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:

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 (

This product includes cryptographic software written by Eric Young (


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



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:

The default algorithm is "totp".


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.


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.


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:


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.


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.


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 (
    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 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

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 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

Secure/Paranoid Configurations
Security-conscious adminstrators 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:

  AuthOTPOption 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

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:

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:

Frequently Asked Questions


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 staticly linked module:
  $ ./configure --enable-openssl --with-modules=mod_sql:mod_sql_sqlite:mod_auth_otp:...
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 2015-2016 TJ Saunders
All Rights Reserved