PAM_KRB5(8) BSD System Manager's Manual PAM_KRB5(8)NAMEpam_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 manage‐
ment. 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 cre‐
dentials (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 appropri‐
ately. 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 pass‐
word, 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 configu‐
ration 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 desig‐
nate the decimal UID of the user; and ‘%p’, to designate
the current process ID; can be used in name.
Kerberos 5 Account Management Module
The Kerberos 5 account management component provides a function to per‐
form 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 pass‐
words fail, the user is prompted for them.
Kerberos 5 Session Management Module
The Kerberos 5 session management component provides functions to initi‐
ate (pam_sm_open_session()) and terminate (pam_sm_close_session()) ses‐
sions. 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 ALSOkdestroy(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 authenti‐
cation 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.
BSD March 10, 2007 BSD
