PAM_KRB5(5) Standards, Environments, and Macros PAM_KRB5(5)


NAME


pam_krb5 - authentication, account, session, and password management PAM
modules for Kerberos V5

SYNOPSIS


/usr/lib/security/pam_krb5.so.1


DESCRIPTION


The Kerberos V5 service module for PAM provides functionality for all
four PAM modules: authentication, account management, session management,
and password management. The service module is a shared object that can
be dynamically loaded to provide the necessary functionality upon demand.
Its path is specified in the PAM configuration file.

Kerberos Authentication Module


The Kerberos V5 authentication component provides functions to verify the
identity of a user, pam_sm_authenticate(), and to manage the Kerberos
credentials cache, pam_sm_setcred().


pam_sm_authenticate() authenticates a user principal through the Kerberos
authentication service. If the authentication request is successful, the
authentication service sends a ticket-granting ticket (TGT) back to the
service module, which then verifies that the TGT came from a valid Key
Distribution Center (KDC) by attempting to get a service ticket for the
local host service. For this to succeed, the local host's keytab file
(/etc/krb5/krb5.keytab) must contain the entry for the local host
service. For example, in the file host/hostname.com@REALM, hostname.com
is the fully qualified local hostname and REALM is the default realm of
the local host as defined in /etc/krb5/krb5.conf. If the host entry is
not found in the keytab file, the authentication fails. Administrators
may optionally disable this "strict" verification by setting
"verify_ap_req_nofail = false" in /etc/krb5/krb5.conf. See krb5.conf(4)
for more details on this option. This allows TGT verification to succeed
in the absence of a keytab host principal entry.


pam_sm_authenticate(3PAM) may be passed the following flag:

PAM_DISALLOW_NULL_AUTHTOK

This flag is ignored. The Kerberos authentication mechanism will not
allow an empty password string by default.


pam_sm_setcred() creates and modifies the user's credential cache. This
function initializes the user's credential cache, if it does not already
exist, and stores the initial credentials for later use by Kerberized
network applications. The following flags may be set in the flags field.
They are best described by their effect on the user's credential cache.

PAM_ESTABLISH_CRED

Stores the initial credentials in the user's credential cache so that
the user may access Kerberos network services. If a successful
authentication pass was made, the new credentials are stored in the
credential cache, overwriting any existing credentials that were
previously stored. If an unsuccessful authentication pass was made,
PAM_CRED_UNAVAIL is returned.


PAM_DELETE_CRED

This flag has no effect on the credential cache and always returns
PAM_SUCCESS. The credential cache is not deleted because there is no
accurate method to determine if the credentials are needed by another
process. The credential cache may be deleted with the kdestroy(1)
command.


PAM_REINITIALIZE_CRED

Deletes the user's existing credential cache, if it exists, and
creates a new credential cache. The new credentials are stored in the
new cache and the user's ticket lifetime and renewable life time
values are reset.


PAM_REFRESH_CRED

Does not require a previous authentication pass, but if a successful
one is made, the new credentials are stored in the credential cache.
If a previous authentication pass was not made or was unsuccessful,
an attempt to renew the existing credentials is made. Note that this
function fails if the user's renewable ticket lifetime is expired.


The following options can be passed to the Kerberos V5 authentication
module:

debug
Provides syslog(3C) debugging information at LOG_DEBUG level.


nowarn
Turns off warning messages.


Kerberos V5 Account Management Module
The Kerberos account management component provides a function to perform
account management, pam_sm_acct_mgmt(). This function checks to see if
the pam_krb5 authentication module has noted that the user's password has
not expired. The following options may be passed in to the Kerberos V5
account management module:

debug
Provides syslog(3C) debugging information at LOG_DEBUG level


nowarn
Turns off warning messages. Also, does not query KDC for
impending password expiration information used to warn the
user.


Kerberos V5 Session Management Module
The Kerberos V5 session management component provides functions to
initiate pam_sm_open_session() and terminate pam_sm_close_session()
Kerberos sessions. For Kerberos V5, both pam_sm_open_session and
pam_sm_close_session() are null functions, returning PAM_IGNORE.

Kerberos V5 Password Management Module
The Kerberos V5 password management component provides a function to
change passwords, pam_sm_chauthtok(), in the Key Distribution Center
(KDC) database. The following flags may be passed to
pam_sm_chauthtok(3PAM):

PAM_CHANGE_EXPIRED_AUTHTOK

The password service should only update the user's Kerberos password
if it is expired. Otherwise, this function returns PAM_IGNORE. The
default behaviour is to always change the user's Kerberos password.


PAM_PRELIM_CHECK

This is a null function that always returns PAM_IGNORE.


PAM_UPDATE_AUTHTOK

This flag is necessary to change the user's Kerberos password. If
this flag is not set, pam_krb5 returns PAM_SYSTEM_ERR.


The following option can be passed to the Kerberos V5 password module:

debug
Provides syslog(3C) debugging information at LOG_DEBUG level.


ERRORS


The following error codes are returned for pam_sm_authenticate():

PAM_AUTH_ERR
Authentication failure


PAM_BUF_ERR
Memory buffer error.


PAM_IGNORE
The user is "root" and the root key exists in the
default keytab.


PAM_SUCCESS
Successfully obtained Kerberos credentials .


PAM_SYSTEM_ERR
System error.


PAM_USER_UNKNOWN
An unknown Kerberos principal was requested.


The following error codes are returned for pam_sm_setcred():

PAM_AUTH_ERR
Authentication failure.


PAM_BUF_ERR
Memory buffer error.


PAM_IGNORE
The user is "root" and the root key exists in the
default keytab.


PAM_SYSTEM_ERR
System error.


PAM_SUCCESS
Successfully modified the Kerberos credential cache.


The following error codes are returned for pam_sm_acct_mgmt():

PAM_AUTH_ERR
Authentication failure.


PAM_IGNORE
Kerberos service module pam_sm_authenticate() was
never called, or the user is "root" and the root
key exists in the default keytab.


PAM_NEW_AUTHTOK_REQD
Obtain new authentication token from the user.


PAM_SERVICE_ERR
Error in underlying service module.


PAM_SUCCESS
Kerberos principal account is valid.


PAM_SYSTEM_ERR
System error.


PAM_USER_UNKNOWN
An unknown Kerberos principal was requested.


The following error code is returned for pam_sm_open_session() and
pam_sm_close_session():

PAM_IGNORE
These two functions are null functions in pam_krb5:


The following error codes are returned for pam_sm_chauthtok():

PAM_AUTH_ERR
Authentication failure.


PAM_IGNORE
The user has not been authenticated by Kerberos
service module pam_sm_authenticate(), or the user
is "root" and the root key exists in the default
keytab.


PAM_NEW_AUTHTOK_REQD
User's Kerberos password has expired.


PAM_SERVICE_ERR
Error in module. At least one input parameter is
missing.


PAM_SYSTEM_ERR
System error.


PAM_USER_UNKNOWN
An unknown Kerberos principal was requested.


PAM_SUCCESS
Successfully changed the user's Kerberos
password.


EXAMPLES


Example 1: Authenticate Users Through Kerberos as First Choice




The following is an excerpt of a sample pam.conf configuration file that
authenticates users through the Kerberos authentication service and
authenticates through the Unix login only if the Kerberos authentication
fails. This arrangement is helpful when a majority of the users are
networked by means of Kerberos and when there are only a few non-Kerberos
type user accounts, such as root. The service illustrated below is for
dtlogin.


dtlogin auth requisite pam_smartcard.so.1
dtlogin auth requisite pam_authtok_get.so.1
dtlogin auth required pam_dhkeys.so.1
dtlogin auth required pam_unix_cred.so.1
dtlogin auth sufficient pam_krb5.so.1
dtlogin auth required pam_unix_auth.so.1


Note that these changes should not be made to the existing krlogin, krsh,
and ktelnet service entries. Those services require Kerberos
authentication, so using a seemingly sufficient control flag would not
provide the necessary functionality for privacy and integrity. There
should be no need to change those entries.


The following entries check for password expiration when dealing with
Kerberos and Unix password aging policies:


other account requisite pam_roles.so.1
other account required pam_unix_account.so.1
other account required pam_krb5.so.1


The following entries would change the Kerberos password of the user and
continue to change the Unix login password only if the Kerberos password
change had failed:


other password required pam_dhkeys.so.1
other password requisite pam_authtok_get.so.1
other password requisite pam_authtok_check.so.1
other password sufficient pam_krb5.so.1
other password required pam_authtok_store.so.1


When changing Kerberos based user's password, use kpasswd(1). When
changing a non-Kerberos user's password, it is recommended that the
repository is specified (-r) with the passwd(1) command.


Example 2: Authenticate Users Through Kerberos Only




The following example allows authentication only to users that have
Kerberos-based accounts.


dtlogin auth requisite pam_smartcard.so.1
dtlogin auth requisite pam_authtok_get.so.1
dtlogin auth required pam_dhkeys.so.1
dtlogin auth required pam_unix_cred.so.1
dtlogin auth binding pam_krb5.so.1
dtlogin auth required pam_unix_auth.so.1


Typically, you would have another service specified in the pam.conf file
that would allow local users, such as database, web server, system
administrator accounts, to log in to the host machine. For example, the
service name "login" could be used for these users. Note that these users
should not belong to any roles.


The rest of the module types look similar to that shown in the previous
example:


other account requisite pam_roles.so.1
other account required pam_unix_account.so.1
other account required pam_krb5.so.1


With binding specified in the following, it is important that non-
Kerberos users specify the repository in which they reside using the -r
option with the passwd(1) command. This configuration is also based on
the assumptions that:


o Kerberos users maintain only their Kerberos passwords;

o changing their Unix password is not necessary, given that they
are authenticated only through their Kerberos passwords when
logging in.

other password required pam_dhkeys.so.1
other password requisite pam_authtok_get.so.1
other password requisite pam_authtok_check.so.1
other password binding pam_krb5.so.1
other password required pam_authtok_store.so.1


Example 3: Authenticate Through Kerberos Optionally




This configuration is helpful when the majority of users are non-Kerberos
users and would like to authenticate through Kerberos if they happened to
exist in the Kerberos database. The effect of this is similar to users
voluntarily executing kinit(1) after they have successfully logged in:


dtlogin auth requisite pam_smartcard.so.1
dtlogin auth requisite pam_authtok_get.so.1
dtlogin auth required pam_dhkeys.so.1
dtlogin auth required pam_unix_cred.so.1
dtlogin auth required pam_unix_auth.so.1
dtlogin auth optional pam_krb5.so.1


The rest of the configuration is as follows:


other account requisite pam_roles.so.1
other account required pam_unix_account.so.1
other account required pam_krb5.so.1

other password required pam_dhkeys.so.1
other password requisite pam_authtok_get.so.1
other password requisite pam_authtok_check.so.1
other password required pam_authtok_store.so.1
other password optional pam_krb5.so.1


Non-Kerberos users should specify their respective repositories by using
the -r option when changing their password with the passwd(1) command.


ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Evolving |
+--------------------+-----------------+

SEE ALSO


kdestroy(1), kinit(1), kpasswd(1), passwd(1), ktkt_warnd(1M),
libpam(3LIB), pam(3PAM), pam_sm(3PAM), pam_sm_acct_mgmt(3PAM),
pam_sm_authenticate(3PAM), pam_sm_chauthtok(3PAM),
pam_sm_close_session(3PAM), pam_sm_open_session(3PAM),
pam_sm_setcred(3PAM), syslog(3C), pam.conf(4), attributes(5),
kerberos(5), krb5envvar(5)

NOTES


The interfaces in libpam(3LIB) are MT-Safe only if each thread within the
multi-threaded application uses its own PAM handle.


On successful acquisition of initial credentials (ticket-granting
ticket), ktkt_warnd(1M) will be notified, to alert the user when the
initial credentials are about to expire.


April 8, 2008 PAM_KRB5(5)