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PAM_KRB5(8) System Manager's Manual 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.
NetBSD 10.99 March 10, 2007 NetBSD 10.99