Large-Scale Mail with Postfix, OpenLDAP and Courier

The first LDAP source definition is for virtual aliases. We've named this LDAP source aliases. In our configuration, our LDAP server is running on localhost. The search base is the top of the hosting subtree we defined in our LDAP server. We're querying for items where the mail elements match the e-mail recipient as well as items that are of the courierMailAlias object class. The destination of the alias is stored in the maildrop attribute. Postfix won't bind using an account, instead it will do an anonymous lookup:

aliases_server_host = localhost
aliases_search_base =
  o=hosting,dc=myhosting,dc=example
aliases_query_filter =
  (&(mail=%s)(objectClass=CourierMailAlias))
aliases_result_attribute = maildrop
aliases_bind = no

When using the accounts source we're looking for entries that have an object class of courierMailAccount. We request the mailbox attribute as the result:

accounts_server_host = localhost
accounts_search_base =
  o=hosting,dc=myhosting,dc=example
accounts_query_filter =
  (&(mail=%s)(objectClass=CourierMailAccount))
accounts_result_attribute = mailbox
accounts_bind = no

A second source for accounts, accountsmap, also needs to be defined to help locate accounts when a catchall is used. Without this lookup, a catchall in the aliases would override virtual accounts in a domain:

accountsmap_server_host = localhost
accountsmap_search_base = o=hosting,dc=myhosting,dc=example
accountsmap_query_filter =
(&(mail=%s)(objectClass=CourierMailAccount
accountsmap_result_attribute = mail
accountsmap_bind = no

virtual_maps = ldap:aliases

:virtual_mailbox_base = /home/vmail/domains
virtual_mailbox_maps = ldap:accounts
virtual_minimum_uid = 125
virtual_uid_maps = static:125
virtual_gid_maps = static:120

We also need to edit the local_recipient_maps parameter to look at the virtual_mailbox_maps so Postfix knows what accounts are valid. This is needed so Postfix can reject mail for unknown accounts:

local_recipient_maps = $alias_maps
  unix:passwd.byname $virtual_mailbox_maps

There aren't any special instructions for installing Courier, so see its documentation for full instructions. It should autodetect LDAP and build it in. You should seriously consider passing the --enable-workarounds-for-imap-client-bugs option to ./configure, otherwise Netscape mail users may have trouble interacting with your server. This bends the IMAP protocol a little bit, but it's better to have happy users than a perfect protocol with unhappy users.

Courier uses an authentication dæmon to keep authentication separate from the other parts of the system. Configure it so that a valid e-mail account is either found in either LDAP or PAM. Specify this in authdaemonrc using the authmodulelist parameter:

authmodulelist="authldap authpam"

All LDAP parameters are in authldaprc. Most parameters are self-explanatory. To use the Courier schema, you actually have a few modifications to make, though. You also need to map all virtual accounts to the vmail account. Here is a summary of the updates you need to make to authldaprc:

LDAP_GLOB_UID           vmail
LDAP_GLOB_GID           vmail
LDAP_HOMEDIR            homeDirectory
LDAP_MAILDIR            mailbox
LDAP_CRYPTPW            userPassword

If you use LDAP_BINDDN and LDAP_BINDPW, you are limited to the crypt, MD5 and SHA algorithms for passwords. SMD5 and SSHA are not available. Also, you must put the root LDAP password in clear text in authldaprc when defining LDAP_BINDPW. There are security issues with putting the root LDAP password in clear text, so definitely use LDAP_AUTHBIND if you can.

The last change is to enable the IMAP server by setting the IMAPDSTART parameter to YES. You should now be able to use the courier-imap.sysvinit startup script to start and stop the IMAP dæmon.