PAM

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 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:

Restrict IP-Addresses allowed to connect via PAM

You can restrict the IP-Addresses allowed to connect via PAM:

passdb {
  driver = pam
  override_fields = allow_nets=10.1.100.0/23,2001:db8:a0b:12f0::/64
}

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 Config 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 {
  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 might look like this:

auth        required       pam_opendirectory.so try_first_pass
account     required       pam_nologin.so
account     required       pam_opendirectory.so
password    required       pam_opendirectory.so

…which, as the equivalent of /etc/pam.d/login on OS X 10.9. For very old versions of OS X (e.g. 10.4), can be represented (where?) as the following in the on that OS:

passdb {
  driver = pam
  args = login
}

On older versions of 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
}