X display access control
X provides mechanism for implementing many access control systems.
Release 5 includes four mechanisms:
Simple host-based access control.
Shared plain-text ``cookies''
Secure DES based private-keys.
Based on Sun's secure rpc system.
Access system descriptions
Any client on a host in the host access control list is allowed access to
the X server. This system can work reasonably well in an environment
where everyone trusts everyone, or when only a single person can log in
to a given machine, and is easy to use when the list of hosts used is small.
This system does not work well when multiple people can log in to a single
machine and mutual trust does not exist.
The list of allowed hosts is stored in the X server and can be changed with
the xhost command.
When using the more secure mechanisms listed below, the host list is
normally configured to be the empty list, so that only authorized
programs can connect to the display.
When using MIT-MAGIC-COOKIE-1,
the client sends a 128 bit ``cookie''
along with the connection setup information.
If the cookie presented by the client matches one
that the X server has, the connection is allowed access.
The cookie is chosen so that it is hard to guess;
xdm generates such cookies automatically when
this form of access control is used.
The user's copy of
the cookie is usually stored in the .Xauthority file in the home
directory, although the environment variable XAUTHORITY can
be used to specify an alternate location.
xdm automatically passes a cookie to the server for each new
login session, and stores the cookie in the user file at login.
The cookie is transmitted on the network without encryption, so
there is nothing to prevent a network snooper from obtaining the data
and using it to gain access to the X server. This system is useful in an
environment where many users are running applications on the same machine
and want to avoid interference from each other, with the caveat that this
control is only as good as the access control to the physical network.
In environments where network-level snooping is difficult, this system
can work reasonably well.
For sites in the US, Release 5 contains a DES-based access control
mechanism called XDM-AUTHORIZATION-1.
It is similar in usage to MIT-MAGIC-COOKIE-1 in that a key is
stored in the .Xauthority file and is shared with the X server.
this key consists of two parts - a 56 bit DES encryption key and 64 bits of
random data used as the authenticator.
When connecting to the X server, the application generates 192 bits of data
by combining the current time in seconds (since 00:00 1/1/1970 GMT) along
with 48 bits of ``identifier''. For TCP/IP connections, the identifier is
the address plus port number; for local connections it is the process ID
and 32 bits to form a unique id (in case multiple connections to the same
server are made from a single process). This 192 bit packet is then
encrypted using the DES key and sent to the X server, which is able to
verify if the requester is authorized to connect by decrypting with the
same DES key and validating the authenticator and additional data.
This system is useful in many environments where host-based access control
is inappropriate and where network security cannot be ensured.
Recent versions of SunOS (and some other systems) have included a
secure public key remote procedure call system. This system is based
on the notion of a network principal; a user name and NIS domain pair.
Using this system, the X server can securely discover the actual user
name of the requesting process. It involves encrypting data with the
X servers public key, and so the identity of the user who started the
X server is needed for this; this identity is stored in the
By extending the semantics of ``host address'' to include this notion of
network principal, this form of access control is very easy to use.
To allow access by a new user, use xhost. For example,
xhost keith@ email@example.com
adds ``keith'' from the NIS domain of the local machine, and ``joe'' in
the ``mit.edu'' NIS domain. For keith or joe to successfully connect
to the display, they must add the principal who started the server to
their .Xauthority file. For example:
xauth add expo.lcs.mit.edu:0 SUN-DES-1 unix.expo.lcs.mit.edu:x.lcs.mit.edu
This system only works on machines which support Secure RPC, and only for
users which have set up the appropriate public/private key pairs on their
system. See the Secure RPC documentation for details.
The authorization file
Except for Host Access control, each of these systems uses data stored in
the .Xauthority file to generate the correct authorization information
to pass along to the X server at connection setup. MIT-MAGIC-COOKIE-1 and
XDM-AUTHORIZATION-1 store secret data in the file; so anyone who can read
the file can gain access to the X server. SUN-DES-1 stores only the
identity of the principal who started the server
(unix.hostname@domain when the server is started by xdm),
and so it is not useful to anyone not authorized to connect to the server.
Each entry in the .Xauthority file matches a certain connection family
(TCP/IP, DECnet or local connections) and X display name (hostname plus display
number). This allows multiple authorization entries for different displays
to share the same data file. A special connection family (FamilyWild, value
65535) causes an entry to match every display, allowing the entry to be used
for all connections. Each entry additionally contains the authorization
name and whatever private authorization data is needed by that authorization
type to generate the correct information at connection setup time.
The xauth program manipulates the .Xauthority file format.
It understands the semantics of the connection families and address formats,
displaying them in an easy to understand format. It also understands that
SUN-DES-1 uses string values for the authorization data, and displays
The X server (when running on a workstation) reads authorization
information from a file name passed on the command line with the
The authorization entries in
the file are used to control access to the server. In each of the
authorization schemes listed above, the data needed by the server to initialize
an authorization scheme is identical to the data needed by the client to
generate the appropriate authorization information, so the same file can be
used by both processes. This is especially useful when xinit is used.
This system uses 128 bits of data shared between the user and the X server.
Any collection of bits can be used. xdm generates these keys using a
cryptographically secure pseudo random number generator, and so the key to
the next session cannot be computed from the current session key.
This system uses two pieces of information. First, 64 bits of random data,
second a 56 bit DES encryption key (again, random data) stored
in 8 bytes, the last byte of which is ignored. xdm generates these keys
using the same random number generator as is used for MIT-MAGIC-COOKIE-1.
This system needs a string representation of the principal which identifies
the associated X server. When xdm starts the X server, it uses
the root principal for the machine on which it is running
(unix.hostname@domain, for example
``firstname.lastname@example.org''). Putting the correct principal
name in the .Xauthority file causes Xlib to generate the appropriate
authorization information using the secure RPC library.
© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003