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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:
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
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".
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:
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.
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 (
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:
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
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
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