PAM_KRB5(7) Standards, Environments, and Macros PAM_KRB5(7)

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

pam_krb5.so.1

The Kerberos V5 service module for PAM, pam_krb5.so.1, provides functionality for all four PAM modules: authentication, account management, session management, and password management.

The Kerberos V5 authentication component provides functions to verify the identity of a user, pam_sm_authenticate(3PAM), and to manage the Kerberos credentials cache, pam_sm_setcred(3PAM).

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(5) 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.

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.

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.

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.

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.

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:

Kerberos users maintain only their Kerberos passwords;
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.

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE ATTRIBUTE VALUE
Interface Stability Evolving

kdestroy(1), kinit(1), kpasswd(1), passwd(1), syslog(3C), 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), pam.conf(5), attributes(7), kerberos(7), krb5envvar(7), ktkt_warnd(8)

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(8) will be notified, to alert the user when the initial credentials are about to expire.

August 19, 2023 OmniOS