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  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

  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: