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XSECURITY(7)	       Miscellaneous Information Manual		  XSECURITY(7)

       Xsecurity - X display access control

       X provides mechanism for	implementing many access control systems.  The
       sample implementation includes six 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.
	   Server Interpreted		 Server-dependent methods of access control
       Not all of these	are available in all builds or implementations.

       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 mu-
	      tual trust does not exist.  The list of allowed hosts is	stored
	      in the X server and can be changed with the xhost	command.   The
	      list is stored in	the server by network address, not host	names,
	      so  is not automatically updated if a host changes address while
	      the server is running.  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.	  See  the  GRANTING ACCESS section of the Xserver man
	      page for details on how  this  list  is  initialized  at	server

	      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 automati-
	      cally 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	appli-
	      cations  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  who  compile  with	DES support 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 .Xau-
	      thority 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  combining the	current	time in	seconds	(since
	      00:00 1/1/1970 GMT) along	with 48	 bits  of  "identifier".   For
	      TCP/IPv4	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 requestor is authorized to  connect  by
	      decrypting  with the same	DES key	and validating the authentica-
	      tor and additional data.	This system is useful in many environ-
	      ments 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 in-
	      volves  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 ex-
	      tending 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@
	      adds "keith" from	the NIS	 domain	 of  the  local	 machine,  and
	      "ruth"  in  the ""	NIS domain.  For keith or ruth to suc-
	      cessfully	connect	to the display,	they must  add	the  principal
	      who started the server to	their .Xauthority file.	 For example:
		  xauth	add SUN-DES-1
	      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  princi-
	      pals  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	 pass-
	      word.  Users gain	access to services 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  cre-
	      dentials cache file and sets the environment 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 expire.

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

       Server Interpreted
	      The Server Interpreted method provides  two  strings  to	the  X
	      server  for  entry in the	access control list.  The first	string
	      represents the type of entry, and	the second string contains the
	      value of the entry.  These strings are interpreted by the	server
	      and different implementations and	builds may  support  different
	      types of entries.	 The types supported in	the sample implementa-
	      tion are defined in the SERVER INTERPRETED ACCESS	TYPES  section
	      below.   Entries of this type can	be manipulated via xhost.  For
	      example to add a Server Interpreted entry	of type	localuser with
	      a	value of root, the command is xhost +si:localuser:root.

       Except  for  Host Access	control	and Server Interpreted Access Control,
       each of these systems uses data stored in the .Xauthority file to  gen-
       erate  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 dif-
       ferent displays to share	the same data file.  A special connection fam-
       ily  (FamilyWild,  value	65535) causes an entry to match	every display,
       allowing	the entry to be	used for all connections.   Each  entry	 addi-
       tionally	 contains  the	authorization name and whatever	private	autho-
       rization	data is	needed by that authorization type to generate the cor-
       rect information	at connection setup time.

       The  xauth  program manipulates the .Xauthority file format.  It	under-
       stands 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  authoriza-
       tion data, and displays them appropriately.

       The X server (when running on a workstation) reads authorization	infor-
       mation from a file name passed on the command line with the  -auth  op-
       tion  (see  the Xserver manual page).  The authorization	entries	in the
       file are	used to	control	access to the server.  In each of  the	autho-
       rization	 schemes  listed  above, the data needed by the	server to ini-
       tialize 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	gener-
	      ates  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.  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 (
	      name@domain,  e.g.,   "").
	      Putting  the  correct  principal	name  in  the .Xauthority file
	      causes Xlib to generate the appropriate  authorization  informa-
	      tion using the secure RPC	library.

	      Kerberos	reads tickets from the cache pointed to	by the KRB5CC-
	      NAME 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 credentials cache file	created	by xdm.	  Note
	      again that this form is not used by clients.

       The  sample  implementation  includes several Server Interpreted	mecha-
	   IPv6				 IPv6 literal addresses
	   hostname			 Network host name
	   localuser			 Local connection user id
	   localgroup			 Local connection group	id

       IPv6   A	literal	IPv6 address as	defined	in IETF	RFC 3513.

	      The value	must be	a hostname as defined in IETF RFC 2396.	Due to
	      Mobile IP	and dynamic DNS, the name service is consulted at con-
	      nection authentication time, unlike the traditional host	access
	      control  list which only contains	numeric	addresses and does not
	      automatically update when	a host's address changes.   Note  that
	      this definition of hostname does not allow use of	literal	IP ad-

       localuser & localgroup
	      On systems which can determine in	a secure fashion  the  creden-
	      tials  of	a client process, the "localuser" and "localgroup" au-
	      thentication methods provide access based	on those  credentials.
	      The  format  of  the  values provided is platform	specific.  For
	      POSIX & UNIX platforms, if the value starts with	the  character
	      '#',  the	rest of	the string is treated as a decimal uid or gid,
	      otherwise	the string is defined as a user	name or	group name.

	      If your system supports this method and you use  it,  be	warned
	      that some	programs that proxy connections	and are	setuid or set-
	      gid may get authenticated	 as  the  uid  or  gid	of  the	 proxy
	      process.	 For  instance,	some versions of ssh will be authenti-
	      cated as the user	root, no matter	what user is running  the  ssh
	      client,  so on systems with such software, adding	access for lo-
	      caluser:root may allow wider access than intended	to the X  dis-


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



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