In many situations, HTTP services are public and intended to be accessed by anyone with the ability to connect to the server. However, there are a number of cases where site administrators need to have some additional control over which users can access the server. In these contexts, it is useful to require users to submit authentication credentials (e.g. usernames and passwords) to a site before gaining access to a resource.
This guide provides an overview of both credential-based and rule-based access control tools for the Apache HTTP server. We assume that you have a working installation of Apache and have access to modify configuration files.
Contents
- Configuring HTTP Authentication
- Generating HTTP AUTH Passwords
- Access Control Lists with Groups
- The Caveats of HTTP Authentication
- More Information
Configuring HTTP Authentication
To enable passwords for a directory, insert the following lines into the appropriate <Directory> section of an Apache configuration file. You may also insert authentication information in an .htaccess file or in a virtual host configuration section. The required directives are:
File excerpt:Apache Configuration File
AuthType Basic AuthUserFile /srv/auth/.htpasswd AuthName "Sign In Here To Gain Access To the Site" Require valid-user
The AuthType directive specifies which authentication method Apache should use when connecting with clients. Basic requires that passwords be sent as clear text over the network. As a result we don't recommend using this to protect sensitive resources.
The AuthUserFile specifies the path (in full) to the password file where the passwords are stored. The AuthName directive contains the message which the browser uses to inform the user of what resource they're authenticating to. The value is arbitrary. The "Require valid-user" setting simply tells Apache that any valid user can authenticate.
At this point we need to create a password file. While this file can be located anywhere on the filesystem, we strongly recommend that you not place them in a web accessible directory. By default, all files beginning with .ht are not web-accessible in most default configurations of Apache, but this should not be assumed.
Generating HTTP AUTH Passwords
To generate passwords, we need the htpasswd tool. For many distributions, this tool may have been installed when you installed Apache itself. Debian and Ubuntu users will have to update their system and install the apache2-utils package with the following commands:
apt-get update apt-get upgrade apt-get install apache2-utils
To create a new file with a single user, issue the following command:
htpasswd -c /srv/auth/.htpasswd squire
In this example, we instruct the program to create a new AuthUserFile with the -c option. The file is to be located at /srv/auth/.htpasswd and the user name is squire. htpasswd will prompt you to enter a password and then confirm the password.
If you have an existing file, omit the -c option. The -b option allows you to enter the password as the last parameter of the command, as in this example
htpasswd -b /srv/auth/.htpasswd squire 5t1ck6
The AuthUserFile will, when populated look something like this:
File excerpt:/srv/auth/.htpasswd
hobby:isiA3Q4djD/.Q
fore:{SHA}x9VvwHI6dmgk9VTE0A8o6hbCw2s=
squire:$apr1$vVzQJxvX$6EyHww61nnZr6IdQv0pVx/
Each user is specified on their own line. Each line follows the form [username]:[hash], where the [hash] is a cryptographic hash of the users' password. This provides one-way encryption and some small measure of additional security.
In the above example, the first hobby user's password is hashed using the "CRYPT" method, which is the default. This is not considered a secure encryption mechanism. If you specify the -s option in the htpasswd command, the password will be hashed with the SHA algorithm as in the second line of the above example. Finally, if you specify the -m option, htpasswd will use the MD5 hash to store the password. We recommend using either the SHA or the MD5 hash.
Additionally, if you would prefer to organize and maintain the AuthUserFile yourself, you can still use the htpasswd tool to generate the user entries. By specifying the -n option the program will output the appropriate line in the terminal. In the following example, the htpasswd entry is followed by the output of the command:
$ htpasswd -nbs betty pr3ty1np1nk
betty:{SHA}5PPXgshSpwiyJHxbz1i1LVijfKo=
You can now append the betty:{SHA}5PPXgshSpwiyJHxbz1i1LVijfKo= line to your AuthUserFile manually. Once this line is in the password file, the betty user credentials will be able to authenticate the HTTP server.
Access Control Lists with Groups
In the Require directive above we specified the "valid-user". This told Apache that any user who could authenticate against one of the users specified in the UserAuthFile could gain access to the site. While you can maintain separate password files for different resources, this is difficult to maintain for deployments with even mildly complex authentication needs.
To address this need, Apache allows you to use a single UserAuthFile, containing all users that will need to authenticate to the server. To limit the set of valid credentials to a specific subset of the users listed in the .htpasswd file, we must specify users in the Require directive. Only users specified after the "Require user" directive will be permitted to access the specified resource. For example:
File excerpt:Apache configuration option
Require user squire fore
Given this directive, the users squire and fore will be able to log into the resource. Any subset of users can be specified on the Require line. Apache also provides the ability to organize users into groups, and then permit access to resources based on group membership. The configuration directives for this setup would look like this:
File excerpt:Apache configuration file
AuthType Basic AuthUserFile /srv/auth/.htpasswd AuthGroupFile /srv/auth/.htpgroup Require group Authorized
In this example, we cite the same AuthUserFile, but we add an AuthGroupFile that specifies user groups. The group file contains a list of user groups and the usernames associated with each group. The htgroup file, like the htpasswd file, can be located anywhere on the file system. For clarity's sake, we recommend that htgroup be in the same directory as the htpasswd file. Here is an example of an htgroup file:
File excerpt:/srv/auth/.htgroup
Authorized: squire betty Team: fore hobby
Given this htgroup file, only the users squire and betty will have access to the above listed resource. The syntax of the group file follows a simple "[groupname]: [username 1] [username 2] [...]". You can put as many usernames from your AuthUserFile into a group entry as you need for the particular resource.
The Caveats of HTTP Authentication
- In "Basic" HTTP AUTH credentials are sent unencrypted over the wire, which makes HTTP AUTH particularly subject to so called "man-in-the-middle" attacks. As a result, this authentication method shouldn't be used for protecting sensitive information.
- In HTTP AUTH session authentication credentials must be exchanged between the client and the server for every request. While most client software can cache this information so that the user only has to enter the username and password once, the authentication credentials must be passed for every request. This can add additional network overhead.
- When Apache processes an HTTP AUTH request it must parse through the entire htpasswd file. When the file only stores a few passwords the processing time is negligible, but when password files grow, requests can longer to process.
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