Tribblix: manual page: Xvnc.1

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
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.

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 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 RGB888.

-interface IP address
Listen on interface. By default Xvnc listens on all available
interfaces.

-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 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 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.

-UseIPv4
Use IPv4 for incoming and outgoing connections. Default is on.

-UseIPv6
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.

-AcceptCutText
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.

-SendCutText
Send clipboard changes to clients. Default is on.

-SendPrimary
Send the primary selection and cut buffer to the server as
well as the clipboard selection. Default is on.

-AcceptPointerEvents
Accept pointer press and release events from clients. Default
is on.

-AcceptKeyEvents
Accept key press and release events from clients. Default is
on.

-AcceptSetDesktopSize
Accept requests to resize the size of the desktop. 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.

-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 2.

-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.

-ImprovedHextile
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 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.

-UseBlacklist
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 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.

-QueryConnect
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.

-localhost
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:

RemapKeys=0x22<>0x40

-AvoidShiftNumLock
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).

-RawKeyboard
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.

-AllowOverride
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.

AUTHOR

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.

TigerVNC Xvnc(1)