/usr/lib/security/pam_krb5.so.1
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.
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 can 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.
If pam_sm_authenticate() is called and the pkinit module option is set, the Kerberos V5 authentication module tries to do PKINIT authentication, assuming that both the system and the KDC are configured to support this type of authentication. This form of authentication uses a user's certificate and private key to acquire the user's initial Kerberos credential (TGT). One of the keystore formats supported is PKCS11 which supports use of any PKCS11 compatible keystore capable of storing the required credential and private key needed for PKINIT authentication (PKCS11 compatible smartcards are an example). See krb5.conf (4) for more details on PKINIT configuration. This form of authentication is typically useful for services where the system on which the auth stack is being processed has access to the user's certificate and private key.
If pam_sm_authenticate() is called and the pkinit module option is not set then the Kerberos V5 authentication module does password based authentication.
In either case, if the PAM_AUTHTOK password item has been set when pam_sm_authenticate() is called, which is the case when pam_krb5 is stacked after pam_authtok_get in the auth stack, the Kerberos V5 authentication module uses that PAM_AUTHTOK password for either PKINIT or password based Kerberos authentication.
If the PAM_USER item is not set pam_krb5 with the pkinit option prompts for and set that item.
If the PAM_AUTHTOK password item has not been set when pam_sm_authenticate() is called, which is the case when pam_krb5 is stacked before pam_authtok_get in the auth stack, and the pkinit option is present the Kerberos V5 authentication module allows the Kerberos pkinit preauth plugin to prompt for whatever information is needed to perform PKINIT (typically this is for the user's PIN). No PAM items are set by way of this prompting. See krb5.conf(4) for more information on PKINIT configuration options.
The pam_krb5 module sets the KRB5CCNAME shell environment variable upon successful authentication or password change to FILE:/tmp/krb5cc_uid where uid is the UID of the user that pam_krb5 authenticated. KRB5CCNAME is documented in krb5envvar(5).
If it is desirable to initially have the Kerberos V5 authentication module try PKINIT Kerberos authentication and fall back to password based Kerberos authentication then either the sufficient or optional control flags must be provided for the instance of pam_krb5 with the pkinit module option set and another instance of pam_krb5 without the pkinit module option must be stacked below pam_authtok_get. If there are PAM modules other than pam_krb5 that must be evaluated below pam_authtok_get then the control flag should be set to optional for the instance of pam_krb5 with the pkinit module option set otherwise the control flag should be set to sufficient.
Only two instances of pam_krb5 are supported in a auth stack.
pam_sm_authenticate(3PAM) can be passed the following flag:
This flag is ignored. The Kerberos authentication mechanism does 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 Kerberos network applications. The following flags can be set in the flags field. They are best described by their effect on the user's credential cache.
Stores the initial credentials in the user's credential cache so that the user can 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.
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 can be deleted with the kdestroy(1) command.
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.
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. This function fails if the user's renewable ticket lifetime is expired.
The following options can be passed to the Kerberos V5 authentication module:
Provides syslog(3C) debugging information at LOG_DEBUG level.
Turns off warning messages.
Indicates that the Kerberos V5 authentication module should try Kerberos PKINIT authentication instead of the default password based Kerberos authentication.
Indicates the realm name used to authenticate the user rather than the system's default_realm as defined in /etc/krb5/krb5.conf.
Given that the system may not have host credentials for every realm configured to perform initial authentication with, they will not have the ability to prevent KDC spoofing.
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 can be passed in to the Kerberos V5 account management module:
Provides syslog(3C) debugging information at LOG_DEBUG level
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.
If the Kerberos V5 authentication module used PKINIT authentication in the auth stack then the Kerberos V5 password management module returns PAM_IGNORE in the following cases:
The new password is NULL.
The old password is NULL.
Verification of the old password fails.
The rationale behind this is that the KDC can not allow a PKINIT user to change/set a password since the user can be expected to use PKINIT only. If all of the cases above are false the Kerberos V5 password management module tries to change the user's password in the KDC database.
If the KDC only supports PKINIT authentication then the Kerberos V5 password management module should not be present in any password stacks.
Related to PKINIT the Kerberos V5 password management module does not support changing the key store PIN used to access a user's private key and certificate.
The following flags can be passed to pam_sm_chauthtok(3PAM):
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.
This is a null function that always returns PAM_IGNORE.
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:
Provides syslog(3C) debugging information at LOG_DEBUG level.
The following error codes are returned for pam_sm_authenticate():
Authentication failure
Memory buffer error.
The user is “root” and the root key exists in the default keytab.
Successfully obtained Kerberos credentials .
System error.
An unknown Kerberos principal was requested.
The following error codes are returned for pam_sm_setcred():
Authentication failure.
Memory buffer error.
The user is “root” and the root key exists in the default keytab.
System error.
Successfully modified the Kerberos credential cache.
The following error codes are returned for pam_sm_acct_mgmt():
Authentication failure.
Kerberos service module pam_sm_authenticate() was never called, or the user is “root” and the root key exists in the default keytab.
Obtain new authentication token from the user.
Error in underlying service module.
Kerberos principal account is valid.
System error.
An unknown Kerberos principal was requested.
The following error code is returned for pam_sm_open_session() and pam_sm_close_session():
These two functions are null functions in pam_krb5:
The following error codes are returned for pam_sm_chauthtok():
Authentication failure.
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.
User's Kerberos password has expired.
Error in module. At least one input parameter is missing.
System error.
An unknown Kerberos principal was requested.
Successfully changed the user's Kerberos password.
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 gdm.
gdm auth requisite pam_authtok_get.so.1 gdm auth required pam_dhkeys.so.1 gdm auth required pam_unix_cred.so.1 gdm auth sufficient pam_krb5.so.1 gdm auth required pam_unix_auth.so.1
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.1Example 2 Authenticating Users Through Kerberos Only Using Password-based Authentication
The following example allows authentication only to users that have Kerberos-based accounts.
gdm auth requisite pam_authtok_get.so.1 gdm auth required pam_dhkeys.so.1 gdm auth required pam_unix_cred.so.1 gdm auth required pam_krb5.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. 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.1Example 3 Authenticating Through Kerberos Optionally Using Password-based Authentication
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:
gdm auth requisite pam_authtok_get.so.1 gdm auth required pam_dhkeys.so.1 gdm auth required pam_unix_cred.so.1 gdm auth required pam_unix_auth.so.1 gdm 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 optional 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.
Example 4 Authenticating Users Through Kerberos PKINIT as First ChoiceThe 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 (using PKINIT) 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 login. The user is prompted once for the PIN by pam_krb5.
login auth required pam_unix_cred.so.1 login auth sufficient pam_krb5.so.1 pkinit login auth requisite pam_authtok_get.so.1 login auth required pam_dhkeys.so.1 login auth required pam_unix_auth.so.1Example 5 Authenticating Users Through Kerberos PKINIT Only
The following example allows authentication only to users that have kerberos-based accounts requiring PKINIT authentication.
login auth required pam_unix_cred.so.1 login auth required pam_krb5.so.1 pkinitExample 6 Authenticating Users Through Kerberos PKINIT Optionally
The following example allows users to acquire a Kerberos credential using PKINIT authentication if they have a Kerberos account. Whether pam_krb5 succeeds or fails the user must provide their Unix password to login.
login auth required pam_unix_cred.so.1 login auth optional pam_krb5.so.1 pkinit login auth requisite pam_authtok_get.so.1 login auth required pam_unix_auth.so.1Example 7 Authenticating Users Through Kerberos PKINIT as a Requirement
The following example allows users to login if pam_krb5 is able to acquire a Kerberos credential using PKINT authentication and in addition must provide their Unix password to pam_unix_auth.
login auth required pam_unix_cred.so.1 login auth required pam_krb5.so.1 pkinit login auth requisite pam_authtok_get.so.1 login auth required pam_unix_auth.so.1Example 8 Authenticating Users Through Kerberos PKINIT as a Requirement
The following example allows users to login using their PAM_AUTHTOK password acquired by pam_authtok_get. This password is used by pam_krb5 to try PKINIT authentication and is also used by pam_unix_auth to authenticate the user using the user's Unix account. If PKINIT requires a password/PIN that differs from the user's Unix password then pam_krb5 must be stacked above pam_authtok_get.
login auth required pam_unix_cred.so.1 login auth requisite pam_authtok_get.so.1 login auth required pam_krb5.so.1 pkinit login auth required pam_unix_auth.so.1Example 9 Authenticating Users Through Kerberos PKINIT with a Fall Back to Password-based krb auth
The following example allows users to acquire a Kerberos credential using PKINIT authentication or using password based authentication if PKINIT fails. If PKINIT succeeds the user is not prompted for their password. If pam_krb5 PKINIT succeeds, the second instance of pam_krb5 does not try password authentication and returns success. If PKINIT fails the user is prompted for their Kerberos password.
login auth required pam_unix_cred.so.1 login auth sufficient pam_krb5.so.1 pkinit login auth requisite pam_authtok_get.so.1 login auth required pam_krb5.so.1Example 10 Authenticating Users Through Kerberos Requiring Users to Authenticate Either through Kerberos PKINIT or Fall Back to Password-based krb auth
The following example allows users to acquire a Kerberos credential using PKINIT authentication or using password based authentication if PKINIT fails. If pam_krb5 PKINIT succeeds, the second instance of pam_krb5 does not try password authentication and returns ignore. If pam_krb5 PKINIT fails the second instance of pam_krb5 tries password based authentication and return success or failure.
login auth required pam_unix_cred.so.1 login auth optional pam_krb5.so.1 pkinit login auth requisite pam_authtok_get.so.1 login auth required pam_krb5.so.1 login auth required pam_dhkeys.so.1 login auth required pam_unix_auth.so.1Example 11 Authenticating Users Through Kerberos Requiring Users to Authenticate Either through Kerberos PKINIT or Fall Back to pam_pkcs11
The following example allows users to acquire a Kerberos credential using PKINIT authentication or if that fails use pam_pkcs11 to validate the user's PIN using their certificate and private key.
login auth required pam_unix_cred.so.1 login auth sufficient pam_krb5.so.1 pkinit login auth sufficient pam_pkcs11.soExample 12 Authenticating Users Through Multiple Realms as First Choice Using Password-based Authentication
This configuration can be used in combination with multiple pam_krb5 in which different realms can be specified for one authentication pass. For example:
gdm auth definitive pam_user_policy.so.1 gdm auth required pam_dhkeys.so.1 gdm auth required pam_unix_cred.so.1 gdm auth sufficient pam_krb5.so.1 gdm auth sufficient pam_krb5.so.1 realm=EXAMPLE1.COM gdm auth required pam_unix_auth.so.1
This will cause an initial authentication attempt for default_realm, as configured in the /etc/krb5/krb5.conf file, realm first. If this fails then an initial authentication attempt for the EXAMPLE1.COM is attempted.
See attributes(5) for descriptions of the following attributes:
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kdestroy(1), kinit(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), krb5.conf(4) , pam.conf(4), attributes(5), kerberos(5), krb5envvar(5), pam_krb5_migrate(5)
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) is notified, to alert the user when the initial credentials are about to expire.