nxagent(1) NX Agent (Xserver) nxagent(1)
NAME
nxagent - nested Xserver optimized for remote computing
SYNOPSIS
nxagent [options]
DESCRIPTION
nxagent is an X server for remote application/desktop access similar to
Xnest or Xephyr.
nxagent implements a very efficient compression of the X11 protocol,
called the NX protocol.
The NX protocol increases performance when using X applications over
high latency and low bandwidth networks, while providing a local (LAN-
like) usage experience even if connecting from off-site locations (via
cable modem or GSM).
nxagent can be used standalone as a nested X server (with NX protocol
disabled), but its real benefits are gained when using it over remote
connections via the nxcomp compression library. The counterpart appli-
cation on the other end (i.e. the client) is called nxproxy.
When used in proxy <-> agent mode, nxagent adds the feature of being
suspendible. Sessions can be started from one client, suspended and
then resumed from another (or the same) client.
nxagent and nxproxy are utilized by various remote application/desktop
frameworks for providing server-side GUI application access from remote
client systems.
Currently, nxagent is co-maintained by three of these projects: The
Arctica Project, TheQVD and X2Go.
STARTING THE SERVER
nxagent should be run in user space. Other than the system's local
X.org server, nxagent does not require to be run as root. When bundled
with a remote application framework, you normally don't have to launch
nxagent manually. nxagent startup is usually managed by the underlying
framework (e.g. Arctica Session Manager, X2Go Server, etc.).
When nxagent starts up (e.g. by typing 'nxagent -ac :1' in a terminal
window), it typically launches in "windowed desktop" mode. On your lo-
cal X server a new window appears being an X server itself.
However, nxagent also supports rootless (or seamless) application mode
and a shadow session mode (similar to what VNC does).
Example: You can launch a complete desktop session inside this nested X
server now:
The Debian way...
$ export DISPLAY=:1
$ STARTUP=mate-session /etc/X11/Xsession
The Fedora / Gentoo / openSUSE way...
### FIXME / TODO ###
However, nxagent also supports rootless (or seamless) application mode
and a shadow session mode (similar to what VNC does).
OPTIONS
nxagent accepts a range of default X server options as described below.
Those default options have to be provided via the command line.
Furthermore, nxagent accepts some nx-X11 specific options, described
further below.
Last but not least, nxagent accepts several more options, the so-called
nx/nx options, provided via the $DISPLAY environment variable or the
-options command line option. See below for further details.
STANDARD XSERVER OPTIONS
:displaynumber
The X server runs as the given displaynumber, which by default
is 0. If multiple X servers are to run simultaneously on a
host, each must have a unique display number. See the DISPLAY
NAMES section of the X(__miscmansuffix__) manual page to learn
how to specify which display number clients should try to use.
-a number
sets pointer acceleration (i.e. the ratio of how much is re-
ported to how much the user actually moved the pointer).
-ac disables host-based access control mechanisms. Enables access
by any host, and permits any host to modify the access control
list. Use with extreme caution. This option exists primarily
for running test suites remotely.
-audit level
sets the audit trail level. The default level is 1, meaning
only connection rejections are reported. Level 2 additionally
reports all successful connections and disconnects. Level 4
enables messages from the SECURITY extension, if present, in-
cluding generation and revocation of authorizations and viola-
tions of the security policy. Level 0 turns off the audit
trail. Audit lines are sent as standard error output.
-auth authorization-file
specifies a file which contains a collection of authorization
records used to authenticate access. See also the xdm(1) and
Xsecurity(__miscmansuffix__) manual pages.
-bs disables backing store support on all screens.
-br sets the default root window to solid black (default).
-wr sets the default root window to solid white.
-c turns off key-click.
c volume
sets key-click volume (allowable range: 0-100).
-cc class
sets the visual class for the root window of color screens.
The class numbers are as specified in the X protocol. Not
obeyed by all servers.
-co filename
This used to be the option for specifying the path to the RGB
color database file. As the RGB color database is now embedded
into the binary this option has no effect but is kept for com-
patibility. Deprecated.
-core causes the server to generate a core dump on fatal errors.
-displayfd fd
specifies a file descriptor in the launching process. Rather
than specifying a display number, the X server will attempt to
listen on successively higher display numbers, and upon finding
a free one, will write the port number back on this file de-
scriptor as a newline-terminated string. The -pn option is ig-
nored when using -displayfd.
nxagent specific:
(1) Other than in X.org's Xserver, you can use -displayfd in
conjunction with an explicit display number. If the explicit
display number is not available (i.e., already in use), nxagent
tries to figure out the next available display number,
e.g.:
nxagent -displayfd 2 :50
(2) If -displayfd <X> is given with <X> equaling 2 (STDERR),
then the display number string written to STDERR is beautified
with some human-readable (machine-parseable) text.
-sync This option tells nxagent to synchronize its window and graph-
ics operations with the real server. This is a useful option
for debugging, but it will slow down nxagent's performance con-
siderably. It should not be used unless absolutely necessary.
-full This option tells nxagent to utilize full regeneration of real
server objects and reopen a new connection to the real server
each time nxagent regenerates. The sample server implementa-
tion regenerates all objects in the server when the last client
of this server terminates. When this happens, nxagent by de-
fault maintains the same top-level window and the same real
server connection in each new generation. If the user selects
full regeneration, even the top-level window and the connection
to the real server will be regenerated for each server genera-
tion.
-class string
This option specifies the default visual class of the nested
server. It is similar to the -cc option from the set of stan-
dard options except that it will accept a string rather than a
number for the visual class specification. The string must be
one of the following six values: StaticGray, GrayScale, Static-
Color, PseudoColor, TrueColor, or DirectColor. If both the
-class and -cc options are specified, the last instance of ei-
ther option takes precedence. The class of the default visual
of the nested server need not be the same as the class of the
default visual of the real server, but it must be supported by
the real server. Use xdpyinfo(__appmansuffix__) to obtain a
list of supported visual classes on the real server before
starting nxagent. If the user chooses a static class, all the
colors in the default color map will be preallocated. If the
user chooses a dynamic class, colors in the default color map
will be available to individual clients for allocation.
-deferglyphs whichfonts
specifies the types of fonts for which the server should at-
tempt to use deferred glyph loading. whichfonts can be all
(all fonts), none (no fonts), or 16 (16 bit fonts only).
-depth int
This option specifies the default visual depth of the nested
server. The depth of the default visual of the nested server
need not be the same as the depth of the default visual of the
real server, but it must be supported by the real server. Use
xdpyinfo(__appmansuffix__) to obtain a list of supported visual
depths on the real server before starting nxagent.
-geometry WxH+X+Y
This option specifies the geometry parameters for the top-level
nxagent window. See “GEOMETRY SPECIFICATIONS” in X(__miscman-
suffix__) for a discusson of this option's syntax. This window
corresponds to the root window of the nested server. The width
W and height H specified with this option will be the maximum
width and height of each top-level nxagent window. nxagent
will allow the user to make any top-level window smaller, but
it will not actually change the size of the nested server root
window. If this option is not specified, nxagent will choose W
and H to be 3/4ths the dimensions of the root window of the
real server. For further values accepted see the documentation
of geometry=<string> below.
-dpi resolution
sets the resolution for all screens, in dots per inch. If this
option is not specified nxagent will assume 96. There's also
-autodpi which will clone the real server's dpi. Note that the
resolution specified via -dpi is a per session setting. It can-
not be changed on reconnect! This means that clients may look
"wrong" when reconnecting a session that had been started with
a different dpi than the current real xserver.
dpms enables DPMS (display power management services), where sup-
ported. The default state is platform and configuration spe-
cific.
-dpms disables DPMS (display power management services). The default
state is platform and configuration specific.
-f volume
sets feep (bell) volume (allowable range: 0-100).
-fc cursorFont
sets default cursor font.
-fn font
sets the default font.
-fp fontPath
sets the search path for fonts. This path is a comma separated
list of directories which the X server searches for font data-
bases. See the FONTS section of this manual page for more in-
formation and the default list.
-help prints a usage message.
-I causes all remaining command line arguments to be ignored.
-maxbigreqsize size
sets the maximum big request to size MB.
-name string
This option specifies the name of the top-level nxagent window
as string. The default value is the program name.
-nolisten trans-type
disables a transport type. For example, TCP/IP connections can
be disabled with -nolisten tcp. This option may be issued mul-
tiple times to disable listening to different transport types.
-noreset
prevents a server reset when the last client connection is
closed. This overrides a previous -terminate command line op-
tion.
-p minutes
sets screen-saver pattern cycle time in minutes.
-pn permits the server to continue running if it fails to establish
all of its well-known sockets (connection points for clients),
but establishes at least one. This option is set by default.
-nopn causes the server to exit if it fails to establish all of its
well-known sockets (connection points for clients).
-r turns off auto-repeat.
r turns on auto-repeat.
-s minutes
sets screen-saver timeout time in minutes.
-su disables save under support on all screens.
-t number
sets pointer acceleration threshold in pixels (i.e. after how
many pixels pointer acceleration should take effect).
-terminate
causes the server to terminate at server reset, instead of con-
tinuing to run. This overrides a previous -noreset command
line option.
-to seconds
sets default connection timeout in seconds.
-tst disables all testing extensions.
v sets video-off screen-saver preference.
-v sets video-on screen-saver preference.
-wm forces the default backing-store of all windows to be When-
Mapped. This is a backdoor way of getting backing-store to ap-
ply to all windows. Although all mapped windows will have
backing store, the backing store attribute value reported by
the server for a window will be the last value established by a
client. If it has never been set by a client, the server will
report the default value, NotUseful. This behavior is required
by the X protocol, which allows the server to exceed the
client's backing store expectations but does not provide a way
to tell the client that it is doing so.
[+-]xinerama
enables(+) or disables(-) XINERAMA provided via the PanoramiX
extension. This is set to off by default.
[+-]rrxinerama
enables(+) or disables(-) XINERAMA provided via the RandR ex-
tension. By default, this feature is enabled. To disable XIN-
ERAMA completely, make sure to use both options (-xinerama and
-rrxinerama) on the command line.
SERVER DEPENDENT OPTIONS
nxagent additionally accepts the following non-standard options:
-logo turns on the X Window System logo display in the screen-saver.
There is currently no way to change this from a client.
nologo turns off the X Window System logo display in the screen-saver.
There is currently no way to change this from a client.
-render
default|mono|gray|color
sets the color allocation policy that will be used by the ren-
der extension.
default selects the default policy defined for the display
depth of the X server.
mono don't use any color cell.
gray use a gray map of 13 color cells for the X render ex-
tension.
color use a color cube of at most 4*4*4 colors (that is 64
color cells).
-dumbSched
disables smart scheduling on platforms that support the smart
scheduler.
-schedInterval interval
sets the smart scheduler's scheduling interval to interval mil-
liseconds.
NXAGENT SPECIFIC OPTIONS
The nx-X11 system adds the following command line arguments:
-forcenx
force use of NX protocol messages assuming communication
through nxproxy
-id string
The session id.
-autograb
enable autograb mode on nxagent startup. The autograb feature
can be toggled via nxagent keystrokes
-nxrealwindowprop
set property NX_REAL_WINDOW for each X11 client inside nxagent,
providing the window XID of the corresponding window object on
the X server that nxagent runs on
-reportwids
explicitly tell nxagent to report its externally exposed X11
window IDs to the session log (in machine readable form), so
that external parsers can obtain that information from there
-reportprivatewids
explicitly tell nxagent to report X11 window IDs of internally
created window objects to the session log (in machine readable
form), so that external parsers can obtain that information
from there; this creates a lot of output and may affect perfor-
mance
-timeout int
auto-disconnect timeout in seconds (minimum allowed: 60). De-
fault is 0 (no timeout).
-norootlessexit
don't exit if there are no clients in rootless mode
-autodpi
detect real server's DPI and set it in the agent session; the
-dpi cmdline option overrides -autodpi. Note that using -au-
todpi will also adapt the DPI on reconnect which will cause
newly started clients respecting the new DPI while clients that
had been started before the reconnect still use the old DPI.
This may lead to applications looking "weird".
-nomagicpixel
disable magic pixel support at session startup, can be re-en-
abled via nx/nx option on session resumption
-norender
disable the use of the render extension
-nocomposite
disable the use of the composite extension
-nopersistent
disable disconnection/reconnection to the X display on SIGHUP.
Non-persistent sessions will terminate on SIGHUP.
-noshmem
disable use of shared memory extension
-shmem enable use of shared memory extension (default)
-noshpix
disable use of shared pixmaps
-shpix enable use of shared pixmaps (default)
-noignore
don't ignore pointer and keyboard configuration changes man-
dated by clients. As a result, configuration commands like dis-
abling the keyboard bell (xset -b) will also affect the real X
server.
-nokbreset
don't reset keyboard device if the session is resumed
-noxkblock
this is only relevant if you also specify -keyboard=query. In
that case nxagent will lock the keyboard settings and clients
will get an error when trying to change keyboard settings via
XKEYBOARD. With -noxkblock the lock is not applied and clients
are allowed to change the keyboard settings through XKEYBOARD.
-tile WxH
maximum size of the tile used when sending an image to the re-
mote display (minimum allowed: 32x32). The default depends on
the link type: 64x64 for modem and isdn, 4096x4096 for all
other link types)
-irlimit
maximum image data rate to the encoder input in kB/s. The de-
fault is no limit.
-D enable desktop mode (default)
-R enable rootless mode
-S enable shadow mode
-B enable proxy binding mode
-keystrokefile
define path to a keyboard shortcut definitions file. Default is
~/.nx/keystrokes.cfg and /etc/nxagent/keystroke.cfg (first ex-
isting file is taken). If nxagent is run as x2goagent the de-
faults are ~/.x2go/keystrokes.cfg and /etc/x2go/keystrokes.cfg
Any keystrokes nxagent knows about that are not defined in this
file are ignored. (Only) if no file is found built-in defaults
are used. The keystroke file can be re-read by a keystroke
(ctrl-alt-k by default). See README.keystrokes and README.key-
strokes.debug for all keystrokes nxagent knows. At startup the
active keystrokes are printed to the session output.
-version
show version information and exit
-options filepath|string
path to an options file containing nx/nx options (see below).
Instead of a path the options can be specified diretly on the
commandline by prefixing the options strings with nx/nx, which
is mostly useful for testing/debugging.
In addition to the command line options, nxagent can be configured at
session startup and at runtime (i.e. when resuming a suspended session)
by so-called nx/nx options. The options file is read on startup. It can
be modified during runtime (but it must stay at the same path). On re-
connect the modified file is then read and the changed options are ap-
plied.
As nx/nx options all options supported by nxcomp (see nxproxy man page)
and all nxagent nx/nx options (see below) can be used. When launching
an nxcomp based nxagent session (i.e. proxy <-> agent), you will nor-
mally set the $DISPLAY variable like this:
$ export DISPLAY=nx/nx,listen=<proxy-port>,options=<options.file>:<nx-display-port>
$ nxagent <command-line-options> :<nx-display-port>
The value for <nx-display-port> is some value of a not-yet-used X11
display (e.g. :50).
Using an options file is recommended, but you can also put available
nx/nx options (see below) into the DISPLAY variable directly. Note,
that the $DISPLAY variable field is of limited length.
As <proxy-port> you can pick an arbitrary (unused) TCP port or Unix
socket file path. This is the port / socket that you have to connect to
with the nxproxy application.
The right hand side of an option (the part following the "=" character)
can include URL encoded characters. It is required to URL encode at
least "," (as %2D) and "=" (as %3D) to avoid wrong parsing of the op-
tions string.
Available nxagent options (as an addition to nx/nx options supported by
nxcomp already):
options=<string>
read options from file, this text file can contain a single
loooong line with comma-separated nx/nx options
rootless=<bool>
start nxagent in rootless mode, matches -R given on the command
line, no-op when resuming (default: 0, disabled)
geometry=<string>
desktop geometry when starting or resuming a session, no-op in
rootless mode (default 66% of the underlying X server geome-
try). You can either specify a standard X geometry string
(WxH+X+Y) or allscreens for a window covering all available
screens or onescreen for a window covering only one screen. For
historical reasons fullscreen (as a synonym to allscreens) is
also accepted.
fullscreen=<int>
start or resume a session in fullscreen mode (default: 0, off).
Specify 1 for a fullscreen window covering all available
screens or 2 for a fullscreen window covering only the first
screen.
resize=<bool>
set resizing support (default: 1, enabled)
keyboard=<string> or kbtype=<string>
query|clone|<model>/<layout>|rmlvo/<rules>#<model>#<lay-
out>#<variant>#<options>
query use the default XKB keyboard layout (see below) and
only allow clients to query the settings but prevent
any changes. query is especially helpful for setups
where you need to set/modify the actual keyboard layout
using core X protocol functions (e.g. via xmodmap). It
is used for MacOS X clients to handle some keyboard
problems that are special for this platform. Note that
in this case XKEYBOARD will always report the default
layout which will most likely not match the experienced
settings.
clone ask the real X server for the keyboard settings using
XKEYBOARD protocol functions and clone them. This is
the recommended setting. For compatibility reasons it
is not the default.
<model>/<layout>
use the given model and layout. A value of null/null is
equivalent to clone. You can not modify keyboard rules,
variant or options this way. Instead preset values are
used. These are base for rules and empty strings for
variant and options.
rmlvo/<rules>#<model>#<layout>#<variant>#<options>
configure the keyboard according to the rmlvo
(Rules+Model+Layout+Variant+Options) description given
after the / and separated by #. This can be used to
fully pass the keyboard configuration of nxagent right
after the start. Example: rm-
lvo/base#pc105#de,us#nodeadkeys#lv3:rwin_switch
If keyboard is omitted the internal defaults of nxagent will be
used (rules: base, layout: us, model: pc102, empty variant and
options).
keyconv=<string>
set keycode conversion mode
auto|on|off
by default (auto) nxagent will activate keycode conversion if
it detects an evdev XKEYBOARD setup on the nxproxy side (the
standard on Linux systems nowadays). Keycode conversion means
that certain keycodes are mapped to make the keyboard appear as
an pc105 model. Using off this conversion can be suppressed and
with on it will be forced.
clipboard=<string>
both|client|server|none
both Allow clipboard data exchange both from nxagent to real
X server and vice-versa. This is the default.
client Limit clipboard data exchange to work only in one di-
rection: from real X server to nxagent. Clipboard will
still work inside nxagent. This setting effectively
prevents data leakage from the nxagent session to the
outside.
server Limit clipboard data exchange to work only in one di-
rection: from nxagent to real X server.
none Disable any clipboard data exchange. Clipboard will
still work inside the nxagent and on the real X server,
but no data exchange will be possible.
streaming=<bool>
enable (set to 1) or disable (set to 0) streaming support for
images, not fully implemented yet and thus non-functional. (de-
fault: disabled)
backingstore=<bool>
disable (set to 0) or enforce (set to 1) backing store support
(default: enforced). In rootless mode backingstore is always
disabled.
composite=<bool>
enable (set to 1) or disable (set to 0) Composite support in
nxagent (default: enabled)
xinerama=<bool>
enable (set to 1) or disable (set to 0) XINERAMA support in nx-
agent (default: enabled)
shmem=<bool>
enable/disable using shared memory. Accepted values: 1 (enable,
default), 0 (disable)
shpix=<bool>
enable/disable shared pixmaps support. Accepted values: 1 (en-
able, default), 0 (disable)
client=<string>
type of connecting operating system (supported: linux, windows,
solaris and macosx)
clients=<string>
filename where to log output of the nxagent's clients. This is
ignored if no session id has been provided. It then points to
stderr. Default: <sessiondir>/clients.
shadow=<string>
define the display that should be shadowed
shadowuid=<int>
unique identifier for the shadow session
shadowmode=<bool>
full access (set to 1) or viewing-only (set to 0, default)
state=<string>
filename where to store the state of the nxagent (for easier
interoperation with software like x2go. Default: ses-
siondir/state.
defer=<int>
defer image updates (enabled for all connection types except
LAN), accepts values 0, 1 and 2
The default value can be set via the command line (-defer). The
value provided as nx/nx option is set when resuming a session,
thus it overrides the command line default.
The default depends on the link type (see man nxproxy).
Each defer level adds the following rules to the previous ones:
0 Eager encoding.
Default for link speed lan and local.
1 No data is put or copied on pixmaps, marking them al-
ways as corrupted and synchronizing them on demand,
i.e. when a copy area to a window is requested, the
source is synchronized before copying it.
Default for link speed wan.
2 The put images over the windows are skipped marking the
destination as corrupted. The same happens for copy
area and composite operations, spreading the corrupted
regions of involved drawables.
Default for link speed adsl, isdn and modem.
tile=<string>
set the maximum tile size in pixels (<W>x<H>) for bitmap data
sent over the wire
The default value can be set via the command line (-tile). The
value provided as nx/nx option is set when resuming a session,
thus it overrides the command line default.
menu=<bool>
support pulldown menu in nxagent session (only available on
proxy <-> agent remote sessions) (default: 1, enabled)
magicpixel=<bool>
enable/disable magic pixel support in fullscreen mode (default:
1, enabled)
copysize=<int>
Maximum number of bytes that can be pasted from an NX session
into an external application. Default is unlimited.
autodpi=<bool>
enable/disable deriving session DPI automatically from real
server (default: 0, disabled); only takes effect on session
startups, gets ignored when reconnecting to a suspended session
sleep=<int>
delay X server operations when suspended (provided in millisec-
onds), set to 0 to keep nxagent session fully functional when
suspended (e.g. useful when mirroring an nxagent session via
VNC). Graphic intensive applications will be affected by this
more than others. The default is 50ms.
tolerancechecks=<string>
strict|safe|risky|bypass
strict means that the number of internal and external pixmap
formats must match exactly and every internal pixmap
format must be available in the external pixmap format
array. This is the default.
safe means that the number of pixmap formats might diverge,
but all internal pixmap formats must also be included
in the external pixmap formats array. This is recom-
mended, because it allows clients with more pixmap for-
mats to still connect, but not lose functionality.
risky means that the internal pixmap formats array is allowed
to be smaller than the external pixmap formats array,
but at least one pixmap format must be included in
both. This is potentially unsafe.
bypass means that all of these checks are essentially deacti-
vated. This is a very bad idea.
autograb=<bool>
enable or disable autograb (default: 0, disabled). Can be tog-
gled during session via keystroke.
If you want to use nxagent as a replacement for Xnest or Xephyr you can
pass options like this:
$ echo nx/nx,fullscreen=1$DISPLAY >/tmp/opt
$ nxagent <command-line-options> -options /tmp/opt :<nx-display-port>
XDMCP OPTIONS
X servers that support XDMCP have the following options. See the X
Display Manager Control Protocol specification for more information.
-query hostname
enables XDMCP and sends Query packets to the specified host-
name.
-broadcast
enable XDMCP and broadcasts BroadcastQuery packets to the net-
work. The first responding display manager will be chosen for
the session.
-multicast [address [hop count]]
Enable XDMCP and multicast BroadcastQuery packets to the net-
work. The first responding display manager is chosen for the
session. If an address is specified, the multicast is sent to
that address. If no address is specified, the multicast is
sent to the default XDMCP IPv6 multicast group. If a hop count
is specified, it is used as the maximum hop count for the mul-
ticast. If no hop count is specified, the multicast is set to
a maximum of 1 hop, to prevent the multicast from being routed
beyond the local network.
-indirect hostname
enables XDMCP and send IndirectQuery packets to the specified
hostname.
-port port-number
uses the specified port-number for XDMCP packets, instead of
the default. This option must be specified before any -query,
-broadcast, -multicast, or -indirect options.
-from local-address
specifies the local address to connect from (useful if the con-
necting host has multiple network interfaces). The local-ad-
dress may be expressed in any form acceptable to the host plat-
form's gethostbyname(3) implementation.
-once causes the server to terminate (rather than reset) when the
XDMCP session ends.
-class display-class
XDMCP has an additional display qualifier used in resource
lookup for display-specific options. This option sets that
value, by default it is "MIT-Unspecified" (not a very useful
value).
-cookie xdm-auth-bits
When testing XDM-AUTHENTICATION-1, a private key is shared be-
tween the server and the manager. This option sets the value
of that private data (not that it is very private, being on the
command line!).
-displayID display-id
Yet another XDMCP specific value, this one allows the display
manager to identify each display so that it can locate the
shared key.
XKEYBOARD OPTIONS
X servers that support the XKEYBOARD (a.k.a. "XKB") extension accept
the following options. All layout files specified on the command line
must be located in the XKB base directory or a subdirectory, and speci-
fied as the relative path from the XKB base directory. The default XKB
base directory is /usr/share/X11/xkb.
[+-]kb enables(+) or disables(-) the XKEYBOARD extension.
[+-]accessx [ timeout [ timeout_mask [ feedback [ options_mask ] ] ] ]
enables(+) or disables(-) AccessX key sequences.
-xkbdir directory
base directory for keyboard layout files. This option is not
available for setuid X servers (i.e., when the X server's real
and effective uids are different).
-ardelay milliseconds
sets the autorepeat delay (length of time in milliseconds that
a key must be depressed before autorepeat starts).
-arinterval milliseconds
sets the autorepeat interval (length of time in milliseconds
that should elapse between autorepeat-generated keystrokes).
-xkbmap filename
loads keyboard description in filename on server startup.
SECURITY EXTENSION OPTIONS
X servers that support the SECURITY extension accept the following op-
tion:
-sp filename
causes the server to attempt to read and interpret filename as
a security policy file with the format described below. The
file is read at server startup and reread at each server reset.
The syntax of the security policy file is as follows. Notation: "*"
means zero or more occurrences of the preceding element, and "+" means
one or more occurrences. To interpret <foo/bar>, ignore the text after
the /; it is used to distinguish between instances of <foo> in the next
section.
<policy file> ::= <version line> <other line>*
<version line> ::= <string/v> '\n'
<other line > ::= <comment> | <access rule> | <site policy> | <blank line>
<comment> ::= # <not newline>* '\n'
<blank line> ::= <space> '\n'
<site policy> ::= sitepolicy <string/sp> '\n'
<access rule> ::= property <property/ar> <window> <perms> '\n'
<property> ::= <string>
<window> ::= any | root | <required property>
<required property> ::= <property/rp> | <property with value>
<property with value> ::= <property/rpv> = <string/rv>
<perms> ::= [ <operation> | <action> | <space> ]*
<operation> ::= r | w | d
<action> ::= a | i | e
<string> ::= <dbl quoted string> | <single quoted string> | <unquoted string>
<dbl quoted string> ::= <space> " <not dqoute>* " <space>
<single quoted string> ::= <space> ' <not squote>* ' <space>
<unquoted string> ::= <space> <not space>+ <space>
<space> ::= [ ' ' | '\t' ]*
Character sets:
<not newline> ::= any character except '\n'
<not dqoute> ::= any character except "
<not squote> ::= any character except '
<not space> ::= any character except those in <space>
The semantics associated with the above syntax are as follows.
<version line>, the first line in the file, specifies the file format
version. If the server does not recognize the version <string/v>, it
ignores the rest of the file. The version string for the file format
described here is "version-1" .
Once past the <version line>, lines that do not match the above syntax
are ignored.
<comment> lines are ignored.
<sitepolicy> lines are currently ignored. They are intended to specify
the site policies used by the XC-QUERY-SECURITY-1 authorization method.
<access rule> lines specify how the server should react to untrusted
client requests that affect the X Window property named <property/ar>.
The rest of this section describes the interpretation of an <access
rule>.
For an <access rule> to apply to a given instance of <property/ar>,
<property/ar> must be on a window that is in the set of windows speci-
fied by <window>. If <window> is any, the rule applies to <prop-
erty/ar> on any window. If <window> is root, the rule applies to
<property/ar> only on root windows.
If <window> is <required property>, the following apply. If <required
property> is a <property/rp>, the rule applies when the window also has
that <property/rp>, regardless of its value. If <required property> is
a <property with value>, <property/rpv> must also have the value speci-
fied by <string/rv>. In this case, the property must have type STRING
and format 8, and should contain one or more null-terminated strings.
If any of the strings match <string/rv>, the rule applies.
The definition of string matching is simple case-sensitive string com-
parison with one elaboration: the occurrence of the character '*' in
<string/rv> is a wildcard meaning "any string." A <string/rv> can con-
tain multiple wildcards anywhere in the string. For example, "x*"
matches strings that begin with x, "*x" matches strings that end with
x, "*x*" matches strings containing x, and "x*y*" matches strings that
start with x and subsequently contain y.
There may be multiple <access rule> lines for a given <property/ar>.
The rules are tested in the order that they appear in the file. The
first rule that applies is used.
<perms> specify operations that untrusted clients may attempt, and the
actions that the server should take in response to those operations.
<operation> can be r (read), w (write), or d (delete). The following
table shows how X Protocol property requests map to these operations in
The Open Group server implementation.
GetProperty r, or r and d if delete = True
ChangeProperty w
RotateProperties r and w
DeleteProperty d
ListProperties none, untrusted clients can always list all properties
<action> can be a (allow), i (ignore), or e (error). Allow means exe-
cute the request as if it had been issued by a trusted client. Ignore
means treat the request as a no-op. In the case of GetProperty, ignore
means return an empty property value if the property exists, regardless
of its actual value. Error means do not execute the request and return
a BadAtom error with the atom set to the property name. Error is the
default action for all properties, including those not listed in the
security policy file.
An <action> applies to all <operation>s that follow it, until the next
<action> is encountered. Thus, irwad means ignore read and write, al-
low delete.
GetProperty and RotateProperties may do multiple operations (r and d,
or r and w). If different actions apply to the operations, the most
severe action is applied to the whole request; there is no partial re-
quest execution. The severity ordering is: allow < ignore < error.
Thus, if the <perms> for a property are ired (ignore read, error
delete), and an untrusted client attempts GetProperty on that property
with delete = True, an error is returned, but the property value is
not. Similarly, if any of the properties in a RotateProperties do not
allow both read and write, an error is returned without changing any
property values.
Here is an example security policy file.
version-1
# Allow reading of application resources, but not writing.
property RESOURCE_MANAGER root ar iw
property SCREEN_RESOURCES root ar iw
# Ignore attempts to use cut buffers. Giving errors causes apps to crash,
# and allowing access may give away too much information.
property CUT_BUFFER0 root irw
property CUT_BUFFER1 root irw
property CUT_BUFFER2 root irw
property CUT_BUFFER3 root irw
property CUT_BUFFER4 root irw
property CUT_BUFFER5 root irw
property CUT_BUFFER6 root irw
property CUT_BUFFER7 root irw
# If you are using Motif, you probably want these.
property _MOTIF_DEFAULT_BINDINGS rootar iw
property _MOTIF_DRAG_WINDOW root ar iw
property _MOTIF_DRAG_TARGETS any ar iw
property _MOTIF_DRAG_ATOMS any ar iw
property _MOTIF_DRAG_ATOM_PAIRS anyar iw
# The next two rules let xwininfo -tree work when untrusted.
property WM_NAME any ar
# Allow read of WM_CLASS, but only for windows with WM_NAME.
# This might be more restrictive than necessary, but demonstrates
# the <required property> facility, and is also an attempt to
# say "top level windows only."
property WM_CLASS WM_NAME ar
# These next three let xlsclients work untrusted. Think carefully
# before including these; giving away the client machine name and command
# may be exposing too much.
property WM_STATE WM_NAME ar
property WM_CLIENT_MACHINE WM_NAME ar
property WM_COMMAND WM_NAME ar
# To let untrusted clients use the standard colormaps created by
# xstdcmap, include these lines.
property RGB_DEFAULT_MAP root ar
property RGB_BEST_MAP root ar
property RGB_RED_MAP root ar
property RGB_GREEN_MAP root ar
property RGB_BLUE_MAP root ar
property RGB_GRAY_MAP root ar
# To let untrusted clients use the color management database created
# by xcmsdb, include these lines.
property XDCCC_LINEAR_RGB_CORRECTION rootar
property XDCCC_LINEAR_RGB_MATRICES rootar
property XDCCC_GRAY_SCREENWHITEPOINT rootar
property XDCCC_GRAY_CORRECTION rootar
# To let untrusted clients use the overlay visuals that many vendors
# support, include this line.
property SERVER_OVERLAY_VISUALS rootar
# Dumb examples to show other capabilities.
# oddball property names and explicit specification of error conditions
property "property with spaces" 'property with "'aw er ed
# Allow deletion of Woo-Hoo if window also has property OhBoy with value
# ending in "son". Reads and writes will cause an error.
property Woo-Hoo OhBoy = "*son"ad
NETWORK CONNECTIONS
The X server supports client connections via a platform-dependent sub-
set of the following transport types: TCPIP, Unix Domain sockets and
several varieties of SVR4 local connections. See the DISPLAY NAMES
section of the X(__miscmansuffix__) manual page to learn how to specify
which transport type clients should try to use.
GRANTING ACCESS
The X server implements a platform-dependent subset of the following
authorization protocols: MIT-MAGIC-COOKIE-1, XDM-AUTHORIZATION-1, XDM-
AUTHORIZATION-2, SUN-DES-1, and MIT-KERBEROS-5. See the Xsecu-
rity(__miscmansuffix__) manual page for information on the operation of
these protocols.
Authorization data required by the above protocols is passed to the
server in a private file named with the -auth command line option.
Each time the server is about to accept the first connection after a
reset (or when the server is starting), it reads this file. If this
file contains any authorization records, the local host is not automat-
ically allowed access to the server, and only clients which send one of
the authorization records contained in the file in the connection setup
information will be allowed access. See the Xau manual page for a de-
scription of the binary format of this file. See xauth(1) for mainte-
nance of this file, and distribution of its contents to remote hosts.
The X server also uses a host-based access control list for deciding
whether or not to accept connections from clients on a particular ma-
chine. If no other authorization mechanism is being used, this list
initially consists of the host on which the server is running as well
as any machines listed in the file /etc/Xn.hosts, where n is the dis-
play number of the server. Each line of the file should contain either
an Internet hostname (e.g. expo.lcs.mit.edu) or a complete name in the
format family:name as described in the xhost(1) manual page. There
should be no leading or trailing spaces on any lines. For example:
joesworkstation
corporate.company.com
star::
inet:bigcpu
local:
Users can add or remove hosts from this list and enable or disable ac-
cess control using the xhost command from the same machine as the
server.
If the X FireWall Proxy (xfwp) is being used without a sitepolicy,
host-based authorization must be turned on for clients to be able to
connect to the X server via the xfwp. If xfwp is run without a config-
uration file and thus no sitepolicy is defined, if xfwp is using an X
server where xhost + has been run to turn off host-based authorization
checks, when a client tries to connect to this X server via xfwp, the X
server will deny the connection. See xfwp(1) for more information
about this proxy.
The X protocol intrinsically does not have any notion of window opera-
tion permissions or place any restrictions on what a client can do; if
a program can connect to a display, it has full run of the screen. X
servers that support the SECURITY extension fare better because clients
can be designated untrusted via the authorization they use to connect;
see the xauth(1) manual page for details. Restrictions are imposed on
untrusted clients that curtail the mischief they can do. See the SECU-
RITY extension specification for a complete list of these restrictions.
Sites that have better authentication and authorization systems might
wish to make use of the hooks in the libraries and the server to pro-
vide additional security models.
SIGNALS
The X server attaches special meaning to the following signals:
SIGHUP This signal causes the server to close all existing connec-
tions, free all resources, and restore all defaults. It is
sent by the display manager whenever the main user's main ap-
plication (usually an xterm or window manager) exits to force
the server to clean up and prepare for the next user.
SIGTERM This signal causes the server to exit cleanly.
SIGUSR1 This signal is used quite differently from either of the above.
When the server starts, it checks to see if it has inherited
SIGUSR1 as SIG_IGN instead of the usual SIG_DFL. In this case,
the server sends a SIGUSR1 to its parent process after it has
set up the various connection schemes. Xdm uses this feature
to recognize when connecting to the server is possible.
FONTS
The X server can obtain fonts from directories and/or from font
servers. The list of directories and font servers the X server uses
when trying to open a font is controlled by the font path.
The default font path is __default_font_path__ .
The font path can be set with the -fp option or by xset(1) after the
server has started.
FILES
/etc/Xn.hosts Initial access control list for display
number n
/usr/share/fonts/X11/misc,
/usr/share/fonts/X11/75dpi,
/usr/share/fonts/X11/100dpi Bitmap
font directories
/usr/share/fonts/X11/Type1 Outline font directories
/usr/share/nx/rgb Color database
/tmp/.X11-unix/Xn Unix domain socket for display number n
/tmp/rcXn Kerberos 5 replay cache for display num-
ber n
SEE ALSO
Protocols: X Window System Protocol, NX Compression Protocol, The X
Font Service Protocol, X Display Manager Control Protocol
Fonts: bdftopcf(1), mkfontdir(1), mkfontscale(1), xfs(1), xlsfonts(1),
xfontsel(1), xfd(1), X Logical Font Description Conventions
Security: Xsecurity(__miscmansuffix__), xauth(1), Xau(1), xdm(1),
xhost(1), xfwp(1), Security Extension Specification
Starting the server: xdm(1), xinit(1)
Controlling the server once started: xset(1), xsetroot(1), xhost(1)
Server-specific man pages: Xdec(1), XmacII(1), Xsun(1), Xnest(1),
Xvfb(1), XFree86(1), XDarwin(1).
Server internal documentation: Definition of the Porting Layer for the
X v11 Sample Server
AUTHORS
The first sample X server was originally written by Susan Angebranndt,
Raymond Drewry, Philip Karlton, and Todd Newman, from Digital Equipment
Corporation, with support from a large cast. It has since been exten-
sively rewritten by Keith Packard and Bob Scheifler, from MIT. Dave
Wiggins took over post-R5 and made substantial improvements.
The first implementation of nx-X11 (version 1.x up to 3.5.x) was writ-
ten by NoMachine (maintained until 2011).
The current implementation of nx-X11 is maintained by various projects,
amongst others The Arctica Project, TheQVD (Qindel Group) and X2Go.
This manual page was written by Per Hansen <spamhans@yahoo.de>, and
modified by Marcelo Boveto Shima <marceloshima@gmail.com> and Mike
Gabriel <mike.gabriel@das-netzwerkteam.de>. In 2016, the original
Xserver.man page shipped with nx-X11 was merged into the nxagent man
page and received a major update by Mike Gabriel <mike.gabriel@das-net-
zwerkteam.de>.
Version 3.5.99.26 Feb 2021 nxagent(1)
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