PAM_KRB5(8)

NAME
pam_krb5 - Kerberos 5 PAM module

SYNOPSIS
[service-name] module-type control-flag pam_krb5 [arguments]

DESCRIPTION
The Kerberos 5 service module for PAM provides functionality for three PAM categories: authentication, account management, and password management. It also provides null functions for session management.

  Kerberos 5 Authentication Module
The Kerberos 5 authentication component provides functions to verify the identity of a user (pam_sm_authenticate()) and to set user specific credentials (pam_sm_setcred()). pam_sm_authenticate() converts the supplied username into a Kerberos principal, by appending the default local realm name. It also supports usernames with explicit realm names. If a realm name is supplied, then upon a successful return, it changes the username by mapping the principal name into a local username (calling krb5_aname_to_localname()). This typically just means the realm name is stripped.

It prompts the user for a password and obtains a new Kerberos TGT for the principal. The TGT is verified by obtaining a service ticket for the local host.

When prompting for the current password, the authentication module will use the prompt “Password for <principal>:”.

The pam_sm_setcred() function stores the newly acquired credentials in a credentials cache, and sets the environment variable KRB5CCNAME appropriately. The credentials cache should be destroyed by the user at logout with kdestroy(1).

The following options may be passed to the authentication module:

debug
    syslog(3) debugging information at LOG_DEBUG level.

no_warn
    suppress warning messages to the user. These messages include reasons why the user's authentication attempt was declined.

use_first_pass
    If the authentication module is not the first in the stack, and a previous module obtained the user's password, that password is used to authenticate the user. If this fails, the authentication module returns failure without prompting the user for a password. This option has no effect if the authentication module is the first in the stack, or if no previous modules obtained the user's password.

try_first_pass
    This option is similar to the use_first_pass option, except that if the previously obtained password fails, the user is prompted for another password.

renewable=timeperiod
    Obtain renewable Kerberos credentials for the user. The renewable time can be specified, or it defaults to one month. Since spaces are not allowed in the pam configuration time, underscores are used to form parseable times (e.g., 1_month).

forwardable
    Obtain forwardable Kerberos credentials for the user.

no_ccache
    Do not save the obtained credentials in a credentials cache. This is a useful option if the authentication module is used for services such as ftp or pop, where the user would not be able to destroy them. [This is not a recommendation to use the module for those services.]

ccache=name
    Use name as the credentials cache. name must be in the form type:residual. The special tokens ‘%u’, to designate the decimal UID of the user; and ‘%p’, to designate the current process ID; can be used in name.

allow_kdc_spoof
    Allow pam_krb5 to succeed even if there is no host or service key available in a keytab to authenticate the Kerberos KDC's ticket. If there is no such key, for example on a host with no keytabs, pam_krb5 will fail immediately without prompting the user.

    Warning: If the host has not been configured with a keytab from the KDC, setting this option makes it vulnerable to malicious KDCs, e.g. via DNS flooding, because pam_krb5 has no way to distinguish the legitimate KDC from a spoofed KDC.

  Kerberos 5 Account Management Module
The Kerberos 5 account management component provides a function to perform account management, pam_sm_acct_mgmt(). The function verifies that the authenticated principal is allowed to login to the local user account by calling krb5_kuserok() (which checks the user's .k5login file).

  Kerberos 5 Password Management Module
The Kerberos 5 password management component provides a function to change passwords (pam_sm_chauthtok()). The username supplied (the user running the passwd(1) command, or the username given as an argument) is mapped into a Kerberos principal name, using the same technique as in the authentication module. Note that if a realm name was explicitly supplied during authentication, but not during a password change, the mapping done by the password management module may not result in the same principal as was used for authentication.

Unlike when changing a UNIX password, the password management module will allow any user to change any principal's password (if the user knows the principal's old password, of course). Also unlike UNIX, root is always prompted for the principal's old password.

The password management module uses the same heuristics as kpasswd(1) to determine how to contact the Kerberos password server.

The following options may be passed to the password management module:

debug
    syslog(3) debugging information at LOG_DEBUG level.

use_first_pass
    If the password management module is not the first in the stack, and a previous module obtained the user's old password, that password is used to authenticate the user. If this fails, the password management module returns failure without prompting the user for the old password. If successful, the new password entered to the previous module is also used as the new Kerberos password. If the new password fails, the password management module returns failure without prompting the user for a new password.

try_first_pass
    This option is similar to the use_first_pass option, except that if the previously obtained old or new passwords fail, the user is prompted for them.

  Kerberos 5 Session Management Module
The Kerberos 5 session management component provides functions to initiate (pam_sm_open_session()) and terminate (pam_sm_close_session()) sessions. Since session management is not defined under Kerberos 5, both of these functions simply return success. They are provided only because of the naming conventions for PAM modules.

ENVIRONMENT

KRB5CCNAME
    Location of the credentials cache.

FILES
/tmp/krb5cc_uid
    default credentials cache (uid is the decimal UID of the user).
$HOME/.k5login
    file containing Kerberos principals that are allowed access.

SEE ALSO
kdestroy(1), passwd(1), syslog(3), pam.conf(5), pam(8)

NOTES
Applications should not call pam_authenticate() more than once between calls to pam_start() and pam_end() when using the Kerberos 5 PAM module.

SECURITY CONSIDERATIONS
The pam_krb5 module implements what is fundamentally a password authentication scheme. It does not use a Kerberos 5 exchange between client and server, but rather authenticates the password provided by the client against the Kerberos KDC. Therefore, care should be taken to only use this module over a secure session ( secure TTY, encrypted session, etc.), otherwise the user's Kerberos 5 password could be compromised.