected into the Xserver using the XTEST extension.
To inject a keyboard event into the X server you invoke
XTestFakeKeyEvent with the appropriate keycode. The X server then maps
this keycode, according to the current modifier state, to a keysym. We
need to make sure that they keycode we pass to the X server maps to
the same keysym as the keysym we received from the VNC client. We can
reverse map a keysym to a keycode, but we also need to make sure the
modifier state is such that the keycode will map back to that
keycode. Both krfb and x11vnc use the same code for ensuring this and
we can just copy that.
You can inject into the X server button presses/releases using
XTestFakeButtonEvent() and pointer movement using
XTestFakeMotionEvent(). The PointerEvent you receive from the client
contains information on the state of each button and the current
pointer location. The only slightly difficult part here is translating
the button state information to button presses/releases, but it merely
involves keeping track of the previous button state.
7.4. Cursor Handling
====================
The basic problem here is how to allow the VNC client to see
the cursor. There are several possible approaches:
1. Draw the cursor directly to the VNC server's copy of the
framebuffer and send framebuffer updates as the cursor is moved
around. The client will see the cursor being moved in all cases.
2. Provide the cursor image to the client using the RichCursor or
XCursor psuedo encoding and let the client draw the cursor
locally. The client only sees cursor movement when that client is
the one moving the cursor.
3. Again provide the cursor image to the client, but also send the
client updates on the pointer position using the PointerPos
pseudo-encoding which the client can use to update the position
of its locally drawn cursor. Again, the client will see the
cursor movement no matter who is moving it.
Approach (2) isn't very useful - the client needs to be able to
see cursor movement on the host. For that reason, we will not
advertise the cursor image to the client unless it supports *both*
cursor position and cursor shape updates. If the client only supports
one or the other, we ignore the support for that encoding and always
draw the cursor to the framebuffer.
One problem here is that, in the screen polling case, because
we will be comparing the local copy of the framebuffer (which may,
with approach (1) above, have the cursor drawn in it) to the actual
framebuffer (which will not contain the cursor image) we will need to
undraw the cursor before doing any comparison. Instead of complicating
the screen polling code with this detail we will draw the cursor image
to the framebuffer just before sending a frame buffer update to the
client and then immediately undraw it.
Of course, with either approach (1) or (2) we need some
mechanism by which we can determine the current cursor position and
shape. The only way to determine the current cursor position is by
regularily polling using XQueryPointer(). Determining the current
cursor shape is not possible but such support is to be added to the
XFIXES extension. [FIXME: more details on this].
7.5. libvncserver Work
======================
libvncserver contains lots of code from different VNC server
implementations. The intent is to bring all that code together under
one API which makes it easy to write VNC servers. However, rather than
being a library, it seems more like a full VNC server implementation
around which you can wrap a main function.
There are a number of problems with the library which can be
fixed in a fairly straightforward manner, by extending the API
slightly and cleaning bits up.
Other concerns around the library containing way more
implementation that we would like/need, many private functions exposed
in the API, structures that will likely need to be expanded being
exposed in the API and a general feeling that the library cannot
hope to maintain ABI compatibility are much harder to address. We have
the option of just statically linking to the library, and so, the
project will not be held up by these problems, but we should continue
to consider coming up with a plan to fix these problems.
Initially, the project will contain a copy of libvncserver
with the following changes:
+ API to allow glib mainloop integration.
+ Support for version 3.7 of the RFB protocol - this adds security
type negotiation.
+ A hook so that we can allow the user to decide whether or not an
authenticated client should be allowed to connect.
+ Re-worked cursor handling so that the cursor remains drawn to the
server's copy of the framebuffer only for the duration of
rfbSendFrameBufferUpdate().
+ API to set the current cursor postion.
+ Only send cursor shape updates when the client also has support
for cursor position updates and vice versa.
+ Removed default implementations of hooks and make that the default
action when the hook isn't implemented.
+ TRUE == 1 instead of -1 to match glib.
+ Warnings fixes.
+ LIBVNCSERVER prefix removed from the #ifdefs so that we can use a
normal config.h. libvncserver really shouldn't have platform
dependant defines in its header anyway.
+ Threading support disabled but not removed.
+ Lots of code which we don't need removed:
- UDP support
- Vestiges of CORBA support
- Back channel support
- setTranslateFunction(), getCursorPtr and displayHook hooks
- HTTP server implementation
- Support for drawing GUIs to the framebuffer on the server
side.
- Command line argument processing.
7.6. Service Discovery
======================
In order to implement the ability to browse the network for
available remote desktop servers there must be some way to enumerate
the available servers. One possible mechanism for doing this is DNS
Based Service Discovery[17], a draft of which is currently on the IETF
standards track.
DNS-SD is a convention for naming and structuring DNS resource
records such that a client can query the DNS for a list of named
instances of a particular service. The client can then resolve one of
these named instances to an address and port number via a SRV[18]
record.
In the remote desktop case, a client could query the DNS for
PTR records of _rfb._tcp.<domain> and would be returned a list of
named instances of RFB servers, using TCP, on the domain. For example:
PTR:_rfb._tcp.ireland.sun.com -> SRV:Mark's Desktop._rfb._tcp.ireland.sun.com
SRV:Gman's Desktop._rfb._tcp.ireland.sun.com
(Note the way the Service Instance Name is a user-friendly
name containing arbitrary UTF-8 encoded text. It is not a host name.)
The client would then display the list of available remote
desktop servers - i.e. "Mark's Desktop" and "Gman's Desktop" - and
allow the user to choose one. If the user chooses "Mark's Desktop" the
client can then resolve that SRV record associated with the remote
desktop instance.
SRV:Mark's Desktop._rfb._tcp.ireland.sun.com -> markmc-box.ireland.sun.com:5900
The client can then resolve the "markmc-box.ireland.sun.com"
hostname and using the resulting ip address connect to the remote
desktop server on port 5900.
While DNS-SD seems like a perfect mechanism by which remote
desktop instances may be queried for, there remains the problem of how
the DNS is populated with the details of these services to begin with.
A related draft proposal on the IETF standards track is
Multicast DNS[19]. The idea behind Multicast DNS is to allow a group
of hosts on a local link, in the absence of a convetionally managed
DNS server, to co-operatively manage a collection of DNS records and
allow clients on that same local link query those records.
The scheme works by each client connecting to the mDNS
multicast IPv4 address and sending/receiving DNS-like queries/answers
to port 5353. Between them, the clients manage the top-level ".local."
domain and negotiate any conflicts that arise. So, for example, the
host referenced by "markmc-box.ireland.sun.com" in the above example
could also be resolved using the host name "markmc-box.local" by other
Multicast DNS clients on the same link.
In order to be queriable by Multicast DNS, our remote desktop
server could act as a Multicast DNS Responder and Querier and register
the remote desktop service there. Here's how the example above would
look like if we were using mDNS:
Client queries the local link for remote desktop servers ...
PTR:_rfb._tcp.local
... and receives a reply first from markmc-box ...
-> SRV:Mark's Desktop._rfb._tcp.local
... and then a reply from gman-box:
-> SRV:Gman's Desktop._rfb._tcp.local
Once the user has selected "Mark's Desktop" from the displayed
list, the client resolves that service and receives a reply once again
from markmc-box:
SRV:Mark's Desktop._rfb._tcp.ireland.sun.com -> markmc-box.local:5900
The client then resolves "markmc-box.local" to an ip address
(still using Multicast DNS) and connects to that address on port 5900.
Luckily, implementing this won't require writing an mDNS
implementation from scratch. There is an existing implementation in
GNOME CVS which integrates nicely with glib's main loop and there are
plans to centralise this in a desktop service advertisement and
discovery daemon.
Another possible mechanism for making remote desktop service
information available via DNS is to use Dynamic DNS Updates[20] to add
DNS-SD records to a conventional DNS server. However, the majority of
DNS server deployments restrict (for obvious security reasons) the
ability to update DNS records completely or to only a few known
hosts. Because using this mechanism would require installation sites
to change their DNS administration policies, this is obviously not an
attractive option.
7.7. Security Considerations
============================
7.7.1. VNC Authentication
=========================
VNC uses a simple DES based challenge-response authentication
scheme. In order to authenticate the client, the server sends a random
16 byte challenge and the client then encrypts the challenge with DES
using the user supplied password as a key. If the response matches the
expected result, the client is authenticated. Otherwise, the server
closes the connection. There are a number of possible vulnerabilities
with this mechanism.
Firstly, the password, being limited to 8 characters, could be
brute force guessed by an attacker who continually tries to
authenticate using different passwords[21]. The standard way of
making such attacks unfeasible is to enforce a delay between failed
authentication attempts - i.e. if there has been a failed
authentication attempt, delay sending the challenge to the next client
who connects for a number of seconds.
Another possible vulnerability is the predictability of the
random challenge sent by the server. If the server, under any
circumstances, sends a challenge which has previously been used in a
successful authentication attempt there is the possibility that an
attacker may use the previously observered valid response again. An
example[22] of such is if the server re-seeds the random number
generator used to produce the challenge with the current time on each
connection attempt. In this case, if an attacker connects to the VNC
server within the same one second window as a valid client, then the
attacker will receive the same challenge as the valid client and use
the response from that client to authenticate. To avoid such a
vulnerability the server should produce highly unpredictable
challenges using the cryptographically strong random number generator
providied with the GNU TLS library.
Challenge-response authentication schemes are inherently
susceptible to man-in-the-middle attacks. The basic idea is that
attacker uses a client to generate a valid response for a given
challenge. One way[23] of carrying out such an attack is if the attack
can intercept and modify the packets flowing between the client and
the server. The attacker can then replace the challenge from the
server with a challenge the attacker has received in a pending
authentication attempt. The client then returns a valid response for
that challenge with which the attacker can use to complete its
authentication.
Given that this tool is aimed mainly at system administrators
administering a network of many desktop machines, and given that an
administrator would likely set the same password for the remote
desktop server on each of these machines, a more worrying
man-in-the-middle attack is:
("C" is the administrator using a VNC client, "S" is the VNC
server under attack and "M" is the attacker.)
1. M starts a modified VNC server which advertises itself on
the local link using mDNS and DNS-SD (see the "Service
Discovery" above)
2. C connects to M's modified VNC server by selecting it from
the list of available VNC servers
3. M then connects to S and receives a challenge
4. M sends this challenge to C
5. C sends back a valid response to M
6. M sends a failed authentication message to C.
7. M uses this response to authenticate against S and is
granted access.
There is no way to protect VNC's challenge response
authentication mechanism from such an attack.
DES[24], by today's standards, is quite a weak encryption
mechanism. Given that in this case that both plaintext and ciphertext
(the challenge and response) are both available a brute force attack
to find the key (the password in the VNC case) is possible. Brute
force cracking of DES is a much discussed[25]. A large amount of
computing power would be required for such an attack and given that
this tool would only deployed on private networks, it is perhaps not
an immediate concern. However, in the years to come it is to be
expected that such attacks would beome much more common and easy to
perform.
7.7.2. Encryption
=================
RFB protocol messages are sent across the network
unencrypted. This is an obvious security concern because an attacker
may snoop the protocol packets and, using a modified VNC viewer,
observe a VNC session in progress. Even more worrying, is that all key
presses are sent in the open and may be snooped. Considering that
system administrators are the primary target audience and that they
are likely to enter the root password when running some system
utility, the password could be snooped and used to gain root-level
access to the machine.
In order to protect the VNC session from such attacks, the
protocol should be extended to allow the stream to be
encrypted. Luckily, the RFB protocol was designed to allow such
extensions while maintaining compatibility.
The encryption of the RFB stream will be implemented with
TLS/SSL[26] using the gnutls[27] library and, for the Java client, the
Java Secure Socket Extension (JSSE)[28].
TLS is a protocol designed to provide privacy, data integrity,
compression and, optionally, peer authentication using public key
cryptography. The protocol mainly consists of two parts - the Record
Protocol and the Handshake Protocol. The Record Protocol is
responsible for fragmenting, compressing, hashing and encrypting the
data to be transmitted. The Handshake Protocol involves the peers
agreeing on a protocol version, cipher suite and compression method,
generating a shared secret and, optionally, exchanging certificates to
allow the peers to authenticate one another (either or both peers may
be authenticated).
New security types will be added (see below) which will cause
the client and server to begin the TLS handshaking protocol
immediately after one of those security types has been agreed upon. If
VNC authentication is required, that challenge-response exchange will
happen immediately after the TLS handshake has completed.
The peer authentication which may take place as part of the
TLS handshake involves the peers exchanging certificates (currently
only X.509[29] certificates are supported by the protocol but support
for OpenPGP[30] certificates has been proposed[31]) and verifying
their identity. In order to support server certificate authentication
the VNC client will need have some sort of certificate store which
contains the server certificates the client trusts - this is useful
because it prevents a man-in-the middle attack. To support client
certificate authentication, the VNC server will also require a
certificate store listing the clients who are authorised to connect -
this is useful because the password is no longer a weak point, but
also that it would be generally more convenient for a system
administrator to distribute his certicate to each of the desktop
systems he administers and never have to type in a password.
If certificate based peer authentication is not used the
client and server agree on a secret using anonymous Diffie-Hellman key
exchange.
TLS supports compression of the communication stream. Some
investigation should be carried out to see if using this compression
mechanism is with uncompressed RFB tiles results in better bandwidth
usage than no TLS compression and compressed RFB tiles.
7.7.3. New RFB Security Types
=============================
The negotiated security-type in the RFB protocol is an 8 bit
unsigned integer. Currently there are only two possible values:
"None"(1) to indicate no authentication is needed and "VNC
authentication"(2) to indicate that the client is to be authenticated
using the challenge-response scheme detailed above. 0 indicates and
error condition.
We will add a further four security types:
+ Anonymous TLS (3)
+ TLS With VNC Authentication (4)
+ TLS With Server Certificate and VNC Authentication (5)
+ TLS With Server and Client Certificate Authentication (6)
In order to ensure interoperability with other
implementations, these security types must be registered with RealVNC
who maintain the RFB protocol specification.
7.7.4. Security Related Preferences
===================================
A number of preferences will be provided which will have a
direct impact of the security of the system. Their meaning and
rationale for their existance is detailed below:
/desktop/gnome/remote_access/enabled (boolean, default=false)
If false, the Remote Desktop server will not be started at
login time and if running will not allow any new remote
connections, put all existing clients on hold and exit after a 30
second timeout.
The rationale for the preference is that unless Remote Desktop
access is to actually be used, it is much more preferable to not
have the server running for both resource consumption and security
reasons.
The default for the preference is false under the assumption
that Remote Desktop access will not be used by the majority of the
user base.
/desktop/gnome/remote_access/prompt_enabled (boolean, default=true)
If true, once a client has connected and been authenticated the
user is prompted on whether the client connection should be
allowed to be completed.
The rationale here is that in the majority of scenarios, the
user on the host machine should be in control of whether or not
remote users are allowed to connect to their desktop. Even if the
host user has enabled remote access, set a password and informed
some other user of that password, the host user may still want to
both be aware of another user connecting and also decide whether
that particular time is suitable for the remote user to connect.
The default is true for two reasons. Firstly, it is assumed that
most users will want to vet new connections and, secondly, because
by default there is no authentication required to connect this
prompt will provide some level of manual authentication.
/desktop/gnome/remote_access/view_only (boolean, default=false)
If true, remote users will only be allowed to view the remote
desktop and all keyboard, pointer and clipboard events from the
remote user will be ignored.
The view only preference is intended to support the simple
collaboration scenario where a number of remote users will connect
to a single host to observe something happening on the host
machine. In this scenario the host will no want any of the remote
users to use the pointer or keyboard on the host.
The default is false because this is not the primary scenario we
are expecting to target.
/desktop/gnome/remote_access/require_encryption (bool, default=true)
If true, the host requires that all connecting client use TLS
encryption. Any clients attempting to use the "None" and "VNC
Authentication" security types will be refused. This will only have
affect the cases where the VNC viewer being used by remote user
does not have support for TLS encryption. If the viewer does have
support, it will always be used.
The preferences is provided so that the host may make the policy
decision on whether unencrypted connections should be allowed.
However, in most cases it is expected that the host will require
that only encrypted connections be allowed so as to not allow any
information on the host to be compromised. For this reason, the
default value for the preference is true.
/desktop/gnome/remote_access/authentication_methods (list, default=[none])
The list of authentication which the server will
advertise. Currently the supported values are "none" and "vnc",
but when certificate based authentication is implemented
"server-cert-with-vnc" and "server-cert-with-client-cert" will
also be supported.
The preference is provided so as to allow the host user decide
on how remote users should be authenticated. The host may decide
that no authentication is required, that password or certificate
based authentication should be used.
The default value is "none" because there is no point in having
the default value be "vnc" because no password would be set.
/desktop/gnome/remote_access/vnc_password (base64 encoded string, default=<unset>)
The password used to authenticate the remote user when VNC
authentication is being used. The password is stored in GConf
base64 encoded to provide an extra level of secrecy. However, the
secrecy of the password is guaranteed by the fact that the files
which GConf stores preference values in are only readable by the
user in question.
7.7.5. Summary of Security Considerations
=========================================
(
How to put this ? There must be some standard methodology to lay
out the specific types of attacks you are and are not protecting
against with different configurations e.g. given the following
configuration:
+ allowed = true
+ require_encryption = true
+ authentication_methods = [vnc]
+ vnc_password = strong-password
and based on the following assumptions about potential attackers:
+ Has the ability to snoop the intervening network stream.
- Does not have the ability to modify the intervening network
stream.
- Does not have the ability to brute force crack the DES key
given the challenge and response.
- Does not have the ability to spoof an IP address.
+ Does not have access to the host machine as either root or the
user who is running the server.
+ Does not have any prior knowledge of what the password may be.
+ Does not have access to the user's X display.
+ Does not have access to the user's GConf daemon.
then no attacker should be able to:
+ Snoop the host's screen contents from the VNC session.
+ Snoop keyboard/pointer/clipboard events from the client.
+ Obtain access to the VNC session.
+ Obtain shell access to the host machine as root or the user
who is running the server.
But you are also making assumptions about the behaviour of the
user on both sides - e.g. that the remote user has the correct IP
address and port number for the host she wishes to connect to, and
not the IP address and port number of an attacker.
)
What problems remain ?
+ No peer authentication - possibility of man-in-the-middle attacks.
8. Host User Interface
======================
8.1. Preferences Dialog
=======================
Menu Entry:
Name = Remote Desktop
Comment = Set your remote desktop access preferences
Categories = GNOME;Application;Settings;
+-+-------------------------------------------------------+-+-+
|#| Remote Desktop Preferences |_|X|
+-+-------------------------------------------------------+-+-+
| |
| ! Some of these preferences are locked down |
| |
| Sharing |
| ------- |
| [ ] Allow other users to view my desktop on their computer |
| [ ] Also allow other users to control my desktop |
| |
| Security |
| -------- |
| When a user tries to view or control my desktop: |
| [ ] Ask me for confirmation |
| [ ] Ensure they are using encryption |
| [ ] Require them to enter this password: |
| Password: ________ |
| |
+-------------------------------------------------------------+
| <Help> <Close> |
+-------------------------------------------------------------+
Notes:
o The "locked down" warning is only displayed if one of the GConf
keys associated with the preferences isn't writable. Each such
non-writable key also causes the appropriate control to be made
insensitive.
o When "Enable remote desktop access" is inactive, the rest of the
controls are insensitive.
o When "Require password for remote access" is inactive, the
"Password" entry is insensitive.
8.2. Connection Query Dialog
============================
This dialog appears when the "Prompt me before allowing
access" preference is set and a remote user connects to the server and
is authenticated.
+-+----------------------------------------------------------+-+-+
|#| Question |_|X|
+-+----------------------------------------------------------+-+-+
| |
| ??? An attempt is being made to access your deskop remotely |
| ??? from '<hostname>' |
| ??? |
| Allow this connection to be completed? |
| |
+----------------------------------------------------------------+
| <Refuse> <Allow> |
+----------------------------------------------------------------+
8.3. Notification Area Icon
===========================
Icon which will appear in the notification area when there are
any remote users connected. Clicking on the icon will show the
connections details dialog.
8.4. Connection Details Dialog
==============================
Dialog will show the list of remote desktop users, the host
they are connected from and how long they have been connected. You
will be able to disconnect a given user using the dialog.
9. Client User Interface
========================
+ Initially a patched version of RealVNC's Java vncviewer.
+ Later possibly a integrated into a lockhart type GUI.
10. See Also
============
[1] - The RFB Protocol Specification
http://www.uk.research.att.com/vnc/rfbproto.pdf
[2] - libvncserver:
http://libvncserver.sourceforge.net/
[3] - xf4vnc:
http://xf4vnc.sourceforge.net/
[4] - realVNC:
http://www.realvnc.com/
[5] - krfb:
http://www.tjansen.de/krfb/
[6] - VNC: Where it came from, where it's going:
http://cyberelk.net/tim/articles/VNC/
[7] - Using Remote Desktop:
https://www.microsoft.com/windowsxp/pro/using/howto/gomobile/remotedesktop/default.asp
[8] - Using Remote Assitance:
https://www.microsoft.com/windowsxp/pro/using/howto/gethelp/remoteassist/default.asp
[9] - FAQ on Remote Desktop:
https://www.microsoft.com/windowsxp/remotedesktop/faq.asp
[10] - ITU-T T.128 Application Sharing Protocol:
http://www.rdesktop.org/docs/t128.zip
[11] - rdesktop: A Remote Desktop Protocol Client:
http://www.rdesktop.org/
[12] - tsclient: A Frontend for rdesktop:
http://www.gnomepro.com/tsclient/
[13] - The Interactive Performace of SLIM: A Stateless, Thin-Client
Architecture:
http://research.sun.com/research/features/tenyears/volcd/papers/nrthcutt.htm
[14] - XTEST Extension Protocol:
http://www.x.org/pub/R6.6/xc/doc/hardcopy/Xext/xtest.PS.gz
[15] - XTEST Extension Library:
http://www.x.org/pub/R6.6/xc/doc/hardcopy/Xext/xtestlib.PS.gz
[16] - XDAMAGE Extension Wiki
http://xlibs.freedesktop.org/Software/DamageExt
[17] - DNS-Based Service Discovery:
http://files.dns-sd.org/draft-cheshire-dnsext-dns-sd.txt
[18] - A DNS RR for specifying the location of services (DNS SRV):
http://www.ietf.org/rfc/rfc2782.txt
[19] - Performing DNS Queries via IP Multicast:
http://files.multicastdns.org/draft-cheshire-dnsext-multicastdns.txt
[20] - Dynamic Updates in the Domain Name System (DNS UPDATE):
http://www.ietf.org/rfc/rfc2136.txt
[21] - Example of a brute force VNC passwords cracking tool:
http://www.securiteam.com/tools/Brute_forcing_VNC_passwords.html
[22] - Example of VNC challenge predictability vulnerability:
http://www.securityfocus.com/archive/1/284064/2003-09-02/2003-09-08/2
[23] - Details of how a man-in-the-middle attack on VNC might be
performed:
http://www1.corest.com/common/showdoc.php?idx=117&idxseccion=10
[24] - A nice overview of DES encryption:
http://www.tropsoft.com/strongenc/des.htm
[25] - Cracking DES: Secrets of Encryption Research, Wiretap Politics
& Chip Design
http://cryptome.org/cracking-des.htm
[26] - Transport Layer Security (TLS) - IETF standardisation of SSL
http://www.ietf.org/html.charters/tls-charter.html
[27] - The GNU Transport Layer Security Library (gnutls)
http://www.gnu.org/software/gnutls/
[28] - Java Secure Sockets Extension
http://java.sun.com/products/jsse/index-14.html
[29] - Public-Key Infrastructure (X.509) (pkix)
http://www.ietf.org/html.charters/pkix-charter.html
[30] - OpenPGP Message Format (rfc2440)
http://www.ietf.org/rfc/rfc2440.txt
[31] - Using OpenPGP keys for TLS authentication
http://www.ietf.org/internet-drafts/draft-ietf-tls-openpgp-keys-03.txt
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:: Command execute :: | |
:: Shadow's tricks :D :: | |
Useful Commands
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:: Preddy's tricks :D :: | |
Php Safe-Mode Bypass (Read Files)
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