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                     XSECURITY(7)

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