[Dovecot] No passdbs specified in configuration file with passdb/userdb in protocol sections

[btb at bitrate.net]

hi-

i have a configuration in which i'm using different passdb/userdb settings for each of imap, lmtp and smtp [without getting too far off on a tangent, this is so ldap group membership can be used to independently control authorization for receiving [lmtp], retrieving [imap], and sending [smtp/postfix] of email.  each passdb/userdb uses a different search filter.]

when i use this as my config:

# 2.0.13: /etc/dovecot/dovecot.conf
# OS: Linux 3.0.0-15-generic-pae i686 Ubuntu 11.10 
auth_debug = yes
first_valid_gid = 2000
first_valid_uid = 2000
log_timestamp = "%d.%m.%Y %H.%M.%S "
login_greeting = dovecot ready
mail_debug = yes
protocols = " imap lmtp"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0666
    user = postfix
  }
}
service lmtp {
  inet_listener lmtp {
    address = 127.0.0.1 ::1
    port = 10026
  }
}
ssl_cert = </etc/dovecot/pki/mra.example.com-cert.pem
ssl_key = </etc/dovecot/pki/mra.example.com-key.pem
verbose_proctitle = yes
protocol imap {
  mail_max_userip_connections = 20
  passdb {
    args = /etc/dovecot/dovecot-ldap-imap.conf.ext
    driver = ldap
  }
  userdb {
    driver = prefetch
  }
  userdb {
    args = /etc/dovecot/dovecot-ldap-imap.conf.ext
    driver = ldap
  }
}
protocol lmtp {
  userdb {
    args = /etc/dovecot/dovecot-ldap-lmtp.conf.ext
    driver = ldap
  }
}
protocol smtp {
  passdb {
    args = /etc/dovecot/dovecot-ldap-smsp.conf.ext
    driver = ldap
  }
}

dovecot complains:

dovecot: auth: Fatal: No passdbs specified in configuration file. PLAIN mechanism needs one

which is a little confusing, since there are multiple passdbs specified, in the various protocol sections.  i found that adding another "global" passdb to the config allows dovecot to work - e.g.:

# 2.0.13: /etc/dovecot/dovecot.conf
# OS: Linux 3.0.0-15-generic-pae i686 Ubuntu 11.10 
auth_debug = yes
first_valid_gid = 2000
first_valid_uid = 2000
log_timestamp = "%d.%m.%Y %H.%M.%S "
login_greeting = dovecot ready
mail_debug = yes
passdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
protocols = " imap lmtp"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0666
    user = postfix
  }
}
service lmtp {
  inet_listener lmtp {
    address = 127.0.0.1 ::1
    port = 10026
  }
}
ssl_cert = </etc/dovecot/pki/mra.example.com-cert.pem
ssl_key = </etc/dovecot/pki/mra.example.com-key.pem
verbose_proctitle = yes
protocol imap {
  mail_max_userip_connections = 20
  passdb {
    args = /etc/dovecot/dovecot-ldap-imap.conf.ext
    driver = ldap
  }
  userdb {
    driver = prefetch
  }
  userdb {
    args = /etc/dovecot/dovecot-ldap-imap.conf.ext
    driver = ldap
  }
}
protocol lmtp {
  userdb {
    args = /etc/dovecot/dovecot-ldap-lmtp.conf.ext
    driver = ldap
  }
}
protocol smtp {
  passdb {
    args = /etc/dovecot/dovecot-ldap-smsp.conf.ext
    driver = ldap
  }
}

the problem with this is that while each of the passdb/userdb configs for the various protocols does indeed work, if a result is not found in one of them, the global passdb appears to then function as a catch-all.

how can i tell dovecot it doesn't need a global passdb?  each of the protocols' passdb/userdb configs is functioning as desired, but having dovecot look elsewhere upon failure ends up defeating the purpose.

thanks
-ben