pam-krb5/docs/pam_krb5.pod

=for stopwords
KRB5CCNAME ChallengeResponseAuthentication GSS-API Heimdal KDC PKINIT
PasswordAuthentication SRV Solaris Sourceforge aname appdefaults auth
canonicalized ccache krb5.conf forwardable kdestroy keytab libdefaults
logout pam-krb5 preauth 0.8rc1 screensaver screensavers sshd localname
krb5.conf. 0.8rc1. Allbery Cusack Salomon FSFAP SPDX-License-Identifier
responder

=head1 NAME

pam_krb5 - Kerberos PAM module

=head1 SYNOPSIS

  auth            sufficient      pam_krb5.so minimum_uid=1000
  session         required        pam_krb5.so minimum_uid=1000
  account         required        pam_krb5.so minimum_uid=1000
  password        sufficient      pam_krb5.so minimum_uid=1000

=head1 DESCRIPTION

The Kerberos service module for PAM, typically installed at
F</lib/security/pam_krb5.so>, provides functionality for the four PAM
operations: authentication, account management, session management, and
password management.  F<pam_krb5.so> is a shared object that is
dynamically loaded by the PAM subsystem as necessary, based on the system
PAM configuration.  PAM is a system for plugging in external
authentication and session management modules so that each application
doesn't have to know the best way to check user authentication or create a
user session on that system.  For details on how to configure PAM on your
system, see the PAM man page, often pam(7).

Here are the actions of this module when called from each group:

=over 4

=item auth

Provides implementations of pam_authenticate() and pam_setcred().  The
former takes the username from the PAM session, prompts for the user's
password (unless configured to use an already-entered password), and then
performs a Kerberos initial authentication, storing the obtained
credentials (if successful) in a temporary ticket cache.  The latter,
depending on the flags it is called with, either takes the contents of the
temporary ticket cache and writes it out to a persistent ticket cache
owned by the user or uses the temporary ticket cache to refresh an
existing user ticket cache.

Passwords as long or longer than PAM_MAX_RESP_SIZE octets (normally 512
octets) will be rejected, since excessively long passwords can be used as
a denial of service attack.

After doing the initial authentication, the Kerberos PAM module will
attempt to obtain tickets for a key in the local system keytab and then
verify those tickets.  Unless this step is performed, the authentication
is vulnerable to KDC spoofing, but it requires that the system have a
local key and that the PAM module be running as a user that can read the
keytab file (normally F</etc/krb5.keytab>.  You can point the Kerberos PAM
module at a different keytab with the I<keytab> option.  If that keytab
cannot be read or if no keys are found in it, the default behavior is to
fail authentication. If you want to skip this check, set the
C<allow_kdc_spoof> option to true either in the [appdefaults] section of
F</etc/krb5.conf> or in the PAM policy.

By default, whenever the user is authenticated, a basic authorization
check will also be done using krb5_kuserok().  The default behavior of
this function is to check the user's account for a F<.k5login> file and,
if one is present, ensure that the user's principal is listed in that
file.  If F<.k5login> is not present, the default check is to ensure that
the user's principal is in the default local realm and the user portion of
the principal matches the account name (this can be changed by configuring
a custom aname to localname mapping in F<krb5.conf>; see the Kerberos
documentation for details).  This can be customized with several
configuration options; see below.

If the username provided to PAM contains an C<@> and Kerberos can,
treating the username as a principal, map it to a local account name,
pam_authenticate() will change the PAM user to that local account name.
This allows users to log in with their Kerberos principal and let Kerberos
do the mapping to an account.  This can be disabled with the
I<no_update_user> option.  Be aware, however, that this facility cannot be
used with OpenSSH.  OpenSSH will reject usernames that don't match local
accounts before this remapping can be done and will pass an invalid
password to the PAM module.  Also be aware that several other common PAM
modules, such as pam_securetty, expect to be able to look up the user with
getpwnam() and cannot be called before pam_krb5 when using this feature.

When pam_setcred() is called to initialize a new ticket cache, the
environment variable KRB5CCNAME is set to the path to that ticket cache.
By default, the cache will be named F</tmp/krb5cc_UID_RANDOM> where UID is
the user's UID and RANDOM is six randomly-chosen letters.  This can be
configured with the I<ccache> and I<ccache_dir> options.

pam-krb5 does not use the default ticket cache location or
I<default_cc_name> in the C<[libdefaults]> section of F<krb5.conf>.  The
default cache location would share a cache for all sessions of the same
user, which causes confusing behavior when the user logs out of one of
multiple sessions.

If pam_setcred() initializes a new ticket cache, it will also set up that
ticket cache so that it will be deleted when the PAM session is closed.
Normally, the calling program (B<login>, B<sshd>, etc.) will run the
user's shell as a sub-process, wait for it to exit, and then close the PAM
session, thereby cleaning up the user's session.

=item session

Provides implementations of pam_open_session(), which is equivalent to
calling pam_setcred() with the PAM_ESTABLISH_CRED flag, and
pam_close_session(), which destroys the ticket cache created by
pam_setcred().

=item account

Provides an implementation of pam_acct_mgmt().  All it does is do the same
authorization check as performed by the pam_authenticate() implementation
described above.

=item password

Provides an implementation of pam_chauthtok(), which implements password
changes.  The user is prompted for their existing password (unless
configured to use an already entered one) and the PAM module then obtains
credentials for the special Kerberos principal C<kadmin/changepw>.  It
then prompts the user for a new password, twice to ensure that the user
entered it properly (again, unless configured to use an already entered
password), and then does a Kerberos password change.

Passwords as long or longer than PAM_MAX_RESP_SIZE octets (normally 512
octets) will be rejected, since excessively long passwords can be used as
a denial of service attack.

Unlike the normal Unix password module, this module will allow any user to
change any other user's password if they know the old password.  Also,
unlike the normal Unix password module, root will always be prompted for
the old password, since root has no special status in Kerberos.  (To
change passwords in Kerberos without knowing the old password, use
kadmin(8) instead.)

=back

Both the account and session management calls of the Kerberos PAM module
will return PAM_IGNORE if called in the context of a PAM session for a
user who did not authenticate with Kerberos (a return code of C<ignore> in
the Linux PAM configuration language).

Note that this module assumes the network is available in order to do a
Kerberos authentication.  If the network is not available, some Kerberos
libraries have timeouts longer than the timeout imposed by the login
process.  This means that using this module incautiously can make it
impossible to log on to console as root.  For this reason, you should
always use the I<ignore_root> or I<minimum_uid> options, list a local
authentication module such as B<pam_unix> first with a control field of
C<sufficient> so that the Kerberos PAM module will be skipped if local
password authentication was successful.

This is not the same PAM module as the Kerberos PAM module available from
Sourceforge, or the one included on Red Hat systems.  It supports many of
the same options, has some additional options, and doesn't support some of
the options those modules do.

=head1 CONFIGURATION

The Kerberos PAM module takes many options, not all of which are relevant
to every PAM group; options that are not relevant will be silently
ignored.  Any of these options can be set in the PAM configuration as
arguments listed after C<pam_krb5.so>.  Some of the options can also be
set in the system F<krb5.conf> file; if this is possible, it will be noted
below in the option description.

To set a boolean option in the PAM configuration file, just give the name
of the option in the arguments.  To set an option that takes an argument,
follow the option name with an equal sign (=) and the value, with no
separating whitespace.  Whitespace in option arguments is not supported in
the PAM configuration.

To set an option for the PAM module in the system F<krb5.conf> file, put
that option in the C<[appdefaults]> section.  All options must be followed
by an equal sign (=) and a value, so for boolean options add C<= true>.
The Kerberos PAM module will look for options either at the top level of
the C<[appdefaults]> section or in a subsection named C<pam>, inside or
outside a section for the realm.  For example, the following fragment of a
F<krb5.conf> file would set I<forwardable> to true, I<minimum_uid> to
1000, and set I<ignore_k5login> only if the realm is EXAMPLE.COM.

    [appdefaults]
        forwardable = true
        pam = {
            minimum_uid = 1000
            EXAMPLE.COM = {
                ignore_k5login = true
            }
        }

For more information on the syntax of F<krb5.conf>, see krb5.conf(5).
Note that options that depend on the realm will be set only on the basis
of the default realm, either as configured in krb5.conf(5) or as set by
the I<realm> option described below.  If the user authenticates to an
account qualified with a realm, that realm will not be used when
determining which options will apply.

There is no difference to the PAM module whether options are specified at
the top level or in a C<pam> section; the C<pam> section is supported in
case there are options that should be set for the PAM module but not for
other applications.

If the same option is set in F<krb5.conf> and in the PAM configuration,
the latter takes precedent.  Note, however, that due to the configuration
syntax, there's no way to turn off a boolean option in the PAM
configuration that was turned on in F<krb5.conf>.

The start of each option description is annotated with the version of
pam-krb5 in which that option was added with the current meaning.

=head2 Authorization

=over 4

=item allow_kdc_spoof

Allow authentication to succeed even if there is no host or service
key available in a keytab to authenticate the Kerberos KDC's ticket.

=item alt_auth_map=<format>

[3.12] This functions similarly to the I<search_k5login> option.  The
<format> argument is used as the authentication Kerberos principal, with
any C<%s> in <format> replaced with the username.  If the username
contains an C<@>, only the part of the username before the realm is used
to replace C<%s>.  If <format> contains a realm, it will be used;
otherwise, the realm of the username (if any) will be appended to the
result.  There is no quote removal.

If this option is present, the default behavior is to try this alternate
principal first and then fall back to the standard behavior if it fails.
The primary usage is to allow alternative principals to be used for
authentication in programs like B<sudo>.  Most examples will look like:

    alt_auth_map=%s/root

which attempts authentication as the root instance of the username first
and then falls back to the regular username (but see I<force_alt_auth> and
I<only_alt_auth>).

This option also allows a cheap way to attempt authentication in an
alternative realm first and then fall back to the primary realm.  A
setting like:

    alt_auth_map=%s@EXAMPLE.COM

will attempt authentication in the EXAMPLE.COM realm first and then fall
back on the local default realm.  This is more convenient than running the
module multiple times with multiple default realms set with I<realm>, but
it is very limited: only two realms can be tried, and the alternate realm
is always tried first.

This option can be set in C<[appdefaults]> in F<krb5.conf>, although
normally it doesn't make sense to do that; normally it is used in the PAM
options of configuration for specific programs.  It is only applicable to
the auth and account groups.  If this option is set for the auth group, be
sure to set it for the account group as well or account authorization may
fail.

=item force_alt_auth

[3.12] This option is used with I<alt_auth_map> and forces authentication
as the mapped principal if that principal exists in the KDC.  Only if the
KDC returns principal unknown does the Kerberos PAM module fall back to
normal authentication.  This can be used to force authentication with an
alternate instance.  If I<alt_auth_map> is not set, it has no effect.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth group.

=item ignore_k5login

[2.0] Never look for a F<.k5login> file in the user's home directory.
Instead, only check that the Kerberos principal maps to the local account
name.  The default check is to ensure the realm matches the local realm
and the user portion of the principal matches the local account name, but
this can be customized by setting up an aname to localname mapping in
F<krb5.conf>.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and account groups.

=item ignore_root

[1.1] Do not do anything if the username is C<root>.  The authentication
and password calls will silently fail (allowing that status to be ignored
via a control of C<optional> or C<sufficient>), and the account and
session calls (including pam_setcred) will return PAM_IGNORE, telling the
PAM library to proceed as if they weren't mentioned in the PAM
configuration.  This option is supported and will remain, but normally you
want to use I<minimum_uid> instead.

This option can be set in C<[appdefaults]> in F<krb5.conf>.

=item minimum_uid=<uid>

[2.0] Do not do anything if the authenticated account name corresponds to
a local account and that local account has a UID lower than <uid>.  If
both of those conditions are true, the authentication and password calls
will silently fail (allowing that status to be ignored via a control of
C<optional> or C<sufficient>), and the account and session calls
(including pam_setcred) will return PAM_IGNORE, telling the PAM library to
proceed as if they weren't mentioned in the PAM configuration.

Using this option is highly recommended if you don't need to use Kerberos
to authenticate password logins to the root account (which isn't
recommended since Kerberos requires a network connection).  It provides
some defense in depth against user principals that happen to match a
system account incorrectly authenticating as that system account.

This option can be set in C<[appdefaults]> in F<krb5.conf>.

=item only_alt_auth

[3.12] This option is used with I<alt_auth_map> and forces the use of the
mapped principal for authentication.  It disables fallback to normal
authentication in all cases and overrides I<search_k5login> and
I<force_alt_auth>.  If I<alt_auth_map> is not set, it has no effect and
the standard authentication behavior is used.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth group.

=item search_k5login

[2.0] Normally, the Kerberos implementation of pam_authenticate attempts
to obtain tickets for the authenticating username in the local realm.  If
this option is set and the local user has a F<.k5login> file in their home
directory, the module will instead open and read that F<.k5login> file,
attempting to use the supplied password to authenticate as each principal
listed there in turn.  If any of those authentications succeed, the user
will be successfully authenticated; otherwise, authentication will fail.
This option is useful for allowing password authentication (via console or
B<sshd> without GSS-API support) to shared accounts.  If there is no
F<.k5login> file, the behavior is the same as normal.  Using this option
requires that the user's F<.k5login> file be readable at the time of
authentication.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth group.

=back

=head2 Kerberos Behavior

=over 4

=item anon_fast

[4.6] Attempt to use Flexible Authentication Secure Tunneling (FAST) by
first authenticating as the anonymous user (WELLKNOWN/ANONYMOUS) and using
its credentials as the FAST armor.  This requires anonymous PKINIT be
enabled for the local realm, that PKINIT be configured on the local
system, and that the Kerberos library support FAST and anonymous PKINIT.

FAST is a mechanism to protect Kerberos against password guessing attacks
and provide other security improvements.  To work, FAST requires that a
ticket be obtained with a strong key to protect exchanges with potentially
weaker user passwords.  This option uses anonymous authentication to
obtain that key and then uses it to protect the subsequent authentication.

If anonymous PKINIT is not available or fails, FAST will not be used and
the authentication will proceed as normal.

To instead use an existing ticket cache for the FAST credentials, use
I<fast_ccache> instead of this option.  If both I<fast_ccache> and
I<anon_fast> are set, the ticket cache named by I<fast_ccache> will be
tried first, and the Kerberos PAM module will fall back on attempting
anonymous PKINIT if that cache could not be used.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and password groups.

The operation is the same as if using the I<fast_ccache> option, but the
cache is created and destroyed automatically.  If both I<fast_ccache> and
I<anon_fast> options are used, the I<fast_ccache> takes precedent and no
anonymous authentication is done.

=item fast_ccache=<ccache_name>

[4.3] The same as I<anon_fast>, but use an existing Kerberos ticket cache
rather than anonymous PKINIT.  This allows use of FAST with a realm that
doesn't support PKINIT or doesn't support anonymous authentication.

<ccache_name> should be a credential cache containing a ticket obtained
using a strong key, such as the randomized key for the host principal of
the local system.  If <ccache_name> names a ticket cache that is readable
by the authenticating process and has tickets then FAST will be attempted.
The easiest way to use this option is to use a program like B<k5start> to
maintain a ticket cache using the host's keytab.  This ticket cache should
normally only be readable by root, so this option will not be able to
protect authentications done as non-root users (such as screensavers).

If no credentials are present in the ticket cache, or if the ticket cache
does not exist or is not readable, FAST will not used and authentication
will proceed as normal.  However, if the credentials in that ticket cache
are expired, authentication will fail if the KDC supports FAST.

To use anonymous PKINIT to protect the FAST exchange, use the I<anon_fast>
option instead.  I<anon_fast> is easier to configure, since no existing
ticket cache is required, but requires PKINIT be available and configured
and that the local realm support anonymous authentication.  If both
I<fast_ccache> and I<anon_fast> are set, the ticket cache named by
I<fast_ccache> will be tried first, and the Kerberos PAM module will fall
back on attempting anonymous PKINIT if that cache could not be used.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and password groups.

=item forwardable

[1.0] Obtain forwardable tickets.  If set (to either true or false,
although it can only be set to false in F<krb5.conf>), this overrides the
Kerberos library default set in the [libdefaults] section of F<krb5.conf>.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth group.

=item keytab=<path>

[3.0] Specifies the keytab to use when validating the user's credentials.
The default is the default system keytab (normally F</etc/krb5.keytab>),
which is usually only readable by root.  Applications not running as root
that use this PAM module for authentication may wish to point it to
another keytab the application can read.  The first principal found in the
keytab will be used as the principal for credential verification.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth group.

=item realm=<realm>

[2.2] Set the default Kerberos realm and obtain credentials in that realm,
rather than in the normal default realm for this system.  If this option
is used, it should be set for all groups being used for consistent
results.  This setting will affect authorization decisions since it
changes the default realm.  This setting will also change the service
principal used to verify the obtained credentials to be in the specified
realm.

If you only want to set the realm assumed for user principals without
changing the realm for authorization decisions or the service principal
used to verify credentials, see the I<user_realm> option.

=item renew_lifetime=<lifetime>

[2.0] Obtain renewable tickets with a maximum renewable lifetime of
<lifetime>.  <lifetime> should be a Kerberos lifetime string such as
C<2d4h10m> or a time in minutes.  If set, this overrides the Kerberos
library default set in the [libdefaults] section of F<krb5.conf>.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth group.

=item ticket_lifetime=<lifetime>

[3.0] Obtain tickets with a maximum lifetime of <lifetime>.  <lifetime>
should be a Kerberos lifetime string such as C<2d4h10m> or a time in
minutes.  If set, this overrides the Kerberos library default set in the
[libdefaults] section of F<krb5.conf>.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth group.

=item user_realm

[4.6] Obtain credentials in the specified realm rather than in the default
realm for this system.  If this option is used, it should be set for all
groups being used for consistent results (although the account group
currently doesn't care about realm).  This will not change authorization
decisions.  If the obtained credentials are supposed to allow access to a
shell account, the user will need an appropriate F<.k5login> file entry or
the system will have to have a custom aname_to_localname mapping.

=back

=head2 PAM Behavior

=over 4

=item clear_on_fail

[3.9] When changing passwords, PAM first does a preliminary check through
the complete password stack, and then calls each module again to do the
password change.  After that preliminary check, the order of module
invocation is fixed.  This means that even if the Kerberos password change
fails (or if one of the other password changes in the stack fails), other
password PAM modules in the stack will still be called even if the failing
module is marked required or requisite.  When using multiple password PAM
modules to synchronize passwords between multiple systems when they
change, this behavior can cause unwanted differences between the
environments.

Setting this option provides a way to work around this behavior.  If this
option is set and a Kerberos password change is attempted and fails (due
to network errors or password strength checking on the KDC, for example),
this module will clear the stored password in the PAM stack.  This will
force any subsequent modules that have I<use_authtok> set to fail so that
those environments won't get out of sync with the password in Kerberos.
The Kerberos PAM module will not meddle with the stored password if it
skips the user due to configuration such as minimum_uid.

Unfortunately, setting this option interferes with other desirable PAM
configurations, such as attempting to change the password in Kerberos
first and falling back on the local Unix password database if that fails.
It therefore isn't the default.  Turn it on (and list pam_krb5 first after
pam_cracklib if used) when synchronizing passwords between multiple
environments.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the password group.

=item debug

[1.0] Log more verbose trace and debugging information to syslog at
LOG_DEBUG priority, including entry and exit from each of the external PAM
interfaces (except pam_close_session).

This option can be set in C<[appdefaults]> in F<krb5.conf>.

=item defer_pwchange

[3.11] By default, pam-krb5 lets the Kerberos library handle prompting for
a password change if an account's password is expired during the auth
group.  If this fails, pam_authenticate() returns an error.

According to the PAM standard, this is not the correct way to handle
expired passwords.  Instead, pam_authenticate() should return success
without attempting a password change, and then pam_acct_mgmt() should
return PAM_NEW_AUTHTOK_REQD, at which point the calling application is
responsible for either rejecting the authentication or calling
pam_chauthtok().  However, following the standard requires that all
applications call pam_acct_mgmt() and check its return status; otherwise,
expired accounts may be able to successfully authenticate.  Many
applications do not do this.

If this option is set, pam-krb5 uses the fully correct PAM mechanism for
handling expired accounts instead of failing in pam_authenticate().  Due
to the security risk of widespread broken applications, be very careful
about enabling this option.  It should normally only be turned on to solve
a specific problem (such as using Solaris Kerberos libraries that don't
support prompting for password changes during authentication), and then
only for specific applications known to call pam_acct_mgmt() and check its
return status properly.

This option is only supported when pam-krb5 is built with MIT Kerberos.
If built against Heimdal, this option does nothing and normal expired
password change handling still happens.  (Heimdal is missing the required
API to implement this option, at least as of version 1.6.)

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth group.

=item fail_pwchange

[4.2] By default, pam-krb5 lets the Kerberos library handle prompting for
a password change if an account's password is expired during the auth
group.  If this option is set, expired passwords are instead treated as an
authentication failure identical to an incorrect password.  Also see
I<defer_pwchange> and I<force_pwchange>.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth group.

=item force_pwchange

[3.11] If this option is set and authentication fails with a Kerberos
error indicating the user's password is expired, attempt to immediately
change their password during the authenticate step.  Under normal
circumstances, this is unnecessary.  Most Kerberos libraries will do this
for you, and setting this option will prompt the user twice to change
their password if the first attempt (done by the Kerberos library) fails.
However, some system Kerberos libraries (such as Solaris's) have password
change prompting disabled in the Kerberos library; on those systems, you
can set this option to simulate the normal library behavior.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth group.

=item no_update_user

[4.7] Normally, if pam-krb5 is able to canonicalize the principal to a
local name using krb5_aname_to_localname() or similar calls, it changes
the PAM_USER variable for this PAM session to the canonicalized local
name.  Setting this option disables this behavior and leaves PAM_USER set
to the initial authentication identity.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth group.

=item silent

[1.0] Don't show messages and errors from Kerberos, such as warnings of
expiring passwords, to the user via the prompter.  This is equivalent to
the behavior when the application passes in PAM_SILENT, but can be set in
the PAM configuration.

This option is only applicable to the auth and password groups.

=item trace=<log-file>

[4.6] Enables Kerberos library trace logging to the specified log file if
it is supported by the Kerberos library.  This is intended for temporary
debugging.  The specified file will be appended to without further
security checks, so do not specify a file in a publicly writable directory
like F</tmp>.

=back

=head2 PKINIT

=over 4

=item pkinit_anchors=<anchors>

[3.0] When doing PKINIT authentication, use <anchors> as the client trust
anchors.  This is normally a reference to a file containing the trusted
certificate authorities.  This option is only used if I<try_pkinit> or
I<use_pkinit> are set.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and password groups.

=item pkinit_prompt

[3.0] Before attempting PKINIT authentication, prompt the user to insert a
smart card.  You may want to set this option for programs such as
B<gnome-screensaver> that call PAM as soon as the mouse is touched and
don't give the user an opportunity to enter the smart card first.  Any
information entered at the first prompt is ignored.  If I<try_pkinit> is
set, a user who wishes to use a password instead can just press Enter and
then enter their password as normal.  This option is only used if
I<try_pkinit> or I<use_pkinit> are set.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and password groups.

=item pkinit_user=<userid>

[3.0] When doing PKINIT authentication, use <userid> as the user ID.  The
value of this string is highly dependent on the type of PKINIT
implementation you're using, but will generally be something like:

    PKCS11:/usr/lib/pkcs11/lib/soft-pkcs11.so

to specify the module to use with a smart card.  It may also point to a
user certificate or to other types of user IDs.  See the Kerberos library
documentation for more details.  This option is only used if I<try_pkinit>
or I<use_pkinit> are set.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and password groups.

=item preauth_opt=<option>

[3.3] Sets a preauth option (currently only applicable when built with MIT
Kerberos).  <option> is either a key/value pair with the key separated
from the value by C<=> or a boolean option (in which case it's turned on).
In F<krb5.conf>, multiple options should be separated by whitespace.  In
the PAM configuration, this option can be given multiple times to set
multiple options.  In either case, <option> may not contain whitespace.

The primary use of this option, at least in the near future, will be to
set options for the MIT Kerberos PKINIT support.  For the full list of
possible options, see the PKINIT plugin documentation.  At the time of
this writing, C<X509_user_identity> is equivalent to I<pkinit_user> and
C<X509_anchors> is equivalent to I<pkinit_anchors>.  C<flag_DSA_PROTOCOL>
can only be set via this option.

Any settings made with this option are applied after the I<pkinit_anchors>
and I<pkinit_user> options, so if an equivalent setting is made via
I<preauth_opt>, it will probably override the other setting.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and password groups.  Note that there is no way to
remove a setting made in F<krb5.conf> using the PAM configuration, but
options set in the PAM configuration are applied after options set in
F<krb5.conf> and therefore may override earlier settings.

=item try_pkinit

[3.0] Attempt PKINIT authentication before trying a regular password.  You
will probably also need to set the I<pkinit_user> configuration option.
If PKINIT fails, the PAM module will fall back on regular password
authentication.  This option is currently only supported if pam-krb5 was
built against Heimdal 0.8rc1 or later or MIT Kerberos 1.6.3 or later.

If this option is set and pam-krb5 is built against MIT Kerberos, and
PKINIT fails and the module falls back to password authentication, the
user's password will not be stored in the PAM stack for subsequent
modules.  This is a bug in the interaction between the module and MIT
Kerberos that requires some reworking of the PKINIT authentication method
to fix.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and password groups.

=item use_pkinit

[3.0, 4.9 for MIT Kerberos] Require PKINIT authentication.  You will
probably also need to set the I<pkinit_user> configuration option.  If
PKINIT fails, authentication will fail.  This option is only supported if
pam-krb5 was built against Heimdal 0.8rc1 or later or MIT Kerberos 1.12 or
later.

Be aware that, with MIT Kerberos, this option is implemented by using a
responder without a prompter, and thus any informational messages from the
Kerberos libraries or KDC during authentication will not be displayed.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and password groups.

=back

=head2 Prompting

=over 4

=item banner=<banner>

[3.0] By default, the prompts when a user changes their password are:

    Current Kerberos password:
    Enter new Kerberos password:
    Retype new Kerberos password:

The string "Kerberos" is inserted so that users aren't confused about
which password they're changing.  Setting this option replaces the word
"Kerberos" with whatever this option is set to.  Setting this option to
the empty string removes the word before "password:" entirely.

If set in the PAM configuration, <banner> may not contain whitespace.  If
you want a value containing whitespace, set it in F<krb5.conf>.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the password group.

=item expose_account

[3.0] By default, the Kerberos PAM module password prompt is simply
"Password:".  This avoids leaking any information about the system realm
or account to principal conversions.  If this option is set, the string
"for <principal>" is added before the colon, where <principal> is the
user's principal.  This string is also added before the colon on prompts
when changing the user's password.

Enabling this option with ChallengeResponseAuthentication enabled in
OpenSSH may cause problems for some ssh clients that only recognize
"Password:" as a prompt.  This option is automatically disabled if
I<search_k5login> is enabled since the principal displayed would be
inaccurate.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and password groups.

=item force_first_pass

[4.0] Use the password obtained by a previous authentication or password
module to authenticate the user without prompting the user again.  If no
previous module obtained the user's password, fail without prompting the
user.  Also see I<try_first_pass> and I<use_first_pass> for weaker
versions of this option.

This option is only applicable to the auth and password groups.  For the
password group, it applies only to the old password.  See I<use_authtok>
for a similar setting for the new password.

=item no_prompt

[4.6] Never prompt for the current password.  Instead, pass in a NULL
password to the Kerberos library and let the Kerberos library do the
prompting.  This may be needed if, for example, the Kerberos library is
configured to use other authentication mechanisms than passwords and needs
full control over the prompting process.

The major disadvantage of this option is that it means the PAM module will
never see the user's password and therefore cannot save it in the PAM
module data for any subsequent modules.  In other words, this option
cannot be used if another module is in the stack behind the Kerberos PAM
module and wants to use I<use_first_pass>.  The Kerberos library also
usually includes the principal in the prompt, and therefore this option
implies behavior similar to I<expose_account>.  Similar to
I<expose_account>, this can cause problems with OpenSSH if
ChallengeResponseAuthentication is enabled, since clients may not
recognize password prompts other than "Password:".

Using this option with I<search_k5login> would result in a password prompt
for every principal listed in the user's F<.k5login> file.  This is
probably not desired behavior, although it's not prohibited by the module.

This option is only applicable to the auth and password groups.  For the
password group, it applies only to the authentication process; the user
will still be prompted for a new password.

=item prompt_principal

[3.6] Before prompting for the user's password (or using the previously
entered password, if I<try_first_pass>, I<use_first_pass>, or
I<force_first_pass> are set), prompt the user for the Kerberos principal
to use for authentication.  This allows the user to authenticate with a
different principal than the one corresponding to the local username,
provided that either a F<.k5login> file or local Kerberos principal to
account mapping authorize that principal to access the local account.

Be cautious when using this configuration option and don't use it with
OpenSSH PasswordAuthentication, only ChallengeResponseAuthentication.
Some PAM-enabled applications expect PAM modules to only prompt for
passwords and may even blindly give the password to the first prompt, no
matter what it is.  Such applications, in combination with this option,
may expose the user's password in log messages and Kerberos requests.

=item try_first_pass

[1.0] If the authentication module isn't the first on the stack, and a
previous module obtained the user's password, use that password to
authenticate the user without prompting them again.  If that
authentication fails, fall back on prompting the user for their password.
This option has no effect if the authentication module is first in the
stack or if no previous module obtained the user's password.  Also see
I<use_first_pass> and I<force_first_pass> for stronger versions of this
option.

This option is only applicable to the auth and password groups.  For the
password group, it applies only to the old password.

=item use_authtok

[4.0] Use the new password obtained by a previous password module when
changing passwords rather than prompting for the new password.  If the new
password isn't available, fail.  This can be used to require passwords be
checked by another, prior module, such as B<pam_cracklib>.

This option is only applicable to the password group.

=item use_first_pass

[1.0] Use the password obtained by a previous authentication module to
authenticate the user without prompting the user again.  If no previous
module obtained the user's password for either an authentication or
password change, fall back on prompting the user.  If a previous module
did obtain the user's password but authentication with that password
fails, fail without further prompting the user.  Also see
I<try_first_pass> and I<force_first_pass> for other versions of this
option.

This option is only applicable to the auth and password groups.  For the
password group, it applies only to the old password.  See I<use_authtok>
for a similar setting for the new password.

=back

=head2 Ticket Caches

=over 4

=item ccache=<pattern>

[2.0] Use <pattern> as the pattern for creating credential cache names.
<pattern> must be in the form <type>:<residual> where <type> and the
following colon are optional if a file cache should be used.  The special
token C<%u>, anywhere in <pattern>, is replaced with the user's numeric
UID.  The special token C<%p>, anywhere in <pattern>, is replaced with the
current process ID.

If <pattern> ends in the literal string C<XXXXXX> (six X's), that string
will be replaced by randomly generated characters and the ticket cache
will be created using mkstemp(3).  This is strongly recommended if
<pattern> points to a world-writable directory.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and session groups.

=item ccache_dir=<directory>

[1.2] Store both the temporary ticket cache used during authentication and
user ticket caches in <directory> instead of in F</tmp>.  The algorithm
for generating the ticket cache name is otherwise unchanged.  <directory>
may be prefixed with C<FILE:> to make the cache type unambiguous (and this
may be required on systems that use a cache type other than file as the
default).

Be aware that pam_krb5 creates and stores a temporary ticket cache file
owned by root during the login process.  If you set I<ccache> above to
avoid using the system F</tmp> directory for user ticket caches, you may
also want to set I<ccache_dir> to move those temporary caches to some
other location.  This will allow pam_krb5 to continue working even if the
system F</tmp> directory is full.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and session groups.

=item no_ccache

[1.0] Do not create a ticket cache after authentication.  This option
shouldn't be set in general, but is useful as part of the PAM
configuration for a particular service that uses PAM for authentication
but isn't creating user sessions and doesn't want the overhead of ever
writing the user credentials to disk.  When using this option, the
application should only call pam_authenticate(); other functions like
pam_setcred(), pam_start_session(), and pam_acct_mgmt() don't make sense
with this option.  Don't use this option if the application needs PAM
account and session management calls.

This option is only applicable to the auth group.

=item retain_after_close

[2.3] Normally, the user's ticket cache is destroyed when either pam_end()
or pam_close_session() is called by the authenticating application so that
ticket caches aren't left behind after the user logs out.  In some cases,
however, this isn't desirable.  (On Solaris 8, for instance, the default
behavior means login will destroy the ticket cache before running the
user's shell.)  If this option is set, the PAM module will never destroy
the user's ticket cache.  If you set this, you may want to call
B<kdestroy> in the shell's logout configuration or run a temporary file
removal program to avoid accumulating hundreds of ticket caches in
F</tmp>.

This option can be set in C<[appdefaults]> in F<krb5.conf> and is only
applicable to the auth and session groups.

=back

=head1 ENVIRONMENT

=over 4

=item KRB5CCNAME

Set by pam_setcred() with the PAM_ESTABLISH_CRED option, and therefore
also by pam_open_session(), to point to the new credential cache for the
user.  See the I<ccache> and I<ccache_dir> options.  By default, the cache
name will be prefixed with C<FILE:> to make the cache type unambiguous.

=item PAM_KRB5CCNAME

Set by pam_authenticate() to point to the temporary ticket cache used for
authentication (unless the I<no_ccache> option was given).  pam_setcred()
then uses that environment variable to locate the temporary cache even if
it was not called in the same PAM session as pam_authenticate() (a problem
with B<sshd> running in some modes).  This environment variable is only
used internal to the PAM module.

=back

=head1 FILES

=over 4

=item F</tmp/krb5cc_UID_RANDOM>

The default credential cache name.  UID is the decimal UID of the local
user and RANDOM is a random six-character string.  The pattern may be
changed with the I<ccache> option and the directory with the I<ccache_dir>
option.

=item F</tmp/krb5cc_pam_RANDOM>

The credential cache name used for the temporary credential cache created
by pam_authenticate().  This cache is removed again when the PAM session
is ended or when pam_setcred() is called and will normally not be
user-visible.  RANDOM is a random six-character string.

=item F<~/.k5login>

File containing Kerberos principals that are allowed access to that
account.

=back

=head1 BUGS

If I<try_pkinit> is set and pam-krb5 is built with MIT Kerberos, the
user's password is not saved in the PAM data if PKINIT fails and the
module falls back to password authentication.

=head1 CAVEATS

Be sure to list this module in the session group as well as the auth group
when using it for interactive logins.  Otherwise, some applications (such
as OpenSSH) will not set up the user's ticket cache correctly.

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.

pam_authenticate() returns failure when called for an ignored account,
requiring the system administrator to use C<optional> or C<sufficient> to
ignore the module and move on to the next module.  It's arguably more
correct to return PAM_IGNORE, which causes the module to be ignored as if
it weren't in the configuration, but this increases the risk of
inadvertent security holes when listing pam-krb5 as the only
authentication module.

This module treats the empty password as an authentication failure
rather than attempting to use that password to avoid unwanted prompting
behavior in the Kerberos libraries.  If you have a Kerberos principal that
intentionally has an empty password, it won't work with this module.

This module will not refresh an existing ticket cache if called with an
effective UID or GID different than the real UID or GID, since refreshing
an existing ticket cache requires trusting the KRB5CCNAME environment
variable and the environment should not be trusted in a setuid context.

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, 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.

=head1 AUTHOR

pam-krb5 was originally written by Frank Cusack.  Andres Salomon made
extensive modifications, and then Russ Allbery <eagle@eyrie.org> adopted
it and made even more extensive modifications.  Russ Allbery currently
maintains the module.

=head1 COPYRIGHT AND LICENSE

Copyright 2005-2010, 2014, 2020 Russ Allbery <eagle@eyrie.org>

Copyright 2008-2014 The Board of Trustees of the Leland Stanford Junior
University

Copying and distribution of this file, with or without modification, are
permitted in any medium without royalty provided the copyright notice and
this notice are preserved.  This file is offered as-is, without any
warranty.

SPDX-License-Identifier: FSFAP

=head1 SEE ALSO

kadmin(8), kdestroy(1), krb5.conf(5), pam(7), passwd(1), syslog(3)

The current version of this module is available from its web page at
L<https://www.eyrie.org/~eagle/software/pam-krb5/>.

=cut