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 (NSS) or static 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 * 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 for your dovecot.conf:

passdb pam {
  # use /etc/pam.d/imap and /etc/pam.d/pop3
  args = *
}

passdb pam {
  # use /etc/pam.d/mail
  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 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.

Non-forking PAM lookups (v1.0-only)

Note that v1.1+ uses this by default.

By default Dovecot v1.0's dovecot-auth forks a new process for each PAM lookup, which is then destroyed after the lookup is done. This may have some problems however because the forked processes share all the file descriptors with the parent process. For example if you're using nss_ldap and your PAM plugin does a NSS lookup, it's entirely possible that two PAM child processes are using the same LDAP connection to do the lookup at the same time and they get their replies mixed, causing wrong user's information to be used.

Setting blocking=yes uses the alternative way: dovecot-auth worker processes do the PAM lookups. See the next section for problems with this.

passdb pam {
  # v1.0 only:
  args = blocking=yes
}

Limiting the number of PAM lookups

Dovecot v1.1+ or v1.0 with blocking=yes enabled uses auth worker processes to do PAM lookups. Usually PAM is used to do only a single lookup in a process, so this may cause memory leaks in PAM plugins or maybe other problems. 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:

# v1.1:
auth_worker_max_request_count = 100

# v1.2+:
passdb pam {
  args = max_requests = 100
}

Note that v1.1's auth_worker_max_request_count applies to all lookups, not just PAM lookups. So for example if you've configured Dovecot to use both PAM and MySQL, both of the lookups are done in the same auth worker processes and both of them are counted as requests. This is never really wanted, which is why this was changed in v1.2.

Username changing

A PAM module can change the username. This requires either Dovecot v1.1+ or blocking=yes with v1.0.

Making PAM plugin failure messages visible

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

passdb 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 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 pam {
  # username and service
  args = cache_key=%u%s *
}

# 1MB auth cache size
auth_cache_size = 1024
passdb 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 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 passwd {
  args =
}