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Xvnc(1) Virtual Network Computing Xvnc(1)
NAME
Xvnc - the X VNC server
SYNOPSIS
Xvnc [options] :display#
DESCRIPTION
Xvnc is the X VNC (Virtual Network Computing) server. It is based on
a standard X server, but it has a "virtual" screen rather than a phys-
ical one. X applications display themselves on it as if it were a
normal X display, but they can only be accessed via a VNC viewer - see
vncviewer(1).
So Xvnc is really two servers in one. To the applications it is an X
server, and to the remote VNC users it is a VNC server. By convention
we have arranged that the VNC server display number will be the same
as the X server display number, which means you can use eg. snoopy:2
to refer to display 2 on machine "snoopy" in both the X world and the
VNC world.
The best way of starting Xvnc is via the vncserver script. This sets
up the environment appropriately and runs some X applications to get
you going. See the manual page for vncserver(1) for more information.
OPTIONS
Xvnc takes lots of options - running Xvnc -help gives a list. Many of
these are standard X server options, which are described in the
Xserver(1) manual page. In addition to options which can only be set
via the command-line, there are also "parameters" which can be set
both via the command-line and through the vncconfig(1) program.
-geometry widthxheight
Specify the size of the desktop to be created. Default is
1024x768.
-depth depth
Specify the pixel depth in bits of the desktop to be created.
Default is 16, other possible values are 8, 15, and 24 - any-
thing else is likely to cause strange behaviour by applica-
tions.
-pixelformat format
Specify pixel format for server to use (BGRnnn or RGBnnn). The
default for depth 8 is BGR233 (meaning the most significant two
bits represent blue, the next three green, and the least sig-
nificant three represent red), the default for depth 16 is
RGB565 and for depth 24 is RGB888.
-cc 3 As an alternative to the default TrueColor visual, this allows
you to run an Xvnc server with a PseudoColor visual (i.e. one
which uses a colour map or palette), which can be useful for
running some old X applications which only work on such a dis-
play. Values other than 3 (PseudoColor) and 4 (TrueColor) for
the -cc option may result in strange behaviour, and PseudoColor
desktops must be 8 bits deep (i.e. -depth 8).
-inetd This significantly changes Xvnc's behaviour so that it can be
launched from inetd. See the section below on usage with
inetd.
-help List all the options and parameters
PARAMETERS
VNC parameters can be set both via the command-line and through the
vncconfig(1) program, and with a VNC-enabled XFree86 server via
Options entries in the XF86Config file.
Parameters can be turned on with -param or off with -param=0. Parame-
ters which take a value can be specified as -param value. Other valid
forms are param=value -param=value --param=value. Parameter names are
case-insensitive.
-desktop desktop-name
Each desktop has a name which may be displayed by the viewer.
It defaults to "x11".
-rfbport port
Specifies the TCP port on which Xvnc listens for connections
from viewers (the protocol used in VNC is called RFB - "remote
framebuffer"). The default is 5900 plus the display number.
-rfbwait time, -ClientWaitTimeMillis time
Time in milliseconds to wait for a viewer which is blocking
Xvnc. This is necessary because Xvnc is single-threaded and
sometimes blocks until the viewer has finished sending or
receiving a message - note that this does not mean an update
will be aborted after this time. Default is 20000 (20 sec-
onds).
-httpd directory
Run a mini-HTTP server which serves files from the given direc-
tory. Normally the directory will contain the classes for the
Java viewer. In addition, files with a .vnc extension will
have certain substitutions made so that a single installation
of the Java VNC viewer can be served by separate instances of
Xvnc.
-httpPort port
Specifies the port on which the mini-HTTP server runs. Default
is 5800 plus the display number.
-rfbauth passwd-file, -PasswordFile passwd-file
Specifies the file containing the password used to authenticate
viewers. The file is accessed each time a connection comes in,
so it can be changed on the fly via vncpasswd(1).
-deferUpdate time
Xvnc uses a "deferred update" mechanism which enhances perfor-
mance in many cases. After any change to the framebuffer, Xvnc
waits for this number of milliseconds (default 40) before send-
ing an update to any waiting clients. This means that more
changes tend to get coalesced together in a single update.
Setting it to 0 results in the same behaviour as earlier ver-
sions of Xvnc, where the first change to the framebuffer causes
an immediate update to any waiting clients.
-SendCutText
Send clipboard changes to clients (default is on). Note that
you must also run vncconfig(1) to get the clipboard to work.
-AcceptCutText
Accept clipboard updates from clients (default is on). Note
that you must also run vncconfig(1) to get the clipboard to
work.
-AcceptPointerEvents
Accept pointer press and release events from clients (default
is on).
-AcceptKeyEvents
Accept key press and release events from clients (default is
on).
-DisconnectClients
Disconnect existing clients if an incoming connection is non-
shared (default is on). If DisconnectClients is false, then a
new non-shared connection will be refused while there is a
client active. When combined with NeverShared this means only
one client is allowed at a time.
-NeverShared
Never treat incoming connections as shared, regardless of the
client-specified setting (default is off).
-AlwaysShared
Always treat incoming connections as shared, regardless of the
client-specified setting (default is off).
-Protocol3.3
Always use protocol version 3.3 for backwards compatibility
with badly-behaved clients (default is off).
-CompareFB
Perform pixel comparison on framebuffer to reduce unnecessary
updates (default is on).
-SecurityTypes sec-types
Specify which security schemes to use separated by commas. At
present only "None" and "VncAuth" are supported. The default
is "VncAuth" - note that if you want a server which does not
require a password, you must set this parameter to "None".
-IdleTimeout seconds
The number of seconds after which an idle VNC connection will
be dropped (default is 3600 i.e. an hour).
-localhost
Only allow connections from the same machine. Useful if you use
SSH and want to stop non-SSH connections from any other hosts.
See the guide to using VNC with SSH on the web site.
-log logname:dest:level
Configures the debug log settings. dest can currently be
stderr or stdout, and level is between 0 and 100, 100 meaning
most verbose output. logname is usually * meaning all, but you
can target a specific source file if you know the name of its
"LogWriter". Default is *:stderr:30.
USAGE WITH INETD
By configuring the inetd(1) service appropriately, Xvnc can be
launched on demand when a connection comes in, rather than having to
be started manually. When given the -inetd option, instead of listen-
ing for TCP connections on a given port it uses its standard input and
standard output. There are two modes controlled by the wait/nowait
entry in the inetd.conf file.
In the nowait mode, Xvnc uses its standard input and output directly
as the connection to a viewer. It never has a listening socket, so
cannot accept further connections from viewers (it can however connect
out to listening viewers by use of the vncconfig program). Further
viewer connections to the same TCP port result in inetd spawning off a
new Xvnc to deal with each connection. When the connection to the
viewer dies, the Xvnc and any associated X clients die. This
behaviour is most useful when combined with the XDMCP options -query
and -once. An typical example in inetd.conf might be (all on one
line):
5950 stream tcp nowait nobody /usr/local/bin/Xvnc Xvnc -inetd
-query localhost -once securitytypes=none
In this example a viewer connection to :50 will result in a new Xvnc
for that connection which should display the standard XDM login screen
on that machine. Because the user needs to login via XDM, it is usu-
ally OK to accept connections without a VNC password in this case.
In the wait mode, when the first connection comes in, inetd gives the
listening socket to Xvnc. This means that for a given TCP port, there
is only ever one Xvnc at a time. Further viewer connections to the
same port are accepted by the same Xvnc in the normal way. Even when
the original connection is broken, the Xvnc will continue to run. If
this is used with the XDMCP options -query and -once, the Xvnc and
associated X clients will die when the user logs out of the X session
in the normal way. It is important to use a VNC password in this
case. A typical entry in inetd.conf might be:
5951 stream tcp wait james /usr/local/bin/Xvnc Xvnc -inetd
-query localhost -once passwordFile=/home/james/.vnc/passwd
In fact typically, you would have one entry for each user who uses VNC
regularly, each of whom has their own dedicated TCP port which they
use. In this example, when user "james" connects to :51, he enters
his VNC password, then gets the XDM login screen where he logs in in
the normal way. However, unlike the previous example, if he discon-
nects, the session remains persistent, and when he reconnects he will
get the same session back again. When he logs out of the X session,
the Xvnc will die, but of course a new one will be created automati-
cally the next time he connects.
SEE ALSO
vncconfig(1), vncpasswd(1), vncserver(1), vncviewer(1), Xserver(1),
inetd(1)
http://www.realvnc.com
AUTHOR
Tristan Richardson, RealVNC Ltd.
VNC was originally developed by the RealVNC team while at Olivetti
Research Ltd / AT&T Laboratories Cambridge. It is now being main-
tained by RealVNC Ltd. See http://www.realvnc.com for details.
RealVNC Ltd 18 May 2004 Xvnc(1)
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