pam_krb5 - Kerberos 5 PAM module

auth            sufficient      pam_krb5.so ignore_root
account         required        pam_krb5.so ignore_root
password        optional        pam_krb5.so ignore_root
session         optional        pam_krb5.so ignore_root

The Kerberos 5 service module for PAM, typically /lib/security/pam_krb5.so, provides functionality for three PAM categories: authentication, account management, session management, and password management.

The pam_krb5.so 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 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.

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.

The pam_sm_setcred() function stores the newly acquired credentials in a credentials cache, and sets the environment variable KRB5CCNAME appropriately. This module destroys the credentials cache when the PAM data is cleaned up. Thus, you should set close_sessions to yes in /etc/login.defs.

If the authentication is done as root and a local keytab is available, pam_krb5.so will do a proper Kerberos authentication by using the Kerberos TGT acquired to obtain a service ticket and then validate the service ticket against a local keytab. If there is no local keytab or if the keytab file (/etc/krb5.keytab) is not readable, this step will be skipped. Without performing this step, authentication is vulnerable to KDC spoofing. If a keytab is available, the module first looks for a principal named host/machine where machine is the fully qualified local hostname. If that principal isn't available, it looks for service/machine where service is the PAM service used (the first argument to pam_start()) and machine is the same as above.

The following options may be passed to the authentication module:

debug

syslog(3) debugging information at level.

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.

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.]

ignore_root

Never attempt to authenticate the user "root". Always fail immediately if that user is being authenticated; presumably the PAM stack will fall back to some other mechanism.

search_k5login

Normally, the authentication module attempts to authenticate as a Kerberos principal derived from the username by appending the default realm. Verification of the presence of that Kerberos principal in the user's .k5login file can then be done via the account management module. When this option is given and a .k5login file is present, the provided password is instead used to attempt to authenticate as each Kerberos principal listed in the .k5login file in turn. If there is no .k5login file, the behavior is the same as normal. This may be useful when there is no simple mapping from local username to Kerberos principal name, but requires the .k5login file be readable at the time of authentication (and therefore may not work with schemes such as sshd's privilege separation).

The Kerberos 5 session management component provides functions to initiate (pam_sm_open_session()) and terminate (pam_sm_close_session()) sessions. This module takes all of the same options as the authentication module and will authenticate the user if authentication has not already been established. In addition, it takes the following options specific to this module:

ccache=<name>

Use <name> as the credentials cache. <name> must be in the form type:residual (where type is optional if a file cache is used). 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>. If <name> ends with the literal string XXXXXX (six X's), that string will be replaced by randomly generated characters and the ticket cache opened via mkstemp. This is strongly recommended if <name> points to a world-writable directory.

ccache_dir=<dir>

Put the ticket cache in <dir> instead of the default of /tmp. Ignored if ccache is also set.

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). Note that this is also done by pam_sm_authenticate() and pam_sm_setcred().

If the user didn't log on using krb5, this function silently succeeds.

The following options may be passed to the account module:

ignore_root

Skip any attempt to validate the root account and just succeed quietly. Presumably whatever validation is necessary will be handled by another module in the PAM stack.

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 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.

ignore_root

Do not try to change passwords for the root user, just fail immediately.

KRB5CCNAME

Location of the credentials cache.

/tmp/krb5cc_[uid]_[rand]

default credentials cache ([uid] is the decimal UID of the user and [rand] is a random string). The directory in which it is stored may be changed with the ccache_dir option.

~/.k5login

file containing Kerberos principals that are allowed access.

kdestroy(1), passwd(1), pam(7), syslog(3).

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