ProFTPD module mod_auth_otp

## ProFTPD module `mod_auth_otp`

The purpose of the `mod_auth_otp` module is to enable the use of one-time-passwords (OTP) for proftpd authentication. There have been multiple different OTP algorithms devised over the years; this module implements the HOTP and TOTP algorithms. Note that `mod_auth_otp` requires storage/retrieval for per-user shared keys and counters, and thus this module currently requires mod_sql.

One-Time Password RFCs
For those wishing to learn more about these one-time password algorithms, see:

- HOTP: An HMAC-Based One-Time Password Algorithm (RFC 4226)
- TOTP: Time-Based One-Time Password Algorithm(RFC 6238)

Note that `Google Authenticator` is based on the TOTP algorithm; `mod_auth_otp` thus enables use of Google Authenticator for ProFTPD authentication.

Installation instructions are discussed here; detailed notes on best practices for using this module are here.

The most current version of `mod_auth_otp` is distributed with the ProFTPD source code.

This product includes software developed by the OpenSSL Project for use in the OpenSSL Toolkit (http://www.openssl.org/).

This product includes cryptographic software written by Eric Young (eay@cryptsoft.com).

## Author

Please contact TJ Saunders <tj at castaglia.org> with any questions, concerns, or suggestions regarding this module.

## Directives

- AuthOTPAlgorithm
- AuthOTPEngine
- AuthOTPLog
- AuthOTPOptions
- AuthOTPTable
- AuthOTPTableLock

### AuthOTPAlgorithm

Syntax: AuthOTPAlgorithm hotp|totp
Default: AuthOTPAlgorithm totp
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth_otp
Compatibility: 1.3.5rc4 and later

The `AuthOTPAlgorithm` directive configures which one-time password algorithm will be used when calculating codes for connections to the virtual host.

The supported algorithm names are:

- `hotp` (counter-based codes)
- `totp` (time-based codes, using HMAC-SHA1)
- `totp-sha256` (time-based codes, using HMAC-SHA256)
- `totp-sha512` (time-based codes, using HMAC-SHA512)

The default algorithm is "totp".

### AuthOTPEngine

Syntax: AuthOTPEngine on|off
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth_otp
Compatibility: 1.3.5rc4 and later

The `AuthOTPEngine` directive enables the handling of one-time password codes for authentication, both for FTP/FTPS as well as SFTP/SCP sessions. By default, use of one-time passwords is disabled.

### AuthOTPLog

Syntax: AuthOTPLog path|"none"
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth_otp
Compatibility: 1.3.5rc4 and later

The `AuthOTPLog` directive is used to specify a log file for `mod_auth_otp`'s reporting on a per-server basis. The path parameter given must be the full path to the file to use for logging.

Note that this path must not be to a world-writable directory and, unless `AllowLogSymlinks` is explicitly set to on (generally a bad idea), the path must not be a symbolic link.

### AuthOTPOptions

Syntax: AuthOTPOptions opt1 ...
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth_otp
Compatibility: 1.3.5rc4 and later

The `AuthOTPOptions` directive is used to configure various optional behavior of `mod_auth_otp`.

For example:

      AuthOTPOptions StandardResponse

The currently implemented options are:

- `DisplayVerificationCode`
  When `mod_auth_otp` prompts the user for the OTP code, it requests that the client not echo/display the verification code as it is entered by the user. In some cases, however, administrators may wish to have the OTP code be displayed. For these situations, use this option, e.g.:

            AuthOTPOptions DisplayVerificationCode

- `RequireTableEntry`

  When `mod_auth_otp` requests information for a user from the `AuthOTPTable` and no information is found, it will normally allow other auth modules to handle the login attempt, even if `mod_auth_otp` is authoritative. This behavior allows for a seamless transition of your user base, provisioning users with shared keys/secrets for their one-time passwords as time allows.

  However, there may be sites which require the use of one-time passwords; any login attempt which does not use a valid one-time password must be rejected. Thus the lack of an entry for a user in the `AuthOTPTable` is, for this policy, a fatal error and should be handled as such. For this kind of very secure configuration, use this option, in conjunction with the `AuthOrder` directive, e.g.:

            AuthOrder mod_auth_otp.c* ...
            AuthOTPOptions RequireTableEntry StandardResponse

- `StandardResponse`

  When `mod_auth_otp` is handling FTP sessions, it will respond to a `USER` command with a response message indicating the expectation of a one-time password:

            331 One-time password required for user

  However, this change of the response message "leaks" information about the server configuration, i.e. that OTPs will be used. To tell `mod_auth_otp` to continue using the standard/normal response message, use this option.

### AuthOTPTable

Syntax: AuthOTPTable table-info
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth_otp
Compatibility: 1.3.5rc4 and later

The `AuthOTPTable` directive configures the information necessary for `mod_auth_otp` to retrieve the shared key/secret and current counter, on a per-user basis; this directive is required for `mod_auth_otp` to function. If `AuthOTPTable` is not configured, `mod_auth_otp` will refuse to work.

The `mod_auth_otp` module currently expects/uses SQL tables for retrieval/storage of its data on a per-user basis. Thus the `AuthOTPTable` directives requires two separate `SQLNamedQuery` directives: one for looking up the needed data, the other for updating that data. The table-info parameter encodes these `SQLNamedQuery` names like so:

      SQLNamedQuery get-user-totp SELECT ...
      SQLNamedQuery update-user-totp UPDATE ...

      AuthOTPTable sql:/get-user-totp/update-user-totp

See the usage section for a more detailed example.

### AuthOTPTableLock

Syntax: AuthOTPTableLock path
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth_otp
Compatibility: 1.3.5rc4 and later

The `AuthOTPTableLock` directive sets the path for a synchronization lockfile which `mod_auth_otp` needs when updating the `AuthOTPTable` for e.g. counter-based codes. Use of `AuthOTPTableLock` is recommended, but not required.

If `AuthOTPTableLock` is used, it is strongly advised that the configured path not be on an NFS (or any other network) filesystem.

## Usage

Note that the following examples assume the existing of an SQL table whose schema looks like the following (using the SQLite schema syntax):

      CREATE TABLE auth_otp (
        user TEXT PRIMARY KEY,
        secret TEXT,
        counter INTEGER
      );

The `auth_otp.secret` column must contain the base32-encoded shared key for the user. Why Base32-encoding? That is what Google Authenticator expects/uses for its shared key storage; its `google-authenticator` command-line tool generates a Base32-encoded string for entering into the Google Authenticator app on your mobile device.

To get the base32-encoded shared key using `google-authenticator`:

      $ ./google-authenticator
      Do you want authentication tokens to be time-based (y/n) y
      Here you will see generated QR code
      Your new secret key is: base32-encoded secret here
      ...

Example Time-based (TOTP) Configuration

      <IfModule mod_auth_otp.c>
        AuthOTPEngine on

        # Use time-based codes (TOTP)
        AuthOTPAlgorithm totp

        AuthOTPTable sql:/get-user-totp/update-user-totp
      </IfModule>

      <IfModule mod_sql.c>
        ...

        # Notice that for time-based counters, we do not need to retrieve
        # the auth_otp.counter column; the counter value is determined from the
        # system clock.
        SQLNamedQuery get-user-totp SELECT "secret FROM auth_otp WHERE user = \'%{0}\'"
        SQLNamedQuery update-user-totp UPDATE "counter = %{1} WHERE user = \'%{0}\'" auth_otp
      </IfModule>

Example Counter-based (HOTP) Configuration

      <IfModule mod_auth_otp.c>
        AuthOTPEngine on

        # Use counter-based codes (HOTP)
        AuthOTPAlgorithm hotp

        AuthOTPTable sql:/get-user-hotp/update-user-hotp
      </IfModule>

      <IfModule mod_sql.c>
        ...
        SQLNamedQuery get-user-hotp SELECT "secret, counter FROM auth_otp WHERE user = \'%{0}\'"
        SQLNamedQuery update-user-hotp UPDATE "counter = %{1} WHERE user = \'%{0}\'" auth_otp
      </IfModule>

Secure/Paranoid Configurations
Security-conscious administrators may not want users to notice if/when they have started expecting one-time passwords for their logins; not all users may have been provisioned with the necessary shared key. To prevent `mod_auth_otp` from "leaking" its presence/usage and instead to continue using the standard FTP response messages, use the following in your configuration for `mod_auth_otp`:

      AuthOTPOptions StandardResponse

If, on the other hand, you have successfully provisioned all of your users with OTP shared keys, and now require that all logins use a one-time password (but still want to not leak this information), then you would use:

      # Make mod_auth_otp authoritative; if it fails to handle a login attempt,
      # that login attempt MUST fail.
      AuthOrder mod_auth_otp.c* ...

      # Use the standard FTP response message, and fail the login if we find
      # a user that has not been provisioned.
      AuthOTPOptions RequireTableEntry StandardResponse

SFTP/SCP Support
One-time passwords can also be used for `mod_sftp` sessions, i.e. for SFTP and SCP clients. The SSH RFCs define any non-standard "password-like" authentication method as "keyboard-interactive". Thus to use `mod_auth_otp` for your SFTP connections, simply include both `mod_sftp` and `mod_auth_otp` in your build. That's it.

Now, if you want `mod_sftp` to only try to use one-time passwords (or public keys), and not normal passwords, then you might use a `mod_sftp` configuration like this:

      SFTPAuthMethods publickey keyboard-interactive

Module Load Order and `mod_sftp`
In order for `mod_auth_otp` to work its magic, it must come after the `mod_sftp` module in the module load order. To do this as a static module, you would use something like this when building proftpd:

      $ ./configure --with-modules=...:mod_sftp:mod_auth_otp:...

ensuring that `mod_auth_otp` comes after `mod_sftp` in your `--with-modules` list.

As a shared module, configuring `mod_auth_otp` to be after `mod_sftp` is much easier. Your configuration will have a list of `LoadModule` directives; make sure `mod_auth_otp` appears after `mod_sftp`:

      LoadModule mod_sftp.c
      ...
      LoadModule mod_auth_otp.c
      ...

You will know if the module load ordering is wrong if you see the following log message appear in your logs:

      proftpd[87129]: mod_auth_otp/0.0: mod_sftp not loaded, skipping keyboard-interactive support

Logging
The `mod_auth_otp` module supports different forms of logging. The main module logging is done via the `AuthOTPLog` directive. This log is used for successes/failures. For example, if the user provides an OTP code, but that user is not configured in the `AuthOTPTable`, you would see a log message such as:

      2016-01-18 12:35:46,725 mod_auth_otp/0.2[27192]: user 'foobar' has no OTP info in AuthOTPTable
      2016-01-18 12:36:47,152 mod_auth_otp/0.2[27192]: FAILED: user 'foobar' provided invalid OTP code

If the user is provisioned in the `AuthOTPTable`, but the OTP code is invalid, you would see just this message:

      2016-01-18 12:40:09,500 mod_auth_otp/0.2[27235]: FAILED: user 'foobar' provided invalid OTP code

And finally, for valid OTP codes, the following is logged:

      2016-01-18 12:42:40,115 mod_auth_otp/0.2[27484]: SUCCESS: user 'foobar' provided valid OTP code

For debugging purposes, the module also uses trace logging, via the module-specific channels:

- auth_otp

Thus for trace logging, to aid in debugging, you would use the following in your `proftpd.conf`:

      TraceLog /path/to/auth-trace.log
      Trace auth_otp:20

This trace logging can generate large files; it is intended for debugging use only, and should be removed from any production configuration.

Suggested Future Features
The following lists the features I hope to add to `mod_auth_otp`, according to need, demand, inclination, and time:

- Configurable number of digits in the expected code (currently hardcoded as 6)
- Support "emergency recovery" codes
- Support resynchronization with clients

Frequently Asked Questions

## Installation

The `mod_auth_otp` module is distributed with ProFTPD. Simply follow the normal steps for using third-party modules in ProFTPD. For including `mod_auth_otp` as a statically linked module:

      $ ./configure --enable-openssl --with-modules=mod_sql:mod_sql_sqlite:mod_auth_otp:...

Note the ordering of modules in the above `--with-modules` list; `mod_sql` must precede `mod_auth_otp`, otherwise you will see errors like this:

      mod_auth_otp/0.2: Missing required 'mod_sql.c'; HOTP/TOTP logins will FAIL

To build `mod_auth_otp` as a DSO module:

      $ ./configure --enable-dso --enable-openssl --with-shared=mod_auth_otp:...

Then follow the usual steps:

      $ make
      $ make install

© Copyright 2015-2019 TJ Saunders
All Rights Reserved