xref: /dports/security/pam_krb5/pam-krb5-4.9/README

                               pam-krb5 4.9
                 (PAM module for Kerberos authentication)
               Maintained by Russ Allbery <eagle@eyrie.org>

  Copyright 2005-2010, 2014-2015, 2017, 2020 Russ Allbery
  <eagle@eyrie.org>.  Copyright 2009-2011 The Board of Trustees of the
  Leland Stanford Junior University.  Copyright 2005 Andres Salomon
  <dilinger@debian.org>.  Copyright 1999-2000 Frank Cusack
  <fcusack@fcusack.com>.  This software is distributed under a BSD-style
  license.  Please see the section LICENSE below for more information.

BLURB

  pam-krb5 is a Kerberos PAM module for either MIT Kerberos or Heimdal.
  It supports ticket refreshing by screen savers, configurable
  authorization handling, authentication of non-local accounts for network
  services, password changing, and password expiration, as well as all the
  standard expected PAM features.  It works correctly with OpenSSH, even
  with ChallengeResponseAuthentication and PrivilegeSeparation enabled,
  and supports extensive configuration either by PAM options or in
  krb5.conf or both.  PKINIT is supported with recent versions of both MIT
  Kerberos and Heimdal and FAST is supported with recent MIT Kerberos.

DESCRIPTION

  pam-krb5 provides a Kerberos PAM module that supports authentication,
  user ticket cache handling, simple authorization (via .k5login or
  checking Kerberos principals against local usernames), and password
  changing.  It can be configured through either options in the PAM
  configuration itself or through entries in the system krb5.conf file,
  and it tries to work around PAM implementation flaws in commonly-used
  PAM-enabled applications such as OpenSSH and xdm.  It supports both
  PKINIT and FAST to the extent that the underlying Kerberos libraries
  support these features.

  This is not the Kerberos PAM module maintained on Sourceforge and used
  on Red Hat systems.  It is an independent implementation that, if it
  ever shared any common code, diverged long ago.  It supports some
  features that the Sourceforge module does not (particularly around
  authorization), and does not support some options (particularly ones not
  directly related to Kerberos) that it does.  This module will never
  support Kerberos v4 or AFS.  For an AFS session module that works with
  this module (or any other Kerberos PAM module), see pam-afs-session [1].

  [1] https://www.eyrie.org/~eagle/software/pam-afs-session/

  If there are other options besides AFS and Kerberos v4 support from the
  Sourceforge PAM module that you're missing in this module, please let me
  know.

REQUIREMENTS

  Either MIT Kerberos (or Kerberos implementations based on it) or Heimdal
  are supported.  MIT Keberos 1.3 or later may be required; this module
  has not been tested with earlier versions.

  For PKINIT support, Heimdal 0.8rc1 or later or MIT Kerberos 1.6.3 or
  later are required.  Earlier MIT Kerberos 1.6 releases have a bug in
  their handling of PKINIT options.  MIT Kerberos 1.12 or later is
  required to use the use_pkinit PAM option.

  For FAST (Flexible Authentication Secure Tunneling) support, MIT
  Kerberos 1.7 or higher is required.  For anonymous FAST support,
  anonymous authentication (generally anonymous PKINIT) support is
  required in both the Kerberos libraries and in the local KDC.

  This module should work on Linux and build with gcc or clang.  It may
  still work on Solaris and build with the Sun C compiler, but I have only
  tested it on Linux recently.  There is beta-quality support for the AIX
  NAS Kerberos implementation that has not been tested in years.  Other
  PAM implementations will probably require some porting, although
  untested build system support is present for FreeBSD, Mac OS X, and
  HP-UX.  I personally can only test on Linux and rely on others to report
  problems on other operating systems.

  Old versions of OpenSSH are known to call pam_authenticate followed by
  pam_setcred(PAM_REINITIALIZE_CRED) without first calling
  pam_open_session, thereby requesting that an existing ticket cache be
  renewed (similar to what a screensaver would want) rather than
  requesting a new ticket cache be created.  Since this behavior is
  indistinguishable at the PAM level from a screensaver, pam-krb5 when
  used with these old versions of OpenSSH will refresh the ticket cache of
  the OpenSSH daemon rather than setting up a new ticket cache for the
  user.  The resulting ticket cache will have the correct permissions
  (this is not a security concern), but will not be named correctly or
  referenced in the user's environment and will be overwritten by the next
  user login.  The best solution to this problem is to upgrade OpenSSH.
  I'm not sure exactly when this problem was fixed, but at the very least
  OpenSSH 4.3 and later do not exhibit it.

  To bootstrap from a Git checkout, or if you change the Automake files
  and need to regenerate Makefile.in, you will need Automake 1.11 or
  later.  For bootstrap or if you change configure.ac or any of the m4
  files it includes and need to regenerate configure or config.h.in, you
  will need Autoconf 2.64 or later.  Perl is also required to generate
  manual pages from a fresh Git checkout.

BUILDING AND INSTALLATION

  You can build and install pam-krb5 with the standard commands:

      ./configure
      make
      make install

  If you are building from a Git clone, first run ./bootstrap in the
  source directory to generate the build files.  make install will
  probably have to be done as root.  Building outside of the source
  directory is also supported, if you wish, by creating an empty directory
  and then running configure with the correct relative path.

  The module will be installed in /usr/local/lib/security by default,
  except on 64-bit versions of Linux which will use
  /usr/local/lib64/security to match the default PAM configuration.  You
  can change the installation locations with the --prefix, --mandir, and
  --libdir options to configure.  The module will always be installed in a
  subdirectory named security under the specified libdir.  On Linux, use
  --prefix=/usr to install the man page into /usr/share/man and the PAM
  module in /lib/security or /lib64/security.

  Normally, configure will use krb5-config to determine the flags to use
  to compile with your Kerberos libraries.  To specify a particular
  krb5-config script to use, either set the PATH_KRB5_CONFIG environment
  variable or pass it to configure like:

      ./configure PATH_KRB5_CONFIG=/path/to/krb5-config

  If krb5-config isn't found, configure will look for the standard
  Kerberos libraries in locations already searched by your compiler.  If
  the the krb5-config script first in your path is not the one
  corresponding to the Kerberos libraries you want to use, or if your
  Kerberos libraries and includes aren't in a location searched by default
  by your compiler, you need to specify a different Kerberos installation
  root via --with-krb5=PATH.  For example:

      ./configure --with-krb5=/usr/pubsw

  You can also individually set the paths to the include directory and the
  library directory with --with-krb5-include and --with-krb5-lib.  You may
  need to do this if Autoconf can't figure out whether to use lib, lib32,
  or lib64 on your platform.

  To not use krb5-config and force library probing even if there is a
  krb5-config script on your path, set PATH_KRB5_CONFIG to a nonexistent
  path:

      ./configure PATH_KRB5_CONFIG=/nonexistent

  krb5-config is not used and library probing is always done if either
  --with-krb5-include or --with-krb5-lib are given.

  Pass --enable-silent-rules to configure for a quieter build (similar to
  the Linux kernel).  Use make warnings instead of make to build with full
  compiler warnings (requires either GCC or Clang and may require a
  relatively current version of the compiler).

  You can pass the --enable-reduced-depends flag to configure to try to
  minimize the shared library dependencies encoded in the binaries.  This
  omits from the link line all the libraries included solely because other
  libraries depend on them and instead links the programs only against
  libraries whose APIs are called directly.  This will only work with
  shared libraries and will only work on platforms where shared libraries
  properly encode their own dependencies (this includes most modern
  platforms such as all Linux).  It is intended primarily for building
  packages for Linux distributions to avoid encoding unnecessary shared
  library dependencies that make shared library migrations more difficult.
  If none of the above made any sense to you, don't bother with this flag.

TESTING

  pam-krb5 comes with a comprehensive test suite, but it requires some
  configuration in order to test anything other than low-level utility
  functions.  For the full test suite, you will need to have a running KDC
  in which you can create two test accounts, one with admin access to the
  other.  Using a test KDC environment, if you have one, is recommended.

  Follow the instructions in tests/config/README to configure the test
  suite.

  Now, you can run the test suite with:

      make check

  If a test fails, you can run a single test with verbose output via:

      tests/runtests -o <name-of-test>

  Do this instead of running the test program directly since it will
  ensure that necessary environment variables are set up.

  The default libkadm5clnt library on the system must match the
  implementation of your KDC for the module/expired test to work, since
  the two kadmin protocols are not compatible.  If you use the MIT library
  against a Heimdal server, the test will be skipped; if you use the
  Heimdal library against an MIT server, the test suite may hang.

  Several module/expired tests are expected to fail with Heimdal 1.5 due
  to a bug in Heimdal with reauthenticating immediately after a
  library-mediated password change of an expired password.  This is fixed
  in later releases of Heimdal.

  To run the full test suite, Perl 5.8 or later is required.  The
  following additional Perl modules will be used if present:

  * Test::Pod
  * Test::Spelling

  All are available on CPAN.  Those tests will be skipped if the modules
  are not available.

CONFIGURING

  Just installing the module does not enable it or change anything about
  your system authentication configuration.  To use the module for all
  system authentication on Debian systems, put something like:

      auth  sufficient   pam_krb5.so minimum_uid=1000
      auth  required     pam_unix.so try_first_pass nullok_secure

  in /etc/pam.d/common-auth, something like:

      session  optional  pam_krb5.so minimum_uid=1000
      session  required  pam_unix.so

  in /etc/pam.d/common-session, and something like:

      account  required  pam_krb5.so minimum_uid=1000
      account  required  pam_unix.so

  in /etc/pam.d/common-account.  The minimum_uid setting tells the PAM
  module to pass on any users with a UID lower than 1000, thereby
  bypassing Kerberos authentication for the root account and any system
  accounts.  You normally want to do this since otherwise, if the network
  is down, the Kerberos authentication can time out and make it difficult
  to log in as root and fix matters.  This also avoids problems with
  Kerberos principals that happen to match system accounts accidentally
  getting access to those accounts.

  Be sure to include the module in the session group as well as the auth
  group.  Without the session entry, the user's ticket cache will not be
  created properly for ssh logins (among possibly others).

  If your users should normally all use Kerberos passwords exclusively,
  putting something like:

      password sufficient pam_krb5.so minimum_uid=1000
      password required   pam_unix.so try_first_pass obscure md5

  in /etc/pam.d/common-password will change users' passwords in Kerberos
  by default and then only fall back on Unix if that doesn't work.  (You
  can make this tighter by using the more complex new-style PAM
  configuration.)  If you instead want to synchronize local and Kerberos
  passwords and change them both at the same time, you can do something
  like:

      password required   pam_unix.so obscure sha512
      password required   pam_krb5.so use_authtok minimum_uid=1000

  If you have multiple environments that you want to synchronize and you
  don't want password changes to continue if the Kerberos password change
  fails, use the clear_on_fail option.  For example:

      password required   pam_krb5.so clear_on_fail minimum_uid=1000
      password required   pam_unix.so use_authtok obscure sha512
      password required   pam_smbpass.so use_authtok

  In this case, if pam_krb5 cannot change the password (due to password
  strength rules on the KDC, for example), it will clear the stored
  password (because of the clear_on_fail option), and since pam_unix and
  pam_smbpass are both configured with use_authtok, they will both fail.
  clear_on_fail is not the default because it would interfere with the
  more common pattern of falling back to local passwords if the user
  doesn't exist in Kerberos.

  If you use a more complex configuration with the Linux PAM [] syntax for
  the session and account groups, note that pam_krb5 returns a status of
  ignore, not success, if the user didn't log on with Kerberos.  You may
  need to handle that explicitly with ignore=ignore in your action list.

  There are many, many other possibilities.  See the Linux PAM
  documentation for all the configuration options.

  On Red Hat systems, modify /etc/pam.d/system-auth instead, which
  contains all of the configuration for the different stacks.

  You can also use pam-krb5 only for specific services.  In that case,
  modify the files in /etc/pam.d for that particular service to use
  pam_krb5.so for authentication.  For services that are using passwords
  over TLS to authenticate users, you may want to use the ignore_k5login
  and no_ccache options to the authenticate module.  .k5login
  authorization is only meaningful for local accounts and ticket caches
  are usually (although not always) only useful for interactive sessions.

  Configuring the module for Solaris is both simpler and less flexible,
  since Solaris (at least Solaris 8 and 9, which are the last versions of
  Solaris with which this module was extensively tested) use a single
  /etc/pam.conf file that contains configuration for all programs.  For
  console login on Solaris, try something like:

      login auth sufficient /usr/local/lib/security/pam_krb5.so minimum_uid=100
      login auth required /usr/lib/security/pam_unix_auth.so.1 use_first_pass
      login account required /usr/local/lib/security/pam_krb5.so minimum_uid=100
      login account required /usr/lib/security/pam_unix_account.so.1
      login session required /usr/local/lib/security/pam_krb5.so retain_after_close minimum_uid=100
      login session required /usr/lib/security/pam_unix_session.so.1

  A similar configuration could be used for other services, such as ssh.
  See the pam.conf(5) man page for more information.  When using this
  module with Solaris login (at least on Solaris 8 and 9), you will
  probably also need to add retain_after_close to the PAM configuration to
  avoid having the user's credentials deleted before they are logged in.

  The Solaris Kerberos library reportedly does not support prompting for a
  password change of an expired account during authentication.  Supporting
  password change for expired accounts on Solaris with native Kerberos may
  therefore require setting the defer_pwchange or force_pwchange option
  for selected login applications.  See the description and warnings about
  that option in the pam_krb5(5) man page.

  Some configuration options may be put in the krb5.conf file used by your
  Kerberos libraries (usually /etc/krb5.conf or /usr/local/etc/krb5.conf)
  instead or in addition to the PAM configuration.  See the man page for
  more details.

  The Kerberos library, via pam-krb5, will prompt the user to change their
  password if their password is expired, but when using OpenSSH, this will
  only work when ChallengeResponseAuthentication is enabled.  Unless this
  option is enabled, OpenSSH doesn't pass PAM messages to the user and can
  only respond to a simple password prompt.

  If you are using MIT Kerberos, be aware that users whose passwords are
  expired will not be prompted to change their password unless the KDC
  configuration for your realm in [realms] in krb5.conf contains a
  master_kdc setting or, if using DNS SRV records, you have a DNS entry
  for _kerberos-master as well as _kerberos.

DEBUGGING

  The first step when debugging any problems with this module is to add
  debug to the PAM options for the module (either in the PAM configuration
  or in krb5.conf).  This will significantly increase the logging from the
  module and should provide a trace of exactly what failed and any
  available error information.

  Many Kerberos authentication problems are due to configuration issues in
  krb5.conf.  If pam-krb5 doesn't work, first check that kinit works on
  the same system.  That will test your basic Kerberos configuration.  If
  the system has a keytab file installed that's readable by the process
  doing authentication via PAM, make sure that the keytab is current and
  contains a key for host/<system> where <system> is the fully-qualified
  hostname.  pam-krb5 prevents KDC spoofing by checking the user's
  credentials when possible, but this means that if a keytab is present it
  must be correct or authentication will fail.  You can check the keytab
  with klist -k and kinit -k.

  Be sure that all libraries and modules, including PAM modules, loaded by
  a program use the same Kerberos libraries.  Sometimes programs that use
  PAM, such as current versions of OpenSSH, also link against Kerberos
  directly.  If your sshd is linked against one set of Kerberos libraries
  and pam-krb5 is linked against a different set of Kerberos libraries,
  this will often cause problems (such as segmentation faults, bus errors,
  assertions, or other strange behavior).  Similar issues apply to the
  com_err library or any other library used by both modules and shared
  libraries and by the application that loads them.  If your OS ships
  Kerberos libraries, it's usually best if possible to build all Kerberos
  software on the system against those libraries.

IMPLEMENTATION NOTES

  The normal sequence of actions taken for a user login is:

      pam_authenticate
      pam_setcred(PAM_ESTABLISH_CRED)
      pam_open_session
      pam_acct_mgmt

  and then at logout:

      pam_close_session

  followed by closing the open PAM session.  The corresponding pam_sm_*
  functions in this module are called when an application calls those
  public interface functions.  Not all applications call all of those
  functions, or in particularly that order, although pam_authenticate is
  always first and has to be.

  When pam_authenticate is called, pam-krb5 creates a temporary ticket
  cache in /tmp and sets the PAM environment variable PAM_KRB5CCNAME to
  point to it.  This ticket cache will be automatically destroyed when the
  PAM session is closed and is there only to pass the initial credentials
  to the call to pam_setcred.  The module would use a memory cache, but
  memory caches will only work if the application preserves the PAM
  environment between the calls to pam_authenticate and pam_setcred.  Most
  do, but OpenSSH notoriously does not and calls pam_authenticate in a
  subprocess, so this method is used to pass the tickets to the
  pam_setcred call in a different process.

  pam_authenticate does a complete authentication, including checking the
  resulting TGT by obtaining a service ticket for the local host if
  possible, but this requires read access to the system keytab.  If the
  keytab doesn't exist, can't be read, or doesn't include the appropriate
  credentials, the default is to accept the authentication.  This can be
  controlled by setting verify_ap_req_nofail to true in [libdefaults] in
  /etc/krb5.conf.  pam_authenticate also does a basic authorization check,
  by default calling krb5_kuserok (which uses ~/.k5login if available and
  falls back to checking that the principal corresponds to the account
  name).  This can be customized with several options documented in the
  pam_krb5(5) man page.

  pam-krb5 treats pam_open_session and pam_setcred(PAM_ESTABLISH_CRED) as
  synonymous, as some applications call one and some call the other.  Both
  copy the initial credentials from the temporary cache into a permanent
  cache for this session and set KRB5CCNAME in the environment.  It will
  remember when the credential cache has been established and then avoid
  doing any duplicate work afterwards, since some applications call
  pam_setcred or pam_open_session multiple times (most notably X.Org 7 and
  earlier xdm, which also throws away the module settings the last time it
  calls them).

  pam_acct_mgmt finds the ticket cache, reads it in to obtain the
  authenticated principal, and then does is another authorization check
  against .k5login or the local account name as described above.

  After the call to pam_setcred or pam_open_session, the ticket cache will
  be destroyed whenever the calling application either destroys the PAM
  environment or calls pam_close_session, which it should do on user
  logout.

  The normal sequence of events when refreshing a ticket cache (such as
  inside a screensaver) is:

      pam_authenticate
      pam_setcred(PAM_REINITIALIZE_CRED)
      pam_acct_mgmt

  (PAM_REFRESH_CRED may be used instead.)  Authentication proceeds as
  above.  At the pam_setcred stage, rather than creating a new ticket
  cache, the module instead finds the current ticket cache (from the
  KRB5CCNAME environment variable or the default ticket cache location
  from the Kerberos library) and then reinitializes it with the
  credentials from the temporary pam_authenticate ticket cache.  When
  refreshing a ticket cache, the application should not open a session.
  Calling pam_acct_mgmt is optional; pam-krb5 doesn't do anything
  different when it's called in this case.

  If pam_authenticate apparently didn't succeed, or if an account was
  configured to be ignored via ignore_root or minimum_uid, pam_setcred
  (and therefore pam_open_session) and pam_acct_mgmt return PAM_IGNORE,
  which tells the PAM library to proceed as if that module wasn't listed
  in the PAM configuration at all.  pam_authenticate, however, returns
  failure in the ignored user case by default, since otherwise a
  configuration using ignore_root with pam-krb5 as the only PAM module
  would allow anyone to log in as root without a password.  There doesn't
  appear to be a case where returning PAM_IGNORE instead would improve the
  module's behavior, but if you know of a case, please let me know.

  By default, pam_authenticate intentionally does not follow the PAM
  standard for handling expired accounts and instead returns failure from
  pam_authenticate unless the Kerberos libraries are able to change the
  account password during authentication.  Too many applications either do
  not call pam_acct_mgmt or ignore its exit status.  The fully correct PAM
  behavior (returning success from pam_authenticate and
  PAM_NEW_AUTHTOK_REQD from pam_acct_mgmt) can be enabled with the
  defer_pwchange option.

  The defer_pwchange option is unfortunately somewhat tricky to implement.
  In this case, the calling sequence is:

      pam_authenticate
      pam_acct_mgmt
      pam_chauthtok
      pam_setcred
      pam_open_session

  During the first pam_authenticate, we can't obtain credentials and
  therefore a ticket cache since the password is expired.  But
  pam_authenticate isn't called again after pam_chauthtok, so
  pam_chauthtok has to create a ticket cache.  We however don't want it to
  do this for the normal password change (passwd) case.

  What we do is set a flag in our PAM data structure saying that we're
  processing an expired password, and pam_chauthtok, if it sees that flag,
  redoes the authentication with password prompting disabled after it
  finishes changing the password.

  Unfortunately, when handling password changes this way, pam_chauthtok
  will always have to prompt the user for their current password again
  even though they just typed it.  This is because the saved
  authentication tokens are cleared after pam_authenticate returns, for
  security reasons.  We could hack around this by saving the password in
  our PAM data structure, but this would let the application gain access
  to it (exactly what the clearing is intended to prevent) and breaks a
  PAM library guarantee.  We could also work around this by having
  pam_authenticate get the kadmin/changepw authenticator in the expired
  password case and store it for pam_chauthtok, but it doesn't seem worth
  the hassle.

HISTORY AND ACKNOWLEDGEMENTS

  Originally written by Frank Cusack <fcusack@fcusack.com>, with the
  following acknowledgement:

    Thanks to Naomaru Itoi <itoi@eecs.umich.edu>, Curtis King
    <curtis.king@cul.ca>, and Derrick Brashear <shadow@dementia.org>, all
    of whom have written and made available Kerberos 4/5 modules.
    Although no code in this module is directly from these author's
    modules, (except the get_user_info() routine in support.c; derived
    from whichever of these authors originally wrote the first module the
    other 2 copied from), it was extremely helpful to look over their code
    which aided in my design.

  The module was then patched for the FreeBSD ports collection with
  additional modifications by unknown maintainers and then was modified by
  Joel Kociolek <joko@logidee.com> to be usable with Debian GNU/Linux.

  It was packaged by Sam Hartman as the Kerberos v5 PAM module for Debian
  and improved and modified by him and later by Russ Allbery to fix bugs
  and add additional features.  It was then adopted by Andres Salomon, who
  added support for refreshing credentials.

  The current distribution is maintained by Russ Allbery, who also added
  support for reading configuration from krb5.conf, added many features
  for compatibility with the Sourceforge module, commented and
  standardized the formatting of the code, and overhauled the
  documentation.

  Thanks to Douglas E. Engert for the initial implementation of PKINIT
  support.  I have since modified and reworked it extensively, so any bugs
  or compilation problems are my fault.

  Thanks to Markus Moeller for lots of debugging and multiple patches and
  suggestions for improved portability.

  Thanks to Booker Bense for the implementation of the alt_auth_map
  option.

  Thanks to Sam Hartman for the FAST support implementation.

SUPPORT

  The pam-krb5 web page at:

      https://www.eyrie.org/~eagle/software/pam-krb5/

  will always have the current version of this package, the current
  documentation, and pointers to any additional resources.

  For bug tracking, use the issue tracker on GitHub:

      https://github.com/rra/pam-krb5/issues

  However, please be aware that I tend to be extremely busy and work
  projects often take priority.  I'll save your report and get to it as
  soon as I can, but it may take me a couple of months.

SOURCE REPOSITORY

  pam-krb5 is maintained using Git.  You can access the current source on
  GitHub at:

      https://github.com/rra/pam-krb5

  or by cloning the repository at:

      https://git.eyrie.org/git/kerberos/pam-krb5.git

  or view the repository via the web at:

      https://git.eyrie.org/?p=kerberos/pam-krb5.git

  The eyrie.org repository is the canonical one, maintained by the author,
  but using GitHub is probably more convenient for most purposes.  Pull
  requests are gratefully reviewed and normally accepted.

LICENSE

  The pam-krb5 package as a whole is covered by the following copyright
  statement and license:

    Copyright 2005-2010, 2014-2015, 2017, 2020
        Russ Allbery <eagle@eyrie.org>
    Copyright 2009-2011
        The Board of Trustees of the Leland Stanford Junior University
    Copyright 2005 Andres Salomon <dilinger@debian.org>
    Copyright 1999-2000 Frank Cusack <fcusack@fcusack.com>

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are
    met:

    1. Redistributions of source code must retain the above copyright
       notice, and the entire permission notice in its entirety, including
       the disclaimer of warranties.

    2. Redistributions in binary form must reproduce the above copyright
       notice, this list of conditions and the following disclaimer in the
       documentation and/or other materials provided with the distribution.

    3. The name of the author may not be used to endorse or promote products
       derived from this software without specific prior written permission.

    ALTERNATIVELY, this product may be distributed under the terms of the
    GNU General Public License, in which case the provisions of the GPL
    are required INSTEAD OF the above restrictions.  (This clause is
    necessary due to a potential bad interaction between the GPL and the
    restrictions contained in a BSD-style copyright.)

    THIS SOFTWARE IS PROVIDED "AS IS" AND ANY EXPRESS OR IMPLIED
    WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
    MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
    IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
    OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
    ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
    TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
    USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
    DAMAGE.

  Some files in this distribution are individually released under
  different licenses, all of which are compatible with the above general
  package license but which may require preservation of additional
  notices.  All required notices, and detailed information about the
  licensing of each file, are recorded in the LICENSE file.

  Files covered by a license with an assigned SPDX License Identifier
  include SPDX-License-Identifier tags to enable automated processing of
  license information.  See https://spdx.org/licenses/ for more
  information.

  For any copyright range specified by files in this package as YYYY-ZZZZ,
  the range specifies every single year in that closed interval.