heimdal-history - Password history via Heimdal external strength checking
heimdal-history [-chmq] [-b
target-time] [-d database]
[-S length-stats-db] [-s strength-program] [principal]
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 the password is accepted, heimdal-history will assume that it will be used and will update the history database to record the new password. It will also update the password length statistics database to account for the new 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:
- The action performed (currently always "check").
- The principal for which a password was checked.
- 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).
- Either "accepted" or "rejected".
- 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 "".
- -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.
- -c, --check-only
- Check password history and password strength and print the results as normal, but do not update the history or length statistics databases. This is a read-only mode of operation that will not make any changes to the underlying database, only report if a password would currently be accepted.
- -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.
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.
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.
- The default password strength checking program. This program must follow the Heimdal external password strength checking API.
- 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.
- 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".
- 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.
- 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".
Russ Allbery <firstname.lastname@example.org>
COPYRIGHT AND LICENSE¶
Copyright 2016-2017, 2020 Russ Allbery <email@example.com>
Copyright 2013-2014 The Board of Trustees of the Leland Stanford Junior University
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