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|
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Authentication, Authorization, and Access Control</title>
</head>
<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
vlink="#000080" alink="#FF0000">
<!--#include virtual="header.html" -->
<h1 align="center">Authentication, Authorization, and Access
Control</h1>
<a name="TOC"></a>
<ul>
<li><a href="#intro">Introduction</a></li>
<li>
<a href="#basic">Basic authentication</a>
<ul>
<li><a href="#basicworks">How basic
authentication works</a></li>
<li>
<a href="#basicconfig">Configuration:
Protecting content with basic authentication</a>
</li>
<li>
<a href="#basicfaq">Frequently asked
questions about basic auth</a>
</li>
<li><a href="#basiccaveat">Security
caveat</a></li>
</ul>
<br />
</li>
<li>
<a href="#digest">Digest authentication</a>
<ul>
<li><a href="#digestworks">How digest auth
works</a></li>
<li>
<a href="#digestconfig">Configuration:
Protecting content with digest authentication</a>
</li>
<li><a href="#digestcaveat">Caveats</a></li>
</ul>
<br />
</li>
<li>
<a href="#database">Database authentication
modules</a>
<ul>
<li><a href="#modauthdb">mod_auth_db and
mod_auth_dbm</a></li>
<li><a href="#dbfiles">Berkeley DB files</a></li>
<li><a href="#installauthdb">Installing mod_auth_db</a></li>
<li>
<a href="#authdbconfig">Protecting a
directory with mod_auth_db</a>
</li>
</ul>
<br />
</li>
<li>
<a href="#access">Access control</a>
<ul>
<li><a href="#allowdeny">Allow and Deny</a></li>
<li><a href="#satisfy">Satisfy</a></li>
</ul>
<br />
</li>
<li><a href="#summary">Summary</a></li>
</ul>
<!--End of Table of Child-Links-->
<hr />
<h1><a name="auth"></a><br />
Authentication, Authorization, and Access Control</h1>
<h1><a name="intro">Introduction</a></h1>
<p>Apache has three distinct ways of dealing with the question
of whether a particular request for a resource will result in
that resource actually be returned. These criteria are called
<i>Authorization</i>, <i>Authentication</i>, and <i>Access
control</i>.</p>
<p>Authentication is any process by which you verify that
someone is who they claim they are. This usually involves a
username and a password, but can include any other method of
demonstrating identity, such as a smart card, retina scan,
voice recognition, or fingerprints. Authentication is
equivalent to showing your drivers license at the ticket
counter at the airport.</p>
<p>Authorization is finding out if the person, once identified,
is permitted to have the resource. This is usually determined
by finding out if that person is a part of a particular group,
if that person has paid admission, or has a particular level of
security clearance. Authorization is equivalent to checking the
guest list at an exclusive party, or checking for your ticket
when you go to the opera.</p>
<p>Finally, access control is a much more general way of
talking about controlling access to a web resource. Access can
be granted or denied based on a wide variety of criteria, such
as the network address of the client, the time of day, the
phase of the moon, or the browser which the visitor is using.
Access control is analogous to locking the gate at closing
time, or only letting people onto the ride who are more than 48
inches tall - it's controlling entrance by some arbitrary
condition which may or may not have anything to do with the
attributes of the particular visitor.</p>
<p>Because these three techniques are so closely related in
most real applications, it is difficult to talk about them
separate from one another. In particular, authentication and
authorization are, in most actual implementations,
inextricable.</p>
<p>If you have information on your web site that is sensitive,
or intended for only a small group of people, the techniques in
this tutorial will help you make sure that the people that see
those pages are the people that you wanted to see them.</p>
<h1><a name="basic"></a>Basic authentication</h1>
<p>As the name implies, basic authentication is the simplest
method of authentication, and for a long time was the most
common authentication method used. However, other methods of
authentication have recently passed basic in common usage, due
to usability issues that will be discussed in a minute.</p>
<h2><a name="basicworks"></a><br />
How basic authentication works</h2>
<p>When a particular resource has been protected using basic
authentication, Apache sends a <tt>401 Authentication
Required</tt> header with the response to the request, in order
to notify the client that user credentials must be supplied in
order for the resource to be returned as requested.</p>
<p>Upon receiving a <tt>401</tt> response header, the client's
browser, if it supports basic authentication, will ask the user
to supply a username and password to be sent to the server. If
you are using a graphical browser, such as Netscape or Internet
Explorer, what you will see is a box which pops up and gives
you a place to type in your username and password, to be sent
back to the server. If the username is in the approved list,
and if the password supplied is correct, the resource will be
returned to the client.</p>
<p>Because the HTTP protocol is stateless, each request will be
treated in the same way, even though they are from the same
client. That is, every resource which is requested from the
server will have to supply authentication credentials over
again in order to receive the resource.</p>
<p>Fortunately, the browser takes care of the details here, so
that you only have to type in your username and password one
time per browser session - that is, you might have to type it
in again the next time you open up your browser and visit the
same web site.</p>
<p>Along with the <tt>401</tt> response, certain other
information will be passed back to the client. In particular,
it sends a name which is associated with the protected area of
the web site. This is called the <i>realm</i>, or just the
authentication name. The client
browser caches the username and password that you supplied, and
stores it along with the authentication realm, so that if other
resources are requested from the same realm, the same username
and password can be returned to authenticate that request
without requiring the user to type them in again. This caching
is usually just for the current browser session, but some
browsers allow you to store them permanently, so that you never
have to type in your password again.</p>
<p>The authentication name, or realm, will appear in the pop-up
box, in order to identify what the username and password are
being requested for.</p>
<h2><a name="basicconfig"></a>
Configuration: Protecting content with basic
authentication</h2>
<p>There are two configuration steps which you must complete in
order to protect a resource using basic authentication. Or
three, depending on what you are trying to do.</p>
<ol>
<li>Create a password file</li>
<li>Set the configuration to use this password file</li>
<li>Optionally, create a group file</li>
</ol>
<h3><a name="htpasswd"></a><br />
Create a password file</h3>
<p>In order to determine whether a particular username/password
combination is valid, the username and password supplied by the
user will need to be compared to some authoritative listing of
usernames and password. This is the password file, which you
will need to create on the server side, and populate with valid
users and their passwords.</p>
<p>Because this file contains sensitive information, it should
be stored outside of the document directory. Although, as you
will see in a moment, the passwords are encrypted in the file,
if a cracker were to gain access to the file, it would be an
aid in their attempt to figure out the passwords. And, because
people tend to be sloppy with the passwords that they choose,
and use the same password for web site authentication as for
their bank account, this potentially be a very serious breach
of security, even if the content on your web site is not
particularly sensitive.</p>
<p><b>Caution:</b> Encourage your users to use a different
password for your web site than for other more essential
things. For example, many people tend to use two passwords -
one for all of their extremely important things, such as the
login to their desktop computer, and for their bank account,
and another for less sensitive things, the compromise of which
would be less serious.</p>
<p>To create the password file, use the <tt>htpasswd</tt>
utility that came with Apache. This will be located in the
<tt>bin</tt> directory of wherever you installed Apache. For
example, it will probably be located at
<tt>/usr/local/apache/bin/htpasswd</tt> if you installed Apache
from source.</p>
<p>To create the file, type:</p>
<pre>
htpasswd -c /usr/local/apache/passwd/passwords username
</pre>
<p><tt>htpasswd</tt> will ask you for the password, and then
ask you to type it again to confirm it:</p>
<pre>
# htpasswd -c /usr/local/apache/passwd/passwords rbowen
New password: mypassword
Re-type new password: mypassword
Adding password for user rbowen
</pre>
<p>Note that in the example shown, a password file is being
created containing a user called <tt>rbowen</tt>, and this
password file is being placed in the location
<tt>/usr/local/apache/passwd/passwords</tt>. You will
substitute the location, and the username, which you want to
use to start your password file.</p>
<p>If <tt>htpasswd</tt> is not in your path, you will have to
type the full path to the file to get it to run. That is, in
the example above, you would replace <tt>htpasswd</tt> with
<tt>/usr/local/apache/bin/htpasswd</tt></p>
<p>The <tt>-c</tt> flag is used only when you are creating a
new file. After the first time, you will omit the <tt>-c</tt>
flag, when you are adding new users to an already-existing
password file.</p>
<pre>
htpasswd /usr/local/apache/passwd/passwords sungo
</pre>
<p>The example just shown will add a user named <tt>sungo</tt>
to a password file which has already been created earlier. As
before, you will be asked for the password at the command line,
and then will be asked to confirm the password by typing it
again.</p>
<p><b>Caution:</b> Be very careful when you add new users to an
existing password file that you don't use the <tt>-c</tt> flag
by mistake. Using the <tt>-c</tt> flag will create a new
password file, even if you already have an existing file of
that name. That is, it will remove the contents of the file
that is there, and replace it with a new file containing only
the one username which you were adding.</p>
<p>The password is stored in the password file in encrypted
form, so that users on the system will not be able to read the
file and immediately determine the passwords of all the users.
Nevertheless, you should store the file in as secure a location
as possible, with whatever minimum permissions on the file so
that the web server itself can read the file. For example, if
your server is configured to run as user <tt>nobody</tt> and
group <tt>nogroup</tt>, then you should set permissions on the
file so that only the webserver can read the file and only
root can write to it:</p>
<pre>
chown root.nogroup /usr/local/apache/passwd/passwords
chmod 640 /usr/local/apache/passwd/passwords
</pre>
<p>On Windows, a similar precaution should be taken, changing
the ownership of the password file to the web server user, so
that other users cannot read the file.</p>
<h3><a name="htpasswdconfig"></a><br />
Set the configuration to use this password file</h3>
<p>Once you have created the password file, you need to tell
Apache about it, and tell Apache to use this file in order to
require user credentials for admission. This configuration is
done with the following directives:</p>
<table cellpadding="3">
<tr>
<td align="left">AuthType</td>
<td align="left" valign="top" width="360">Authentication
type being used. In this case, it will be set to
<tt>Basic</tt></td>
</tr>
<tr>
<td align="left">AuthName</td>
<td align="left" valign="top" width="360">The
authentication realm or name</td>
</tr>
<tr>
<td align="left">AuthUserFile</td>
<td align="left" valign="top" width="360">The location of
the password file</td>
</tr>
<tr>
<td align="left">AuthGroupFile</td>
<td align="left" valign="top" width="360">The location of
the group file, if any</td>
</tr>
<tr>
<td align="left">Require</td>
<td align="left" valign="top" width="360">The
requirement(s) which must be satisfied in order to grant
admission</td>
</tr>
</table>
<p>These directives may be placed in a <tt>.htaccess</tt> file
in the particular directory being protected, or may go in the
main server configuration file, in a <tt><Directory></tt>
section, or other scope container.</p>
<p>The example shown below defines an authentication realm
called ``By Invitation Only''. The password file located at
<tt>/usr/local/apache/passwd/passwords</tt> will be used to
verify the user's identity. Only users named <tt>rbowen</tt> or
<tt>sungo</tt> will be granted access, and even then only if
they provide a password which matches the password stored in
the password file.</p>
<pre>
AuthType Basic
AuthName "By Invitation Only"
AuthUserFile /usr/local/apache/passwd/passwords
Require user rbowen sungo
</pre>
<p>The phrase ``By Invitation Only'' will be displayed in the
password pop-up box, where the user will have to type their
credentials.</p>
<p>You will need to restart your Apache server in order for the
new configuration to take effect, if these directives were put
in the main server configuration file. Directives placed in
<tt>.htaccess</tt> files take effect immediately, since
<tt>.htaccess</tt> files are parsed each time files are
served.</p>
<p>The next time that you load a file from that directory, you
will see the familiar username/password dialog box pop up,
requiring that you type the username and password before you
are permitted to proceed.</p>
<p>Note that in addition to specifically listing the users to
whom you want to grant access, you can specify that any valid
user should be let in. This is done with the
<tt>valid-user</tt> keyword:</p>
<pre>
Require valid-user
</pre>
<h3><a name="basicgroupfile"></a><br />
Optionally, create a group file</h3>
<p>Most of the time, you will want more than one, or two, or
even a dozen, people to have access to a resource. You want to
be able to define a group of people that have access to that
resource, and be able to manage that group of people, adding
and removing members, without having to edit the server
configuration file, and restart Apache, each time.</p>
<p>This is handled using authentication groups. An
authentication group is, as you would expect, a group name
associated with a list of members. This list is stored in a
group file, which should be stored in the same location as the
password file, so that you are able to keep track of these
things.</p>
<p>The format of the group file is exceedingly simple. A group
name appears first on a line, followed by a colon, and then a
list of the members of the group, separated by spaces. For
example:</p>
<pre>
authors: rich daniel allan
</pre>
<p>Once this file has been created, you can <tt>Require</tt>
that someone be in a particular group in order to get the
requested resource. This is done with the
<tt>AuthGroupFile</tt> directive, as shown in the following
example.</p>
<pre>
AuthType Basic
AuthName "Apache Admin Guide Authors"
AuthUserFile /usr/local/apache/passwd/passwords
AuthGroupFile /usr/local/apache/passwd/groups
Require group authors
</pre>
<p>The authentication process is now one step more involved.
When a request is received, and the requested username and
password are supplied, the group file is first checked to see
if the supplied username is even in the required group. If it
is, then the password file will be checked to see if the
username is in there, and if the supplied password matches the
password stored in that file. If any of these steps fail,
access will be forbidden.</p>
<h2><a name="basicfaq"></a><br />
Frequently asked questions about basic auth</h2>
<p>The following questions tend to get asked very frequently
with regard to basic authentication. It should be understood
that basic authentication is very basic, and so is limited to
the set of features that has been presented above. Most of the
more interesting things that people tend to want, need to be
implemented using some alternate authentication scheme.</p>
<h3><a name="logout"></a><br />
How do I log out?</h3>
<p>Since browsers first started implementing basic
authentication, website administrators have wanted to know how
to let the user log out. Since the browser caches the username
and password with the authentication realm, as described
earlier in this tutorial, this is not a function of the server
configuration, but is a question of getting the browser to
forget the credential information, so that the next time the
resource is requested, the username and password must be
supplied again. There are numerous situations in which this is
desirable, such as when using a browser in a public location,
and not wishing to leave the browser logged in, so that the
next person can get into your bank account.</p>
<p>However, although this is perhaps the most frequently asked
question about basic authentication, thus far none of the major
browser manufacturers have seen this as being a desirable
feature to put into their products.</p>
<p>Consequently, the answer to this question is, you can't.
Sorry.</p>
<h3><a name="passworddialog"></a><br />
How can I change what the password box looks like?</h3>
<p>The dialog that pops up for the user to enter their username
and password is ugly. It contains text that you did not
indicate that you wanted in there. It looks different in
Internet Explorer and Netscape, and contains different text.
And it asks for fields that the user might not understand -
for example, Netscape asks the user to type in their ``User
ID'', and they might not know what that means. Or, you might
want to provide additional explanatory text so that the user
has a better idea what is going on.</p>
<p>Unfortunately, these things are features of the browser, and
cannot be controlled from the server side. If you want the
login to look different, then you will need to implement your
own authentication scheme. There is no way to change what this
login box looks like if you are using basic authentication.</p>
<h3><a name="persistpass"></a><br />
How to I make it not ask me for my password the next
time?</h3>
<p>Because most browsers store your password information only
for the current browser session, when you close your browser it
forgets your username and password. So, when you visit the same
web site again, you will need to re-enter your username and
password.</p>
<p>There is nothing that can be done about this on the server
side.</p>
<p>However, the most recent versions of the major browsers
contain the ability to remember your password forever, so that
you never have to log in again. While it is debatable whether
this is a good idea, since it effectively overrides the entire
point of having security in the first place, it is certainly
convenient for the user, and simplifies the user
experience.</p>
<h3><a name="passwordtwice"></a><br />
Why does it sometimes ask me for my password twice?</h3>
<p>When entering a password-protected web site for the first
time, you will occasionally notice that you are asked for your
password twice. This may happen immediately after you entered
the password the first time, or it may happen when you click on
the first link after authenticating the first time.</p>
<p>This happens for a very simple, but nonetheless confusing,
reason, again having to do with the way that the browser caches
the login information.</p>
<p>Login information is stored on the browser based on the
authentication realm, specified by the <tt>AuthName</tt>
directive, and by the server name. In this way, the browser can
distinguish between the <tt>Private</tt> authentication realm
on one site and on another. So, if you go to a site using one
name for the server, and internal links on the server refer to
that server by a different name, the browser has no way to know
that they are in fact the same server.</p>
<p>For example, if you were to visit the URL
<tt>http://example.com/private/</tt>, which required
authentication, your browser would remember the supplied
username and password, associated with the hostname
<tt>example.com</tt>. If, by virtue of an internal redirect, or
fully-qualified HTML links in pages, you are then sent to the
URL <tt>http://www.example.com/private/</tt>, even though this
is really exactly the same URL, the browser does not know this
for sure, and is forced to request the authentication
information again, since <tt>example.com</tt> and
<tt>www.example.com</tt> are not exactly the same hostname.
Your browser has no particular way to know that these are the
same web site.</p>
<h2><a name="basiccaveat"></a><br />
Security caveat</h2>
<p>Basic authentication should not be considered secure for any
particularly rigorous definition of secure.</p>
<p>Although the password is stored on the server in encrypted
format, it is passed from the client to the server in plain
text across the network. Anyone listening with any variety of
packet sniffer will be able to read the username and password
in the clear as it goes across.</p>
<p>Not only that, but remember that the username and password
are passed with every request, not just when the user first
types them in. So the packet sniffer need not be listening at a
particularly strategic time, but just for long enough to see
any single request come across the wire.</p>
<p>And, in addition to that, the content itself is also going
across the network in the clear, and so if the web site
contains sensitive information, the same packet sniffer would
have access to that information as it went past, even if the
username and password were not used to gain direct access to
the web site.</p>
<p>Don't use basic authentication for anything that requires
real security. It is a detriment for most users, since very few
people will take the trouble, or have the necessary software
and/or equipment, to find out passwords. However, if someone
had a desire to get in, it would take very little for them to
do so.</p>
<p>Basic authentication across an SSL connection, however, will be
secure, since everything is going to be encrypted, including the
username and password.</p>
<h1><a name="digest"></a>Digest authentication</h1>
<p>Addressing one of the security caveats of basic
authentication, digest authentication provides an alternate
method for protecting your web content. However, it to has a
few caveats.</p>
<h2><a name="digestworks">How digest auth works</a></h2>
<p>Digest authentication is implemented by the module
<tt>mod_auth_digest</tt>. There is an older module,
<tt>mod_digest</tt>, which implemented an older version of the
digest authentication specification, but which will probably
not work with newer browsers.</p>
<p>Using digest authentication, your password is never sent
across the network in the clear, but is always transmitted as
an MD5 digest of the user's password. In this way, the password
cannot be determined by sniffing network traffic.</p>
<p>The full specification of digest authentication can be seen
in the internet standards document RFC 2617, which you can see
at <tt>http://www1.ics.uci.edu/pub/ietf/http/rfc2617.txt</tt>.
Additional information and resources about MD5 can be found at
<tt>http://userpages.umbc.edu/ mabzug1/cs/md5/md5.html</tt></p>
<h2><a name="digestconfig"></a>Configuration:
Protecting content with digest authentication</h2>
<p>The steps for configuring your server for digest
authentication are very similar for those for basic
authentication.</p>
<ol>
<li>Create the password file</li>
<li>Set the configuration to use this password file</li>
<li>Optionally, create a group file</li>
</ol>
<h3><a name="htdigest"></a>Creating a password file</h3>
<p>As with basic authentication, a simple utility is provided
to create and maintain the password file which will be used to
determine whether a particular user's name and password are
valid. This utility is called <tt>htdigest</tt>, and will be
located in the <tt>bin</tt> directory of wherever you installed
Apache. If you installed Apache from some variety of package
manager, <tt>htdigest</tt> is likely to have been placed
somewhere in your path.</p>
<p>To create a new digest password file, type:</p>
<pre>
htdigest -c /usr/local/apache/passwd/digest realm username
</pre>
<p><tt>htdigest</tt> will ask you for the desired password, and
then ask you to type it again to confirm it.</p>
<p>Note that the realm for which the authentication will be
required is part of the argument list.</p>
<p>Once again, as with basic authentication, you are encouraged
to place the generated file somewhere outside of the document
directory.</p>
<p>And, as with the <tt>htpasswd</tt> utility, the <tt>-c</tt>
flag creates a new file, or, if a file of that name already
exists, deletes the contents of that file and generates a new
file in its place. Omit the <tt>-c</tt> flag in order to add
new user information to an existing password file.</p>
<h3><a name="htdigestconfig"></a>Set the configuration
to use this password file</h3>
<p>Once you have created a password file, you need to tell
Apache about it in order to start using it as a source of
authenticated user information. This configuration is done with
the following directives:</p>
<table cellpadding="3">
<tr>
<td align="left">AuthType</td>
<td align="left" valign="top" width="360">Authentication
type being used. In this case, it will be set to
<tt>Digest</tt></td>
</tr>
<tr>
<td align="left">AuthName</td>
<td align="left" valign="top" width="360">The
authentication realm or name</td>
</tr>
<tr>
<td align="left">AuthDigestFile</td>
<td align="left" valign="top" width="360">The location of
the password file</td>
</tr>
<tr>
<td align="left">AuthDigestGroupFile</td>
<td align="left" valign="top" width="360">Location of the
group file, if any</td>
</tr>
<tr>
<td align="left">Require</td>
<td align="left" valign="top" width="360">The
requirement(s) which must be satisfied in order to grant
admission</td>
</tr>
</table>
<p>These directives may be placed in a <tt>.htaccess</tt> file
in the particular directory being protected, or may go in the
main server configuration file, in a <tt><Directory></tt>
section, or another scope container.</p>
<p>The following example defines an authentication realm called
"Private". The password file located at
<tt>/usr/local/apache/passwd/digest</tt> will be used to verify
the user's identity. Only users named <tt>drbacchus</tt> or
<tt>dorfl</tt> will be granted access, if they provide a
password that patches the password stored in the password
file.</p>
<pre>
AuthType Digest
AuthName "Private"
AuthDigestFile /usr/local/apache/passwd/digest
Require user drbacchus dorfl
</pre>
<p>The phrase "Private" will be displayed in the password
pop-up box, where the user will have to type their
credentials.</p>
<h3><a name="digestgroup"></a>Optionally, create a group file</h3>
<p>As you have observed, there are not many differences between
this configuration process and that required by basic
authentication, described in the previous section. This is true
also of group functionality. The group file used for digest
authentication is exactly the same as that used for basic
authentication. That is to say, lines in the group file consist
the name of the group, a colon, and a list of the members of
that group. For example:</p>
<pre>
admins: jim roy ed anne
</pre>
<p>Once this file has been created, you can <tt>Require</tt>
that someone be in a particular group in order to get the
requested resource. This is done with the
<tt>AuthDigestGroupFile</tt> directive, as shown in the
following example.</p>
<pre>
AuthType Digest
AuthName "Private"
AuthDigestFile /usr/local/apache/passwd/digest
AuthDigestGroupFile /usr/local/apache/passwd/digest.groups
Require group admins
</pre>
<p>The authentication process is the same as that used by basic
authentication. It is first verified that the user is in the
required group, and, if this is true, then the password is
verified.</p>
<h2><a name="digestcaveat">Caveats</a></h2>
<p>Before you leap into using digest authentication instead of
basic authentication, there are a few things that you should
know about.</p>
<p>Most importantly, you need to know that, although digest
authentication has this great advantage that you don't send
your password across the network in the clear, it is not
supported by all major browsers in use today, and so you should
not use it on a web site on which you cannot control the
browsers that people will be using, such as on your intranet
site. In particular, Opera 4.0 or later, Microsoft Internet
Explorer 5.0 or later, Mozilla 1.0.1 and Netscape 7 or later
as well as Amaya support digest authentication, while various
other browsers do not.</p>
<p>Next, with regard to security considerations, you should
understand two things. Although your password is not passed in
the clear, all of your data is, and so this is a rather small
measure of security. And, although your password is not really
sent at all, but a digest form of it, someone very familiar
with the workings of HTTP could use that information - just
your digested password - and use that to gain access to the
content, since that digested password is really all the
information required to access the web site.</p>
<p>The moral of this is that if you have content that really
needs to be kept secure, use SSL.</p>
<h1><a name="database">Database authentication
modules</a></h1>
<p>Basic authentication and digest authentication both suffer
from the same major flaw. They use text files to store the
authentication information. The problem with this is that
looking something up in a text file is very slow. It's rather
like trying to find something in a book that has no index. You
have to start at the beginning, and work through it one page at
a time until you find what you are looking for. Now imagine
that the next time you need to find the same thing, you don't
remember where it was before, so you have to start at the
beginning again, and work through one page at a time until you
find it again. And the next time. And the time after that.</p>
<p>Since HTTP is stateless, authentication has to be verified
every time that content is requested. And so every time a
document is accessed which is secured with basic or digest
authentication, Apache has to open up those text password files
and look through them one line at a time, until it finds the
user that is trying to log in, and verifies their password. In
the worst case, if the username supplied is not in there at
all, every line in the file will need to be checked. On
average, half of the file will need to be read before the user
is found. This is very slow.</p>
<p>While this is not a big problem for small sets of users,
when you get into larger numbers of users (where "larger" means
a few hundred) this becomes prohibitively slow. In many cases,
in fact, valid username/password combinations will get rejected
because the authentication module just had to spend so much
time looking for the username in the file that Apache will just
get tired of waiting and return a failed authentication.</p>
<p>In these cases, you need an alternative, and that
alternative is to use some variety of database. Databases are
optimized for looking for a particular piece of information in
a very large data set. It builds indexes in order to rapidly
locate a particular record, and they have query languages for
swiftly locating records that match particular criteria.</p>
<p>There are numerous modules available for Apache to
authenticate using a variety of different databases. In this
section, we'll just look at two modules which ship with Apache.
</p>
<h2><a name="modauthdb"></a>mod_auth_db and mod_auth_dbm</h2>
<p><tt>mod_auth_db</tt> and <tt>mod_auth_dbm</tt> are modules
which lets you keep your usernames and passwords in DB or DBM
files. There are few practical differences between DB files and
DBM files. And, on some operating systems, such as various
BSDs, and Linux, they are exactly the same thing. You should
pick whichever of the two modules makes the most sense on your
particular platform of choice. If you do not have DB support on
your platform, you may need to install it. You download an
implementation of DB at <tt>http://www.sleepycat.com/</tt>.
<h2><a name="dbfiles"></a>Berkeley DB files</h2>
<p>DB files, also known as Berkeley database files, are the
simplest form of database, and are rather ideally suited for
the sort of data that needs to be stored for HTTP
authentication. DB files store key/value pairs. That is, the
name of a variable, and the value of that variable. While other
databases allow the storage of many fields in a given record, a
DB file allows only this pairing of key and value.<a
name="foot1_return" href="#foot1"><sup>1</sup></a> This is ideal for
authentication, which requires only the pair of a username and
password.</p>
<h2><a name="installauthdb">Installing mod_auth_db</a></h2>
<p>For the purposes of this tutorial, we'll talk about
installing and configuring <tt>mod_auth_db</tt>. However,
everything that is said here can be directly applied to
<tt>mod_auth_dbm</tt> by simply replacing 'db' with 'dbm' and
'DB' with 'DBM' in the various commands, file names, and
directives.</p>
<p>Since <tt>mod_auth_db</tt> is not compiled in by default,
you will need to rebuild Apache in order to get the
functionality, unless you built in everything when we started.
Note that if you installed Apache with shared object
support, you may be able to just build the module and load it
in to Apache.</p>
<p>To build Apache from scratch with <tt>mod_auth_db</tt> built
in, use the following <tt>./configure</tt> line in your apache
source code directory.</p>
<pre>
./configure --enable-module=auth_db
</pre>
<p>Or, if you had a more complex <tt>configure</tt> command
line, you can just add the <tt>-enable-module=auth_db</tt>
option to that command line, and you'll get
<tt>mod_auth_db</tt> built into your server.</p>
<h2><a name="authdbconfig">Protecting a directory with
mod_auth_db</a></h2>
<p>Once you have compiled the <tt>mod_auth_db</tt> module, and
loaded it into your web server, you'll find that there's very
little difference between using regular authentication and
using <tt>mod_auth_db</tt> authentication. The procedure is the
same as that we went through with basic and digest
authentication:</p>
<ol>
<li>Create the user file.</li>
<li>Configure Apache to use that file for
authentication.</li>
<li>Optionally, create a group file.</li>
</ol>
<h3><a name="dbmmanage"></a>Create the user file</h3>
<p>The user file for authentication is, this time, not a flat
text file, but is a DB file<a name="foot2_return"
href="#foot2"><sup>2</sup></a>. Fortunately, once again,
Apache provides us with a simple utility for the purpose of
managing this user file. This time, the utility is called
<tt>dbmmanage</tt>, and will be located in the <tt>bin</tt>
subdirectory of wherever you installed Apache.</p>
<p><tt>dbmmanage</tt> is somewhat more complicated to use than
<tt>htpasswd</tt> or <tt>htdigest</tt>, but it is still fairly
simple. The syntax which you will usually be using is as
follows:</p>
<pre>
dbmmanage passwords.dat adduser montressor
</pre>
<p>As with <tt>htpasswd</tt>, you will at this point be
prompted for a password, and then asked to confirm that
password by typing it again. The main difference here is that
rather than a text file being created, you are creating a
binary file containing the information that you have
supplied.</p>
<p>Type <tt>dbmmanage</tt> with no arguments to get the full
list of options available with this utility.</p>
<h3><a name="perl_dbfile">Creating your user file with
Perl</a></h3>
<p>Note that, if you are so inclined, you can manage your user
file with Perl, or any other language which has a DB-file
module, for interfacing with this type of database. This covers
a number of popular programming languages.</p>
<p>The following Perl code, for example, will add a user
'rbowen', with password 'mypassword', to your password
file:</p>
<pre>
use DB_File;
tie %database, 'DB_File', "passwords.dat"
or die "Can't initialize database: $!\n";
$username = 'rbowen';
$password = 'mypassword';
@chars=(0..9,'a'..'z');
$salt = $chars[int rand @chars] . $chars[int rand @chars];
$crypt = crypt($password, $salt);
$database{$username} = $crypt;
untie %database;
</pre>
<p>As you can imagine, this makes it very simple to write tools
to manage the user and password information stored in these
files.</p>
<p>Passwords are stored in Unix <tt>crypt</tt> format, just as
they were in the "regular" password files. The 'salt' that is
created in the middle there is part of the process, generating
a random starting point for that encryption. The technique
being used is called a 'tied hash'. The idea is to tie a
built-in data structure to the contents of the file, such that
when the data structure is changed, the file is automatically
modified at the same time.</p>
<h3><a name="authdbuserfile"></a>Configuring Apache
to use this password file</h3>
<p>Once you have created the password file, you need to tell
Apache about it, and tell Apache to use this file to verify
user credentials. This configuration will look almost the same
as that for basic authentication. This configuration can go in
a <tt>.htaccess</tt> file in the directory to be protected, or
can go in the main server configuration, in a
<tt><Directory></tt> section, or other scope container
directive.</p>
<p>The configuration will look something like the
following:</p>
<pre>
AuthName "Members Only"
AuthType Basic
AuthDBUserFile /usr/local/apache/passwd/passwords.dat
require user rbowen
</pre>
<p>Now, users accessing the directory will be required to
authenticate against the list of valid users who are in
<tt>/usr/local/apache/passwd/passwords.dat</tt>.</p>
<h3><a name="authdbgroupfile"></a><br />
Optionally, create a group file</h3>
<p>As mentioned earlier, DB files store a key/value pair. In
the case of group files, the key is the name of the user, and
the value is a comma-separated list of the groups to which the
user belongs.</p>
<p>While this is the opposite of the way that group files are
stored elsewhere, note that we will primarily be looking up
records based on the username, so it is more efficient to index
the file by username, rather than by the group name.</p>
<p>Groups can be added to your group file using
<tt>dbmmanage</tt> and the <tt>add</tt> command:</p>
<pre>
dbmmanage add groupfile rbowen one,two,three
</pre>
<p>In the above example, <tt>groupfile</tt> is the literal name
of the group file, <tt>rbowen</tt> is the user being added, and
<tt>one</tt>, <tt>two</tt>, and <tt>three</tt> are names of
three groups to which this user belongs.</p>
<p>Once you have your groups in the file, you can require a
group in the regular way:</p>
<pre>
AuthName "Members Only"
AuthType Basic
AuthDBUserFile /usr/local/apache/passwd/passwords.dat
AuthDBGroupFile /usr/local/apache/passwd/groups.dat
require group three
</pre>
<p>Note that if you want to use the same file for both password
and group information, you can do so, but this is a little more
complicated to manage, as you have to encrypt the password
yourself before you feed it to the <tt>dbmmanage</tt>
utility.</p>
<h1><a name="access"></a>Access control</h1>
<p>Authentication by username and password is only part of the
story. Frequently you want to let people in based on something
other than who they are. Something such as where they are
coming from. Restricting access based on something other than
the identity of the user is generally referred to as <i>Access
Control</i>.</p>
<h2><a name="allowdeny"></a>Allow and Deny</h2>
<p>The <tt>Allow</tt> and <tt>Deny</tt> directives let you
allow and deny access based on the host name, or host address,
of the machine requesting a document. The directive goes
hand-in-hand with these is the <tt>Order</tt> directive, which
tells Apache in which order to apply the filters.</p>
<p>The usage of these directives is:</p>
<pre>
allow from address
</pre>
<p>where <i>address</i> is an IP address (or a partial IP
address) or a fully qualified domain name (or a partial domain
name); you may provide multiple addresses or domain names, if
desired.</p>
<p>For example, if you have someone spamming your message
board, and you want to keep them out, you could do the
following:</p>
<pre>
deny from 11.22.33.44
</pre>
<p>Visitors coming from that address will not be able to see
the content behind this directive. If, instead, you have a
machine name, rather than an IP address, you can use that.
</p>
<pre>
deny from hostname.example.com
</pre>
<p>And, if you'd like to block access from an entire domain,
or even from an entire tld (top level domain, such as .com or .gov)
you can specify just part of an address or domain name:</p>
<pre>
deny from 192.101.205
deny from exampleone.com exampletwo.com
deny from tld
</pre>
<p>Using <tt>Order</tt> will let you be sure that you are
actually restricting things to the group that you want to let
in, by combining a <tt>deny</tt> and an <tt>allow</tt>
directive:</p>
<pre>
Order Deny,Allow
Deny from all
Allow from hostname.example.com
</pre>
<p>Listing just the <tt>allow</tt> directive would not do what
you want, because it will let users from that host in, in
addition to letting everyone in. What you want is to let in
<i>only</i> users from that host.</p>
<h2><a name="satisfy"></a>Satisfy</h2>
<p>The <tt>Satisfy</tt> directive can be used to specify that
several criteria may be considered when trying to decide if a
particular user will be granted admission. <tt>Satisfy</tt> can
take as an argument one of two options - <tt>all</tt> or
<tt>any</tt>. By default, it is assumed that the value is
<tt>all</tt>. This means that if several criteria are
specified, then all of them must be met in order for someone to
get in. However, if set to <tt>any</tt>, then several criteria
may be specified, but if the user satisfies any of these, then
they will be granted entrance.</p>
<p>A very good example of this is using access control to
assure that, although a resource is password protected from
outside your network, all hosts inside the network will be
given free access to the resource. This would be accomplished
by using the <tt>Satisfy</tt> directive, as shown below.</p>
<pre>
<Directory /usr/local/apache/htdocs/sekrit>
AuthType Basic
AuthName intranet
AuthUserFile /www/passwd/users
AuthGroupFile /www/passwd/groups
Require group customers
Order allow,deny
Allow from internal.com
Satisfy any
</Directory>
</pre>
<p>In this scenario, users will be let in if they either have a
password, or if they are in the internal network.</p>
<h1><a name="summary">Summary</a></h1>
<p>The various authentication modules provide a number of ways
to restrict access to your host based on the identity of the
user. They offer a somewhat standard interface to this
functionality, but provide different back-end mechanisms for
actually authenticating the user.</p>
<p>And the access control mechanism allows you to restrict
access based on criteria unrelated to the identity of the
user.<br />
</p>
<hr />
<h4>Footnotes</h4>
<dl>
<dt><a name="foot1">... value.</a><a
href="#foot1_return"><sup>1</sup></a></dt>
<dd>There are actually a number of implementations that get
around this limitation. MLDBM is one of them, for example.
However, for the purposes of this discussion, we'll just deal
with standard Berkeley DB, which is likely to have shipped
with whatever operating system you are already running.</dd>
<dt><a name="foot2">... file</a><a
href="#foot2_return"><sup>2</sup></a></dt>
<dd>Or, if you are using mod_auth_dbm, a DBM file.</dd>
</dl>
<hr />
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