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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.
           MIT-KERBEROS-5                Kerberos Version 5 user-to-user.


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.  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.

       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.

       MIT-KERBEROS-5
              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 krb5:gildea@x.org
              adds "judy" from the Kerberos realm of the  local  machine,  and
              "gildea" from the "x.org" realm.


THE AUTHORIZATION FILE

       Except  for Host Access control, each of these systems uses data stored
       in the .Xauthority file to generate the correct authorization  informa-
       tion  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 any-
       one  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
       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.

       MIT-KERBEROS-5
              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 credentials cache file created by xdm.  Note
              again that this form is not used by clients.


FILES

       .Xauthority


SEE ALSO

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



X Version 11                      Release 6.6                     Xsecurity(7)

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