Change CrackLib tests for system CrackLib

[kerberos/krb5-strength.git] / tools / heimdal-history

#!/usr/bin/perl
#
# Password history via Heimdal external strength checking.
#
# This script is meant to be called via the Heimdal external password strength
# checking interface and maintains per-user password history.  Password
# history is stored as Crypt::PBKDF2 hashes with random salt for each
# password.

##############################################################################
# Declarations and configuration
##############################################################################

require 5.010;
use autodie;
use strict;
use warnings;

use DB_File::Lock;
use Crypt::PBKDF2;
use Fcntl qw(O_CREAT O_RDWR);
use File::Basename qw(basename);
use Getopt::Long::Descriptive qw(describe_options);
use IPC::Run qw(run);
use JSON qw(encode_json decode_json);
use POSIX qw(setgid setuid);
use Readonly;
use Sys::Syslog qw(openlog syslog LOG_AUTH LOG_INFO LOG_WARNING);

# The most convenient interface to Berkeley DB files is ties.
## no critic (Miscellanea::ProhibitTies)

# The number of PBKDF2 iterations to use when hashing passwords.  This number
# should be chosen so as to force the hash operation to take approximately 0.1
# seconds on current hardware.
Readonly my $HASH_ITERATIONS => 14592;

# Path to the history database.  Currently, this must be a Berkeley DB file in
# the old DB_HASH format.  Keys will be principal names, and values will be a
# JSON array of hashes.  Each hash will have two keys: timestamp, which holds
# the seconds since UNIX epoch at which the history entry was stored, and
# hash, which holds the Crypt::PBKDF2 LDAP-style password hash.
Readonly my $HISTORY_PATH => '/var/lib/heimdal-history/history.db';

# User and group used to do all password history lookups and writes, assuming
# that this program is invoked as root and can therefore change UID and GID.
Readonly my $HISTORY_USER  => '_history';
Readonly my $HISTORY_GROUP => '_history';

# Path to the Berkeley DB file (DB_HASH format) that stores statistics on
# password length of accepted passwords.  Each successful password validation
# will increase the counter for that length.  This is read and written with
# $HISTORY_USER and $HISTORY_GROUP.
Readonly my $LENGTH_STATS_PATH => '/var/lib/heimdal-history/lengths.db';

# The message to return to the user if we reject the password because it was
# found in the user's history.
Readonly my $REJECT_MESSAGE => 'Password was previously used';

# The path to the external strength checking program to run.  This is done
# first before checking history, and if it fails, that failure is returned as
# the failure for this program.
Readonly my $STRENGTH_PROGRAM => '/usr/bin/heimdal-strength';

# User and group used to do password strength checking.  Generally, this
# doesn't require any privileges since the strength dictionary is
# world-readable.
Readonly my $STRENGTH_USER  => 'nobody';
Readonly my $STRENGTH_GROUP => 'nogroup';

# Global boolean variable saying whether to log with syslog.  This is set
# based on the presence of the -q (--quiet) command-line option.
my $SYSLOG = 1;

##############################################################################
# Utility functions
##############################################################################

# Change real and effective UID and GID to those for the given user and group.
# Does nothing if not running as root.
#
# $user  - User to change the UID to
# $group - Group to change the GID to (and clear all supplemental groups)
#
# Returns: undef
#  Throws: Text exception on any failure
sub drop_privileges {
    my ($user, $group) = @_;

    # If running as root, drop privileges.  Fail if we can't get the UID and
    # GID corresponding to our users.
    if ($> == 0 || $< == 0) {
        my $uid = getpwnam($user)
          or die "$0: cannot get UID for $user\n";
        my $gid = getgrnam($group)
          or die "$0: cannot get GID for $group\n";
        setgid($gid) or die "$0: cannot setgid to $gid: $!\n";
        setuid($uid) or die "$0: cannot setuid to $uid: $!\n";
        if ($> == 0 || $< == 0) {
            die "$0: failed to drop permissions\n";
        }
    }
    return;
}

##############################################################################
# Logging
##############################################################################

# Given a list of keys and values for a log message as a hash reference,
# return in encoded format following our logging protocol.  The log format is
# a set of <key>=<value> parameters separated by a space.  Values containing
# whitespace are quoted with double quotes, with any internal double quotes
# doubled.
#
# Here also is defined a custom sort order for the encoded key/value pairs to
# keep them in a reasonable order for a human to read.
#
# $params_ref - Reference to a hash of key/value pairs
#
# Returns: The encoded format as a string
sub encode_log_message {
    my ($params_ref) = @_;

    # Define the custom sort order for keys.
    my $order = 1;
    my %order
      = map { $_ => $order++ } qw(action principal error result reason);

    # Build the message from the parameters.
    my $message;
    for my $key (sort { $order{$a} <=> $order{$b} } keys %{$params_ref}) {
        my $value = $params_ref->{$key};
        $value =~ s{ \" }{\"\"}xmsg;
        if ($value =~ m{ [ \"] }xms) {
            $value = qq{"$value"};
        }
        $message .= qq{$key=$value };
    }
    chomp($message);
    return $message;
}

# Log a non-fatal error encountered while trying to check or store password
# history.  This is used for errors where the password is accepted, but we ran
# into some anomalous event such as corrupted history data that should be
# drawn to the attention of an administrator.  The log format is a set of
# <key>=<value> parameters, with the following keys:
#
# - action:    the action performed (currently always "check")
# - principal: the principal to check a password for
# - error:     an error message explaining the anomalous situation
#
# Values containing whitespace are quoted with double quotes, with any
# internal double quotes doubled.  No logging will be done if $SYSLOG is
# false.
#
# $principal - Principal for which we checked a password
# $error     - The error message
#
# Returns: undef
sub log_error {
    my ($principal, $error) = @_;
    if (!$SYSLOG) {
        return;
    }
    my $message = encode_log_message(
        action    => 'check',
        principal => $principal,
        error     => $error,
    );
    syslog(LOG_WARNING, '%s', $message);
    return;
}

# Log the disposition of a particular password strength checking request.  All
# log messages are logged through syslog at class info.  The log format is a
# set of <key>=<value> parameters, with the following keys:
#
# - action:    the action performed (currently always "check")
# - principal: the principal to check a password for
# - result:    either "accepted" or "rejected"
# - reason:    the reason for a rejection
#
# Values containing whitespace are quoted with double quotes, with any
# internal double quotes doubled.  No logging will be done if $SYSLOG is
# false.
#
# $principal - Principal for which we checked a password
# $result    - "accepted" or "rejected" per above
# $reason    - On rejection, the reason
#
# Returns: undef
sub log_result {
    my ($principal, $result, $reason) = @_;
    if (!$SYSLOG) {
        return;
    }

    # Create the message.
    my %message = (
        action    => 'check',
        principal => $principal,
        result    => $result,
    );
    if ($result eq 'rejected' && defined($reason)) {
        $message{reason} = $reason;
    }
    my $message = encode_log_message(\%message);

    # Log the message.
    syslog(LOG_INFO, '%s', $message);
    return;
}

##############################################################################
# Crypto
##############################################################################

# Given a password, return the hash for that password.  Hashing is done with
# PBKDF2 using SHA-2 as the underlying hash function.  As of version 0.133330,
# this uses SHA-256.
#
# $password   - Password to hash
# $iterations - Optional iteration count, defaulting to $HASH_ITERATIONS
#
# Returns: Hash encoded in the LDAP-compatible Crypt::PBKDF2 format
sub password_hash {
    my ($password, $iterations) = @_;
    $iterations //= $HASH_ITERATIONS;
    my $hasher = Crypt::PBKDF2->new(
        hash_class => 'HMACSHA2',
        iterations => $iterations,
    );
    return $hasher->generate($password);
}

# Given a password and the password history for the user as a reference to a
# array, check whether that password is found in the history.  The history
# array is expected to contain anonymous hashes.  The only key of interest is
# the "hash" key, whose value is expected to be a hash in the LDAP-compatible
# Crypt::PBKDF2 format.
#
# Invalid history entries are ignored for the purposes of this check and
# treated as if the entry did not exist.
#
# $principal   - Principal to check (solely for logging purposes)
# $password    - Password to check
# $history_ref - Reference to array of anonymous hashes with "hash" keys
#
# Returns: True if the password matches one of the history hashes, false
#          otherwise
sub is_in_history {
    my ($principal, $password, $history_ref) = @_;
    my $hasher = Crypt::PBKDF2->new(hash_class => 'HMACSHA2');

    # Walk the history looking at each hash key.
    for my $entry (@{$history_ref}) {
        my $hash = $entry->{hash};
        next if !defined($hash);

        # validate throws an exception if the hash is in an invalid format.
        # Treat that case the same as a miss, but log it.
        if (eval { $hasher->validate($hash, $password) }) {
            return 1;
        } elsif ($@) {
            log_error($principal, "hash validate failed: $@");
        }
    }

    # No match.
    return;
}

##############################################################################
# Benchmarking
##############################################################################

# Perform a binary search for a number of hash iterations that makes password
# hashing take the given target time on the current system.
#
# Assumptions:
#
# * The system load is low enough that this benchmark result is meaningful
#   and not heavily influenced by other programs running on the system.  The
#   binary search may be unstable if the system load is too variable.
#
# * The static "password" string used for benchmarking will exhibit similar
#   performance to the statistically average password.
#
# Information about the iteration search process is printed to standard output
# while the search runs.
#
# $target - The elapsed time, in real seconds, we're aiming for
# $delta  - The permissible delta around the target time
#
# Returns: The number of hash iterations with that performance characteristic
#  Throws: Text exception on failure to write to standard output
sub find_iteration_count {
    my ($target, $delta) = @_;
    my $high = 0;
    my $low  = 0;

    # A static password to use for benchmarking.
    my $password = 'this is a benchmark';

    # Start at the current configured iteration count.  If this doesn't take
    # long enough, it becomes the new low mark and we try double that
    # iteration count.  Otherwise, do binary search.
    #
    # We time twenty iterations each time, chosen because it avoids the
    # warnings from Benchmark about too few iterations for a reliable count.
    require Benchmark;
    my $iterations = $HASH_ITERATIONS;
    while (1) {
        my $hash = sub { password_hash($password, $iterations) };
        my $times = Benchmark::timethis(20, $hash, q{}, 'none');

        # Extract the CPU time from the formatted time string.  This will be
        # the total time for all of the iterations, so divide by the iteration
        # count to recover the time per iteration.
        my $report = Benchmark::timestr($times);
        my ($time) = ($report =~ m{ ([\d.]+) [ ] CPU }xms);
        $time = $time / 20;

        # Tell the user what we discovered.
        say {*STDOUT} "Performing $iterations iterations takes $time seconds"
          or die "$0: cannot write to standard output: $!\n";

        # If this is what we're looking for, we're done.
        if (abs($time - $target) < $delta) {
            last;
        }

        # Determine the new iteration target.
        if ($time > $target) {
            $high = $iterations;
        } else {
            $low = $iterations;
        }
        if ($time < $target && $high == 0) {
            $iterations = $iterations * 2;
        } else {
            $iterations = int(($high + $low) / 2);
        }
    }

    # Report the result and return it.
    say {*STDOUT} "Use $iterations iterations"
      or die "$0: cannot write to standard output: $!\n";
    return $iterations;
}

##############################################################################
# Database
##############################################################################

# Given a principal and a password, determine whether the password was found
# in the password history for that user.
#
# $path      - Path to the history file
# $principal - Principal for which to check history
# $password  - Check history for this password
#
# Returns: True if $password is found in history, false otherwise
#  Throws: On failure to open, lock, or tie the database
sub check_history {
    my ($path, $principal, $password) = @_;

    # Open and lock the database and retrieve the history for the user.
    # We have to lock for write so that we can create the database if it
    # doesn't already exist.  Password change should be infrequent enough
    # and our window is fast enough that it shouldn't matter.  We do this
    # in a separate scope so that the history hash goes out of scope and
    # is freed and unlocked.
    my $history_json;
    {
        my %history;
        my $mode = O_CREAT | O_RDWR;
        tie(%history, 'DB_File::Lock', $path, $mode, oct(600), $DB_HASH,
            'write')
          or die "$0: cannot open $path: $!\n";
        $history_json = $history{$principal};
    }

    # If there is no history for the user, return the trivial false.
    if (!defined($history_json)) {
        return;
    }

    # Decode history from JSON.  If this fails (corrupt history), treat it as
    # if the user has no history, but log the error message.
    my $history_ref = eval { decode_json($history_json) };
    if (!defined($history_ref)) {
        log_error($principal, "history JSON decoding failed: $@");
        return;
    }

    # Finally, check the password against the hashes in history.
    return is_in_history($principal, $password, $history_ref);
}

# Write a new history entry to the database given the principal and the
# password to record.  History records are stored as JSON arrays of objects,
# with keys "timestamp" and "hash".
#
# $path      - Path to the history file
# $principal - Principal for which to check history
# $password  - Check history for this password
#
# Returns: undef
#  Throws: On failure to open, lock, or tie the database
sub write_history {
    my ($path, $principal, $password) = @_;

    # Open and lock the database for write.
    my %history;
    my $mode = O_CREAT | O_RDWR;
    tie(%history, 'DB_File::Lock', $path, $mode, oct(600), $DB_HASH, 'write')
      or die "$0: cannot open $path: $!\n";

    # Read the existing history.  If the existing history is corrupt, treat
    # that as equivalent to not having any history, but log an error.
    my $history_json = $history{$principal};
    my $history_ref;
    if (defined($history_json)) {
        $history_ref = eval { decode_json($history_json) };
        if ($@) {
            log_error($principal, "history JSON decoding failed: $@");
        }
    }
    if (!defined($history_ref)) {
        $history_ref = [];
    }

    # Add a new history entry.
    my $entry = { timestamp => time(), hash => password_hash($password) };
    unshift(@{$history_ref}, $entry);

    # Store the encoded data back in the history database.
    $history{$principal} = encode_json($history_ref);

    # The database is closed and unlocked when %history goes out of scope.
    # Unfortunately, we lose on error detection here, since there doesn't
    # appear to be a way to determine whether all the writes succeeded.  But
    # losing a bit of history in the rare error case of failing to write to
    # local disk is probably not a big deal.
    return;
}

# Write statistics about password length.  Given the length of the password
# and the path to the length statistics database, increments the counter for
# that password length.
#
# Any failure to open or write to the database is ignored, since this is
# considered optional logging and should not block the password change.
#
# $path   - Path to the length statistics file
# $length - Length of the accepted password
#
# Returns: undef
sub update_length_counts {
    my ($path, $length) = @_;

    # Open and lock the database for write.
    my %lengths;
    my $mode = O_CREAT | O_RDWR;
    tie(%lengths, 'DB_File::Lock', $path, $mode, oct(600), $DB_HASH, 'write')
      or return;

    # Write each of the hashes.
    $lengths{$length}++;

    # The database is closed and unlocked when %lengths goes out of scope.
    return;
}

##############################################################################
# Heimdal password quality protocol
##############################################################################

# Run another external password quality checker and return the results.  This
# allows us to chain to another program that handles the actual strength
# checking prior to handling history.
#
# $path      - Password quality check program to run
# $principal - Principal attempting to change their password
# $password  - The new password
#
# Returns: Scalar context: true if the password was accepted, false otherwise
#          List context: whether the password is okay, the exit status of the
#            quality checking program, and the error message if the first
#            element is false
#  Throws: Text exception on failure to execute the program, or read or write
#          from it or to it, or if it fails without an error
sub strength_check {
    my ($path, $principal, $password) = @_;

    # Run the external quality checking program.  If we're root, we'll run it
    # as the strength checking user and group.
    my $in = "principal: $principal\nnew-password: $password\nend\n";
    my $init = sub { drop_privileges($STRENGTH_USER, $STRENGTH_GROUP) };
    my ($out, $err);
    run([$path, $principal], \$in, \$out, \$err, init => $init);
    my $status = ($? >> 8);

    # Check the results.
    my $okay = ($status == 0 && $out eq "APPROVED\n");

    # If the program failed, collect the error message.
    if (!$okay) {
        if ($err) {
            $err =~ s{ \n .* }{}xms;
        } else {
            die "$0: password strength checking failed without an error\n";
        }
    }

    # Return the results.
    return wantarray ? ($okay, $err, $status) : $okay;
}

# Read a Heimdal external password quality checking request from the provided
# file handle and return the principal (ignored for our application) and the
# password.
#
# The protocol expects the following data (without leading whitespace) on
# standard input, in precisely this order:
#
#     principal: <principal>
#     new-password: <password>
#     end
#
# There is one and only one space after the colon, and any subsequent spaces
# are part of the value (such as leading spaces in the password).
#
# $fh - File handle from which to read
#
# Returns: Scalar context: the password
#          List context: a list of the password and the principal
#  Throws: Text exception on any protocol violations or IO errors
sub read_change_data {
    my ($fh) = @_;
    my @keys = qw(principal new-password);
    my %data;

    # Read the data elements we expect.  Verify that they come in the correct
    # order and the correct format.
    local $/ = "\n";
    for my $key (@keys) {
        my $line = readline($fh);
        if (!defined($line)) {
            die "$0: truncated input before $key: $!\n";
        }
        chomp($line);
        if ($line =~ s{ \A \Q$key\E : [ ] }{}xms) {
            $data{$key} = $line;
        } else {
            die "$0: unrecognized input line before $key\n";
        }
    }

    # The final line of input must be a literal "end\n";
    my $line = readline($fh);
    if (!defined($line)) {
        die "$0: truncated input before end: $!\n";
    } elsif ($line ne "end\n") {
        die "$0: unrecognized input line before end\n";
    }

    # Return the results.
    my $password  = $data{'new-password'};
    my $principal = $data{principal};
    return wantarray ? ($password, $principal) : $password;
}

##############################################################################
# Main routine
##############################################################################

# Always flush output.
STDOUT->autoflush;

# Clean up the script name for error reporting.
my $fullpath = $0;
local $0 = basename($0);

# Parse the argument list.
my ($opt, $usage) = describe_options(
    '%c %o',
    ['benchmark|b=f', 'Benchmark hash iterations for this target time'],
    ['database|d=s',  'Path to the history database, overriding the default'],
    ['help|h',        'Print usage message and exit'],
    ['manual|man|m',  'Print full manual and exit'],
    ['quiet|q',       'Suppress logging to syslog'],
    ['stats|S=s',     'Path to hash of length statistics'],
    ['strength|s=s',  'Path to strength checking program to run'],
);
if ($opt->help) {
    print {*STDOUT} $usage->text
      or die "$0: cannot write to standard output: $!\n";
    exit(0);
} elsif ($opt->manual) {
    say {*STDOUT} 'Feeding myself to perldoc, please wait...'
      or die "$0: cannot write to standard output: $!\n";
    exec('perldoc', '-t', $fullpath);
}
my $database = $opt->database || $HISTORY_PATH;
my $stats_db = $opt->stats    || $LENGTH_STATS_PATH;
my $strength = $opt->strength || $STRENGTH_PROGRAM;

# If asked to do benchmarking, ignore other arguments and just do that.
# Currently, we hard-code a 0.005-second granularity on our binary search.
if ($opt->benchmark) {
    find_iteration_count($opt->benchmark, 0.005);
    exit(0);
}

# Open syslog for result reporting.
if ($opt->quiet) {
    $SYSLOG = 0;
} else {
    openlog($0, 'pid', LOG_AUTH);
}

# Read the principal and password that we're supposed to check.
my ($password, $principal) = read_change_data(\*STDIN);

# Delegate to the external strength checking program.
my ($okay, $error, $status) = strength_check($strength, $principal, $password);
if (!$okay) {
    log_result($principal, 'rejected', $error);
    warn "$error\n";
    exit($status);
}

# Drop privileges for the rest of the program.
drop_privileges($HISTORY_USER, $HISTORY_GROUP);

# Hash the password and check history.  Exit if a hash is in history.
if (check_history($database, $principal, $password)) {
    log_result($principal, 'rejected', $REJECT_MESSAGE);
    warn "$REJECT_MESSAGE\n";
    exit(0);
}

# The password is accepted.  Record it, update the length counter, and return
# success.
log_result($principal, 'accepted');
write_history($database, $principal, $password);
say {*STDOUT} 'APPROVED'
  or die "$0: cannot write to standard output: $!\n";
update_length_counts($stats_db, length($password));
exit(0);

__END__

##############################################################################
# Documentation
##############################################################################

=for stopwords
heimdal-history heimdal-strength Heimdal -hmq BerkeleyDB timestamps POSIX
whitespace API Allbery sublicense MERCHANTABILITY NONINFRINGEMENT syslog
pseudorandom JSON LDAP-compatible PBKDF2 SHA-256 KDC

=head1 NAME

heimdal-history - Password history via Heimdal external strength checking

=head1 SYNOPSIS

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

=head1 DESCRIPTION

B<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 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.  C<timestamp> contains the time when the history entry was added (in
POSIX seconds since UNIX epoch), and C<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 L<Crypt::PBKDF2> for more information.

B<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
B</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
L</CONFIGURATION> below.

As with any implementation of the Heimdal external password strength
checking protocol, B<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, B<heimdal-history> will run the external strength
checking program as user C<nobody> and group C<nogroup>, and will check
and write to the history database as user C<_history> and group
C<_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 C<< I<key>=I<value> >>.  The keys are:

=over 4

=item action

The action performed (currently always C<check>).

=item principal

The principal for which a password was checked.

=item 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
C<result> nor C<reason> will be present.  There will be a subsequent log
message from the same invocation giving the final result of the history
check (assuming B<heimdal-history> doesn't exit with a fatal error).

=item result

Either C<accepted> or C<rejected>.

=item reason

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

=back

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 C<">
becomes C<"">.

=head1 OPTIONS

=over 4

=item B<-b> I<target-time>, B<--benchmark>=I<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 I<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 B<heimdal-history> will exit
successfully.

=item B<-d> I<database>, B<--database>=I<database>

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

=item B<-h>, B<--help>

Print a short usage message and exit.

=item B<-m>, B<--manual>, B<--man>

Display this manual and exit.

=item B<-q>, B<--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.

=item B<-S> I<length-stats-db>, B<--stats>=I<length-stats-db>

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

=item B<-s> I<strength-program>, B<--strength>=I<strength-program>

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

=back

=head1 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 B<heimdal-history> program.
By default, it requires a C<_history> user and C<_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 C<nobody> user and
C<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 F</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 C<_history> user.

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

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

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

=head1 RETURN STATUS

On approval of the password, B<heimdal-history> will print C<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, B<heimdal-history> will print the reason for
rejection to standard error and exit with status 0.

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

=head1 FILES

=over 4

=item F</usr/bin/heimdal-strength>

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

=item F</var/lib/heimdal-history/history.db>

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

=item F</var/lib/heimdal-history/history.db.lock>

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

=item F</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
B<heimdal-strength> is run as root, this file needs to be readable and
writable by user C<_history> and group C<_history>.  If it doesn't exist,
it will be created with mode 0600.

=item F</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 B<heimdal-strength>
is run as root, this file needs to be readable and writable by user
C<_history> and group C<_history>.

=back

=head1 AUTHOR

Russ Allbery <eagle@eyrie.org>

=head1 COPYRIGHT AND LICENSE

Copyright 2016 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.

=head1 SEE ALSO

L<Crypt::PBKDF2>, L<heimdal-strength(1)>

=cut