PAM - Pluggable Authentication Modules
======================================

This is the most common way to authenticate system users nowadays. PAM is not
itself a password database, but rather its configuration tells the system how
exactly to do the authentication. Usually this means using the 'pam_unix.so'
module, which authenticates user from the system's shadow password file.

Because PAM is not an actual database, only plaintext authentication mechanisms
can be used with PAM. PAM cannot be used as a user database either (although
static user templates could be used to provide the same effect). Usually PAM is
used with <passwd> [AuthDatabase.Passwd.txt] (NSS) or <static>
[UserDatabase.Static.txt] user databases.

Dovecot should work with Linux PAM, Solaris PAM, OpenPAM (FreeBSD) and ApplePAM
(Mac OS X).

Service name
------------

The PAM configuration is usually in the '/etc/pam.d/' directory, but some
systems may use a single file,'/etc/pam.conf'. By default Dovecot uses
'dovecot' as the PAM service name, so the configuration is read from
'/etc/pam.d/dovecot'. You can change this by giving the wanted service name in
the 'args' parameter. You can also set the service to '%s' in which case
Dovecot automatically uses either 'imap' or 'pop3' as the service, depending on
the actual service the user is logging in to. Here are a few examples:

 * Use '/etc/pam.d/imap' and '/etc/pam.d/pop3':

   ---%<----------------------------------------------------------------------
   passdb {
     driver = pam
     args = %s
   }
   ---%<----------------------------------------------------------------------

 * Use '/etc/pam.d/mail':

   ---%<----------------------------------------------------------------------
   passdb {
     driver = pam
     args = mail
   }
   ---%<----------------------------------------------------------------------

PAM sessions
------------

By giving a 'session=yes' parameter, you can make Dovecot open a PAM session
and close it immediately. Some PAM plugins need this, for instance
'pam_mkhomedir'. With this parameter, 'dovecot.conf' might look something like
this:

---%<-------------------------------------------------------------------------
passdb {
  driver = pam
  args = session=yes dovecot
}
---%<-------------------------------------------------------------------------

PAM credentials
---------------

By giving a 'setcred=yes' parameter, you can make Dovecot create PAM
credentials. Some PAM plugins need this. The credentials are never deleted
however, so using this might cause problems with other PAM plugins.

Limiting the number of PAM lookups
----------------------------------

Usually in other software PAM is used to do only a single lookup in a process,
so PAM plugin writers haven't done much testing on what happens when multiple
lookups are done. Because of this, many PAM plugins leak memory and possibly
have some other problems when doing multiple lookups. If you notice that PAM
authentication stops working after some time, you can limit the number of
lookups done by the auth worker process before it dies:

---%<-------------------------------------------------------------------------
passdb {
  driver = pam
  args = max_requests=100
}
---%<-------------------------------------------------------------------------

The default max_requests value is 100.

Username changing
-----------------

A PAM module can change the username.

Making PAM plugin failure messages visible
------------------------------------------

You can replace the default "Authentication failed" reply with PAM's failure
reply by setting:

---%<-------------------------------------------------------------------------
passdb {
  driver = pam
  args = failure_show_msg=yes
}
---%<-------------------------------------------------------------------------

This can be useful with e.g. pam_opie to find out which one time password
you're supposed to give:

---%<-------------------------------------------------------------------------
1 LOGIN username otp
1 NO otp-md5 324 0x1578 ext, Response:
---%<-------------------------------------------------------------------------

Caching
-------

Dovecot supports caching password lookups by setting 'auth_cache_size' to
non-zero value. For this to work with PAM, you'll also have to give 'cache_key'
parameter. Usually the user is authenticated only based on the username and
password, but PAM plugins may do all kinds of other checks as well, so this
can't be relied on. For this reason the 'cache_key' must contain all the
<variables> [Variables.txt] that may affect authentication. The commonly used
variables are:

 * '%u' - Username. You'll most likely want to use this.
 * '%s' - Service. If you use '*' as the service name you'll most likely want
   to use this.
 * '%r' - Remote IP address. Use this if you do any IP related checks.
 * '%l' - Local IP address. Use this if you do any checks based on the local IP
   address that was connected to.

Examples:

---%<-------------------------------------------------------------------------
# 1MB auth cache size
auth_cache_size = 1024
passdb {
  driver = pam
  # username and service
  args = cache_key=%u%s *
}
---%<-------------------------------------------------------------------------

---%<-------------------------------------------------------------------------
# 1MB auth cache size
auth_cache_size = 1024
passdb {
  driver = pam
  # username, remote IP and local IP
  args = cache_key=%u%r%l dovecot
}
---%<-------------------------------------------------------------------------

Examples
--------

Linux
-----

Here is an example '/etc/pam.d/dovecot' configuration file which uses standard
UNIX authentication:

---%<-------------------------------------------------------------------------
auth    required        pam_unix.so nullok
account required        pam_unix.so
---%<-------------------------------------------------------------------------

Solaris
-------

For Solaris you will have to edit '/etc/pam.conf'. Here is a working Solaris
example (using 'args = *' instead of the default 'dovecot' service):

---%<-------------------------------------------------------------------------
imap    auth    requisite       pam_authtok_get.so.1
imap    auth    required       pam_unix_auth.so.1
imap    account requisite       pam_roles.so.1
imap    account required        pam_unix_account.so.1
imap    session required        pam_unix_session.so.1
pop3   auth    requisite   pam_authtok_get.so.1
pop3   auth    required    pam_unix_auth.so.1
pop3   account requisite   pam_roles.so.1
pop3   account required    pam_unix_account.so.1
pop3   session required    pam_unix_session.so.1
---%<-------------------------------------------------------------------------

Mac OS X
--------

On Mac OS X, the '/etc/pam.d/dovecot' file should look like this:

---%<-------------------------------------------------------------------------
auth       required       pam_nologin.so
auth       sufficient     pam_securityserver.so
auth       sufficient     pam_unix.so
auth       required       pam_deny.so
account    required       pam_permit.so
password   required       pam_deny.so
session    required       pam_uwtmp.so
---%<-------------------------------------------------------------------------

...which, as the equivalent of '/etc/pam.d/login' on OS X 10.4, can be
represented as the following on that OS:

---%<-------------------------------------------------------------------------
passdb {
  driver = pam
  args = login
}
---%<-------------------------------------------------------------------------

On Mac OS X, "passwd" can be used as a userdb to fill in UID, GID, and homedir
information after PAM was used as a passdb, even though Directory Services
prevents "passdb passwd" from working as a username/password authenticator. 
This will provide full system user authentication with true homedir mail
storage, without resorting to a single virtual mail user or LDAP:

---%<-------------------------------------------------------------------------
userdb {
  driver = passwd
}
---%<-------------------------------------------------------------------------

(This file was created from the wiki on 2011-08-29 04:42)