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Apache Authentication Module Functionality

Apache provides the basic framework and directives to perform authentication and access control. The authentication modules provide support for validating passwords against a specific back end method (file, database, and so on). Users can optionally be organized in groups, easing management of access control rules.

Apache provides three built-in directives related to authentication that is used with any of the authentication modules: AuthName, AuthType, and Require.

AuthName accepts a string argument, the name for the authentication realm. A realm is a logical area of the Web server that you are asking the password for. It will be displayed in the browser pop-up window.

AuthType specifies the type of browser authentication: basic or digest.

Require enables you to specify a list of users or groups that will be allowed access. The syntax is Require user followed by one or more usernames, or Require group followed by one or more group names. For example

Require user joe bob


Require group employee contractor

If you want to grant access to anyone who provides a valid username and password, you can do so with

Require valid-user

With the preceding directives, you can control who has access to specific virtual hosts, directories, files, and so on. Although authentication and authorization are separate concepts, in practice they are tied together in Apache. Access is granted based on specific user identity or group membership. Some third-party modules, such as certain LDAP-based modules, allow for clearer separation between authentication and authorization.

The authentication modules included with Apache provide

  • Back-end storage Provides text or database files containing the username and group information

  • User management Supplies tools for creating and managing users and groups in the back-end storage

  • Authoritative information Specifies whether the results of the module are authoritative

By the Way

Sometimes users will not be allowed access to a particular realm because their information is not found in the user database provided by the module, or because no authentication rules matched their information. In that case, one of two situations will occur:

  • If the module specifies its results as authoritative, a user will be denied access and Apache will return an error.

  • If the module specifies its results as not authoritative, other modules can have a chance of authenticating the user.

This enables you to have a main authorization module that knows about most users, and to be able to have additional modules that can authenticate the rest of the users.

File-Based Authentication

The mod_auth Apache module provides basic authentication via text files containing usernames and passwords, similar to how traditional Unix authentication works with the /etc/passwd and /etc/groups files.

Back-End Storage

When using back-end storage methods, you need to specify the file containing the list of usernames and passwords and, optionally, the file containing the list of groups.

The users file is a Unix-style password file, containing names of users and encrypted passwords. The entries look like the following, on Unix, using the crypt algorithm:


and on Windows, using the MD5 algorithm:


The groups file contains a list of groups and the users who belong to each one of them, separated by spaces, such as in the following entry:

web: admin joe Daniel

The AuthUserFile and the AuthGroupFile directives take a path argument, pointing to the users file and the groups file. The groups file is optional.

User Management

The Apache distribution includes the htpasswd utility on Unix and htpasswd.exe on Windows; they are designed to help you manage user password files. Both versions are functionally identical, but the Windows version uses a different method to encrypt the password. The encryption is transparent to the user and administrator. On Linux/Unix, the first time you add a user, you need to type

/usr/local/apache2/bin/htpasswd -c file userid

where file is the password file that will contain the list of usernames and passwords, and userid is the username you want to add. You will be prompted for a password, and the file will be created. For example, on Linux/Unix, the command

/usr/local/apache2/bin/htpasswd -c /usr/local/apache2/conf/htusers admin

will create the password file /usr/local/apache2/conf/htusers and add the admin user.

Similar functionality exists on Windows, where the command-line operation might look something like the following:

htpasswd -c "C:\Program Files\Apache Group\Apache2\conf\htusers" admin

The -c command-line option tells the htpasswd executable that it should create the file. When you want to add users to an existing password file, do not use the -c option; otherwise, the file will be overwritten.

It is important that you store the password file outside the document root and thus make it inaccessible via a Web browser. Otherwise, an attacker could download the file and get a list of your usernames and passwords. Although the passwords are encrypted, when you have the file, it is possible to perform a brute-force attack to try to guess them.


The AuthAuthoritative directive takes a value of on or off. By default, it is on, meaning that the module authentication results are authoritative. That is, if the user is not found or does not match any rules, access will be denied.

Using mod_auth

Listing 24.1 shows a sample configuration, restricting access to the private directory in the document root to authenticated users present in the htusers password file. Note that the optional AuthGroupFile directive is not present.

Listing 24.1. File-Based Authentication Example
1: <Directory /usr/local/apache2/htdocs/private>
2: AuthType Basic
3: AuthName "Private Area"
4: AuthUserFile /usr/local/apache2/conf/htusers
5: AuthAuthoritative on
6: Require valid-user
7: </Directory>

Database File-Based Access Control

Storing usernames and passwords in plain text files is convenient, but this method does not scale well. Apache would need to open and read the files sequentially to look for a particular user. When the number of users grows, this operation becomes very time-consuming. The mod_auth_dbm module enables you to replace the text-based files with indexed database files, which can handle a much greater number of users without performance degradation. The mod_auth_dbm module is included with Apache but is not enabled by default. Enabling this module occurs when configuring Apache to be built, using the --enable-module=dbm option.

Back-End Storage

The mod_auth_dbm module provides two directives, AuthDBMUserFile and AuthDBMGroupFile, that point to the database files containing the usernames and groups. Unlike plain text files, both directives can point to the same file, which combines both users and groups.

User Management

Apache provides a Perl script (dbmmanage on Unix and on Windows) that allows you to create and manage users and groups stored in a database file. Under Unix, you might need to edit the first line of the script to point to the location of the Perl interpreter in your system. On Windows, you need to install the additional MD5 password package. If you are using ActiveState Perl, start the Perl package manager and type

install Crypt-PasswdMD5

To add a user to a database on Linux/Unix, type

dbmmanage dbfile adduser userid

On Windows, type

perl dbfile adduser userid

You will be prompted for the password, and the user will be added to the existing database file or a new file will be created if one does not exist.

When adding a user, you can optionally specify the groups it belongs to as comma-separated arguments. The following command adds the user daniel to the database file /usr/local/apache2/conf/dbmusers and makes it a member of the groups employee and engineering:

dbmmanage /usr/local/apache2/conf/dbmusers adduser daniel employee,engineering

If you ever need to delete the user daniel, you can issue the following command:

dbmmanage dbfile delete daniel

The dbmmanage program supports additional options. You can find complete syntax information in the dbmmanage manual page or by invoking dbmmanage without any arguments.

By the Way

Apache 2.0 provides an additional utility, htdbm, that does not depend on Perl and provides all the functionality that dbmmanage does.

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