This documentation is for Dovecot v2.x, see wiki1 for v1.x documentation.

Master users/passwords

It's possible to configure master users who are able to log in as other users. It's also possible to directly log in as any user using a master password, although this isn't recommended.

Master users

There are two ways for master users to log in as other users:

  1. Give the login username in the SASL mechanism's authorization ID field.

  2. Specify both the master username and the login username in the same username field. The usernames are separated by a string configured by the auth_master_user_separator setting. UW-IMAP uses "*" as the separator, so that could be a good choice. Using "*" as the separator, the master user would log in as "login_user*master_user".

Master users are configured by adding a new passdb with master=yes setting. The users in the master passdb cannot log in as themselves, only as other people. That means they don't need to exist in the userdb, because the userdb lookup is done only for the user they're logging in as.

You should also add the pass=yes setting to the master passdb if possible. It means that Dovecot verifies that the login user really exists before allowing the master user to log in. Without the setting if a nonexistent login username is given, depending on the configuration, it could either return an internal login error (the userdb lookup failed) or create a whole new user (with eg. static userdb). pass=yes doesn't work with PAM or LDAP with auth_bind=yes, because both of them require knowing the user's password.

pass=yes is especially useful with a Checkpassword passdb because the script gets both the login and the master username as environment variables. Other passdbs see only the login username in %u. In the future there will probably be another setting to make the user verification to be done from userdb.

If you want master users to be able to log in as themselves, you'll need to either add the user to the normal passdb or add the passdb to dovecot.conf twice, with and without master=yes. Note that if the passdbs point to different locations, the user can have a different password when logging in as other users than when logging in as himself. This is a good idea since it can avoid accidentally logging in as someone else.

Usually it's better to have only a few special master users that are used only to log in as other people. One example could be a special "spam" master user that trains the users' spam filters by reading the messages from the user's spam mailbox.

ACLs

If ACL plugin is enabled, the Master user is still subject to ACLs just like any other user, which means that by default the master user has no access to any mailboxes of the user. The options for handling this are:

  1. Adding a global ACL for the master user. Unfortunately currently you can only create per-mailbox ACLs, there is no "default ACL" that applies to all mailboxes.

  2. Assigning MASTER_USER=$USER in a postlogin-script. The result is that the login username's ACL is used, rather than the master username's.

  3. Assigning master_user=%u in userdb lookup, which basically does the same as the post-login script.

Example configuration

auth_master_user_separator = *
passdb {
  driver = passwd-file
  args = /etc/dovecot/passwd.masterusers
  master = yes
  pass = yes
}
passdb {
  driver = shadow
}
userdb {
  driver = passwd
}

Where the passwd.masterusers file would contain the master usernames and passwords:

admin:{SHA1}nU4eI71bcnBGqeO0t9tXvY1u5oQ=
admin2:{SHA1}i+UhJqb95FCnFio2UdWJu1HpV50=

One way to create this master file is to use the htaccess program as follows:

htpasswd -b -c -s passwd.masterusers user password

SQL Example

The master passdb doesn't have to be passwd-file, it could be an SQL query as well:

auth_master_user_separator = *
passdb {
  driver = sql
  args = /etc/dovecot/dovecot-sql-master.conf.ext
  master = yes
  pass = yes
}
passdb {
  driver = sql
  args = /etc/dovecot/dovecot-sql.conf.ext
}
userdb {
  driver = sql
  args = /etc/dovecot/dovecot-sql.conf.ext
}
}

dovecot-sql-master.conf.ext would contain all the normal connection settings and a password_query:

password_query = SELECT password FROM users WHERE userid = '%u' and master_user = true

Testing

# telnet localhost 143
* OK Dovecot ready.
1 login loginuser*masteruser masterpass
1 OK Logged in.

If you had any problems, set auth_debug=yes and look at the logs.

Master passwords

The easiest way to implement this is with SQL:

password_query = SELECT user, 'master-password' AS password FROM users WHERE userid = '%u'

If you don't have the users in SQL database, you can still fake it:

password_query = SELECT '%u' AS user, 'master-password' AS password

However note that the above will allow logins for any username using the master password, even those that don't really exist.

Then in your dovecot.conf, have something like:

passdb {
  driver = pam
}
passdb {
  driver = sql
  args = /etc/dovecot-sql-master.conf.ext
}

One way to do this without SQL is to create a passwd-file containing every user:

user1:{plain}master-password
user2:{plain}master-password
..etc..

Advanced SQL Examples

In these example we will create 3 kinds of master users. The first will be users who can read all email for all domains. The next example will be users who can read all email for their domain only. The third example will be users who can read email of domains listed in a separate ownership table. We will use MySQL and create 2 tables with the following structure.

CREATE TABLE `users` (
  `uid` int(4) NOT NULL AUTO_INCREMENT,
  `user_name` varchar(80) NOT NULL,
  `domain_name` varchar(80) NOT NULL,
  `password` varchar(60) DEFAULT NULL,
  `last_login` datetime DEFAULT NULL,
  `masteradmin` tinyint(1) NOT NULL DEFAULT '0',
  `owns_domain` tinyint(1) NOT NULL DEFAULT '0',
  UNIQUE KEY `emaillookup` (`domain_name`,`user_name`),
  UNIQUE KEY `uid` (`uid`)
) ENGINE=MyISAM AUTO_INCREMENT=995 DEFAULT CHARSET=latin

CREATE TABLE `ownership` (
  `login_id` varchar(128) NOT NULL,
  `owned_object` varchar(128) NOT NULL,
  UNIQUE KEY `login_id_full` (`login_id`,`owned_object`),
  KEY `login_id` (`login_id`),
  KEY `owned_object` (`owned_object`),
  KEY `login_id_index` (`login_id`),
  KEY `owned_object_index` (`owned_object`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1

The dovecot.conf file for all 3 master user configurations will be as follows:

passdb {
  driver = sql
  args = /etc/dovecot/ownership-sql.conf
  master = yes
  pass = yes
}

passdb {
  driver = sql
  args = /etc/dovecot/domain-owner-sql.conf
  master = yes
  pass = yes
}

passdb {
  driver = sql
  args = /etc/dovecot/masteradmin-sql.conf
  master = yes
  pass = yes
}
passdb {
  args = /etc/dovecot/sql.conf
  driver = sql
}

Before we get into the master user tricks, we start with normal email authentication. The query for that is as follows:

password_query = SELECT user_name, domain_name, password FROM users WHERE user_name = '%n' AND domain_name = '%d'

In this first example master admin suppose you want to allow a few people to be master users over all domains. These users will have the "masteradmin" field set to 1. The query would be:

password_query = SELECT user_name, domain_name, password FROM users WHERE user_name = '%n' AND domain_name = '%d' AND masteradmin='1'

In the second example suppose you are hosting multiple domains and you want to allow a few users to become master users of their domain only.

Your query would be as follows:

password_query = SELECT user_name, domain_name, password FROM users WHERE user_name = '%n' \
AND domain_name = '%d' AND owns_domain='1' AND '%d'='%{login_domain}'

This will allow you to log in using the following to read Joe's email if master@dovecot.org is flagged as the domain_owner.

joe@dovecot.org*master@dovecot.org

In this third example we have a table of owners. There are a list of pairs between owner email addresses and domains that are owned. That way if a person controls a lot of domains then they can view all the users in all the domains they control. The query would be as follows:

password_query = SELECT user_name, domain_name, password FROM users, ownership WHERE \
user_name = '%n' AND domain_name = '%d' AND login_id='%u' AND owned_object='%{login_domain}'

If you really want to get tricky and efficient you can combine all 3 queries into one giant query that does everything.

password_query = SELECT user_name, domain_name, password FROM users, ownership WHERE \
user_name = '%n' AND domain_name = '%d' AND ( \
(masteradmin='1') OR \
(owns_domain='1' AND '%d'='%{login_domain}') OR \
(login_id='%u' and owned_object='%{login_domain}')) \
group by uid

Authentication/MasterUsers (last edited 2013-03-20 16:51:45 by TimoSirainen)