Nginx Digest Authentication module

The ngx_http_auth_digest module supplements Nginx's built-in Basic Authentication module by providing support for RFC 2617 Digest Authentication. The module is currently functional but has only been tested and reviewed by its author. And given that this is security code, one set of eyes is almost certainly insufficient to guarantee that it's 100% correct. Until a few bug reports come in and some of the ‘unknown unknowns’ in the code are flushed out, consider this module an ‘alpha’ and treat it with the appropriate amount of skepticism.

A listing of known issues with the module can be found in the bugs.txt file as well as in the Issue Tracker. Please do consider contributing a patch if you have the time and inclination. Any help fixing the bugs or changing the implementation to a more idiomatically nginx-y one would be greatly appreciated.

Dependencies

• Sources for Nginx 1.0.x, and its dependencies.

Building

1. Unpack the Nginx sources:

   $ tar zxvf nginx-1.0.x.tar.gz
   

2. Unpack the sources for the digest module:

   $ tar xzvf samizdatco-nginx-http-auth-digest-xxxxxxx.tar.gz
   

3. Change to the directory which contains the Nginx sources, run the configuration script with the desired options and be sure to put an --add-module flag pointing to the directory which contains the source of the digest module:

   $ cd nginx-1.0.x
   $ ./configure --add-module=../samizdatco-nginx-http-auth-digest-xxxxxxx  [other configure options]
   

4. Build and install the software:

   $ make && sudo make install
   

5. Configure Nginx using the module's configuration directives.

Example

You can password-protect a directory tree by adding the following lines into a server section in your Nginx configuration file:

auth_digest_user_file /opt/httpd/conf/passwd.digest; # a file created with htdigest
location /private{
  auth_digest 'this is not for you'; # set the realm for this location block
}

The other directives control the lifespan defaults for the authentication session. The following is equivalent to the previous example but demonstrates all the directives:

auth_digest_user_file /opt/httpd/conf/passwd.digest;
auth_digest_shm_size 4m;   # the storage space allocated for tracking active sessions

location /private {
  auth_digest 'this is not for you';
  auth_digest_timeout 60s; # allow users to wait 1 minute between receiving the
                           # challenge and hitting send in the browser dialog box
  auth_digest_expires 10s; # after a successful challenge/response, let the client
                           # continue to use the same nonce for additional requests
                           # for 10 seconds before generating a new challenge
  auth_digest_replays 20;  # also generate a new challenge if the client uses the
                           # same nonce more than 20 times before the expire time limit
}

Adding digest authentication to a location will affect any uris that match that block. To disable authentication for specific sub-branches off a uri, set auth_digest to off:

location / {
  auth_digest 'this is not for you';
  location /pub {
    auth_digest off; # this sub-tree will be accessible without authentication
  }
}

Directives

auth_digest

Syntax:	auth_digest [realm-name | off]
Default:	off
Context:	server, location
Description:	Enable or disable digest authentication for a server or location block. The realm name should correspond to a realm used in the user file. Any user within that realm will be able to access files after authenticating. To selectively disable authentication within a protected uri hierarchy, set auth_digest to “off” within a more-specific location block (see example).

auth_digest_user_file

Syntax:	auth_digest_user_file /path/to/passwd/file
Default:	unset
Context:	server, location
Description:	The password file should be of the form created by the apache htdigest command (or the included htdigest.py script). Each line of the file is a colon-separated list composed of a username, realm, and md5 hash combining name, realm, and password. For example: joi:enfield:ef25e85b34208c246cfd09ab76b01db7

auth_digest_timeout

Syntax:	auth_digest_timeout delay-time
Default:	60s
Context:	server, location
Description:	When a client first requests a protected page, the server returns a 401 status code along with a challenge in the www-authenticate header. At this point most browsers will present a dialog box to the user prompting them to log in. This directive defines how long challenges will remain valid. If the user waits longer than this time before submitting their name and password, the challenge will be considered ‘stale’ and they will be prompted to log in again.

auth_digest_expires

Syntax:	auth_digest_expires lifetime-in-seconds
Default:	10s
Context:	server, location
Description:	Once a digest challenge has been successfully answered by the client, subsequent requests will attempt to re-use the ‘nonce’ value from the original challenge. To complicate MitM attacks, it's best to limit the number of times a cached nonce will be accepted. This directive sets the duration for this re-use period after the first successful authentication.

auth_digest_replays

Syntax:	auth_digest_replays number-of-uses
Default:	20
Context:	server, location
Description:	Nonce re-use should also be limited to a fixed number of requests. Note that increasing this value will cause a proportional increase in memory usage and the shm_size may have to be adjusted to keep up with heavy traffic within the digest-protected location blocks.

auth_digest_shm_size

Syntax:	auth_digest_shm_size size-in-bytes
Default:	4096k
Context:	server
Description:	The module maintains a fixed-size cache of active digest sessions to save state between authenticated requests. Once this cache is full, no further authentication will be possible until active sessions expire. As a result, choosing the proper size is a little tricky since it depends upon the values set in the expiration-related directives. Each stored challenge takes up 48 + ceil(replays/8) bytes and will live for up to auth_digest_timeout + auth_digest_expires seconds. When using the default module settings this translates into allowing around 82k non-replay requests every 70 seconds.