Master Users/Passwords

It is 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.

Master Users

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

1. Give the login username in the authentication mechanisms authorization ID field.

2. Specify both the master username and the login username in the same username field. See auth_master_user_separator for the format of the string.

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 result_success=continue 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, e.g., static authentication database).

result_success=continue doesn't work with PAM or LDAP without auth_bind=yes, because both of them require knowing the user's password.

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

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 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 plugin for the master user.

   You can create a default ACL, that applies to all mailboxes. See example below.

2. Set plugin { acl_user=%u }. This preserves the master_user for other purposes (e.g. %{master_user} variable).

3. Set plugin { master_user=%u }. This fully hides that master user login is being used.

Example configuration:

auth_master_user_separator = *

passdb {
  driver = passwd-file
  args = /etc/dovecot/passwd.masterusers
  master = yes
  result_success = continue
}

userdb {
  driver = passwd
}

To grant the master user access to all Mailboxes, the dovecot-acl file can contain:

* user=masteruser lr

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
  result_success = continue
}

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

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

If you had any problems, set log_debug = category=auth and look at the logs.

Master Passwords

You can configure a passdb which first performs authentication using the master password. Then it continues to the primary passdb to verify that the user exists and get other extra fields.

# master password passdb
passdb {
  driver = static
  args = password=master-password
  result_success = continue
}

# primary passdb
passdb {
  driver = pam
}

Advanced SQL Examples

For these examples, we will create 3 kinds of master users:

• Users who can read all email for all domains
• Users who can read all email for their domain only
• 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=InnoDB AUTO_INCREMENT=995 DEFAULT CHARSET=utf8

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=InnoDB DEFAULT CHARSET=utf8

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
  result_success = continue
}

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

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

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, 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 joe@dovecot.org*master@dovecot.org to read Joe's email if master@dovecot.org is flagged as the domain_owner.

In the 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