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auth-digest: Digest Authentication for NGINX

Debian/Ubuntu installation

These docs apply to the APT package nginx-module-auth-digest provided by the GetPageSpeed Extras repository.

  1. Configure the APT repository as described in APT repository setup.
  2. Install the module:
sudo apt-get update
sudo apt-get install nginx-module-auth-digest
Show suites and architectures 
| Distro   | Suite             | Component   | Architectures   |
|----------|-------------------|-------------|-----------------|
| debian   | bookworm          | main        | amd64, arm64    |
| debian   | bookworm-mainline | main        | amd64, arm64    |
| debian   | trixie            | main        | amd64, arm64    |
| debian   | trixie-mainline   | main        | amd64, arm64    |
| ubuntu   | focal             | main        | amd64, arm64    |
| ubuntu   | focal-mainline    | main        | amd64, arm64    |
| ubuntu   | jammy             | main        | amd64, arm64    |
| ubuntu   | jammy-mainline    | main        | amd64, arm64    |
| ubuntu   | noble             | main        | amd64, arm64    |
| ubuntu   | noble-mainline    | main        | amd64, arm64    |

Changes from other forks

Bug fixes 1, 2, 3

Added log message for invalid login attempts

Description

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.

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_evasion_time

Syntax
auth_digest_evasion_time time-in-seconds

Default
300s

Context
server, location

Description
The amount of time for which the server will ignore authentication requests from a client address once the number of failed authentications from that client reaches auth_digest_maxtries.

auth_digest_maxtries

Syntax
auth_digest_maxtries number-of-attempts

Default
5

Context
server, location

Description
The number of failed authentication attempts from a client address before the module enters evasive tactics. For evasion purposes, only network clients are tracked, and only by address (not including port number). A successful authentication clears the counters.

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.