[edit] Name

ngx_http_auth_digest - HTTP Digest Authentication support for Nginx.

This module is not distributed with the Nginx source. See the installation instructions.

[edit] Status

The module is feature-complete with respect to the RFC but is in need of broader testing before it can be considered secure enough for use in production. See the bugs.txt file and the github issue tracker for the current set of caveats.

[edit] Synopsis

You can limit access to 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
    }
  }

[edit] Directives

[edit] auth_digest

Syntax: auth_digest [realm-name | off ]

Default: off

Context: server, location

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).

[edit] auth_digest_user_file

Syntax: auth_digest_user_file /path/to/passwd/file

Default: none

Context: server, location

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  

[edit] auth_digest_timeout

Syntax: auth_digest_timeout delay-time

Default: 60s

Context: server, location

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.

[edit] auth_digest_expires

Syntax: auth_digest_expires lifetime-in-seconds

Default: 10s

Context: server, location

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.

[edit] auth_digest_replays

Syntax: auth_digest_replays number-of-uses

Default: 20

Context: server, location

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.

[edit] auth_digest_shm_size

Syntax: auth_digest_shm_size size-in-bytes

Default: 4096k

Context: server

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(auth_digest_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.

[edit] Source Repository

Available on github at samizdatco/nginx-http-auth-digest.

[edit] Author

Christian Swinehart / Samizdat Drafting Co.

[edit] Copyright & License

The basic request-handling and password-file-parsing is based on the ngx_http_auth_basic module in the Nginx 1.0.8 sources. The original code is copyright Igor Sysoev.

Copyright (c) 2011, Christian Swinehart

This module is licensed under the terms of the BSD license.

[edit] See Also

• The RFC 2617 definition of basic and digest authentication.
• Shane Holloway's werkzeug module which was used as a reference implementation.