Ubuntu Manpage: krb5-strength - Kerberos password strength checking plugin

NAME

       krb5-strength - Kerberos password strength checking plugin

SYNOPSIS

       MIT Kerberos:

           [plugins]
             pwqual = {
               module = strength:/usr/lib/x86_64-linux-gnu/krb5/plugins/pwqual/strength.so
             }

       Heimdal:

           [password_quality]
               policies         = krb5-strength
               policy_libraries = /usr/lib/x86_64-linux-gnu/krb5/plugins/pwqual/strength.so

DESCRIPTION

       strength.so is a KDC plugin for Kerberos password strength checking for either MIT
       Kerberos or Heimdal provided as part of the krb5-strength package.  For MIT Kerberos KDCs
       (or, more to the point, kadmind servers), this plugin is the recommended way of enabling
       strength checking.  For Heimdal KDCs, you normally should use the heimdal-strength
       external program instead, but the plugin is a supported option if you want to avoid
       external programs for some reason.

       For this module to be effective for either Heimdal or MIT Kerberos, you will also need to
       construct a dictionary.  What type of dictionary you create depends on what backends you
       want to use: CrackLib, CDB, or SQLite.

       For CrackLib, on Debian systems, you can install the cracklib-runtime package and use the
       cracklib-format and cracklib-packer utilities that come with it.  The former takes a set
       of wordlists and outputs a wordlist in the format required by cracklib-packer, and the
       latter turns this into a CrackLib dictionary.  Alternately, you can use the mkdict and
       packer utilities, which are included in the krb5-strength package but not installed by
       default.  You can run them out of the cracklib directory of the source tree after
       building.  (mkdict is the equivalent of cracklib-format.)

       For building a CDB or SQLite dictionary, use krb5-strength-wordlist.

CONFIGURATION

First, build and install either a CrackLib dictionary as described above. The CrackLib
dictionary will consist of three files, one each ending in "*.hwm", "*.pwd", and "*.pwi".
The CDB and SQLite dictionaries will be single files, conventionally ending in "*.cdb" and
"*.sqlite" respectively. Install those files somewhere on your system. Then, follow the
relevant instructions below for either "Heimdal" or "MIT Kerberos".

See "Other Settings" below for additional krb5.conf setting supported by both Heimdal and
MIT Kerberos.

Heimdal
There are two options: using an external password check program, or using the plugin. I
recommend the external password check program unless you encounter speed problems with
that approach that cause kpasswd to time out. If you choose to use the external program,
read the heimdal-strength documentation instead of this documentation.

If using the module, first add a stanza like the following to the "[appdefaults]" section
of your /etc/krb5.conf (or wherever your krb5.conf file is located):

krb5-strength = {
password_dictionary = /path/to/cracklib/dictionary
password_dictionary_cdb = /path/to/cdb/dictionary.cdb
password_dictionary_sqlite = /path/to/sqlite/dictionary.sqlite
}

The first setting configures a CrackLib dictionary, the second a CDB dictionary, and the
third a SQLite dictionary. The provided path should be the full path to the dictionary
files, omitting the trailing "*.hwm", "*.pwd", and "*.pwi" extensions for the CrackLib
dictionary (but including the extensions for the other types). You can use any
combination of the three settings. If you use more than one, CrackLib will be checked
first, then CDB, and then SQLite as appropriate.

When checking against a CDB database, the password, the password with the first character
removed, the last character removed, the first and last characters removed, the first two
characters removed, and the last two characters removed will all be checked against the
dictionary.

When checking a SQLite database, the password will be rejected if it is within edit
distance one of any word in the dictionary, meaning that the database word can be formed
from the password by deleting, adding, or changing a single character.

Then, add a new section (or modify the existing "[password_quality]" section) like the
following:

[password_quality]
policies = krb5-strength
policy_libraries = /usr/lib/x86_64-linux-gnu/krb5/plugins/pwqual/strength.so

in either krb5.conf or kdc.conf. Note that some older versions of Heimdal have a bug in
the support for loading modules when "policy_libraries" is set. If you get an error like:

didn't find `kadm5_password_verifier' symbol in `(null)'

you may have to omit "policy_libraries" in your configuration and instead pass the
"--check-library argument" to kpasswdd specifying the library to load.

If you want to also enable history checking, see heimdal-history(1) for further
instructions.

MIT Kerberos
To add this module to the list of password quality checks, add a section to krb5.conf (or
to a separate kdc.conf if you use that) like:

[plugins]
pwqual = {
module = strength:/usr/lib/x86_64-linux-gnu/krb5/plugins/pwqual/strength.so
}

to register the plugin.

There are two ways to tell where the dictionary is. One option is to use krb5.conf (and
in this case you must use krb5.conf, even if you use a separate kdc.conf file). For this
approach, add the following to the "[appdefaults]" section:

krb5-strength = {
password_dictionary = /path/to/cracklib/dictionary
password_dictionary_cdb = /path/to/cdb/dictionary.cdb
password_dictionary_sqlite = /path/to/sqlite/dictionary.sqlite
}

The second option is to use the normal "dict_path" setting. In the "[realms]" section of
your krb5.conf or kdc.conf, under the appropriate realm or realms, specify the path to the
dictionary:

dict_file = /path/to/cracklib/dictionary

This will be taken as a CrackLib dictionary path, the same as the setting for
password_dictionary above. The provided path should be the full path to the dictionary
files, omitting the trailing "*.hwm", "*.pwd", or "*.pwi" extension. However, be aware
that, if you use this approach, you will probably want to disable the built-in standard
dict pwqual plugin by adding the line:

disable = dict

to the "pwqual" block of the "[plugins]" section as shown above. Otherwise, it will also
try to load a dictionary at the same path to do simple dictionary matching.

You can also mix and match these settings, by using "dict_path" for the CrackLib
dictionary path and krb5.conf for the CDB or SQLite dictionary paths. If both settings
are used for the CrackLib path, krb5.conf overrides the "dict_path" setting (so that
"dict_path" can be used for other password quality modules). There is no way to specify a
CDB or SQLite dictionary via the "dict_path" setting.

Other Settings
The following additional settings are supported in the "[appdefaults]" section of
krb5.conf when running under either Heimdal or MIT Kerberos.

cracklib_maxlen
Normally, all passwords are checked with CrackLib if a CrackLib dictionary is defined.
However, CrackLib's rules were designed for a world in which most passwords were four
to eight characters long, and tends to spuriously reject a lot of passphrases. If
this option is set to something other than its default of 0, passwords longer than
that length bypass CrackLib checks. (Using a SQLite dictionary for longer passwords
is strongly recommended.)

minimum_different
If set to a numeric value, passwords with fewer than this number of unique characters
will be rejected. This can be used to reject, for example, passwords that are long
strings of the same character or repetitions of small numbers of characters, which may
be too easy to guess.

minimum_length
If set to a numeric value, passwords with fewer than that number of characters will be
rejected, independent of any length restrictions in CrackLib. Note that this setting
does not bypass the minimum length requirements in CrackLib itself (which, for the
version embedded in this package, is eight characters).

require_ascii_printable
If set to a true boolean value, rejects any password that contains non-ASCII
characters or ASCII control characters. Spaces are allowed; tabs are not (at least
assuming the POSIX C locale). No canonicalization or character set is defined for
Kerberos passwords in general, so you may want to reject non-ASCII characters to avoid
interoperability problems with computers with different default character sets or
Unicode normalization forms.

require_classes
This option allows specification of more complex character class requirements. The
value of this parameter should be one or more whitespace-separated rule. Each rule
has the syntax:

[<min>-<max>:]<class>[,<class>...]

where <class> is one of "upper", "lower", "digit", or "symbol" (without quote marks),
or an integer representing a minimum number of character classes. The symbol class
includes all characters other than alphanumeric characters, including space. The
listed classes must appear in the password. Separate multiple required classes with a
comma (and no space).

The character class checks will be done in whatever locale the plugin or password
check program is run in, which will normally be the POSIX C locale but may be
different depending on local configuration.

A simple example:

require_classes = upper,lower,digit

This requires all passwords contain at least one uppercase letter, at least one
lowercase letter, and at least one digit.

If present, <min> and <max> specify the minimum password length and maximum password
length to which this rule applies. This allows one to specify character class
requirements that change with password length. So, for example:

require_classes = 8-19:upper,lower 8-15:digit 8-11:symbol

requires all passwords from 8 to 11 characters long contain all four character
classes, passwords from 12 to 15 characters long contain upper and lower case and a
digit, and passwords from 16 to 19 characters long contain both upper and lower case.
Passwords longer than 20 characters have no character class restrictions. (This
example is probably used in conjunction with "minimum_length = 8".)

"require_classes" also supports specifying the minimum number of character classes a
password should contain. For example:

require_classes = 3

would require all passwords to have a minimum of any three of the character classes.

This can also be used with <min> and <max> ranges, as above. For example:

require_classes = 8-11:3 12-19:2

requires all passwords from 8 to 11 characters contain at least three different
character classes, and passwords from 12 to 19 characters contain at least two
different character classes. Ranges can overlap, as in the examples above, but this
makes less sense when specifying a minimum number of classes.

Minimum numbers of character classes can be combined with specific character classes.
For example:

require_classes = symbol,3

requires all passwords contain three distinct character classes and must contain a
symbol character.

require_non_letter
If set to a true boolean value, the password must contain at least one character that
is not a letter (uppercase or lowercase) or a space. This may be helpful in
combination with passphrases; users may choose a stock English phrase, and this will
force at least some additional complexity.

You can omit any dictionary setting and only use the above settings, in which case only
the above checks and checks for passwords based on the principal will be done, bypassing
any dictionary check. (But for that simple style of password strength checking, there are
probably better strength checking plugins already available.)

AUTHOR

       Russ Allbery <eagle@eyrie.org>

COPYRIGHT AND LICENSE