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           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.


           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
                  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.
                  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
                  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@
                  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.
           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
           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.
                  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.


           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.   This
                  allows adding IPv6 addresses when the X  server  supports  IPv6,
                  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
                  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  pro-
                  cess.   For instance, some versions of ssh will be authenticated
                  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




           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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