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XSECURITY(7)							  XSECURITY(7)

NAME
       Xsecurity - X display access control

SYNOPSIS
       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 1.6			  XSECURITY(7)
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