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XSECURITY(7x)									    XSECURITY(7x)

       Xsecurity - X display access control

       X provides mechanism for implementing many access control systems.  The sample implementa-
       tion includes five mechanisms:
	   Host Access			 Simple host-based access control.
	   MIT-MAGIC-COOKIE-1		 Shared plain-text "cookies".
	   XDM-AUTHORIZATION-1		 Secure DES based private-keys.
	   SUN-DES-1			 Based on Sun's secure rpc system.
	   MIT-KERBEROS-5		 Kerberos Version 5 user-to-user.

       Host Access
	      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 XAUTHOR-
	      ITY 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.

	      Sites in the United States can use 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.  However, 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 com-
	      bining 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 num-
	      ber; 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 requestor 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 server's public key, and so the identity of the user who
	      started the X server is needed for this; this identity is stored in the .Xauthority
	      file.   By extending the semantics of "host address" to include this notion of net-
	      work principal, this form of access control is very easy to use.

	      To allow access by a new user, use xhost.  For example,
		  xhost keith@ ruth@mit.edu
	      adds "keith" from the NIS domain of the local machine, and "ruth" in the	"mit.edu"
	      NIS  domain.   For  keith or ruth 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@our.domain.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.  To access  the  display  from	a  remote
	      host, you may have to do a keylogin on the remote host first.

	      Kerberos	is  a  network-based  authentication  scheme developed by MIT for Project
	      Athena.  It allows mutually suspicious principals to  authenticate  each	other  as
	      long as each trusts a third party, Kerberos.  Each principal has a secret key known
	      only to it and Kerberos.	Principals includes servers, such as an FTP server  or	X
	      server,  and  human  users, whose key is their password.	Users gain access to ser-
	      vices by getting Kerberos tickets for those services from a Kerberos server.  Since
	      the  X  server has no place to store a secret key, it shares keys with the user who
	      logs in.	X authentication thus uses the user-to-user scheme of Kerberos version 5.

	      When you log in via xdm, xdm will use your password to obtain the initial  Kerberos
	      tickets.	 xdm stores the tickets in a credentials cache file and sets the environ-
	      ment variable KRB5CCNAME to point to the file.  The credentials cache is	destroyed
	      when  the session ends to reduce the chance of the tickets being stolen before they

	      Since Kerberos is a user-based authorization protocol, like the SUN-DES-1 protocol,
	      the  owner  of a display can enable and disable specific users, or Kerberos princi-
	      pals.  The xhost client is used to enable or disable authorization.  For example,
		  xhost krb5:judy krb5:gildea@x.org
	      adds "judy" from the Kerberos realm of the local machine,  and  "gildea"	from  the
	      "x.org" realm.

       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  multi-
       ple  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, allow-
       ing the entry to be used for all connections.  Each entry additionally contains the autho-
       rization 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 and MIT-KERBEROS-5 use string values for the
       authorization data, and displays them appropriately.

       The X server (when running on a workstation) reads authorization information from  a  file
       name  passed on the command line with the -auth option (see the Xserver manual page).  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  appro-
       priate 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 genera-
	      tor as is used for MIT-MAGIC-COOKIE-1.

	      This system needs a string representation of the	principal  which  identifies  the
	      associated  X  server.   This information is used to encrypt the client's authority
	      information when it is sent to the X server.  When xdm starts the X server, it uses
	      the  root  principal  for the machine on which it is running (unix.hostname@domain,
	      e.g., "unix.expire.lcs.mit.edu@our.domain.edu").	 Putting  the  correct	principal
	      name  in the .Xauthority file causes Xlib to generate the appropriate authorization
	      information using the secure RPC library.

	      Kerberos reads tickets from the cache pointed  to  by  the  KRB5CCNAME  environment
	      variable,  so  does  not	use any data from the .Xauthority file.  An entry with no
	      data must still exist to tell clients that MIT-KERBEROS-5 is available.

	      Unlike the .Xauthority file for clients, the authority file  passed  by  xdm  to	a
	      local  X	server (with ``-auth filename'', see xdm(1)) does contain the name of the
	      credentials cache, since the X server will  not  have  the  KRB5CCNAME  environment
	      variable	set.   The data of the MIT-KERBEROS-5 entry is the credentials cache name
	      and has the form ``UU:FILE:filename'', where filename is the name  of  the  creden-
	      tials cache file created by xdm.	Note again that this form is not used by clients.


       X(7x), xdm(1), xauth(1), xhost(1), xinit(1), Xserver(1)

X Version 11				   Release 6.6				    XSECURITY(7x)
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