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<chapter id="pam">
<chapterinfo>
<author>
<firstname>John</firstname><surname>Terpstra</surname>
<affiliation>
<orgname>Samba Team</orgname>
<address>
<email>jht@samba.org</email>
</address>
</affiliation>
</author>
<pubdate> (Jun 21 2001) </pubdate>
</chapterinfo>
<title>Configuring PAM for distributed but centrally
managed authentication</title>
<sect1>
<title>Samba and PAM</title>
<para>
A number of Unix systems (eg: Sun Solaris), as well as the
xxxxBSD family and Linux, now utilize the Pluggable Authentication
Modules (PAM) facility to provide all authentication,
authorization and resource control services. Prior to the
introduction of PAM, a decision to use an alternative to
the system password database (<filename>/etc/passwd</filename>)
would require the provision of alternatives for all programs that provide
security services. Such a choice would involve provision of
alternatives to such programs as: <command>login</command>,
<command>passwd</command>, <command>chown</command>, etc.
</para>
<para>
PAM provides a mechanism that disconnects these security programs
from the underlying authentication/authorization infrastructure.
PAM is configured either through one file <filename>/etc/pam.conf</filename> (Solaris),
or by editing individual files that are located in <filename>/etc/pam.d</filename>.
</para>
<note>
<para>
If the PAM authentication module (loadable link library file) is located in the
default location then it is not necessary to specify the path. In the case of
Linux, the default location is <filename>/lib/security</filename>. If the module
is located other than default then the path may be specified as:
<programlisting>
eg: "auth required /other_path/pam_strange_module.so"
</programlisting>
</para>
</note>
<para>
The following is an example <filename>/etc/pam.d/login</filename> configuration file.
This example had all options been uncommented is probably not usable
as it stacks many conditions before allowing successful completion
of the login process. Essentially all conditions can be disabled
by commenting them out except the calls to <filename>pam_pwdb.so</filename>.
</para>
<para><programlisting>
#%PAM-1.0
# The PAM configuration file for the `login' service
#
auth required pam_securetty.so
auth required pam_nologin.so
# auth required pam_dialup.so
# auth optional pam_mail.so
auth required pam_pwdb.so shadow md5
# account requisite pam_time.so
account required pam_pwdb.so
session required pam_pwdb.so
# session optional pam_lastlog.so
# password required pam_cracklib.so retry=3
password required pam_pwdb.so shadow md5
</programlisting></para>
<para>
PAM allows use of replacable modules. Those available on a
sample system include:
</para>
<para><programlisting>
$ /bin/ls /lib/security
pam_access.so pam_ftp.so pam_limits.so
pam_ncp_auth.so pam_rhosts_auth.so pam_stress.so
pam_cracklib.so pam_group.so pam_listfile.so
pam_nologin.so pam_rootok.so pam_tally.so
pam_deny.so pam_issue.so pam_mail.so
pam_permit.so pam_securetty.so pam_time.so
pam_dialup.so pam_lastlog.so pam_mkhomedir.so
pam_pwdb.so pam_shells.so pam_unix.so
pam_env.so pam_ldap.so pam_motd.so
pam_radius.so pam_smbpass.so pam_unix_acct.so
pam_wheel.so pam_unix_auth.so pam_unix_passwd.so
pam_userdb.so pam_warn.so pam_unix_session.so
</programlisting></para>
<para>
The following example for the login program replaces the use of
the <filename>pam_pwdb.so</filename> module which uses the system
password database (<filename>/etc/passwd</filename>,
<filename>/etc/shadow</filename>, <filename>/etc/group</filename>) with
the module <filename>pam_smbpass.so</filename> which uses the Samba
database which contains the Microsoft MD4 encrypted password
hashes. This database is stored in either
<filename>/usr/local/samba/private/smbpasswd</filename>,
<filename>/etc/samba/smbpasswd</filename>, or in
<filename>/etc/samba.d/smbpasswd</filename>, depending on the
Samba implementation for your Unix/Linux system. The
<filename>pam_smbpass.so</filename> module is provided by
Samba version 2.2.1 or later. It can be compiled by specifying the
<command>--with-pam_smbpass</command> options when running Samba's
<filename>configure</filename> script. For more information
on the <filename>pam_smbpass</filename> module, see the documentation
in the <filename>source/pam_smbpass</filename> directory of the Samba
source distribution.
</para>
<para><programlisting>
#%PAM-1.0
# The PAM configuration file for the `login' service
#
auth required pam_smbpass.so nodelay
account required pam_smbpass.so nodelay
session required pam_smbpass.so nodelay
password required pam_smbpass.so nodelay
</programlisting></para>
<para>
The following is the PAM configuration file for a particular
Linux system. The default condition uses <filename>pam_pwdb.so</filename>.
</para>
<para><programlisting>
#%PAM-1.0
# The PAM configuration file for the `samba' service
#
auth required /lib/security/pam_pwdb.so nullok nodelay shadow audit
account required /lib/security/pam_pwdb.so audit nodelay
session required /lib/security/pam_pwdb.so nodelay
password required /lib/security/pam_pwdb.so shadow md5
</programlisting></para>
<para>
In the following example the decision has been made to use the
smbpasswd database even for basic samba authentication. Such a
decision could also be made for the passwd program and would
thus allow the smbpasswd passwords to be changed using the passwd
program.
</para>
<para><programlisting>
#%PAM-1.0
# The PAM configuration file for the `samba' service
#
auth required /lib/security/pam_smbpass.so nodelay
account required /lib/security/pam_pwdb.so audit nodelay
session required /lib/security/pam_pwdb.so nodelay
password required /lib/security/pam_smbpass.so nodelay smbconf=/etc/samba.d/smb.conf
</programlisting></para>
<note><para>PAM allows stacking of authentication mechanisms. It is
also possible to pass information obtained within one PAM module through
to the next module in the PAM stack. Please refer to the documentation for
your particular system implementation for details regarding the specific
capabilities of PAM in this environment. Some Linux implmentations also
provide the <filename>pam_stack.so</filename> module that allows all
authentication to be configured in a single central file. The
<filename>pam_stack.so</filename> method has some very devoted followers
on the basis that it allows for easier administration. As with all issues in
life though, every decision makes trade-offs, so you may want examine the
PAM documentation for further helpful information.
</para></note>
</sect1>
<sect1>
<title>Distributed Authentication</title>
<para>
The astute administrator will realize from this that the
combination of <filename>pam_smbpass.so</filename>,
<command>winbindd</command>, and a distributed
passdb backend, such as ldap, will allow the establishment of a
centrally managed, distributed
user/password database that can also be used by all
PAM (eg: Linux) aware programs and applications. This arrangement
can have particularly potent advantages compared with the
use of Microsoft Active Directory Service (ADS) in so far as
reduction of wide area network authentication traffic.
</para>
</sect1>
<sect1>
<title>PAM Configuration in smb.conf</title>
<para>
There is an option in smb.conf called <ulink
url="smb.conf.5.html#OBEYPAMRESTRICTIONS">obey pam restrictions</ulink>.
The following is from the on-line help for this option in SWAT;
</para>
<para>
When Samba is configured to enable PAM support (i.e.
<constant>--with-pam</constant>), this parameter will
control whether or not Samba should obey PAM's account
and session management directives. The default behavior
is to use PAM for clear text authentication only and to
ignore any account or session management. Note that Samba always
ignores PAM for authentication in the case of
<ulink url="smb.conf.5.html#ENCRYPTPASSWORDS">encrypt passwords = yes</ulink>.
The reason is that PAM modules cannot support the challenge/response
authentication mechanism needed in the presence of SMB
password encryption.
</para>
<para>Default: <command>obey pam restrictions = no</command></para>
</sect1>
</chapter>
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