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.Dd March 10, 2007
.Dt PAM_KRB5 8
.Os
.Sh NAME
.Nm pam_krb5
.Nd Kerberos 5 PAM module
.Sh SYNOPSIS
.Op Ar service-name
.Ar module-type
.Ar control-flag
.Pa pam_krb5
.Op Ar arguments
.Sh 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.
.Ss Kerberos 5 Authentication Module
The Kerberos 5 authentication component
provides functions to verify the identity of a user
.Pq Fn pam_sm_authenticate
and to set user specific credentials
.Pq Fn pam_sm_setcred .
.Fn 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
.Fn krb5_aname_to_localname ) .
This typically just means
the realm name is stripped.
.Pp
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.
.Pp
When prompting for the current password, the authentication
module will use the prompt
.Dq Li "Password for <principal>:" .
.Pp
The
.Fn pam_sm_setcred
function stores the newly acquired credentials in a credentials cache,
and sets the environment variable
.Ev KRB5CCNAME
appropriately.
The credentials cache should be destroyed by the user at logout with
.Xr kdestroy 1 .
.Pp
The following options may be passed to the authentication module:
.Bl -tag -width ".Cm use_first_pass"
.It Cm debug
.Xr syslog 3
debugging information at
.Dv LOG_DEBUG
level.
.It Cm no_warn
suppress warning messages to the user.
These messages include
reasons why the user's
authentication attempt was declined.
.It Cm 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.
.It Cm try_first_pass
This option is similar to the
.Cm use_first_pass
option, except that if the previously obtained password fails, the
user is prompted for another password.
.It Cm renewable Ns = Ns Ar 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).
.It Cm forwardable
Obtain forwardable Kerberos credentials for the user.
.It Cm 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.]
.It Cm ccache Ns = Ns Ar name
Use
.Ar name
as the credentials cache.
.Ar name
must be in the form
.Ar type : Ns Ar residual .
The special tokens
.Ql %u ,
to designate the decimal UID of the user;
and
.Ql %p ,
to designate the current process ID; can be used in
.Ar name .
.It Cm allow_kdc_spoof
Allow
.Nm
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,
.Nm
will fail immediately without prompting the user.
.Pp
.Sy 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
.Nm
has no way to distinguish the legitimate KDC from a spoofed KDC.
.El
.Ss Kerberos 5 Account Management Module
The Kerberos 5 account management component
provides a function to perform account management,
.Fn pam_sm_acct_mgmt .
The function verifies that the authenticated principal is allowed
to login to the local user account by calling
.Fn krb5_kuserok
(which checks the user's
.Pa .k5login
file).
.Ss Kerberos 5 Password Management Module
The Kerberos 5 password management component
provides a function to change passwords
.Pq Fn pam_sm_chauthtok .
The username supplied (the
user running the
.Xr 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.
.Pp
Unlike when
changing a
.Ux
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
.Ux ,
root
is always prompted for the principal's old password.
.Pp
The password management module uses the same heuristics as
.Xr kpasswd 1
to determine how to contact the Kerberos password server.
.Pp
The following options may be passed to the password management
module:
.Bl -tag -width ".Cm use_first_pass"
.It Cm debug
.Xr syslog 3
debugging information at
.Dv LOG_DEBUG
level.
.It Cm 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.
.It Cm try_first_pass
This option is similar to the
.Cm use_first_pass
option, except that if the previously obtained old or new passwords fail,
the user is prompted for them.
.El
.Ss Kerberos 5 Session Management Module
The Kerberos 5 session management component
provides functions to initiate
.Pq Fn pam_sm_open_session
and terminate
.Pq Fn 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.
.Sh ENVIRONMENT
.Bl -tag -width "KRB5CCNAME"
.It Ev KRB5CCNAME
Location of the credentials cache.
.El
.Sh FILES
.Bl -tag -width ".Pa /tmp/krb5cc_ Ns Ar uid" -compact
.It Pa /tmp/krb5cc_ Ns Ar uid
default credentials cache
.Ar ( uid
is the decimal UID of the user).
.It Pa $HOME/.k5login
file containing Kerberos principals that are allowed access.
.El
.Sh SEE ALSO
.Xr kdestroy 1 ,
.Xr passwd 1 ,
.Xr syslog 3 ,
.Xr pam.conf 5 ,
.Xr pam 8
.Sh NOTES
Applications should not call
.Fn pam_authenticate
more than once between calls to
.Fn pam_start
and
.Fn pam_end
when using the Kerberos 5 PAM module.
.Sh SECURITY CONSIDERATIONS
The
.Nm
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
.Po
secure TTY, encrypted session, etc.
.Pc ,
otherwise the user's Kerberos 5 password could be compromised.