Section: Virtual Network Computing (1)
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
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 vncsession. This sets up the
environment appropriately and starts a desktop environment. See the manual
page for vncsession(8) 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
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
- -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 24,
other possible values are 16 and 32. Anything else is likely to cause strange
behaviour by applications and may prevent the server from starting at all.
- -pixelformat format
Specify pixel format for server to use (BGRnnn or RGBnnn). The default for
depth 16 is RGB565 and for depth 24 and 32 is RGB888.
- -interface IP address
Listen on interface. By default Xvnc listens on all available interfaces.
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
(1) 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=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.
Use IPv4 for incoming and outgoing connections. Default is on.
Use IPv6 for incoming and outgoing connections. Default is on.
- -rfbunixpath path
Specifies the path of a Unix domain socket on which Xvnc listens for
connections from viewers, instead of listening on a TCP port.
- -rfbunixmode mode
Specifies the mode of the Unix domain socket. The default is 0600.
- -rfbwait time, -ClientWaitTimeMillis time
Time in milliseconds to wait for a viewer which is blocking the server. This is
necessary because the server 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).
- -rfbauth passwd-file, -PasswordFile passwd-file
Password file for VNC authentication. There is no default, you should
specify the password file explicitly. Password file should be created with
the vncpasswd(1) utility. The file is accessed each time a connection
comes in, so it can be changed on the fly.
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 is on.
Accept requests to resize the size of the desktop. 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.
- -FrameRate fps
The maximum number of updates per second sent to each client. If the screen
updates any faster then those changes will be aggregated and sent in a single
update to the client. Note that this only controls the maximum rate and a
client may get a lower rate when resources are limited. Default is 60.
- -CompareFB mode
Perform pixel comparison on framebuffer to reduce unnecessary updates. Can
be either 0 (off), 1 (always) or 2 (auto). Default is
- -ZlibLevel level
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) compression library.
Use improved compression algorithm for Hextile encoding which achieves better
compression ratios by the cost of using slightly more CPU time. Default is
- -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
- -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 name
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.
- -GnuTLSPriority priority
GnuTLS priority string that controls the TLS session’s handshake algorithms.
See the GnuTLS manual for possible values. Default is NORMAL.
Temporarily reject connections from a host if it repeatedly fails to
authenticate. Default is on.
- -BlacklistThreshold count
The number of unauthenticated connection attempts allowed from any individual
host before that host is black-listed. Default is 5.
- -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
- -MaxConnectionTime seconds
Terminate when a client has been connected for N seconds. Default is
- -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. Default is off.
The vncconfig(1) program must be running on the desktop in order for
QueryConnect to be supported.
- -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
- -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:
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).
Send keyboard events straight through and avoid mapping them to the current
keyboard layout. This effectively makes the keyboard behave according to the
layout configured on the server instead of the layout configured on the
client. Default is off.
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.
Default is desktop,AcceptPointerEvents,SendCutText,AcceptCutText,SendPrimary,SetPrimary.
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. 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 suite.