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  Samba Authenticated Gateway HOWTO
  Ricardo Alexandre Mattar
  v1.3, 2005-01-06

  This document intends to show how to build a Firewall/Gateway with
  rules set on user basis having the users authenticated by a Samba Pri-
  mary Domain Controller

  �[1m1.  Introduction�[0m

  As you can see by the poorness of my language, English is not my
  native language. I am writing this document in English for the sake of
  the Linux community. So, please, excuse me for my poor English.  And,
  please, if you speak Portuguese, address me in this language.

  This document intends to enlighten you (and myself) in the process of
  building a Linux Gateway or Firewall, which modify rules on demand
  when users log in or out from their Windows workstations.

  In this document, I will try to show how to build a gateway to NAT or
  MASQUERADE Windows workstations. Use your imagination to modify it to
  get any level of network management. You may use it to grant or deny
  access to services, servers or entire subnetworks on your network.

  Imagine that you have to build a gateway to let Windows workstation
  access the Internet and that you need to authenticate each user before
  letting them access the external networks. The first solution you
  think about is Squid. It's indeed a great solution, when http and ftp
  access is enough for your users. When it comes to let them access
  other services like pop, smtp, ssh, a database server or whatever
  else, you immediately think about NAT or MASQUERADE. But what happens
  to the user authentication?

  Well, this is my solution. It gives you user authentication and fine
  grain control over their access to the external networks.

  �[1m1.1.  Overview�[0m

  We know that SAMBA can act as a Domain Controller and so it can
  authenticate users on Windows boxes. As a PDC, SAMBA can push netlogon
  scripts to the Windows workstations. We can use this netlogon scripts
  to force the Windows workstations mounting a given share from our
  Linux PDC. This "forced" share shall have preexec and postexec scripts
  which shall be triggered when the user logs in or out. There is a
  program named smbstatus which lists the shares being used, giving us
  also the username and ip address of the workstation. We just need to
  grep this information from smbstatus output and update our firewall

  �[1m1.2.  Candy�[0m

  If you are impatient and don't like to read, go to
  <>, but in the end you may
  find yourself coming back here to read.

  �[1m1.3.  Disclaimer�[0m

  No liability for the contents of this document can be accepted.  Use
  the concepts, examples and other content at your own risk. As this is
  a new edition of this document, there may be errors and inaccuracies,
  that may of course be damaging to your system. Proceed with caution,
  and although this is highly unlikely, the author(s) do not take any
  responsibility for that.

  All copyrights are held by their respective owners, unless
  specifically noted otherwise. Use of a term in this document should
  not be regarded as affecting the validity of any trademark or service

  Naming of particular products or brands should not be seen as

  �[1m1.4.  New versions�[0m

  The newest release of this document can be found at
  <> or at <>

  Related HOWTOs can be found at the Linux Documentation Project
  homepage at <>.

  �[1m1.5.  Translations�[0m

  A Portuguese version is available.

  A French translation by Guillaume Lelarge is available

  A Hungarian translation is available at

  If you want to contribute with a translation, please do.

  �[1m1.6.  Feedback�[0m

  Contributions and criticism are both welcome.

  Corrections to my English are also very welcome!

  If you find any bugs in the scripts included, please tell me.

  You can find me at or at

  �[1m1.7.  Copyright and trademarks�[0m

  Copyright (c) 2002-2003 Ricardo Alexandre Mattar

  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.2 or
  any later version published by the Free Software Foundation; with no
  Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
  copy of the license is included in the section entitled "GNU Free
  Documentation License".

  �[1m1.8.  Acknowledgments and Thanks�[0m

  Thanks to Carlos Alberto Reis Ribeiro for introducing me to Linux.

  Thanks to Cesar Bremer Pinheiro for motivating me to write this

  Thanks to Guillaume Lelarge for the (continuous) help with the

  Thanks to Erik Esplund for further language corrections.

  Thanks to Albert Teixidr code improvements.

  Thanks to Felipe Cordeiro Caetano for helping on my main testing site.

  Thanks to the secure communications company RASEAC
  <> for sponsoring my work.

  �[1m2.  Requirements�[0m

  �[1m2.1.  Knowledge�[0m

  This document is target at the seasoned systems administrator.

  You must have a fair knowledge about (at least know what these are):

  · TCP/IP;

  · Linux netfilter;

  · A scripting language (bash?);

  · SAMBA and Windows networking and domain controllers;

  Fortunately, there is plenty of documentation on these topics on the

  �[1m2.2.  Software�[0m

  Installed on your server, you will need at least:

  · Samba;

  · Iptables;

  · A scripting language;

  �[1m3.  Linux box setup�[0m

  This Howto assumes you have a kernel from the 2.4 series as it uses
  iptables. Other than that, there are no known issues why this should
  not work on a 2.2 kernel box with the scripts adapted to ipchains.

  Of course, you need to install the iptables userland tools, an apache
  http server if you want to run a CGI tool to change passwords and
  SAMBA. And you will need a kernel compiled with iptables modules.

  You may wish to use DHCP. If so, it is easy to set up. Remember to
  configure the dhcp server to give the nameserver IP address and the
  gateway IP address as well. The Windows machines will make good use of
  this information.

  �[1m3.1.  Basic system setup�[0m

  Generally any basic system setup from the common Linux distributions
  will fit in this gateway example. Just check if you have Samba and

  �[1m3.2.  Additional directory hierarchy�[0m

  The additional directory hierarchy will be required to accomplish the
  example of this howto:

  This is used to keep track of the users and IP addresses:


  This is where I place user specific scripts:


  And group specific scripts:


  Directory for the netlogon share:


  Directory for the tracking share:


  These hierarchies are required by some of the scripts and daemons of
  the example.

  �[1m3.3.  Firewall setup�[0m

  Its very unlikely that your distribution's kernel won't be compiled
  with Iptables and the userland tools won't be installed either.
  Anyway, if you don't have it, refer to
  <> or
  <> to get the software and the documentation.

  You will need a basic firewall setup in order to get the gateway
  working. Take a look at the iptables tutorial at IPTABLES TUTORIAL
  tutorial.html>. It's an interesting reading. Anyway, if you have no
  time to spend, the following code is somewhat (very) loose but it may
  fit your needs:

  /sbin/depmod -a
  /sbin/insmod ip_tables
  /sbin/insmod ip_conntrack
  /sbin/insmod ip_conntrack_ftp
  /sbin/insmod ip_conntrack_irc
  /sbin/insmod iptable_nat
  /sbin/insmod ip_nat_ftp
  echo "1" > /proc/sys/net/ipv4/ip_forward
  echo "1" > /proc/sys/net/ipv4/ip_dynaddr
  $IPTABLES -t nat -F

  You will notice that this code actually does nothing, but load the
  kernel modules related to nat and firewalling and turns the packet
  routing on. You can (and should) place any rules there to give your
  gateway a standard behavior, but the big magic will be done by scripts
  called by the SAMBA daemon.

  Please, remember that this code doesn't have the least bit of
  security! Don't use these examples in production environments. This
  example intends only to be educational. You have to add a firewall
  configuration that suits your systems.
  You have been warned!

  �[1m3.4.  SAMBA setup�[0m

  Check if you have Samba installed. If your distribution doesn't come
  with Samba pre-packaged then refer to
  <> to get the packages and for documentation on
  how to install Samba. Brows around their web site and learn about it.
  The site has plenty of documentation and maybe your LINUX distribution
  also has plenty of SAMBA documentation.

  We will need to setup SAMBA as a Primary Domain Controller. I will
  give an example configuration file here, but you should read the Samba
  HOWTO Collection <
  Collection.html> and learn all you can about a PDC.

  �[1m3.4.1.  Basic SAMBA setup.�[0m

  Since I do not intend to rewrite the SAMBA documentation, here goes a
  sample smb.conf file:

  # Global parameters
  workgroup = DOMAIN
  netbios name = LINUX
  server string = Linux PDC
  encrypt passwords = Yes
  map to guest = Bad Password
  passwd program = /usr/bin/passwd
  unix password sync = Yes
  max log size = 50
  time server = Yes
  socket options = TCP_NODELAY SO_RCVBUF=8192 SO_SNDBUF=8192
  add user script = /usr/sbin/useradd -d /dev/null -g 100 -s /bin/false -M %u
  logon script = %a.bat
  domain logons = Yes
  os level = 64
  lm announce = True
  preferred master = True
  domain master = True
  dns proxy = No
  printing = lprng
  comment = Home Directories
  path = /home/%u
  read only = No
  comment = All Printers
  path = /var/spool/samba
  printable = Yes
  browseable = No
  available = No
  comment = NetLogon ShARE
  path = /home/samba/netlogon
  guest account =
  comment = login tracking share
  path = /home/samba/samba
  browseable = No
  root preexec = /usr/local/bin/ %u %I
  root postexec = /usr/local/bin/ %u

  You will have to do with it or read the SAMBA documentation if you
  really want to control your server and network.

  �[1m3.4.2.  The "logon script"�[0m

  Using "logon script = %a.bat" makes samba evaluate the guest os and
  call an appropriated logon script. If you want a static script, just
  change to "logon script = netlogon.bat". Actually you can do anything
  here and even generate any script during the logon.

  �[1m3.4.3.  The netlogon and the tracking shares�[0m

  The netlogon share is where the Windows workstations download the
  logon script from. We need this share in order to place there a logon
  script, which will tell the workstation to mount a share that will be
  used to track the users ip addresses.

  As you can see, there must be a line like the following in your
  logon script = netlogon.bat

  This line will tell your Windows client to download and execute the
  script named netlogon.bat. This script must be placed at the netlogon
  share. So, we will also need a netlogon.bat script to your Windows
  workstations. You can use the following example and place it at the
  netlogon share, in this case: /home/samba/netlogon/NETLOGON.BAT.

  net use z: \\linux\samba /yes

  This script will tell the Windows workstation to mount the specified
  share, and so we will be able to keep track of the user and
  workstation through the output of the smbstatus program.

  Quite simple! But not enough...

  As you could see, we will need also a tracking share which, in this
  example, I named samba. You can see the tracking share configuration
  in smb.conf:

  comment = login tracking share
  path = /home/samba/samba
  browseable = No
  root preexec = /usr/local/bin/ %u %I
  root postexec = /usr/local/bin/ %u

  As you can guess or know if you read the SAMBA documentation, the root
  preexec and the root postexec lines tell SAMBA to run the indicated
  scripts when a user mounts or unmounts the share. In this case, we are
  passing the username to the script as a parameter. Note the %u at the
  end of the lines. These scripts are the beasts which will call a
  script or program to modify our gateway's packet filtering rules.

  Note that the script must check if the refered workstation
  has already mounted the tracking share.

  Take a look at the and scripts:

  # usage:
  # <username>
  if [ -f /var/run/smbgate/$1 ] ; then
      exit 0
  echo $2 > /var/run/smbgate/$1
  ADDRESS=`cat /var/run/smbgate/$1`
  GROUP=`groups $1 | gawk '// { print $3 }'`
  if [ -f /etc/smbgate/users/$1 ] ; then
      /etc/smbgate/users/$1 $COMMAND $ADDRESS $EXTIF
      if [ -f /etc/smbgate/groups/$GROUP ] ; then
          /etc/smbgate/groups/$GROUP $COMMAND $ADDRESS $EXTIF
          /etc/smbgate/users/ $COMMAND $ADDRESS $EXTIF

  This script ( is intended to run when the user logs in and
  will select the which scripts will be executed based on the user name
  and to which group the user belongs. The user's ip address will be
  written to a file at /var/run/smbgate for tracking purposes.  The file
  will take the user's name and will be later used when the user log
  off. The IP address will be passed as an argument to a script with the
  users' name which will finally update the firewall.

  Notice that this script tries a user script, then if it
  can't find the user script it tries a group script, and finally if it
  can't find the group script it tries the script.  You can
  modify this logic and behavior as you wish and need, just remember to
  modify the others accordingly.

  Chances are if the user belong to more than one that these scripts
  will fail. I did not have time to write a better code.

  # usage:
  # <username>
  ADDRESS=`cat /var/run/smbgate/$1`
  GROUP=`groups $1 | gawk '// { print $3 }'`
  NM=`smbstatus -u $1 | grep $TRACKSHARE | wc -l`
  if [ $NM -gt 0 ]; then
  if [ -f /etc/smbgate/users/$1 ] ; then
      /etc/smbgate/users/$1 $COMMAND $ADDRESS $EXTIF
      if [ -f /etc/smbgate/groups/$GROUP ] ; then
          /etc/smbgate/groups/$GROUP $COMMAND $ADDRESS $EXTIF
          /etc/smbgate/users/ $COMMAND $ADDRESS $EXTIF
  rm -f /var/run/smbgate/$1

  This script ( is intended to run when the user logs off
  and will get the address from the /var/run/smbgate/user file which
  will be passed as an argument to the /etc/smbgate/users/user script
  which will update the firewall to the state desired when the user is
  not logged in.

  Some versions of Windows, such as Windows 2000, mount the tracking
  share more than once per login. This may cause problems with the and, triggering the scripts more the once.
  This can make a real mess. So, you may prefer to use a logout checking
  script at cron instead of a script triggered by SAMBA.
  Here is an example:

  # usage:
  # intended to run at cron (maybe each 10 minutes)

  if [ -d $TRACKDIR ]; then
    for n in $TRACKDIR/*; do
      [ -d $n ] && continue;
      if [ -f $n ] ; then
        IPADDRESS=`cat $n`
        NMS=`smbstatus -u $USERNAME | grep $TRACKSHARE | grep $IPADDRESS | grep -v grep | wc -l`
        if [ $NMS == 0 ] ; then
          rm -f $n
          GROUP=`groups $USERNAME | gawk '// { print $3 }'`
          if [ -f /etc/smbgate/users/$USERNAME ] ; then
            /etc/smbgate/users/$USERNAME $COMMAND $IPADDRESS $EXTIF
            if [ -f /etc/smbgate/groups/$GROUP ] ; then
              /etc/smbgate/groups/$GROUP $COMMAND $IPADDRESS $EXTIF
              /etc/smbgate/users/ $COMMAND $IPADDRESS $EXTIF
        exit 0

  In that case you should remove the root postexec clause from the
  tracking share on smb.conf:

  root postexec = /usr/local/bin/ %u

  The following is a standard /etc/smbgate/users/user script. This is
  the one which will actually modify the firewall rules.


  We should also have a script at /etc/smbgate/users/ to give
  the gateway a default behavior.

  exit 0

  �[1m4.  An alternative solution�[0m

  The whole scheme of mounting a tracking share and triggering scripts
  to update the firewall and waiting for them to be triggered again on
  unmounting to reset the firewall rule may be too confusing and loose.
  It may become even obsolete as the Samba project release new features.

  The latest Samba release has the capability of listing the logged
  users. I used this feature in a script to track the users and update
  the firewall as they log in and out. This script does not require all
  the work described on this text. It is very easy to use actually.

  You can download the code from the project site at

  �[1m5.  SSH setup�[0m

  You may want to run your PDC on one box and have another box as a
  managed gateway for any reason. If so you must setup your gateway to
  accept rsa authenticated logins without passwords from the PDC.

  Take a look at <>
  for information on how to properly setup your ssh server and client
  for this.

  �[1m5.1.  Important�[0m

  You should read the ssh documentation and make shure that you fully
  understand what you are doing when you setup rsa or any other kind of
  cryptographic authentication.

  If security isn't an issue, just use my example and go on.

  �[1m5.2.  Key pair generation�[0m

  To create a key pair issue the following commands on the manchine
  meant to be the PDC:

  pdc:~# ssh-keygen -t rsa

  Answer the questions and copy the resulting public key to the gateway
  it self. Usually the public key goes to "~.ssh/"
  pdc:~# cd .ssh
  pdc:~# scp root@gateway:/root/.ssh/authorized_keys2

  �[1m5.3.  SSH enabled logon script�[0m

  The following is a standard /etc/smbgate/users/user script modified to
  use the ssh cryptographic authentication.


  Note that the iptables binary in called through ssh at the "gateway".
  Again, make sure that you read the ssh server documentation.

  �[1m6.  Windows workstation setup�[0m

  �[1m6.1.  Introduction�[0m

  We will stick to setting up the network, user management and policies
  on the Windows workstations.

  I will not go through all those steps, naming each dialog box.  I will
  presume that if you can read and understand this document you can find
  your way through that mess.

  �[1m6.2.  Network protocols�[0m

  First, unless you really need, remove all network protocols but
  TCP/IP. Even without their own protocol, Windows machines like to
  broadcast a lot, and this doesn't please anyone. Anyway, with TCP/IP
  who needs anything else?

  �[1m6.3.  DHCP setup�[0m

  If you setup a DHCP server on your Linux box, remember that Windows
  workstations can get the nameservers and gateway's address besides its
  own IP address from it. So, you don't need to set all these items on
  each workstation.

  �[1m6.4.  Join your Linux server domain�[0m

  Configure the Windows workstation to log in a Domain, and give the
  domain name of your Linux server. This is essential to the gateway

  You must know that in order to join some versions of Windows to a
  SAMBA domain controller, you must create machine accounts in your
  Linux PDC. Check the SAMBA documentation on how to setup your PDC to
  the specific version of Windows which you have.
  �[1m6.4.1.  Windows fo workgroups�[0m

  This version seems to need no special configuration to join the Linux
  PDC domain.

  The netlogon script shall be named "WfWg.bat" so when %a is translated
  the right script is chosen.


  net use z: \\linux\samba /yes

  �[1m6.4.2.  Windows 95/98/ME�[0m

  These versions also seems to need no special configuration to join the
  Linux PDC domain.

  The netlogon script shall be named "Win95.bat" so when %a is
  translated the right script is chosen.


  net use z: \\linux\samba /yes

  �[1m6.4.3.  Windows NT�[0m

  This version requires machine accounts at the Linux box. Check the
  SAMBA documentation.

  The netlogon script shall be named "WinNT.bat" so when %a is
  translated the right script is chosen.


  net use z: \\linux\samba /yes /persistent:no

  �[1m6.4.4.  Windows 2000�[0m

  This version requires machine accounts at the Linux box. Again, check
  the SAMBA documentation.

  The netlogon script shall be named "Win2K.bat" so when %a is
  translated the right script is chosen.


  net use z: \\linux\samba /yes /persistent:no

  �[1m6.4.5.  Windows XP�[0m

  This version needs a machine account at the Linux box and a tweak at
  the registry, as follows.

  Locate the key
  The default value is 1. Set it to 0 and it will no more complain about
  joining the domain.

  If you have many workstation to configure create a file named
  anything.reg with the following content and use it to modify the
  "faulty" registry.

  Windows Registry Editor Version 5.00


  This version also needs an adjust at the logon script. Sometimes it
  insists on making the mounting persistent. The netlogon script shall
  be named "WinXP.bat" so when %a is translated the right script is


  net use z: \\linux\samba /yes /persistent:no

  �[1m6.5.  Policy editor�[0m

  There is a utility named policy editor bundled on the Windows CD. The
  file name is poledit.exe. This tool, as the name suggest, allows to
  create a user and system policy file.

  Unfortunately, this tool does not generate a plain text configuration
  file, so I can't place an example here.

  Use the policy editor to create a policy to your workstations and
  users. You should disable the local password cache and domain cache in
  order to get some security. Save the policy file as config.pol and
  place it at the netlogon share of your Linux server. In this way, your
  Windows workstations will download and use the config.pol file to set
  their policy. Of course this task must be done on a Windows machine.
  If you don't use a config.pol file, your Windows workstations will
  annoy you asking for a Windows password and you will become nuts
  trying to synchronize and manage your Domain and Windows passwords.
  It seems that the OS doesn't know that it joined a domain. You must
  tell it and then you have to slap it in the face so it will believe

  �[1m7.  User management�[0m

  �[1m7.1.  Adding users�[0m

  Adding a Linux user by usual means and setting a samba password using
  smbpasswd will work. If you have any doubt, just refer to the SAMBA
  documentation. This is not a difficult issue.

  �[1m7.2.  Password management�[0m

  I am issuing this a major topic because I couldn't learn yet how to
  manage users and users' passwords from a Windows workstation without
  using a web interface. I couldn't find and didn't know how to build
  integrated tools to solve this problem. So, I am using a CGI program
  to get it done.

  Try the package at
  <>, it seems to be a good choice.

  �[1m7.3.  Granting or denying access to users�[0m

  As you could see in a previous section of this howto, the SAMBA daemon
  will call a script every time the tracking share is
  mounted. This script will call a script with the user's
  name giving this script the ip address of the refered workstation as a
  parameter. This user script will apply the desired rules.

  For example if you want to give the user full access to internet:


  If you don't want to change anything to a particular user, just give
  him an empty script:

  exit 0

  Or just don't create any script for the less privileged users, letting
  them have the script, which would be empty as the previous
  or just give limited access as follows:


  Remember that this script requires you to modify all the previous
  scripts to include the extra parameter ou just modify the script
  script. And remember that you will go nowhere whis this howto if you
  don't understand iptables.

  �[1m8.  Group management�[0m

  �[1m8.1.  Creating groups�[0m

  Just create your user groups in the Linux PDC and add the users to the
  groups. This is it.

  Remember that the example scripts in this howto will probably fail if
  you have users belonging to more than one group. If you need this,
  remember to adjust the scripts.

  �[1m8.2.  Group policy�[0m

  You will need to define group specific scripts and place them in the
  directory "/etc/smbgate/groups/". Remember that the script must be
  named as the group, at least if you want to follow the examples in
  this howto.

  The default scheme of this howto is to check for a user script, then
  for a group script and finally for the default script. If you want to
  modify this behavior remember to adapt the,
  (or the scripts. The whole logic is in these scripts.

  �[1m9.  Bibliography�[0m

  tutorial.html> by Oskar Andreasson

  Samba HOWTO Collection <
  HOWTO-Collection.html> by the SAMBA Team

  �[1m10.  GNU Free Documentation License�[0m

  GNU Free Documentation License Version 1.2, November 2002

  Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. 59 Temple
  Place, Suite 330, Boston, MA 02111-1307 USA Everyone is permitted to
  copy and distribute verbatim copies of this license document, but
  changing it is not allowed.


  The purpose of this License is to make a manual, textbook, or other
  functional and useful document "free" in the sense of freedom: to
  assure everyone the effective freedom to copy and redistribute it,
  with or without modifying it, either commercially or noncommercially.
  Secondarily, this License preserves for the author and publisher a way
  to get credit for their work, while not being considered responsible
  for modifications made by others.

  This License is a kind of "copyleft", which means that derivative
  works of the document must themselves be free in the same sense. It
  complements the GNU General Public License, which is a copyleft
  license designed for free software.

  We have designed this License in order to use it for manuals for free
  software, because free software needs free documentation: a free
  program should come with manuals providing the same freedoms that the
  software does. But this License is not limited to software manuals; it
  can be used for any textual work, regardless of subject matter or
  whether it is published as a printed book. We recommend this License
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  This License applies to any manual or other work, in any medium, that
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  that says that the Document is released under this License. If a
  section does not fit the above definition of Secondary then it is not
  allowed to be designated as Invariant. The Document may contain zero
  Invariant Sections. If the Document does not identify any Invariant
  Sections then there are none.

  The "Cover Texts" are certain short passages of text that are listed,
  as Front-Cover Texts or Back-Cover Texts, in the notice that says that
  the Document is released under this License.  A Front-Cover Text may
  be at most 5 words, and a Back-Cover Text may be at most 25 words.

  A "Transparent" copy of the Document means a machine-readable copy,
  represented in a format whose specification is available to the
  general public, that is suitable for revising the document
  straightforwardly with generic text editors or (for images composed of
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  drawing editor, and that is suitable for input to text formatters or
  for automatic translation to a variety of formats suitable for input
  to text formatters.  A copy made in an otherwise Transparent file
  format whose markup, or absence of markup, has been arranged to thwart
  or discourage subsequent modification by readers is not Transparent.
  An image format is not Transparent if used for any substantial amount
  of text. A copy that is not "Transparent" is called "Opaque".

  Examples of suitable formats for Transparent copies include plain
  ASCII without markup, Texinfo input format, LaTeX input format, SGML
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  transparent image formats include PNG, XCF and JPG. Opaque formats
  include proprietary formats that can be read and edited only by
  proprietary word processors, SGML or XML for which the DTD and/or
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  generated HTML, PostScript or PDF produced by some word processors for
  output purposes only.

  The "Title Page" means, for a printed book, the title page itself,
  plus such following pages as are needed to hold, legibly, the material
  this License requires to appear in the title page. For works in
  formats which do not have any title page as such, "Title Page" means
  the text near the most prominent appearance of the work's title,
  preceding the beginning of the body of the text.

  A section "Entitled XYZ" means a named subunit of the Document whose
  title either is precisely XYZ or contains XYZ in parentheses following
  text that translates XYZ in another language. (Here XYZ stands for a
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  of such a section when you modify the Document means that it remains a
  section "Entitled XYZ" according to this definition.

  The Document may include Warranty Disclaimers next to the notice which
  states that this License applies to the Document. These Warranty
  Disclaimers are considered to be included by reference in this
  License, but only as regards disclaiming warranties: any other
  implication that these Warranty Disclaimers may have is void and has
  no effect on the meaning of this License.


  You may copy and distribute the Document in any medium, either
  commercially or noncommercially, provided that this License, the
  copyright notices, and the license notice saying this License applies
  to the Document are reproduced in all copies, and that you add no
  other conditions whatsoever to those of this License. You may not use
  technical measures to obstruct or control the reading or further
  copying of the copies you make or distribute. However, you may accept
  compensation in exchange for copies. If you distribute a large enough
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  You may also lend copies, under the same conditions stated above, and
  you may publicly display copies.


  If you publish printed copies (or copies in media that commonly have
  printed covers) of the Document, numbering more than 100, and the
  Document's license notice requires Cover Texts, you must enclose the
  copies in covers that carry, clearly and legibly, all these Cover
  Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
  the back cover. Both covers must also clearly and legibly identify you
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  You may add other material on the covers in addition. Copying with
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  the Document and satisfy these conditions, can be treated as verbatim
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  If the required texts for either cover are too voluminous to fit
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  reasonably) on the actual cover, and continue the rest onto adjacent

  If you publish or distribute Opaque copies of the Document numbering
  more than 100, you must either include a machine-readable Transparent
  copy along with each Opaque copy, or state in or with each Opaque copy
  a computer-network location from which the general network-using
  public has access to download using public-standard network protocols
  a complete Transparent copy of the Document, free of added material.
  If you use the latter option, you must take reasonably prudent steps,
  when you begin distribution of Opaque copies in quantity, to ensure
  that this Transparent copy will remain thus accessible at the stated
  location until at least one year after the last time you distribute an
  Opaque copy (directly or through your agents or retailers) of that
  edition to the public.

  It is requested, but not required, that you contact the authors of the
  Document well before redistributing any large number of copies, to
  give them a chance to provide you with an updated version of the


  You may copy and distribute a Modified Version of the Document under
  the conditions of sections 2 and 3 above, provided that you release
  the Modified Version under precisely this License, with the Modified
  Version filling the role of the Document, thus licensing distribution
  and modification of the Modified Version to whoever possesses a copy
  of it. In addition, you must do these things in the Modified Version:

  A. Use in the Title Page (and on the covers, if any) a title distinct
  from that of the Document, and from those of previous versions (which
  should, if there were any, be listed in the History section of the
  Document). You may use the same title as a previous version if the
  original publisher of that version gives permission. B. List on the
  Title Page, as authors, one or more persons or entities responsible
  for authorship of the modifications in the Modified Version, together
  with at least five of the principal authors of the Document (all of
  its principal authors, if it has fewer than five), unless they release
  you from this requirement. C. State on the Title page the name of the
  publisher of the Modified Version, as the publisher.  D. Preserve all
  the copyright notices of the Document. E. Add an appropriate copyright
  notice for your modifications adjacent to the other copyright notices.
  F. Include, immediately after the copyright notices, a license notice
  giving the public permission to use the Modified Version under the
  terms of this License, in the form shown in the Addendum below. G.
  Preserve in that license notice the full lists of Invariant Sections
  and required Cover Texts given in the Document's license notice. H.
  Include an unaltered copy of this License.  I. Preserve the section
  Entitled "History", Preserve its Title, and add to it an item stating
  at least the title, year, new authors, and publisher of the Modified
  Version as given on the Title Page. If there is no section Entitled
  "History" in the Document, create one stating the title, year,
  authors, and publisher of the Document as given on its Title Page,
  then add an item describing the Modified Version as stated in the
  previous sentence. J. Preserve the network location, if any, given in
  the Document for public access to a Transparent copy of the Document,
  and likewise the network locations given in the Document for previous
  versions it was based on. These may be placed in the "History"
  section. You may omit a network location for a work that was published
  at least four years before the Document itself, or if the original
  publisher of the version it refers to gives permission. K. For any
  section Entitled "Acknowledgements" or "Dedications", Preserve the
  Title of the section, and preserve in the section all the substance
  and tone of each of the contributor acknowledgements and/or
  dedications given therein. L.  Preserve all the Invariant Sections of
  the Document, unaltered in their text and in their titles. Section
  numbers or the equivalent are not considered part of the section
  titles. M. Delete any section Entitled "Endorsements". Such a section
  may not be included in the Modified Version. N. Do not retitle any
  existing section to be Entitled "Endorsements" or to conflict in title
  with any Invariant Section. O. Preserve any Warranty Disclaimers.

  If the Modified Version includes new front-matter sections or
  appendices that qualify as Secondary Sections and contain no material
  copied from the Document, you may at your option designate some or all
  of these sections as invariant. To do this, add their titles to the
  list of Invariant Sections in the Modified Version's license notice.
  These titles must be distinct from any other section titles.

  You may add a section Entitled "Endorsements", provided it contains
  nothing but endorsements of your Modified Version by various
  parties--for example, statements of peer review or that the text has
  been approved by an organization as the authoritative definition of a

  You may add a passage of up to five words as a Front-Cover Text, and a
  passage of up to 25 words as a Back-Cover Text, to the end of the list
  of Cover Texts in the Modified Version. Only one passage of Front-
  Cover Text and one of Back-Cover Text may be added by (or through
  arrangements made by) any one entity. If the Document already includes
  a cover text for the same cover, previously added by you or by
  arrangement made by the same entity you are acting on behalf of, you
  may not add another; but you may replace the old one, on explicit
  permission from the previous publisher that added the old one.

  The author(s) and publisher(s) of the Document do not by this License
  give permission to use their names for publicity for or to assert or
  imply endorsement of any Modified Version.


  You may combine the Document with other documents released under this
  License, under the terms defined in section 4 above for modified
  versions, provided that you include in the combination all of the
  Invariant Sections of all of the original documents, unmodified, and
  list them all as Invariant Sections of your combined work in its
  license notice, and that you preserve all their Warranty Disclaimers.

  The combined work need only contain one copy of this License, and
  multiple identical Invariant Sections may be replaced with a single
  copy. If there are multiple Invariant Sections with the same name but
  different contents, make the title of each such section unique by
  adding at the end of it, in parentheses, the name of the original
  author or publisher of that section if known, or else a unique number.
  Make the same adjustment to the section titles in the list of
  Invariant Sections in the license notice of the combined work.

  In the combination, you must combine any sections Entitled "History"
  in the various original documents, forming one section Entitled
  "History"; likewise combine any sections Entitled "Acknowledgements",
  and any sections Entitled "Dedications". You must delete all sections
  Entitled "Endorsements".


  You may make a collection consisting of the Document and other
  documents released under this License, and replace the individual
  copies of this License in the various documents with a single copy
  that is included in the collection, provided that you follow the rules
  of this License for verbatim copying of each of the documents in all
  other respects.

  You may extract a single document from such a collection, and
  distribute it individually under this License, provided you insert a
  copy of this License into the extracted document, and follow this
  License in all other respects regarding verbatim copying of that


  A compilation of the Document or its derivatives with other separate
  and independent documents or works, in or on a volume of a storage or
  distribution medium, is called an "aggregate" if the copyright
  resulting from the compilation is not used to limit the legal rights
  of the compilation's users beyond what the individual works permit.
  When the Document is included an aggregate, this License does not
  apply to the other works in the aggregate which are not themselves
  derivative works of the Document.

  If the Cover Text requirement of section 3 is applicable to these
  copies of the Document, then if the Document is less than one half of
  the entire aggregate, the Document's Cover Texts may be placed on
  covers that bracket the Document within the aggregate, or the
  electronic equivalent of covers if the Document is in electronic form.
  Otherwise they must appear on printed covers that bracket the whole


  Translation is considered a kind of modification, so you may
  distribute translations of the Document under the terms of section 4.
  Replacing Invariant Sections with translations requires special
  permission from their copyright holders, but you may include
  translations of some or all Invariant Sections in addition to the
  original versions of these Invariant Sections. You may include a
  translation of this License, and all the license notices in the
  Document, and any Warrany Disclaimers, provided that you also include
  the original English version of this License and the original versions
  of those notices and disclaimers. In case of a disagreement between
  the translation and the original version of this License or a notice
  or disclaimer, the original version will prevail.

  If a section in the Document is Entitled "Acknowledgements",
  "Dedications", or "History", the requirement (section 4) to Preserve
  its Title (section 1) will typically require changing the actual


  You may not copy, modify, sublicense, or distribute the Document
  except as expressly provided for under this License. Any other attempt
  to copy, modify, sublicense or distribute the Document is void, and
  will automatically terminate your rights under this License. However,
  parties who have received copies, or rights, from you under this
  License will not have their licenses terminated so long as such
  parties remain in full compliance.


  The Free Software Foundation may publish new, revised versions of the
  GNU Free Documentation License from time to time. Such new versions
  will be similar in spirit to the present version, but may differ in
  detail to address new problems or concerns. See

  Each version of the License is given a distinguishing version number.
  If the Document specifies that a particular numbered version of this
  License "or any later version" applies to it, you have the option of
  following the terms and conditions either of that specified version or
  of any later version that has been published (not as a draft) by the
  Free Software Foundation. If the Document does not specify a version
  number of this License, you may choose any version ever published (not
  as a draft) by the Free Software Foundation.

  ADDENDUM: How to use this License for your documents

  To use this License in a document you have written, include a copy of
  the License in the document and put the following copyright and
  license notices just after the title page:

  Copyright (c) YEAR YOUR NAME. Permission is granted to copy,
  distribute and/or modify this document under the terms of the GNU Free
  Documentation License, Version 1.2 or any later version published by
  the Free Software Foundation; with no Invariant Sections, no Front-
  Cover Texts, and no Back-Cover Texts. A copy of the license is
  included in the section entitled "GNU Free Documentation License".

  If you have Invariant Sections, Front-Cover Texts and Back-Cover
  Texts, replace the "with...Texts." line with this:

  with the Invariant Sections being LIST THEIR TITLES, with the Front-
  Cover Texts being LIST, and with the Back-Cover Texts being LIST.

  If you have Invariant Sections without Cover Texts, or some other
  combination of the three, merge those two alternatives to suit the

  If your document contains nontrivial examples of program code, we
  recommend releasing these examples in parallel under your choice of
  free software license, such as the GNU General Public License, to
  permit their use in free software.

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