Section: Virtual Network Computing (1)
Updated: 03 Mar 2005
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Xvnc - the X VNC server
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 physical 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.
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 - anything else is likely to cause
strange behaviour by applications.
- -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 significant 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 display. 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).
This significantly changes Xvnc's behaviour so that it can be launched from
inetd. See the section below on usage with inetd.
List all the options and 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. Parameters which take a value can be specified as
-param value. Other valid forms are param=value
-param=value --param=value. Parameter names are
- -desktop desktop-name
Each desktop has a name which may be displayed by the viewer. It defaults to
- -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 seconds).
- -httpd directory
Run a mini-HTTP server which serves files from the given directory. 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
- -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 performance in many
cases. After any change to the framebuffer, Xvnc waits for this number of
milliseconds (default 40) before sending 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 versions of
Xvnc, where the first change to the framebuffer causes an immediate update to
any waiting clients.
Send clipboard changes to clients (default is on). Note that you must also run
vncconfig(1) to get the clipboard to work.
Accept clipboard updates from clients (default is on). Note that you must also
run vncconfig(1) to get the clipboard to work.
Accept pointer press and release events from clients (default is on).
Accept key press and release events from clients (default is on).
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.
Never treat incoming connections as shared, regardless of the client-specified
setting (default is off).
Always treat incoming connections as shared, regardless of the client-specified
setting (default is off).
Always use protocol version 3.3 for backwards compatibility with badly-behaved
clients (default is off).
Perform pixel comparison on framebuffer to reduce unnecessary updates (default
- -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).
Prompts the user of the desktop to explicitly accept or reject incoming
connections. This is most useful when using the vnc.so module or
x0vncserver(1) program to access an existing X desktop via VNC.
The vncconfig(1) program must be running on the desktop in order for
QueryConnect to be supported by the vnc.so(1) module or
Xvnc(1) program. The x0vncserver(1) program does not require
vncconfig(1) to be running.
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
- -RemapKeys mapping
Sets up a keyboard mapping.
is a comma-separated string of character mappings, each of the form
is a hexadecimal keysym. For example, to exchange the " and @ symbols you would specify the following:
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 listening 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 usually 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 disconnects, 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 automatically the next time he connects.
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 maintained by RealVNC Ltd. See
http://www.realvnc.com for details.
- USAGE WITH INETD
- SEE ALSO
linux.jgfs.net manual pages