heimdal-history

(Password history via Heimdal external strength checking)

SYNOPSIS

heimdal-history [-hmq] [-b target-time] [-d database] [-S length-stats-db] [-s strength-program] [principal]

DESCRIPTION

heimdal-history is an implementation of password history via the Heimdal external password strength checking interface. It stores separate history for each principal, hashed using Crypt::PBKDF2 with randomly-generated salt. (The randomness is from a weak pseudorandom number generator, not strongly random.) Password history is stored indefinitely (implementing infinite history); older password hashes are never removed by this program.

Password history is stored in a BerkeleyDB DB_HASH file. The key is the principal. The value is a JSON array of objects, each of which has two keys. timestamp contains the time when the history entry was added (in POSIX seconds since UNIX epoch), and hash contains the hash of a previously-used password in the Crypt::PBKDF2 LDAP-compatible format. Passwords are hashed using PBKDF2 (from PKCS#5) with SHA-256 as the underlying hash function using a number of rounds configured in this script. See Crypt::PBKDF2 for more information.

heimdal-history also checks password strength before checking history. It does so by invoking another program that also uses the Heimdal external password strength checking interface. By default, it runs /usr/bin/heimdal-strength. Only if that program approves the password does it hash it and check history.

For more information on how to set up password history, see CONFIGURATION below.

As with any implementation of the Heimdal external password strength checking protocol, heimdal-history expects, on standard input:

    principal: <principal>
    new-password: <password>
    end

(with no leading whitespace). <principal> is the principal changing its password (passed to the other password strength checking program but otherwise unused here), and <password> is the new password. There must be exactly one space after the colon. Any subsequent spaces are taken to be part of the principal or password.

If invoked as root, heimdal-history will run the external strength checking program as user nobody and group nogroup, and will check and write to the history database as user _history and group _history. These users must exist on the system if it is run as root.

The result of each password check will be logged to syslog (priority LOG_INFO, facility LOG_AUTH). Each log line will be a set of key/value pairs in the format key=value. The keys are:

action

The action performed (currently always check).

principal

The principal for which a password was checked.

error

An internal error message that did not stop the history check, but which may indicate that something is wrong with the history database (such as corrupted entries or invalid hashes). If this key is present, neither result nor reason will be present. There will be a subsequent log message from the same invocation giving the final result of the history check (assuming heimdal-history doesn't exit with a fatal error).

result

Either accepted or rejected.

reason

If the password was rejected, the reason for the rejection.

The value will be surrounded with double quotes if it contains a double quote or space. Any double quotes in the value will be doubled, so " becomes "".

OPTIONS

-b target-time, --benchmark=target-time

Do not do a password history check. Instead, benchmark the hash algorithm with various possible iteration counts and find an iteration count that results in target-time seconds of computation time required to hash a password (which should be a real number). A result will be considered acceptable if it is within 0.005 seconds of the target time. The results will be printed to standard output and then heimdal-history will exit successfully.

-d database, --database=database

Use database as the history database file instead of the default (/var/lib/heimdal-history/history.db). Primarily used for testing, since Heimdal won't pass this argument.

-h, --help

Print a short usage message and exit.

-m, --manual, --man

Display this manual and exit.

-q, --quiet

Suppress logging to syslog and only return the results on standard output and standard error. Primarily used for testing, since Heimdal won't pass this argument.

-S length-stats-db, --stats=length-stats-db

Use length-stats-db as the database file for password length statistics instead of the default (/var/lib/heimdal-history/lengths.db). Primarily used for testing, since Heimdal won't pass this argument.

-s strength-program, --strength=strength-program

Run strength-program as the external strength-checking program instead of the default (/usr/bin/heimdal-strength). Primarily used for testing, since Heimdal won't pass this argument.

CONFIGURATION

Additional setup is required to use this history implementation with your Heimdal KDC.

First, ensure that its dependencies are installed, and then examine the local configuration settings at the top of the heimdal-history program. By default, it requires a _history user and _history group be present on the system, and all history information will be read and written as that user and group. It also requires a nobody user and nogroup group to be present (this should be the default with most variants of UNIX), and all strength checking will be done as that user and group. It uses various files in /var/lib/heimdal-history to store history and statistical information by default, so if using the defaults, create that directory and ensure it is writable by the _history user.

Once that setup is done, change your [password_quality] configuration in krb5.conf or kdc.conf to:

    [password_quality]
        policies         = external-check
        external_program = /usr/local/bin/heimdal-history

The heimdal-history program will automatically also run heimdal-strength as well, looking for it in /usr/bin. Change the $STRENGTH_PROGRAM setting at the top of the script if you have that program in a different location. You should continue to configure heimdal-strength as if you were running it directly.

RETURN STATUS

On approval of the password, heimdal-history will print APPROVED and a newline to standard output and exit with status 0.

If the password is rejected by the strength checking program or if it (or a version with a single character removed) matches one of the hashes stored in the password history, heimdal-history will print the reason for rejection to standard error and exit with status 0.

On any internal error, heimdal-history will print the error to standard error and exit with a non-zero status.

FILES

/usr/bin/heimdal-strength

The default password strength checking program. This program must follow the Heimdal external password strength checking API.

/var/lib/heimdal-history/history.db

The default database path. If heimdal-strength is run as root, this file needs to be readable and writable by user _history and group _history. If it doesn't exist, it will be created with mode 0600.

/var/lib/heimdal-history/history.db.lock

The lock file used to synchronize access to the history database. As with the history database, if heimdal-strength is run as root, this file needs to be readable and writable by user _history and group _history.

/var/lib/heimdal-history/lengths.db

The default length statistics path, which will be a BerkeleyDB DB_HASH file of password lengths to counts of passwords with that length. If heimdal-strength is run as root, this file needs to be readable and writable by user _history and group _history. If it doesn't exist, it will be created with mode 0600.

/var/lib/heimdal-history/lengths.db.lock

The lock file used to synchronize access to the length statistics database. As with the length statistics database, if heimdal-strength is run as root, this file needs to be readable and writable by user _history and group _history.

AUTHOR

Russ Allbery <eagle@eyrie.org>

COPYRIGHT AND LICENSE

Copyright 2016, 2017 Russ Allbery <eagle@eyrie.org>

Copyright 2013, 2014 The Board of Trustees of the Leland Stanford Junior University

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

SEE ALSO

Crypt::PBKDF2, heimdal-strength(1)

Last modified and spun 2017-05-27