modules/pam_krb5/pam-krb5.8

.Dd November 21, 2025
.Dt PAM_KRB5 8
.Os
.Sh NAME
.Nm pam_krb5
.Nd Kerberos PAM module
.Sh SYNOPSIS
.Bd -literal
  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
.Ed
.Sh DESCRIPTION
The Kerberos service module for PAM, typically installed at
.Pa /lib/security/pam_krb5.so ,
provides functionality for the four PAM operations: authentication,
account management, session management, and password management.
.Pa 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.conf(5).
.Pp
Here are the actions of this module when called from each group:
.Bl -tag -width Ds
.It auth
Provides implementations of
.Xr pam_authenticate 3
and
.Xr pam_setcred 3 .
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.
.Pp
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.
.Pp
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
.Pa /etc/krb5.keytab .
You can point the Kerberos PAM module at a different keytab with the
.Em 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
.Qo Li allow_kdc_spoof Qc
option to true either in the [appdefaults] section of
.Pa /etc/krb5.conf
or in the PAM policy.
.Pp
By default, whenever the user is authenticated, a basic authorization
check will also be done using
.Xr krb5_kuserok 3 .
The default behavior of this function is to check the user's account for
a
.Pa .k5login
file and, if one is present, ensure that the user's principal is listed
in that file.
If
.Pa .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
.Pa krb5.conf ;
see the Kerberos documentation for details).
This can be customized with several configuration options; see below.
.Pp
If the username provided to PAM contains an
.Qo Li @ Qc
and Kerberos can, treating the username as a principal, map it to a
local account name,
.Xr pam_authenticate 3
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
.Em 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
.Xr getpwnam 3
and cannot be called before pam_krb5 when using this feature.
.Pp
When
.Xr pam_setcred 3
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
.Pa /tmp/krb5cc_UID_RANDOM
where UID is the user's UID and RANDOM is six randomly-chosen letters.
This can be configured with the
.Em ccache
and
.Em ccache_dir
options.
.Pp
pam-krb5 does not use the default ticket cache location or
.Em default_cc_name
in the
.Qo Li [libdefaults] Qc
section of
.Pa 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.
.Pp
If
.Xr pam_setcred 3
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
.Pf ( Sy login ,
.Sy 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.
.It session
Provides implementations of
.Xr pam_open_session 3 ,
which is equivalent to calling
.Xr pam_setcred 3
with the PAM_ESTABLISH_CRED flag, and
.Xr pam_close_session 3 ,
which destroys the ticket cache created by
.Xr pam_setcred 3 .
.It account
Provides an implementation of
.Xr pam_acct_mgmt 3 .
All it does is do the same authorization check as performed by the
.Xr pam_authenticate 3
implementation described above.
.It password
Provides an implementation of
.Xr pam_chauthtok 3 ,
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
.Qo Li kadmin/changepw Qc .
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.
.Pp
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.
.Pp
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.)
.El
.Pp
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
.Qo Li ignore Qc
in the Linux PAM configuration language).
.Pp
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
.Em ignore_root
or
.Em minimum_uid
options, list a local authentication module such as
.Sy pam_unix
first with a control field of
.Qo Li sufficient Qc
so that the Kerberos PAM module will be skipped if local password
authentication was successful.
.Pp
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.
.Sh 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
.Qo Li pam_krb5.so Qc .
Some of the options can also be set in the system
.Pa krb5.conf
file; if this is possible, it will be noted below in the option
description.
.Pp
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.
.Pp
To set an option for the PAM module in the system
.Pa krb5.conf
file, put that option in the
.Qo Li [appdefaults] Qc
section.
All options must be followed by an equal sign (=) and a value, so for
boolean options add
.Qo Li = true Qc .
The Kerberos PAM module will look for options either at the top level of
the
.Qo Li [appdefaults] Qc
section or in a subsection named
.Qo Li pam Qc ,
inside or outside a section for the realm.
For example, the following fragment of a
.Pa krb5.conf
file would set
.Em forwardable
to true,
.Em minimum_uid
to 1000, and set
.Em ignore_k5login
only if the realm is EXAMPLE.COM.
.Bd -literal
    [appdefaults]
        forwardable = true
        pam = {
            minimum_uid = 1000
            EXAMPLE.COM = {
                ignore_k5login = true
            }
        }
.Ed
.Pp
For more information on the syntax of
.Pa 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
.Em 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.
.Pp
There is no difference to the PAM module whether options are specified
at the top level or in a
.Qo Li pam Qc
section; the
.Qo Li pam Qc
section is supported in case there are options that should be set for
the PAM module but not for other applications.
.Pp
If the same option is set in
.Pa 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
.Pa krb5.conf .
.Pp
The start of each option description is annotated with the version of
pam-krb5 in which that option was added with the current meaning.
.Ss Authorization
.Bl -tag -width Ds
.It 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.
.It alt_auth_map=<format>
[3.12] This functions similarly to the
.Em search_k5login
option.
The <format> argument is used as the authentication Kerberos principal,
with any
.Qo Li %s Qc
in <format> replaced with the username.
If the username contains an
.Qo Li @ Qc ,
only the part of the username before the realm is used to replace
.Qo Li %s Qc .
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.
.Pp
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
.Sy sudo .
Most examples will look like:
.Bd -literal
    alt_auth_map=%s/root
.Ed
.Pp
which attempts authentication as the root instance of the username first
and then falls back to the regular username (but see
.Em force_alt_auth
and
.Em only_alt_auth Ns ).
.Pp
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:
.Bd -literal
    alt_auth_map=%s@EXAMPLE.COM
.Ed
.Pp
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
.Em realm ,
but it is very limited: only two realms can be tried, and the alternate
realm is always tried first.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa 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.
.It force_alt_auth
[3.12] This option is used with
.Em 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
.Em alt_auth_map
is not set, it has no effect.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth group.
.It ignore_k5login
[2.0] Never look for a
.Pa .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
.Pa krb5.conf .
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and account groups.
.It ignore_root
[1.1] Do not do anything if the username is
.Qo Li root Qc .
The authentication and password calls will silently fail (allowing that
status to be ignored via a control of
.Qo Li optional Qc
or
.Qo Li sufficient Qc Ns ),
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
.Em minimum_uid
instead.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf .
.It 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
.Qo Li optional Qc
or
.Qo Li sufficient Qc Ns ),
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.
.Pp
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.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf .
.It only_alt_auth
[3.12] This option is used with
.Em alt_auth_map
and forces the use of the mapped principal for authentication.
It disables fallback to normal authentication in all cases and overrides
.Em search_k5login
and
.Em force_alt_auth .
If
.Em alt_auth_map
is not set, it has no effect and the standard authentication behavior is
used.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth group.
.It 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
.Pa .k5login
file in their home directory, the module will instead open and read that
.Pa .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
.Sy sshd
without GSS-API support) to shared accounts.
If there is no
.Pa .k5login
file, the behavior is the same as normal.
Using this option requires that the user's
.Pa .k5login
file be readable at the time of authentication.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth group.
.El
.Ss Kerberos Behavior
.Bl -tag -width Ds
.It 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.
.Pp
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.
.Pp
If anonymous PKINIT is not available or fails, FAST will not be used and
the authentication will proceed as normal.
.Pp
To instead use an existing ticket cache for the FAST credentials, use
.Em fast_ccache
instead of this option.
If both
.Em fast_ccache
and
.Em anon_fast
are set, the ticket cache named by
.Em 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.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and password groups.
.Pp
The operation is the same as if using the
.Em fast_ccache
option, but the cache is created and destroyed automatically.
If both
.Em fast_ccache
and
.Em anon_fast
options are used, the
.Em fast_ccache
takes precedent and no anonymous authentication is done.
.It fast_ccache=<ccache_name>
[4.3] The same as
.Em 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.
.Pp
<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
.Sy 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).
.Pp
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.
.Pp
To use anonymous PKINIT to protect the FAST exchange, use the
.Em anon_fast
option instead.
.Em 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
.Em fast_ccache
and
.Em anon_fast
are set, the ticket cache named by
.Em 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.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and password groups.
.It forwardable
[1.0] Obtain forwardable tickets.
If set (to either true or false, although it can only be set to false in
.Pa krb5.conf Ns ),
this overrides the Kerberos library default set in the [libdefaults]
section of
.Pa krb5.conf .
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth group.
.It keytab=<path>
[3.0] Specifies the keytab to use when validating the user's
credentials.
The default is the default system keytab (normally
.Pa /etc/krb5.keytab Ns ),
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.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth group.
.It 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.
.Pp
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
.Em user_realm
option.
.It renew_lifetime=<lifetime>
[2.0] Obtain renewable tickets with a maximum renewable lifetime of
<lifetime>. <lifetime> should be a Kerberos lifetime string such as
.Qo Li 2d4h10m Qc
or a time in minutes.
If set, this overrides the Kerberos library default set in the
[libdefaults] section of
.Pa krb5.conf .
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth group.
.It ticket_lifetime=<lifetime>
[3.0] Obtain tickets with a maximum lifetime of <lifetime>. <lifetime>
should be a Kerberos lifetime string such as
.Qo Li 2d4h10m Qc
or a time in minutes.
If set, this overrides the Kerberos library default set in the
[libdefaults] section of
.Pa krb5.conf .
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth group.
.It 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
.Pa .k5login
file entry or the system will have to have a custom aname_to_localname
mapping.
.El
.Ss PAM Behavior
.Bl -tag -width Ds
.It 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.
.Pp
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
.Em 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.
.Pp
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.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the password group.
.It 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).
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf .
.It 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,
.Xr pam_authenticate 3
returns an error.
.Pp
According to the PAM standard, this is not the correct way to handle
expired passwords.
Instead,
.Xr pam_authenticate 3
should return success without attempting a password change, and then
.Xr pam_acct_mgmt 3
should return PAM_NEW_AUTHTOK_REQD, at which point the calling
application is responsible for either rejecting the authentication or
calling
.Xr pam_chauthtok 3 .
However, following the standard requires that all applications call
.Xr pam_acct_mgmt 3
and check its return status; otherwise, expired accounts may be able to
successfully authenticate.
Many applications do not do this.
.Pp
If this option is set, pam-krb5 uses the fully correct PAM mechanism for
handling expired accounts instead of failing in
.Xr pam_authenticate 3 .
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
.Xr pam_acct_mgmt 3
and check its return status properly.
.Pp
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.)
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth group.
.It 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
.Em defer_pwchange
and
.Em force_pwchange .
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth group.
.It 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.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth group.
.It no_update_user
[4.7] Normally, if pam-krb5 is able to canonicalize the principal to a
local name using
.Xr krb5_aname_to_localname 3
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.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth group.
.It 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.
.Pp
This option is only applicable to the auth and password groups.
.It 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
.Pa /tmp .
.El
.Ss PKINIT
.Bl -tag -width Ds
.It 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
.Em try_pkinit
or
.Em use_pkinit
are set.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and password groups.
.It 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
.Sy 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
.Em 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
.Em try_pkinit
or
.Em use_pkinit
are set.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and password groups.
.It 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:
.Bd -literal
    PKCS11:/usr/lib/pkcs11/lib/soft-pkcs11.so
.Ed
.Pp
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
.Em try_pkinit
or
.Em use_pkinit
are set.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and password groups.
.It 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
.Qo Li = Qc
or a boolean option (in which case it's turned on).
In
.Pa 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.
.Pp
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,
.Qo Li X509_user_identity Qc
is equivalent to
.Em pkinit_user
and
.Qo Li X509_anchors Qc
is equivalent to
.Em pkinit_anchors .
.Qo Li flag_DSA_PROTOCOL Qc
can only be set via this option.
.Pp
Any settings made with this option are applied after the
.Em pkinit_anchors
and
.Em pkinit_user
options, so if an equivalent setting is made via
.Em preauth_opt ,
it will probably override the other setting.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and password groups.
Note that there is no way to remove a setting made in
.Pa krb5.conf
using the PAM configuration, but options set in the PAM configuration
are applied after options set in
.Pa krb5.conf
and therefore may override earlier settings.
.It try_pkinit
[3.0] Attempt PKINIT authentication before trying a regular password.
You will probably also need to set the
.Em 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.
.Pp
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.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and password groups.
.It use_pkinit
[3.0, 4.9 for MIT Kerberos] Require PKINIT authentication.
You will probably also need to set the
.Em 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.
.Pp
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.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and password groups.
.El
.Ss Prompting
.Bl -tag -width Ds
.It banner=<banner>
[3.0] By default, the prompts when a user changes their password are:
.Bd -literal
    Current Kerberos password:
    Enter new Kerberos password:
    Retype new Kerberos password:
.Ed
.Pp
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.
.Pp
If set in the PAM configuration, <banner> may not contain whitespace.
If you want a value containing whitespace, set it in
.Pa krb5.conf .
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the password group.
.It 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.
.Pp
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
.Em search_k5login
is enabled since the principal displayed would be inaccurate.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and password groups.
.It 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
.Em try_first_pass
and
.Em use_first_pass
for weaker versions of this option.
.Pp
This option is only applicable to the auth and password groups.
For the password group, it applies only to the old password.
See
.Em use_authtok
for a similar setting for the new password.
.It 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.
.Pp
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
.Em use_first_pass .
The Kerberos library also usually includes the principal in the prompt,
and therefore this option implies behavior similar to
.Em expose_account .
Similar to
.Em expose_account ,
this can cause problems with OpenSSH if ChallengeResponseAuthentication
is enabled, since clients may not recognize password prompts other than
"Password:".
.Pp
Using this option with
.Em search_k5login
would result in a password prompt for every principal listed in the
user's
.Pa .k5login
file.
This is probably not desired behavior, although it's not prohibited by
the module.
.Pp
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.
.It prompt_principal
[3.6] Before prompting for the user's password (or using the previously
entered password, if
.Em try_first_pass ,
.Em use_first_pass ,
or
.Em 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
.Pa .k5login
file or local Kerberos principal to account mapping authorize that
principal to access the local account.
.Pp
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.
.It 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
.Em use_first_pass
and
.Em force_first_pass
for stronger versions of this option.
.Pp
This option is only applicable to the auth and password groups.
For the password group, it applies only to the old password.
.It 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
.Sy pam_cracklib .
.Pp
This option is only applicable to the password group.
.It 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
.Em try_first_pass
and
.Em force_first_pass
for other versions of this option.
.Pp
This option is only applicable to the auth and password groups.
For the password group, it applies only to the old password.
See
.Em use_authtok
for a similar setting for the new password.
.El
.Ss Ticket Caches
.Bl -tag -width Ds
.It 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
.Qo Li %u Qc ,
anywhere in <pattern>, is replaced with the user's numeric UID. The
special token
.Qo Li %p Qc ,
anywhere in <pattern>, is replaced with the current process ID.
.Pp
If <pattern> ends in the literal string
.Qo Li XXXXXX Qc
(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.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and session groups.
.It ccache_dir=<directory>
[1.2] Store both the temporary ticket cache used during authentication
and user ticket caches in <directory> instead of in
.Pa /tmp .
The algorithm for generating the ticket cache name is otherwise
unchanged.
<directory> may be prefixed with
.Qo Li FILE: Qc
to make the cache type unambiguous (and this may be required on systems
that use a cache type other than file as the default).
.Pp
Be aware that pam_krb5 creates and stores a temporary ticket cache file
owned by root during the login process.
If you set
.Em ccache
above to avoid using the system
.Pa /tmp
directory for user ticket caches, you may also want to set
.Em ccache_dir
to move those temporary caches to some other location.
This will allow pam_krb5 to continue working even if the system
.Pa /tmp
directory is full.
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and session groups.
.It 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
.Xr pam_authenticate 3 ;
other functions like
.Xr pam_setcred 3 ,
.Xr pam_start_session 3 ,
and
.Xr pam_acct_mgmt 3
don't make sense with this option.
Don't use this option if the application needs PAM account and session
management calls.
.Pp
This option is only applicable to the auth group.
.It retain_after_close
[2.3] Normally, the user's ticket cache is destroyed when either
.Xr pam_end 3
or
.Xr pam_close_session 3
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
.Sy kdestroy
in the shell's logout configuration or run a temporary file removal
program to avoid accumulating hundreds of ticket caches in
.Pa /tmp .
.Pp
This option can be set in
.Qo Li [appdefaults] Qc
in
.Pa krb5.conf
and is only applicable to the auth and session groups.
.El
.Sh ENVIRONMENT
.Bl -tag -width Ds
.It KRB5CCNAME
Set by
.Xr pam_setcred 3
with the PAM_ESTABLISH_CRED option, and therefore also by
.Xr pam_open_session 3 ,
to point to the new credential cache for the user.
See the
.Em ccache
and
.Em ccache_dir
options.
By default, the cache name will be prefixed with
.Qo Li FILE: Qc
to make the cache type unambiguous.
.It PAM_KRB5CCNAME
Set by
.Xr pam_authenticate 3
to point to the temporary ticket cache used for authentication (unless
the
.Em no_ccache
option was given).
.Xr pam_setcred 3
then uses that environment variable to locate the temporary cache even
if it was not called in the same PAM session as
.Xr pam_authenticate 3
(a problem with
.Sy sshd
running in some modes).
This environment variable is only used internal to the PAM module.
.El
.Sh FILES
.Bl -tag -width Ds
.It Pa /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
.Em ccache
option and the directory with the
.Em ccache_dir
option.
.It Pa /tmp/krb5cc_pam_RANDOM
The credential cache name used for the temporary credential cache
created by
.Xr pam_authenticate 3 .
This cache is removed again when the PAM session is ended or when
.Xr pam_setcred 3
is called and will normally not be user-visible.
RANDOM is a random six-character string.
.It Pa ~/.k5login
File containing Kerberos principals that are allowed access to that
account.
.El
.Sh BUGS
If
.Em 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.
.Sh 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.
.Pp
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.
.Pp
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.
.Pp
.Xr pam_authenticate 3
returns failure when called for an ignored account, requiring the system
administrator to use
.Qo Li optional Qc
or
.Qo Li sufficient Qc
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.
.Pp
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.
.Pp
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.
.Pp
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.
.Sh 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.
.Sh COPYRIGHT AND LICENSE
Copyright 2005-2010, 2014, 2020 Russ Allbery <eagle@eyrie.org>
.Pp
Copyright 2008-2014 The Board of Trustees of the Leland Stanford Junior
University
.Pp
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.
.Pp
SPDX-License-Identifier: FSFAP
.Sh SEE ALSO
kadmin(8), kdestroy(1), krb5.conf(5), pam.conf(5), passwd(1), syslog(3)
.Pp
The current version of this module is available from its web page at
.Lk https://www.eyrie.org/~eagle/software/pam-krb5/ .