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Chapter 4. Configuring PAM for distributed but centrally +managed authentication

4.1. Samba and PAM

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 (/etc/passwd) +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: login, +passwd, chown, etc.

PAM provides a mechanism that disconnects these security programs +from the underlying authentication/authorization infrastructure. +PAM is configured either through one file /etc/pam.conf (Solaris), +or by editing individual files that are located in /etc/pam.d.

The following is an example /etc/pam.d/login 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 pam_pwdb.so.

#%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

PAM allows use of replacable modules. Those available on a +sample system include:

$ /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

The following example for the login program replaces the use of +the pam_pwdb.so module which uses the system +password database (/etc/passwd, +/etc/shadow, /etc/group) with +the module pam_smbpass.so which uses the Samba +database which contains the Microsoft MD4 encrypted password +hashes. This database is stored in either +/usr/local/samba/private/smbpasswd, +/etc/samba/smbpasswd, or in +/etc/samba.d/smbpasswd, depending on the +Samba implementation for your Unix/Linux system. The +pam_smbpass.so module is provided by +Samba version 2.2.1 or later. It can be compiled by specifying the +--with-pam_smbpass options when running Samba's +configure script. For more information +on the pam_smbpass module, see the documentation +in the source/pam_smbpass directory of the Samba +source distribution.

#%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

The following is the PAM configuration file for a particular +Linux system. The default condition uses pam_pwdb.so.

#%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

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.

#%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

Note: 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 pam_stack.so module that allows all +authentication to be configured in a single central file. The +pam_stack.so 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.

4.2. Distributed Authentication

The astute administrator will realize from this that the +combination of pam_smbpass.so, +winbindd, and rsync (see +http://rsync.samba.org/) +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.

4.3. PAM Configuration in smb.conf

There is an option in smb.conf called obey pam restrictions. +The following is from the on-line help for this option in SWAT;

When Samba 2.2 is configure to enable PAM support (i.e. +--with-pam), 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 +encrypt passwords = yes. +The reason is that PAM modules cannot support the challenge/response +authentication mechanism needed in the presence of SMB +password encryption.

Default: obey pam restrictions = no

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