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XSECURITY(7)							  XSECURITY(7)

NAME
       Xsecurity - X display access control

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

ACCESS SYSTEM DESCRIPTIONS
       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 com-
	      mand.   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
	      startup.

       MIT-MAGIC-COOKIE-1
	      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.

       XDM-AUTHORIZATION-1
	      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.

       SUN-DES-1
	      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  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@ 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 suc-
	      cessfully	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.

       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.

THE AUTHORIZATION FILE
       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 uses string values for the authorization	data, and dis-
       plays 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
       option (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.

       MIT-MAGIC-COOKIE-1
	      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.

       XDM-AUTHORIZATION-1
	      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.

       SUN-DES-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.host-
	      name@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 informa-
	      tion using the secure RPC	library.

SERVER INTERPRETED ACCESS TYPES
       The sample implementation includes several  Server  Interpreted	mecha-
       nisms:
	   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.	  This
	      allows adding IPv6 addresses when	the X  server  supports	 IPv6,
	      but the xhost client was compiled	without	IPv6 support.

       hostname
	      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
	      addresses.

       localuser & localgroup
	      On  systems  which can determine in a secure fashion the creden-
	      tials of a client	 process,  the	"localuser"  and  "localgroup"
	      authentication  methods  provide	access	based on those creden-
	      tials.  The format of the	values provided	is platform  specific.
	      For POSIX	& UNIX platforms, if the value starts with the charac-
	      ter '#', 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
	      localuser:root may allow wider access than  intended  to	the  X
	      display.

FILES
       .Xauthority

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

X Version 11			xorg-docs 1.7.1			  XSECURITY(7)

NAME | OVERVIEW | ACCESS SYSTEM DESCRIPTIONS | THE AUTHORIZATION FILE | SERVER INTERPRETED ACCESS TYPES | FILES | SEE ALSO

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