Provided by: nxagent_3.5.99.23-1build1_amd64 
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
application 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 local 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 reported 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, including generation and revocation of
authorizations and violations 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
compatibility. 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 descriptor as a newline-terminated string.
The -pn option is ignored 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 graphics operations with the real server.
This is a useful option for debugging, but it will slow down nxagent's performance considerably.
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 implementation
regenerates all objects in the server when the last client of this server terminates. When this
happens, nxagent by default 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 generation.
-class string
This option specifies the default visual class of the nested server. It is similar to the -cc
option from the set of standard 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, StaticColor, PseudoColor, TrueColor, or DirectColor. If both the -class
and -cc options are specified, the last instance of either 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 attempt 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(__miscmansuffix__) 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.
-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 cannot 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 supported. The default state is platform
and configuration specific.
-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 databases. See the FONTS section of this manual page for more
information 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 multiple 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 option.
-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 continuing 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 WhenMapped. This is a backdoor way of
getting backing-store to apply 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 extension. By default, this feature is
enabled. To disable XINERAMA 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 render 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 extension.
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 milliseconds.
NXAGENT SPECIFIC OPTIONS
The nx-X11 system adds the following command line arguments:
-forcenx
force use of NX protocol messages assuming communication through nxproxy
-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 performance
-timeout int
auto-disconnect timeout in seconds (minimum allowed: 60)
-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 -autodpi 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-enabled 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
-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 mandated by clients. As a result,
configuration commands like disabling 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
size of image tiles (minimum allowed: 32x32)
-D enable desktop mode (default)
-R enable rootless mode
-S enable shadow mode
-B enable proxy binding mode
-version
show version information and exit
-options filename
path to an options file containing nx/nx options (see below).
Other than 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.
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
normally 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 options 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 geometry)
resize=<bool>
set resizing support (default: 1, enabled)
fullscreen=<bool>
start or resume a session in fullscreen mode (default: 0, off)
keyboard=<string> or kbtype=<string>
query|clone|<model>/<layout>|rmlvo/<rules>#<model>#<layout>#<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:
rmlvo/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 client 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 direction: 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 direction: 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. (default: 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 nxagent (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 (enable, default), 0 (disable)
client=<string>
type of connecting operating system (supported: linux, windows, solaris and macosx)
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)
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.
Each defer level adds the following rules to the previous ones:
0 Eager encoding.
1 No data is put or copied on pixmaps, marking them always as corrupted and synchronizing
them on demand, i.e. when a copy area to a window is requested, the source is syn-
chronized before copying it.
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.
tile=<string>
set the 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)
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 msec), set to 0 to keep nxagent session
fully functional when suspended (e.g. useful when mirroring an nxagent session via VNC)
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 recommended, because
it allows clients with more pixmap formats 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 deactivated. This is a very bad idea.
autograb=<bool>
enable or disable autograb (default: 0, disabled)
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 hostname.
-broadcast
enable XDMCP and broadcasts BroadcastQuery packets to the network. The first responding display
manager will be chosen for the session.
-multicast [address [hop count]]
Enable XDMCP and multicast BroadcastQuery packets to the network. 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 multicast. 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 connecting host has multiple network
interfaces). The local-address may be expressed in any form acceptable to the host platform'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 between 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
specified 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 option:
-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 specified by <window>. If <window> is any, the rule applies to
<property/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 specified 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 comparison with one elaboration: the
occurrence of the character '*' in <string/rv> is a wildcard meaning "any string." A <string/rv> can
contain 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 execute 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, allow 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
request 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 subset 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
Xsecurity(__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 automatically 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 description of the binary format of this file. See
xauth(1) for maintenance 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 machine. 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 display 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 access 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
configuration 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 operation 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 SECURITY
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 provide additional security models.
SIGNALS
The X server attaches special meaning to the following signals:
SIGHUP This signal causes the server to close all existing connections, free all resources, and restore
all defaults. It is sent by the display manager whenever the main user's main application
(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 number 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
extensively 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 written 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-netzwerkteam.de>.