Ubuntu Manpage: pam_krb5 - Kerberos PAM module

Provided by: libpam-heimdal_4.8-2ubuntu1_amd64

NAME

       pam_krb5 - Kerberos PAM module

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

DESCRIPTION

The Kerberos service module for PAM, typically installed at /lib/security/pam_krb5.so, provides
functionality for the four PAM operations: authentication, account management, session management, and
password management. 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:

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.

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 /etc/krb5.keytab.
You can point the Kerberos PAM module at a different keytab with the keytab option. If that keytab
cannot be read or if no keys are found in it, the default (potentially insecure) behavior is to skip
this check. If you want to instead fail authentication if the obtained tickets cannot be checked,
set "verify_ap_req_nofail" to true in the [libdefaults] section of /etc/krb5.conf. Note that this
will affect applications other than this PAM module.

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 .k5login
file and, if one is present, ensure that the user's principal is listed in that file. If .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 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 "@" 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 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 /tmp/krb5cc_UID_RANDOM
where UID is the user's UID and RANDOM is six randomly-chosen letters. This can be configured with
the ccache and ccache_dir options.

pam-krb5 does not use the default ticket cache location or default_cc_name in the "[libdefaults]"
section of 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 (login, 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.

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().

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.

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 "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.

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.)

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
"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 ignore_root or minimum_uid options, list a
local authentication module such as pam_unix first with a control field of "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.

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 "pam_krb5.so". Some of the options can also be set in the system 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 krb5.conf file, put that option in the "[appdefaults]"
section. All options must be followed by an equal sign (=) and a value, so for boolean options add "=
true". The Kerberos PAM module will look for options either at the top level of the "[appdefaults]"
section or in a subsection named "pam", inside or outside a section for the realm. For example, the
following fragment of a krb5.conf file would set forwardable to true, minimum_uid to 1000, and set
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 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 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 "pam"
section; the "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 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 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.

Authorization
alt_auth_map=<format>
[3.12] This functions similarly to the search_k5login option. The <format> argument is used as the
authentication Kerberos principal, with any %s in <format> replaced with the username. If the
username contains an "@", only the part of the username before the realm is used to replace %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 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 force_alt_auth and 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 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 "[appdefaults]" in 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.

force_alt_auth
[3.12] This option is used with 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 alt_auth_map is not set, it has no effect.

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

ignore_k5login
[2.0] Never look for a .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 krb5.conf.

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

ignore_root
[1.1] Do not do anything if the username is "root". The authentication and password calls will
silently fail (allowing that status to be ignored via a control of "optional" or "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 minimum_uid instead.

This option can be set in "[appdefaults]" in krb5.conf.

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 "optional"
or "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 "[appdefaults]" in krb5.conf.

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

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

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 .k5login
file in their home directory, the module will instead open and read that .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 sshd without GSS-
API support) to shared accounts. If there is no .k5login file, the behavior is the same as normal.
Using this option requires that the user's .k5login file be readable at the time of authentication.

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

Kerberos Behavior
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 fast_ccache instead of this
option. If both fast_ccache and anon_fast are set, the ticket cache named by 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 "[appdefaults]" in krb5.conf and is only applicable to the auth and
password groups.

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

fast_ccache=<ccache_name>
[4.3] The same as 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 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 anon_fast option instead. 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 fast_ccache and
anon_fast are set, the ticket cache named by 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 "[appdefaults]" in krb5.conf and is only applicable to the auth and
password groups.

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

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

keytab=<path>
[3.0] Specifies the keytab to use when validating the user's credentials. The default is the default
system keytab (normally /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 "[appdefaults]" in krb5.conf and is only applicable to the auth group.

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 user_realm
option.

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

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

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

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

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 .k5login file entry or the system will have to have a custom aname_to_localname mapping.

PAM Behavior
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 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 "[appdefaults]" in krb5.conf and is only applicable to the password group.

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 "[appdefaults]" in krb5.conf.

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 "[appdefaults]" in krb5.conf and is only applicable to the auth group.

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
defer_pwchange and force_pwchange.

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

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 "[appdefaults]" in krb5.conf and is only applicable to the auth group.

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 "[appdefaults]" in krb5.conf and is only applicable to the auth group.

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.

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 /tmp.

PKINIT
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
try_pkinit or use_pkinit are set.

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

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 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 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
try_pkinit or use_pkinit are set.

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

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 try_pkinit or use_pkinit are set.

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

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 "=" or a boolean option (in which
case it's turned on). In 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, "X509_user_identity" is equivalent to pkinit_user and "X509_anchors" is
equivalent to pkinit_anchors. "flag_DSA_PROTOCOL" can only be set via this option.

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

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

try_pkinit
[3.0] Attempt PKINIT authentication before trying a regular password. You will probably also need to
set the 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 "[appdefaults]" in krb5.conf and is only applicable to the auth and
password groups.

use_pkinit
[3.0] Require PKINIT authentication. You will probably also need to set the pkinit_user
configuration option. If PKINIT fails, authentication will fail. This option is currently only
supported if pam-krb5 was built against Heimdal 0.8rc1 or later. MIT Kerberos doesn't provide a
method to enforce use of PKINIT, so try_pkinit must be used with that implementation instead.

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

Prompting
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 krb5.conf.

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

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 search_k5login is enabled since the principal displayed would be inaccurate.

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

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 try_first_pass and 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 use_authtok for a similar setting for the new password.

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 use_first_pass. The Kerberos library also usually includes the principal in the
prompt, and therefore this option implies behavior similar to expose_account. Similar to
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 search_k5login would result in a password prompt for every principal listed in
the user's .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.

prompt_principal
[3.6] Before prompting for the user's password (or using the previously entered password, if
try_first_pass, use_first_pass, or 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 .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.

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 use_first_pass and 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.

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 pam_cracklib.

This option is only applicable to the password group.

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 try_first_pass and 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 use_authtok for a similar setting for the new password.

Ticket Caches
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 %u, anywhere in <pattern>, is replaced with the user's numeric UID. The
special token %p, anywhere in <pattern>, is replaced with the current process ID.

If <pattern> ends in the literal string "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 "[appdefaults]" in krb5.conf and is only applicable to the auth and session
groups.

ccache_dir=<directory>
[1.2] Store both the temporary ticket cache used during authentication and user ticket caches in
<directory> instead of in /tmp. The algorithm for generating the ticket cache name is otherwise
unchanged. <directory> may be prefixed with "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 ccache above to avoid using the system /tmp directory for user ticket
caches, you may also want to set ccache_dir to move those temporary caches to some other location.
This will allow pam_krb5 to continue working even if the system /tmp directory is full.

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

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.

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 kdestroy in the shell's logout configuration or run a temporary file removal program to avoid
accumulating hundreds of ticket caches in /tmp.

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

ENVIRONMENT

       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 ccache and ccache_dir options.  By default,
           the cache name will be prefixed with "FILE:" to make the cache type unambiguous.

       PAM_KRB5CCNAME
           Set by pam_authenticate() to point to the temporary ticket cache used for authentication (unless  the
           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 sshd running in some modes).  This environment variable is only used internal to the PAM module.

FILES

       /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 ccache option and the directory  with  the
           ccache_dir option.

       /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.

       ~/.k5login
           File containing Kerberos principals that are allowed access to that account.

BUGS

       If 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.

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  "optional"  or  "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.

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.

COPYRIGHT AND LICENSE

       Copyright 2005, 2006, 2007, 2008, 2009, 2010, 2014 Russ Allbery <eagle@eyrie.org>

       Copyright  2008,  2009,  2010, 2011, 2012, 2013, 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.

NAME

SYNOPSIS

DESCRIPTION

CONFIGURATION

ENVIRONMENT

FILES

BUGS

CAVEATS

AUTHOR

COPYRIGHT AND LICENSE

SEE ALSO