Softpanorama (slightly skeptical) Open Source Software Educational Society

May the source be with you, but remember the KISS principle ;-)

Selected PAM Modules

Suse 10 comes with the complicated cornucopia of 64 bit PAM modules (located in /lib64/security):

-rwxr-xr-x 1 root root  13648 May  3  2007 pam_access.so
-rwxr-xr-x 1 root root  19440 May  3  2007 pam_chroot.so
-rwxr-xr-x 1 root root  12704 May  3  2007 pam_cracklib.so
-rwxr-xr-x 1 root root   7552 May  3  2007 pam_debug.so
-rwxr-xr-x 1 root root   4648 May  3  2007 pam_deny.so
-rwxr-xr-x 1 root root  15552 May  3  2007 pam_devperm.so
-rwxr-xr-x 1 root root   7368 May  3  2007 pam_echo.so
-rwxr-xr-x 1 root root  12064 May  3  2007 pam_env.so
-rwxr-xr-x 1 root root   8928 May  3  2007 pam_exec.so
drwxr-xr-x 2 root root   4096 Oct 17 14:33 pam_filter
-rwxr-xr-x 1 root root  13024 May  3  2007 pam_filter.so
-rwxr-xr-x 1 root root   6952 May  3  2007 pam_ftp.so
-rwxr-xr-x 1 root root  12792 May  3  2007 pam_group.so
-rwxr-xr-x 1 root root   8872 May  3  2007 pam_homecheck.so
-rwxr-xr-x 1 root root   9080 May  3  2007 pam_issue.so
-rwxr-xr-x 1 root root   9432 May  3  2007 pam_lastlog.so
-rwxr-xr-x 1 root root  13320 May  3  2007 pam_limits.so
-rwxr-xr-x 1 root root  11072 May  3  2007 pam_listfile.so
-rwxr-xr-x 1 root root   5976 May  3  2007 pam_localuser.so
-rwxr-xr-x 1 root root   6896 May  3  2007 pam_loginuid.so
-rwxr-xr-x 1 root root   9680 May  3  2007 pam_mail.so
-rwxr-xr-x 1 root root   8968 May  3  2007 pam_make.so
-rwxr-xr-x 1 root root  17952 May  3  2007 pam_mkhomedir.so
-rwxr-xr-x 1 root root   7504 May  3  2007 pam_mktemp.so
-rwxr-xr-x 1 root root   5688 May  3  2007 pam_motd.so
-rwxr-xr-x 1 root root  66824 May  3  2007 pam_ncp_auth.so
-rwxr-xr-x 1 root root   6872 May  3  2007 pam_nologin.so
-rwxr-xr-x 1 root root  16032 Apr 27  2007 pam_opensc.so
-rwxr-xr-x 1 root root  32600 Jun 16  2006 pam_opie.so
-rwxr-xr-x 1 root root  44088 May  3  2007 pam_passwdqc.so
-rwxr-xr-x 1 root root   5072 May  3  2007 pam_permit.so
-rwxr-xr-x 1 root root  32952 May  3  2007 pam_pwcheck.so
-rwxr-xr-x 1 root root   7984 Jun 16  2006 pam_resmgr.so
-rwxr-xr-x 1 root root   6072 May  3  2007 pam_rhosts.so
-rwxr-xr-x 1 root root  11352 May  3  2007 pam_rhosts_auth.so
-rwxr-xr-x 1 root root   4856 May  3  2007 pam_rootok.so
-rwxr-xr-x 1 root root  18056 May  4  2007 pam_rpasswd.so
-rwxr-xr-x 1 root root   7264 May  3  2007 pam_securetty.so
-rwxr-xr-x 1 root root   5976 May  3  2007 pam_shells.so
-rwxr-xr-x 1 root root  12528 May  3  2007 pam_stress.so
-rwxr-xr-x 1 root root  12928 May  3  2007 pam_succeed_if.so
-rwxr-xr-x 1 root root  11784 May  3  2007 pam_tally.so
-rwxr-xr-x 1 root root  11312 May  3  2007 pam_time.so
-rwxr-xr-x 1 root root   8448 May  3  2007 pam_umask.so
-rwxr-xr-x 2 root root  55872 May  3  2007 pam_unix.so
-rwxr-xr-x 2 root root  55872 May  3  2007 pam_unix2.so
-rwxr-xr-x 4 root root  42712 May  3  2007 pam_unix_acct.so
-rwxr-xr-x 4 root root  42712 May  3  2007 pam_unix_auth.so
-rwxr-xr-x 4 root root  42712 May  3  2007 pam_unix_passwd.so
-rwxr-xr-x 4 root root  42712 May  3  2007 pam_unix_session.so
-rwxr-xr-x 1 root root  11080 May  3  2007 pam_userdb.so
-rwxr-xr-x 1 root root   6192 May  3  2007 pam_userpass.so
-rwxr-xr-x 1 root root   5736 May  3  2007 pam_warn.so
-rwxr-xr-x 1 root root   7248 May  3  2007 pam_wheel.so
-rwxr-xr-x 1 root root  15496 May  3  2007 pam_xauth.so
			

A list from The PAM  reference guide for available modules:

Common Pam modules:

• pam_wheel -- Only permit root access to members of the wheel (gid=0) group. This is similar to theFreeBSD style wheel access, so that you can't su to root unless you are in the wheel group. 
   

• pam_env.so -- Sets environment variables for PAM using the settings in /etc/security/pam_env.conf.
   

• pam_ftp.so -- Allows (forces) anonymous logins for ftp. User must enter ftp or anonymous for the UID and an e-mail address for the password.
   
• pam_group -- This module provides group-settings based on the user's name and the terminal they are requesting a given service from. This allows group settings separate from the /etc/group to be used only in PAM processing. This module does not authenticate the user, but instead it grants group memberships (in the credential setting phase of the authentication module) to the user. Such memberships are based on the service they are applying for. The group memberships are listed in text form in the /etc/security/group.conf file. The pam_group module functions in parallel with the /etc/group file. If the user is granted any groups based on the behavior of this module, they are granted in addition to those entries /etc/group (or equivalent).
   
• pam_homecheck -- Checks the home directory of the user for world writable or dot files not owned by the user. Check, if the home directory is world-writeable. If the home directory is world-writeable, or one of the important dot files is world-writeable or not owned by the user or root, a warning is printed. The abort argument will deny permission to log in in such a case.
   
• pam_limits -- Uses the /etc/security/pam_limits.conf file to enforce resource limitations on users. Through the contents of the configuration file, /etc/security/limits.conf, resource limits are placed on users' sessions. Users of uid=0 are not affected by this restriction.  Two implementations of this have been merged to produce this module. Cristian Gafton <gafton at redhat.com> and Elliot Lee <sopwith at redhat.com>

   

• pam_listfile -- Allows or denys access to a service based on a text file. filename contains one line per item listed. If the item is found, then if sense=allow, PAM_SUCCESS is returned, causing the authorization request to succeed; else if sense=deny, PAM_AUTH_ERR is returned, causing the authorization request to fail.

Here is an example of user the ftpusers file to deny access to those listed in the file:

#
# deny ftp-access to users listed in the /etc/ftpusers file
#
ftp auth required pam_listfile.so \
onerr=succeed item=user sense=deny file=/etc/ftpusers
pam_mail -- [authentication (credential);session (open)]
This module provides the “you have new mail” service to the user. It can be plugged into any application that has credential hooks. It gives a single message indicating the newness of any mail it finds in the user's mail folder. This module also sets the Linux-PAM environment variable, MAIL, to the user's mail directory.

• pam_motd -- Displays the motd file.
   
• pam_nologin -- Provides standard Unix nologin authentication. If the file /etc/nologin exists, only root is allowed to log in; other users are turned away with an error message. All users (root or otherwise) are shown the contents of /etc/nologin. If the file /etc/nologin does not exist, this module succeeds silently. If /etc/nologin exists, its contents are displayed to the user.  Michael K. Johnson <johnsonm at redhat.com>
   
• pam_permit: always allow access; Andrew Morgan
   
• pam_pwcheck -- This is a module for checking passwords. It reads /etc/login.defs and makes the checks the standard Linux shadow suite also does. If configured, it also uses the cracklib library to check the password.
   
• pam_rootok: module to authenticate the user if their (real) uid is root (intended for use with the sufficient control flag); Andrew Morgan
   
• pam_securetty -- Provides standard Unix securetty checking, which causes authentication for root to fail unless PAM_TTY is set to a string listed in the /etc/securetty file. For all other users, it succeeds.
 
• pam_succeed_if - test account characteristics
   
• pam_unix2 -- [account; authentication; password; session]
  The pam_unix2 PAM module is for traditional password authentication. If configured in /etc/nsswitch.conf, this module will make NIS or NIS+ queries. This is the standard Unix authentication module. It uses standard calls from the glibc NSS libraries to retrieve and set account information as well as authentication. Usually this is obtained from the the local files /etc/passwd and /etc/shadow, from an NIS map or from the NIS+ passwd.org_dir table.
  
  The advantages of this particular version are full SecureRPC and NIS+ support, HP-UX password aging, and password encryption with DES, bigcrypt, MD5, or blowfish. It allows the modification of user accounts in the source files for NIS maps on the NIS master server, if they are not the standard files in /etc.
  
  Written by Thorsten Kukuk.

Less common PAM modules

• pam_require Allows you to require a special group or user to access a service. It is an account module that allows you to require a special user or group to access a service.
   
• pam_chroot A PAM module for chrooting.
   
• pam_deny: deny all forms of access; Andrew Morgan
   
• pam_filter: module to allow easy access to the stdin/out of a running process. It can be used to log users input etc.. Current pluggable filters include:
  • upperLOWER: demonstration filter that transposes upper and lower case characters.
  You are encouraged to write your own.. (Email me if you need help.); Andrew Morgan
   
• pam_passwd+: password strength checking; Al Longyear <longyear at netcom.com>
   
• pam_pwdb: plug in replacement for pam_unix_* that uses the Password Database library. Andrew Morgan
   
• pam_radius: RADIUS authentication, using the Password Database library. [Redhat offer fixes and changes to libpwdb, for info on these, take a look here: libpwdb. Debian also have a package for this.]
  Cristian Gafton <gafton at redhat.com>
   
• pam_rootok: module to authenticate the user if their (real) uid is root (intended for use with the sufficient control flag); Andrew Morgan
   
• pam_shells: authenticate users if their shell is listed in the /etc/shells file. Erik Troan <ewt at redhat.com>
   
• pam_stress: stress test your application with this module; Andrew Morgan
   
• pam_tally: this module keeps track of the number of times an attempt is made to access an account. It can deny access after a specified number of failures. Root's account can be treated specially. RPMS for newer versions of this module than the one contained in the source tree are here: ftp://rudy.mif.pg.gda.pl/pub/People/milek.
  Tim Baverstock <warwick at demon.co.uk>
   
• pam_time: authorize users based on when and where they log in (like securetty, but) in a way that is dependent on the service they are requesting; Andrew Morgan

   

• pam_unix_*: standard unix authentication (with some shadow support); This module is being supported by Red Hat. Michael K. Johnson <johnsonm at redhat.com>
   
• pam_warn: provides a diagnostic tool for dumping information to syslog(2) about the service-application.
  Andrew Morgan

Old News ;-)

freshmeat.net Project details for pam_require

pam_require is a simple Plugable Authentication Module (PAM) for Linux (and maybe other systems which use PAM). It is an account module that allows you to require a special group or user to access a service.

freshmeat.net Project details for pam_script

pam_script is a PAM that executes a script at the start and end of a session. Any PAM-aware application can use the module to perform arbitrary operations. It was originally written for cleaning up when a user logs out.

Author:
Izak Burger [contact developer]

Recipe 4.3, Creating Access Control Lists with PAM from Linux Security Cookbook
By Daniel J. Barrett,

Author's note: PAM (Pluggable Authentication Modules) is a flexible infrastructure for controlling authentication on Linux systems. In this recipe, taken from Chapter 4, "Authentication Techniques and Infrastructures," we show you how to restrict authentication to a given set of users by creating an access control list.

Problem

You would like to apply an access control list (ACL) to an existing service that does not explicitly support ACLs (e.g., telnetd, imapd, etc.).

Solution

Use the listfile PAM module.

First, make sure the server in question uses PAM for authentication, and find out which PAM service name it uses. This may be in the server documentation, or it may be clear from examining the server itself and perusing the contents of /etc/pam.d. For example, suppose you're dealing with the IMAP mail server. First notice that there is a file called /etc/pam.d/imap. Further, the result of:

# locate imapd
...
/usr/sbin/imapd

shows that the IMAP server is in /usr/sbin/imapd, and:

# ldd /usr/sbin/imapd
libpam.so.0 => /lib/libpam.so.0 (0x40027000)
...

shows that the server is dynamically linked against the PAM library (libpam.so), also suggesting that it uses PAM. In fact, the Red Hat 8.0 IMAP server uses PAM via that service name and control file ("imap").

Continuing with this example, create an ACL file for the IMAP service, let's say /etc/imapd.acl, and make sure it is not world-writable:

# chmod o-w /etc/imapd.acl

Edit this file, and place in it the usernames of those accounts authorized to use the IMAP server, one name per line. Then, add the following to /etc/pam.d/imap:

account required /lib/security/pam_listfile.so file=/etc/imapd.acl \
item=user sense=allow onerr=fail

With this configuration, only those users listed in the ACL file will be allowed access to the IMAP service. If the ACL file is missing, PAM will deny access for all accounts.

Discussion

The PAM "listfile" module is actually even more flexible than we've indicated. Entries in your ACL file can be not only usernames (item=user), but also:

• Terminal lines (item=tty)
• Remote host (item=rhost)
• Remote user (item=ruser)
• Group membership (item=group)
• Login shell (item=shell)

The sense keyword determines how the ACL file is interpreted. sense=allow means that access will be allowed only if the configured item is in the file, and denied otherwise. sense=deny means the opposite: access will be denied only if the item is in the file, and allowed otherwise.

The onerr keyword indicates what to do if some unexpected error occurs during PAM processing of the listfile module - for instance, if the ACL file does not exist. The values are succeed and fail. fail is a more conservative option from a security standpoint, but can also lock you out of your system because of a configuration mistake!

Another keyword, apply=[user|@group], limits the restriction in question to apply only to particular users or groups. This is intended for use with the tty, rhost, and shell items. For example, using item=rhost and apply=@foo would restrict access to connections from hosts listed in the ACL file, and furthermore only to local accounts in the foo group.

To debug problems with PAM modules, look for PAM-specific error messages in /var/log/messages and /var/log/secure. (If you don't see the expected messages, check your system logger configuration. [Recipe 9.28])

Note that not all module parameters have defaults. Specifically, the file, item, and sense parameters must be supplied; if not, the module will fail with an error message like:

Dec  2 15:49:21 localhost login: PAM-listfile: Unknown sense or sense not specified

You generally do not need to restart servers using PAM: they usually re-initialize the PAM library for every authentication and reread your changed files. However, there might be exceptions.

There is no standard correspondence between a server's name and its associated PAM service. For instance, logins via Telnet are actually mediated by /bin/login, and thus use the login service. The SSH server sshd uses the same-named PAM service (sshd), whereas the IMAP server imapd uses the imap (with no "d") PAM service. And many services in turn depend on other services, notably system-auth.

See Also

See /usr/share/doc/pam-*/txts/README.pam_listfile for a list of parameters to tweak.

Recommended Links

The Linux-PAM Administration and Developer Guides

• The System Administrators' Guide

Last modified: June 02, 2008