>Configuring PAM for distributed but centrally managed authentication

Configuring PAM for distributed but centrally managed authentication

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.

# 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.

# 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.

# 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.

# 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 on 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.

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.

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