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<title>Samba Authenticated Gateway HOWTO
</title><author>Ricardo Alexandre Mattar
</author><date>v1.1.1, 2003-07-14
</date><abstract>This documents intends to show how to build a Firewall/Gateway
with rules set on user basis having the users authenticated by a
Samba Primary Domain Controller
</abstract><sect>Introduction
<p>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.
</p><p>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.
</p><p>I should be writing an application, but I am too lazy. Hopefully
when the idea is out there, people will build a few intelligently
integrated packages. Meanwhile...
</p><p>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.
</p><p>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?
</p><p>Well, this is my solution. It gives you user authentication and
fine grain control over their access to the external networks.
</p><sect1>Overview
<p>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
rules.
</p><sect1>Disclaimer
<p>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.
</p><p>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 mark.
</p><p>Naming of particular products or brands should not be seen as
endorsements.
</p><sect1>New versions
<p>The newest release of this document can be found at <url url="http://ram.eti.br" name="http://ram.eti.br"> or at <url url="http://www.tldp.org" name="http://www.tldp.org">
</p><p>Related HOWTOs can be found at the Linux Documentation Project
homepage at <url url="http://tldp.org" name="http://tldp.org">.
</p><sect1>Translations
<p>A Portuguese version is available.
</p><p>A French translation by Guillaume Lelarge is available at<url url="http://www.traduc.org/docs/HOWTO/lecture/Samba-Authenticated-Gateway-HOWTO.html" name="http://www.traduc.org">
</p><p>If you want to contribute with a translation, please do.
</p><sect1>Feedback
<p>Contributions and criticism are both welcome.
</p><p>Corrections to my English are also very welcome!
</p><p>If you find any bugs in the scripts included, please tell me.
</p><p>You can find me at ricardo@ram.eti.br or ricardo.mattar@bol.com.br
</p><sect1>Copyright and trademarks
<p>Copyright (c) 2002-2003 Ricardo Alexandre Mattar
</p><p>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".
</p><sect1>Acknowledgments and Thanks
<p>Thanks to Carlos Alberto Reis Ribeiro for introducing me to Linux.
</p><p>Thanks to Cesar Bremer Pinheiro for motivating me to write this
document.
</p><p>Thanks to Guillaume Lelarge for the (continuous) help with the
revision.
</p><p>Thanks to Erik Esplund for further language corrections.
</p><p>Thanks to Albert Teixids for code improvements.
</p><sect>Requirements
<sect1>Knowledge
<p>This document is target at the seasoned systems administrator.
</p><p>You must have a fair knowledge about (at least know what these
are):
</p><p><itemize><item>TCP/IP;
<item>Linux netfilter;
<item>A scripting language (bash?);
<item>SAMBA and Windows networking and domain controllers;
</itemize></p><p>Fortunately, there is plenty of documentation on these topics
on the Internet.
</p><sect1>Software
<p>Installed on your server, you will need at least:
</p><p><itemize><item>Samba;
<item>Iptables;
<item>A scripting language;
</itemize></p><sect>Linux box setup
<p>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.
</p><p>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.
</p><p>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.
</p><sect1>Basic system setup
<p>Generally any basic system setup from the common Linux distributions
will fit in this gateway example. Just check if you have Samba and
IPTABLES.
</p><sect1>Additional directory hierarchy
<p>The additional directory hierarchy will be required to accomplish
the example of this howto:
</p><p>This is used to keep track of the users and IP addresses:
</p><p><verb>/var/run/smbgate/
</verb></p><p>This is where I place user specific scripts:
</p><p><verb>/etc/smbgate/users/
</verb></p><p>And group specific scripts:
</p><p><verb>/etc/smbgate/groups/
</verb></p><p>Directory for the netlogon share:
</p><p><verb>/home/samba/netlogon/
</verb></p><p>Directory for the tracking share:
</p><p><verb>/home/samba/samba/
</verb></p><p>These hierarchies are required by some of the scripts and daemons
of the example.
</p><sect1>Firewall setup
<p>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 <url url="http://www.netfilter.org" name="http://www.netfilter.org"> or <url url="http://www.iptables.org" name="http://www.iptables.org"> to get the software and the documentation.
</p><p>You will need a basic firewall setup in order to get the gateway
working. Take a look at the iptables tutorial at <url url="http://www.netfilter.org/documentation/tutorials/blueflux/iptables-tutorial.html" name="IPTABLES TUTORIAL">. 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:
</p><p><verb>#!/bin/sh
IPTABLES=/usr/sbin/iptables
/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 -P INPUT ACCEPT
$IPTABLES -F INPUT
$IPTABLES -P OUTPUT ACCEPT
$IPTABLES -F OUTPUT
$IPTABLES -P FORWARD ACCEPT
$IPTABLES -F FORWARD
$IPTABLES -t nat -F
</verb></p><p>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.
</p><p>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.
</p><p>You have been warned!
</p><sect1>SAMBA setup
<p>Check if you have Samba installed. If your distribution doesn't
come with Samba pre-packaged then refer to <url url="http://www.samba.org" name="http://www.samba.org"> 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.
</p><p>We will need to setup SAMBA as a Primary Domain Controller. I
will give an example configuration file here, but you should read
the <url url="http://www.samba.org/samba/docs/man/Samba-HOWTO-Collection.html" name="Samba HOWTO Collection"> and learn all you can about a PDC.
</p><sect2>Basic SAMBA setup.
<p>Since I do not intend to rewrite the SAMBA documentation, here
goes a sample smb.conf file:
</p><p><verb># Global parameters
[global]
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
logon script = netlogon.bat
domain logons = Yes
os level = 64
lm announce = True
preferred master = True
domain master = True
dns proxy = No
printing = lprng
[homes]
comment = Home Directories
path = /home/%u
read only = No
[printers]
comment = All Printers
path = /var/spool/samba
printable = Yes
browseable = No
available = No
[netlogon]
comment = NetLogon ShARE
path = /home/samba/netlogon
guest account =
[samba]
comment = login tracking share
path = /home/samba/samba
browseable = No
root preexec = /usr/local/bin/netlogon.sh %u %I
root postexec = /usr/local/bin/netlogoff.sh %u
</verb></p><p>You will have to do with it or read the SAMBA documentation if
you really want to control your server and network.
</p><sect2>The netlogon and the tracking shares
<p>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.
</p><p>As you can see, there must be a line like the following in your
smb.conf
</p><p><verb>logon script = netlogon.bat
</verb></p><p>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.
</p><p><verb>REM NETLOGON.BAT
net use z: \\linux\samba /yes
</verb></p><p>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.
</p><p>Quite simple! But not enough...
</p><p>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:
</p><p><verb>[samba]
comment = login tracking share
path = /home/samba/samba
browseable = No
root preexec = /usr/local/bin/netlogon.sh %u %I
root postexec = /usr/local/bin/netlogoff.sh %u
</verb></p><p>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.
</p><p>Note that the netlogon.sh script must check if the refered workstation
has already mounted the tracking share.
</p><p>Take a look at the netlogon.sh and netlogoff.sh scripts:
</p><p><verb>#!/bin/sh
#
# netlogon.sh
#
# usage:
# netlogon.sh <username>
#
if [ -f /var/run/smbgate/$1 ] ; then
exit 0
fi
echo $2 > /var/run/smbgate/$1
IPTABLES='/usr/sbin/iptables'
EXTIF='eth0'
COMMAND='-A'
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 $EXTIF
else
if [ -f /etc/smbgate/groups/$GROUP ] ; then
/etc/smbgate/groups/$GROUP $COMMAND $ADDRESS $EXTIF
else
/etc/smbgate/users/default.sh $COMMAND $ADDRESS $EXTIF
fi
fi
</verb></p><p>This script (netlogon.sh) 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.
</p><p>Notice that this netlogon.sh 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 default.sh script.
You can modify this logic and behavior as you wish and need, just
remember to modify the others accordingly.
</p><p>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.
</p><p><verb>#!/bin/sh
#
# netlogoff.sh
#
# usage:
# netlogoff.sh <username>
#
IPTABLES='/usr/sbin/iptables'
EXTIF='ppp0'
COMMAND='-D'
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 $EXTIF
else
if [ -f /etc/smbgate/groups/$GROUP ] ; then
/etc/smbgate/groups/$GROUP $COMMAND $ADDRESS $EXTIF
else
/etc/smbgate/users/default.sh $COMMAND $ADDRESS $EXTIF
fi
fi
rm -f /var/run/smbgate/$1
</verb></p><p>This script (netlogoff.sh) is intended to run when the user logs
off and will get the address from the /var/run/smbgate/user file
which will be passed as an argument to the /etc/smbgate/users/user
script which will update the firewall to the state desired when the
user is not logged in.
</p><p>Some versions of Windows, such as Windows 2000, mount the tracking
share more than once per login. This may cause problems with the
netlogon.sh and netlogoff.sh, triggering the scripts more the once.
This can make a real mess. So, you may prefer to use a logout checking
script at cron instead of a netlogoff.sh script triggered by SAMBA.
Here is an example:
</p><p><verb>#!/bin/sh
# checklogout.sh
#
# usage:
# intended to run at cron (maybe each 10 minutes)
TRACKDIR="/var/run/smbgate"
DIRLENGTH=${#TRACKDIR}
TRACKSHARE="samba"
EXTIF='eth0'
COMMAND='-D'
if [ -d $TRACKDIR ]; then
for n in $TRACKDIR/*; do
[ -d $n ] && continue;
if [ -f $n ] ; then
IPADDRESS=`cat $n`
USERNAME=${n:$DIRLENGTH+1}
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 $EXTIF
else
if [ -f /etc/smbgate/groups/$GROUP ] ; then
/etc/smbgate/groups/$GROUP $COMMAND $IPADDRESS $EXTIF
else
/etc/smbgate/users/default.sh $COMMAND $IPADDRESS $EXTIF
fi
fi
fi
else
exit 0
fi
done
fi
</verb></p><p>In that case you should remove the root postexec clause from
the tracking share on smb.conf:
</p><p><verb>root postexec = /usr/local/bin/netlogoff.sh %u
</verb></p><p>The following is a standard /etc/smbgate/users/user script. This
is the one which will actually modify the firewall rules.
</p><p><verb>#!/bin/sh
#
COMMAND=$1
ADDRESS=$2
EXTIF=$3
IPTABLES='/usr/sbin/iptables'
$IPTABLES $COMMAND POSTROUTING -t nat -s $ADDRESS -o $EXTIF -j MASQUERADE
</verb></p><p>We should also hase a default.sh script at /etc/smbgate/users/
to give the gateway a default behavior.
</p><p><verb>#!/bin/sh
#
# default.sh
COMMAND=$1
ADDRESS=$2
EXTIF=$3
IPTABLES='/usr/sbin/iptables'
#$IPTABLES $COMMAND POSTROUTING -t nat -s $ADDRESS -o $EXTIF -j MASQUERADE
exit 0
</verb></p><sect>Windows workstation setup
<sect1>Introduction
<p>We will stick to setting up the network, user management and
policies on the Windows workstations.
</p><p>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.
</p><sect1>Network protocols
<p>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?
</p><sect1>DHCP setup
<p>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.
</p><sect1>Join your Linux server domain
<p>Configure the Windows workstation to log in a Domain, and give
the domain name of your Linux server. This is essential to the gateway
work.
</p><p>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.
</p><sect2>Windows 95/98
<p>These versions seems to need no special configuration to join
the Linux PDC domain.
</p><sect2>Windows NT and 2000
<p>These versions require machine accounts at the Linux box. Again,
check the SAMBA documentation.
</p><sect2>Windows XP
<p>This version needs a machine account at the Linux box and a tweak
at the registry, as follows.
</p><p>Locate the key "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Netlogon\Parameters\RequireSignOrSeal".
The default value is 1. Set it to 0 and it will no more complain
about joining the domain.
</p><p>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.
</p><p><verb>Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Netlogon\Parameters]
"requiresignorseal"=dword:00000000
</verb></p><p>This version also needs an adjust at the NETLOGON.BAT script.
Sometimes it insists on making the mounting persistent.
</p><p><verb>REM NETLOGON.BAT
net use z: \\linux\samba /yes /persistent:no
</verb></p><sect1>Policy editor
<p>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.
</p><p>Unfortunately, this tool does not generate a plain text configuration
file, so I can't place an example here.
</p><p>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.
</p><p>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
you.
</p><sect>User management
<sect1>Adding users
<p>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.
</p><sect1>Password management
<p>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.
</p><p>Try the package at <url url="http://changepassword.sourceforge.net" name="http://changepassword.sourceforge.net">, it seems to be a good choice.
</p><sect1>Granting or denying access to users
<p>As you could see in a previous section of this howto, the SAMBA
daemon will call a netlogon.sh script every time the tracking share
is mounted. This netlogon.sh 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.
</p><p>For example if you want to give the user full access to internet:
</p><p><verb>#!/bin/sh
#
COMMAND=$1
ADDRESS=$2
EXTIF=$3
IPTABLES='/usr/sbin/iptables'
$IPTABLES $COMMAND POSTROUTING -t nat -s $ADDRESS -o $EXTIF -j MASQUERADE
</verb></p><p>If you don't want to change anything to a particular user, just
give him an empty script:
</p><p><verb>#/bin/sh
#
exit 0
</verb></p><p>Or just don't create any script for the less privileged users,
letting them have the default.sh script, which would be empty as
the previous or just give limited access as follows:
</p><p><verb>#!/bin/sh
#
COMMAND=$1
ADDRESS=$2
EXTIF=$3
EXTIFADDRESS=$4
IPTABLES='/usr/sbin/iptables'
$IPTABLES $COMMAND POSTROUTING -t nat -s $ADDRESS -o $EXTIF --dport 25 -j SNAT --to-source $EXTIFADDRESS
$IPTABLES $COMMAND POSTROUTING -t nat -s $ADDRESS -o $EXTIF --dport 110 -j SNAT --to-source $EXTIFADDRESS
</verb></p><p>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.
</p><sect>Group management
<sect1>Creating groups
<p>Just create your user groups in the Linux PDC and add the users
to the groups. This is it.
</p><p>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.
</p><sect1>Group policy
<p>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.
</p><p>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 netlogon.sh, netlogoff.sh
(or the checklogout.sh) scripts. The whole logic is in these scripts.
</p><sect>Bibliography
<p><url url="http://www.netfilter.org/documentation/tutorials/blueflux/iptables-tutorial.html" name="IPTABLES TUTORIAL"> by Oskar Andreasson
</p><p><url url="http://www.samba.org/samba/docs/man/Samba-HOWTO-Collection.html" name="Samba HOWTO Collection"> by the SAMBA Team
</p><sect>GNU Free Documentation License
<p>GNU Free Documentation License Version 1.2, November 2002
</p><p>Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. 59
Temple Place, Suite 330, Boston, MA 02111-1307 USA Everyone is permitted
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is a licensee, and is addressed as "you". You accept the
license if you copy, modify or distribute the work in a way requiring
permission under copyright law.
</p><p>A "Modified Version" of the Document means any work
containing the Document or a portion of it, either copied verbatim,
or with modifications and/or translated into another language.
</p><p>A "Secondary Section" is a named appendix or a front-matter
section of the Document that deals exclusively with the relationship
of the publishers or authors of the Document to the Document's overall
subject (or to related matters) and contains nothing that could fall
directly within that overall subject. (Thus, if the Document is in
part a textbook of mathematics, a Secondary Section may not explain
any mathematics.) The relationship could be a matter of historical
connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding
them.
</p><p>The "Invariant Sections" are certain Secondary Sections
whose titles are designated, as being those of Invariant Sections,
in the notice 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.
</p><p>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.
</p><p>A "Transparent" copy of the Document means a machine-readable
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A copy made in an otherwise Transparent file format whose markup,
or absence of markup, has been arranged to thwart or discourage subsequent
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is not "Transparent" is called "Opaque".
</p><p>Examples of suitable formats for Transparent copies include plain
ASCII without markup, Texinfo input format, LaTeX input format, SGML
or XML using a publicly available DTD, and standard-conforming simple
HTML, PostScript or PDF designed for human modification. Examples
of 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 processing
tools are not generally available, and the machine-generated HTML,
PostScript or PDF produced by some word processors for output purposes
only.
</p><p>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.
</p><p>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 specific section name mentioned below, such as "Acknowledgements",
"Dedications", "Endorsements", or "History".)
To "Preserve the Title" of such a section when you modify
the Document means that it remains a section "Entitled XYZ"
according to this definition.
</p><p>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.
</p><p>2. VERBATIM COPYING
</p><p>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
number of copies you must also follow the conditions in section 3.
</p><p>You may also lend copies, under the same conditions stated above,
and you may publicly display copies.
</p><p>3. COPYING IN QUANTITY
</p><p>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 as the publisher of these copies. The front cover must present
the full title with all words of the title equally prominent and
visible. You may add other material on the covers in addition. Copying
with changes limited to the covers, as long as they preserve the
title of the Document and satisfy these conditions, can be treated
as verbatim copying in other respects.
</p><p>If the required texts for either cover are too voluminous to
fit legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.
</p><p>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.
</p><p>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
Document.
</p><p>4. MODIFICATIONS
</p><p>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:
</p><p>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.
</p><p>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.
</p><p>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 standard.
</p><p>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.
</p><p>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.
</p><p>5. COMBINING DOCUMENTS
</p><p>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.
</p><p>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.
</p><p>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".
</p><p>6. COLLECTIONS OF DOCUMENTS
</p><p>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.
</p><p>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
document.
</p><p>7. AGGREGATION WITH INDEPENDENT WORKS
</p><p>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.
</p><p>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 aggregate.
</p><p>8. TRANSLATION
</p><p>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.
</p><p>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 title.
</p><p>9. TERMINATION
</p><p>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.
</p><p>10. FUTURE REVISIONS OF THIS LICENSE
</p><p>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 http://www.gnu.org/copyleft/.
</p><p>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.
</p><p>ADDENDUM: How to use this License for your documents
</p><p>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:
</p><p>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".
</p><p>If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:
</p><p>with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts being
LIST.
</p><p>If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.
</p><p>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.
</p>
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