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.
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
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.
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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:
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.
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.
web: admin joe Daniel
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.
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.
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.
Apache provides a Perl script (dbmmanage on Unix and dbmmanage.pl 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
dbmmanage dbfile adduser userid
On Windows, type
perl dbmmanage.pl 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
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.
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