Authentication, Authorization and Access Control - Apache HTTP Server

Apache HTTP Server Version 2.2

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# Authentication, Authorization and Access Control

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Authentication is any process by which you verify that someone is who they claim they are. Authorization is any process by which someone is allowed to be where they want to go, or to have information that they want to have.

## Related Modules and Directives

There are three types of modules involved in the authentication and authorization process. You will usually need to choose at least one module from each group.

- Authentication type (see the `AuthType` directive)
  - `mod_auth_basic`
  - `mod_auth_digest`
- Authentication provider
  - `mod_authn_alias`
  - `mod_authn_anon`
  - `mod_authn_dbd`
  - `mod_authn_dbm`
  - `mod_authn_default`
  - `mod_authn_file`
  - `mod_authnz_ldap`
- Authorization (see the `Require` directive)
  - `mod_authnz_ldap`
  - `mod_authz_dbm`
  - `mod_authz_default`
  - `mod_authz_groupfile`
  - `mod_authz_owner`
  - `mod_authz_user`

The module `mod_authnz_ldap` is both an authentication and authorization provider. The module `mod_authn_alias` is not an authentication provider in itself, but allows other authentication providers to be configured in a flexible manner.

The module `mod_authz_host` provides authorization and access control based on hostname, IP address or characteristics of the request, but is not part of the authentication provider system.

You probably also want to take a look at the Access Control howto, which discusses the various ways to control access to your server.

## Introduction

If you have information on your web site that is sensitive or intended for only a small group of people, the techniques in this article will help you make sure that the people that see those pages are the people that you wanted to see them.

This article covers the "standard" way of protecting parts of your web site that most of you are going to use.

### Note:

If your data really needs to be secure, consider using `mod_ssl` in addition to any authentication.

## The Prerequisites

The directives discussed in this article will need to go either in your main server configuration file (typically in a `<Directory>` section), or in per-directory configuration files (`.htaccess` files).

If you plan to use `.htaccess` files, you will need to have a server configuration that permits putting authentication directives in these files. This is done with the `AllowOverride` directive, which specifies which directives, if any, may be put in per-directory configuration files.

Since we're talking here about authentication, you will need an `AllowOverride` directive like the following:

`AllowOverride AuthConfig`

Or, if you are just going to put the directives directly in your main server configuration file, you will of course need to have write permission to that file.

And you'll need to know a little bit about the directory structure of your server, in order to know where some files are kept. This should not be terribly difficult, and I'll try to make this clear when we come to that point.

## Getting it working

Here's the basics of password protecting a directory on your server.

First, you need to create a password file. Exactly how you do this will vary depending on what authentication provider you have chosen. More on that later. To start with, we'll use a text password file.

This file should be placed somewhere not accessible from the web. This is so that folks cannot download the password file. For example, if your documents are served out of `/usr/local/apache/htdocs` you might want to put the password file(s) in `/usr/local/apache/passwd`.

To create the file, use the `htpasswd` utility that came with Apache. This will be located in the `bin` directory of wherever you installed Apache. If you have installed Apache from a third-party package, it may be in your execution path.

To create the file, type:

`htpasswd -c /usr/local/apache/passwd/passwords rbowen`

`htpasswd` will ask you for the password, and then ask you to type it again to confirm it:

`# htpasswd -c /usr/local/apache/passwd/passwords rbowen
New password: mypassword
Re-type new password: mypassword
Adding password for user rbowen`

If `htpasswd` is not in your path, of course you'll have to type the full path to the file to get it to run. With a default installation, it's located at `/usr/local/apache2/bin/htpasswd`

Next, you'll need to configure the server to request a password and tell the server which users are allowed access. You can do this either by editing the `httpd.conf` file or using an `.htaccess` file. For example, if you wish to protect the directory `/usr/local/apache/htdocs/secret`, you can use the following directives, either placed in the file `/usr/local/apache/htdocs/secret/.htaccess`, or placed in `httpd.conf` inside a <Directory /usr/local/apache/apache/htdocs/secret> section.

`AuthType Basic
AuthName "Restricted Files"
# (Following line optional)
AuthBasicProvider file
AuthUserFile /usr/local/apache/passwd/passwords
Require user rbowen`

Let's examine each of those directives individually. The `AuthType` directive selects that method that is used to authenticate the user. The most common method is `Basic`, and this is the method implemented by `mod_auth_basic`. It is important to be aware, however, that Basic authentication sends the password from the client to the server unencrypted. This method should therefore not be used for highly sensitive data, unless accompanied by `mod_ssl`. Apache supports one other authentication method: `AuthType Digest`. This method is implemented by `mod_auth_digest` and is much more secure. Most recent browsers support Digest authentication.

The `AuthName` directive sets the Realm to be used in the authentication. The realm serves two major functions. First, the client often presents this information to the user as part of the password dialog box. Second, it is used by the client to determine what password to send for a given authenticated area.

So, for example, once a client has authenticated in the `"Restricted Files"` area, it will automatically retry the same password for any area on the same server that is marked with the `"Restricted Files"` Realm. Therefore, you can prevent a user from being prompted more than once for a password by letting multiple restricted areas share the same realm. Of course, for security reasons, the client will always need to ask again for the password whenever the hostname of the server changes.

The `AuthBasicProvider` is, in this case, optional, since `file` is the default value for this directive. You'll need to use this directive if you are choosing a different source for authentication, such as `mod_authn_dbm` or `mod_authn_dbd`.

The `AuthUserFile` directive sets the path to the password file that we just created with `htpasswd`. If you have a large number of users, it can be quite slow to search through a plain text file to authenticate the user on each request. Apache also has the ability to store user information in fast database files. The `mod_authn_dbm` module provides the `AuthDBMUserFile` directive. These files can be created and manipulated with the `dbmmanage` program. Many other types of authentication options are available from third party modules in the Apache Modules Database.

Finally, the `Require` directive provides the authorization part of the process by setting the user that is allowed to access this region of the server. In the next section, we discuss various ways to use the `Require` directive.

## Satisfy

The `Satisfy` directive can be used to specify that several criteria may be considered when trying to decide if a particular user will be granted admission. Satisfy can take as an argument one of two options - `all` or `any`. By default, it is assumed that the value is `all`. 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 `any`, then several criteria may be specified, but if the user satisfies any of these, then they will be granted entrance.

An 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 unauthenticated access to the resource. This would be accomplished by using the Satisfy directive, as shown below.

`<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>`

## Letting more than one person in

The directives above only let one person (specifically someone with a username of `rbowen`) into the directory. In most cases, you'll want to let more than one person in. This is where the `AuthGroupFile` comes in.

If you want to let more than one person in, you'll need to create a group file that associates group names with a list of users in that group. The format of this file is pretty simple, and you can create it with your favorite editor. The contents of the file will look like this:

`GroupName: rbowen dpitts sungo rshersey`

That's just a list of the members of the group in a long line separated by spaces.

To add a user to your already existing password file, type:

`htpasswd /usr/local/apache/passwd/passwords dpitts`

You'll get the same response as before, but it will be appended to the existing file, rather than creating a new file. (It's the `-c` that makes it create a new password file).

Now, you need to modify your `.htaccess` file to look like the following:

`AuthType Basic
AuthName "By Invitation Only"
# Optional line:
AuthBasicProvider file
AuthUserFile /usr/local/apache/passwd/passwords
AuthGroupFile /usr/local/apache/passwd/groups
Require group GroupName`

Now, anyone that is listed in the group `GroupName`, and has an entry in the `password` file, will be let in, if they type the correct password.

There's another way to let multiple users in that is less specific. Rather than creating a group file, you can just use the following directive:

`Require valid-user`

Using that rather than the `Require user rbowen` line will allow anyone in that is listed in the password file, and who correctly enters their password. You can even emulate the group behavior here, by just keeping a separate password file for each group. The advantage of this approach is that Apache only has to check one file, rather than two. The disadvantage is that you have to maintain a bunch of password files, and remember to reference the right one in the `AuthUserFile` directive.

## Possible problems

Because of the way that Basic authentication is specified, your username and password must be verified every time you request a document from the server. This is even if you're reloading the same page, and for every image on the page (if they come from a protected directory). As you can imagine, this slows things down a little. The amount that it slows things down is proportional to the size of the password file, because it has to open up that file, and go down the list of users until it gets to your name. And it has to do this every time a page is loaded.

A consequence of this is that there's a practical limit to how many users you can put in one password file. This limit will vary depending on the performance of your particular server machine, but you can expect to see slowdowns once you get above a few hundred entries, and may wish to consider a different authentication method at that time.

## Alternate password storage

Because storing passwords in plain text files has the above problems, you may wish to store your passwords somewhere else, such as in a database.

`mod_authn_dbm` and `mod_authn_dbd` are two modules which make this possible. Rather than selecting ``AuthBasicProvider` file`, instead you can choose `dbm` or `dbd` as your storage format.

To select a dbd file rather than a text file, for example:

`<Directory /www/docs/private>
AuthName "Private"
AuthType Basic
AuthBasicProvider dbm
AuthDBMUserFile /www/passwords/passwd.dbm
Require valid-user
</Directory>`

Other options are available. Consult the `mod_authn_dbm` documentation for more details.

## More information

You should also read the documentation for `mod_auth_basic` and `mod_authz_host` which contain some more information about how this all works. `mod_authn_alias` can also help in simplifying certain authentication configurations.

The various ciphers supported by Apache for authentication data are explained in Password Encryptions.

And you may want to look at the Access Control howto, which discusses a number of related topics.

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