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
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
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
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)
- -geometry widthxheight
- Specify the size of the desktop to be created. Default is
- -depth depth
- Specify the pixel depth in bits of the desktop to be
created. Default is 24, other possible values are 8, 15, and 16 - 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
- -interface IP address
- Listen on interface. By default Xvnc listens on all
- 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
program, and with a VNC-enabled Xorg server via Options
entries in the xorg.conf file.
Parameters can be turned on with - param
or off with - param
Parameters which take a value can be specified as - param value
Other valid forms are param=value
. Parameter names are
- -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
- -rfbwait time, -ClientWaitTimeMillis
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
- -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 Xvnc.
- -httpPort port
- Specifies the port on which the mini-HTTP server runs.
Default is 5800 plus the display number.
- -rfbauth passwd-file, -PasswordFile
- 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 1) 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.
- Keep delaying sending the screen update to the client(s)
each time the screen is updated. Otherwise the delay is from the first
update. Default is off.
- Accept clipboard updates from clients (default is on).
- -MaxCutText bytes
- The maximum size of a clipboard update that will be
accepted from a client. Default is 262144.
- Send clipboard changes to clients (default is on).
- Send the primary selection and cut buffer to the server as
well as the clipboard selection. Default is on.
- Accept pointer press and release events from clients
(default is on).
- Accept key press and release events from clients (default
- 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 is on).
- Zlib compression level for ZRLE encoding (it does not
affect Tight encoding). Acceptable values are between 0 and 9. Default is
to use the standard compression level provided by the zlib(3)
- Use improved compression algorithm for Hextile encoding
which achieves better compression ratios by the cost of using slightly
more CPU time. Default is on.
- -SecurityTypes sec-types
- Specify which security scheme to use for incoming
connections. Valid values are a comma separated list of None,
VncAuth, Plain, TLSNone, TLSVnc,
TLSPlain, X509None, X509Vnc and X509Plain.
Default is VncAuth,TLSVnc.
- -Password password
- Obfuscated binary encoding of the password which clients
must supply to access the server. Using this parameter is insecure, use
PasswordFile parameter instead.
- -PlainUsers user-list
- A comma separated list of user names that are allowed to
authenticate via any of the "Plain" security types (Plain,
TLSPlain, etc.). Specify * to allow any user to authenticate using
this security type. Default is to deny all users.
- -pam_service name, -PAMService
- PAM service name to use when authentication users using any
of the "Plain" security types. Default is vnc.
- -X509Cert path
- Path to a X509 certificate in PEM format to be used for all
X509 based security types (X509None, X509Vnc, etc.).
- -X509Key path
- Private key counter part to the certificate given in
X509Cert. Must also be in PEM format.
- -BlacklistThreshold count
- The number of unauthenticated connection attempts allowed
from any individual host before that host is black-listed. Default is
- -BlacklistTimeout seconds
- The initial timeout applied when a host is first
black-listed. The host cannot re-attempt a connection until the timeout
expires. Default is 10.
- -IdleTimeout seconds
- The number of seconds after which an idle VNC connection
will be dropped (default is 0, which means that idle connections will
never be dropped).
- -MaxDisconnectionTime seconds
- Terminate when no client has been connected for N
seconds. Default is 0.
- -MaxConnectionTime seconds
- Terminate when a client has been connected for N
seconds. Default is 0.
- -MaxIdleTime seconds
- Terminate after N seconds of user inactivity.
Default is 0.
- 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
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.
- -QueryConnectTimeout seconds
- Number of seconds to show the Accept Connection dialog
before rejecting the connection. Default is 10.
- Only allow connections from the same machine. Useful if you
use SSH and want to stop non-SSH connections from any other hosts.
- -Log logname:dest:level
- Configures the debug log settings. dest can
currently be stderr, stdout or syslog, 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.
- -RemapKeys mapping
- Sets up a keyboard mapping. mapping is a
comma-separated string of character mappings, each of the form
char->char, or char<>char, where
char is a hexadecimal keysym. For example, to exchange the "
and @ symbols you would specify the following:
- Key affected by NumLock often require a fake Shift to be
inserted in order for the correct symbol to be generated. Turning on this
option avoids these extra fake Shift events but may result in a slightly
different symbol (e.g. a Return instead of a keypad Enter).
- Comma separated list of parameters that can be modified
using VNC extension. Parameters can be modified for example using
vncconfig(1) program from inside a running session.
Allowing override of parameters such as PAMService or
PasswordFile can negatively impact security if Xvnc runs under
different user than the programs allowed to override the parameters.
When NoClipboard parameter is set, allowing override of
SendCutText and AcceptCutText has no effect.
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
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
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
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. and others.
VNC was originally developed by the RealVNC team while at Olivetti Research Ltd
/ AT&T Laboratories Cambridge. TightVNC additions were implemented by
Constantin Kaplinsky. Many other people have since participated in
development, testing and support. This manual is part of the TigerVNC software