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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  mecha-
       nisms:
	   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  sys-
	      tem  can	work  reasonably  well	in an environment
	      where everyone trusts everyone, or when only a sin-
	      gle  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  command.	When using the more secure mecha-
	      nisms listed below, the host list is normally  con-
	      figured  to  be the empty list, so that only autho-
	      rized 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 auto-
	      matically 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  auto-
	      matically 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  net-
	      work  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 run-
	      ning applications 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

X Version 11		Release 6.3				1

XSECURITY(7)					 XSECURITY(7)

	      control  to  the physical network.  In environments
	      where network-level  snooping  is difficult,  this
	      system can work reasonably well.

       XDM-AUTHORIZATION-1
	      Sites  in the  United  States  can use a DES-based
	      access  control  mechanism  called   XDM-AUTHORIZA-
	      TION-1.	It  is	similar in  usage  to MIT-MAGIC-
	      COOKIE-1 in that a key is stored in the .Xauthority
	      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/IP connec-
	      tions, the identifier is the address plus port num-
	      ber; for local connections it is the process ID and
	      32 bits to form a unique id (in case multiple  con-
	      nections	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 authenticator and	additional  data.
	      This  system  is	useful in many environments 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 successfully connect	 to  the
	      display,	they  must  add the principal who started
	      the server to their .Xauthority file.  For example:

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XSECURITY(7)					 XSECURITY(7)

		  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 principals to authenticate each
	      other as long as each trusts a  third  party,  Ker-
	      beros.   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 password.  Users gain access  to
	      services by getting Kerberos tickets for those ser-
	      vices 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 credentials 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 proto-
	      col,  like  the  SUN-DES-1 protocol, the owner of a
	      display can enable and disable 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  cor-
       rect  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

X Version 11		Release 6.3				3

XSECURITY(7)					 XSECURITY(7)

       connection  family  (TCP/IP,  DECnet or local connections)
       and X display name (hostname plus display  number).   This
       allows  multiple authorization entries for different dis-
       plays to share the same data file.  A  special  connection
       family  (FamilyWild, value 65535) causes an entry to match
       every display, allowing the entry to be used for all  con-
       nections.  Each entry additionally contains the authoriza-
       tion name  and  whatever private	 authorization	data  is
       needed  by that authorization type to generate the correct
       information at connection setup time.

       The xauth program manipulates the .Xauthority file format.
       It  understands	the  semantics of the connection families
       and address formats, displaying them in an easy to  under-
       stand format.  It also understands that SUN-DES-1 and MIT-
       KERBEROS-5 use string values for the  authorization  data,
       and displays them appropriately.

       The  X server (when running on a workstation) reads autho-
       rization information from a file name passed on	the  com-
       mand  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 authoriza-
       tion schemes listed above, the data needed by  the  server
       to  initialize 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  generates  these keys using a
	      cryptographically secure pseudo random number  gen-
	      erator,  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 encryp-
	      tion 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  run-
	      ning	  (unix.hostname@domain,	  e.g.,

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XSECURITY(7)					 XSECURITY(7)

	      "unix.expire.lcs.mit.edu@our.domain.edu").  Putting
	      the  correct principal name in the .Xauthority file
	      causes Xlib to generate the appropriate  authoriza-
	      tion information 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 6.3				5

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