[2021-07-28]

Setting up Dovecot

This chapter of our journey leads us to Dovecot – the software that…

  • gets emails from Postfix and saves them to disk
  • executes user-based sieve filter rules (can be used to e.g. move emails to different folders based on certain criteria or to send automated vacation responses)
  • allows the user to fetch emails using POP3 or IMAP

Before we get to the actual configuration for security reasons I recommend that you create a new system user that will own all virtual mailboxes. The following shell commands will create a system group “vmail” with GID (group ID) 5000 and a system user “vmail” with UID (user ID) 5000. (Make sure that UID and GID are not yet used or choose another – the number can be anything between 1000 and 65000 that is not yet used):

groupadd -g 5000 vmail
useradd -g vmail -u 5000 vmail -d /var/vmail -m

If the /var/vmail directory was already there because you assigned it a dedicated mount point then you should make sure that the permissions are set correctly:

chown -R vmail:vmail /var/vmail

The configuration files for Dovecot are found in /etc/dovecot/conf.d/. All these files are loaded by Dovecot. This is done by this magical line in the /etc/dovecot/dovecot.conf file:

!include conf.d/*.conf

It loads all files in /etc/dovecot/conf.d/ that end in “.conf” in alphanumerical order. So “10-auth.conf” is loaded first and “90-sieve-extprograms.conf” is loaded last. The big advantage is that you can edit or replace parts of the configuration without having to overwrite the entire configuration. The main /etc/dovecot/dovecot.conf file does not require any changes. Those other files in conf.d/ however need a few changes…

conf.d/

10-auth.conf

The most common authentication mechanism is called PLAIN. However if you have Outl**k users then you need to add the LOGIN mechanism, too.:

auth_mechanisms = plain login

These two mechanisms would ask for a password without enforcing encryption to secure the password. But don’t worry. By default Dovecot sets “disable_plaintext_auth = yes” which ensures that authentication is only accepted over TLS-encrypted connections.

At the end of this file you will find various authentication backends that Dovecot ships with. By default it will use system users (those from the /etc/passwd). But we want to use the MariaDB database backend so go ahead and change this block to:

#!include auth-system.conf.ext
!include auth-sql.conf.ext
#!include auth-ldap.conf.ext
#!include auth-passwdfile.conf.ext
#!include auth-checkpassword.conf.ext
#!include auth-static.conf.ext

10-mail.conf

Change the mail_location setting to:

mail_location = maildir:~/Maildir

This is the directory where Dovecot will look for the emails of a specific user. The tilde character (~) means the user’s home directory as defined in the previous section.

Further down you will find sections defining the namespaces. Those are folder structures that your email program sees when connecting to the mail server. If you use POP3 you can only access the “inbox” – which is where all incoming email is stored. Using the IMAP protocol you get access to a hierarchy of folders and subfolders. And you can even share folders between users. Or use a public folder that can be accessed by anyone – even anonymously. So IMAP is generally to be preferred.

Look for the “namespace inbox” section. If you already have emails stored on your server from previous versions of this ISPmail guide you need to change:

separator = .

here. By default the separator is “/” which creates a directory structure like “/var/vmail/example.org/john/Maildir/INBOX/staff/marketing/simon”. This is perfectly fine. But previous ISPmail guides used “.” as a separator so the mentioned folder would rather have been “/var/vmail/example.org/john/Maildir/.INBOX.staff.marketing.simon”.

Also edit the “mail_plugins” line to enable the quota plugin we will configure later and turn it into:

mail_plugins = quota

10-master.conf

This configuration file deals with services that allow communication with other processes. For example it enables or disables POP3 or IMAP. Don’t worry about the standard unencrypted TCP ports 110 (for POP3) and 143 (for IMAP). They can be kept accessible. If a user connects to these ports they will have to issue a STARTTLS command to switch into encrypted mode before they are allowed to send their password. There is basically no difference between using an plaintext port like 110 for POP3 and then using STARTTLS – or connecting to the encrypted 995 port for POP3S (=secure). See the Dovecot documentation for another explanation.

So most settings are sane here and do not have to be changed. However one change is required in the “service auth” section because we want Postfix to allow Dovecot as an authentication service. This is what has to be entered:

# Postfix smtp-auth
unix_listener /var/spool/postfix/private/auth {
  mode = 0660
  user = postfix
  group = postfix
}

Why that strange path? Well, Postfix runs in a chroot environment located at /var/spool/postfix. It can’t access anything outside of that directory. So to allow communication with Postfix we tell Dovecot to place a communication socket into that chroot.

10-ssl.conf

Earlier in this guide you created both a key and a certificate file to encrypt the communication with POP3, IMAPs and HTTPS between the users and your mail server. You need to tell Dovecot where to find these files:

ssl_cert = </etc/letsencrypt/live/webmail.example.org/fullchain.pem
ssl_key = </etc/letsencrypt/live/webmail.example.org/privkey.pem

And enforce TLS encryption by setting:

ssl = required

See the Dovecot documentation on SSL encryption for more information.

Next let’s take a look at how Dovecot knows about users and their passwords:

auth-sql.conf.ext

Dovecot reads the auth-sql.conf.ext which defines how to find user information in your database. Open the file. There are two sections:

  • userdb: where to find a user’s mailbox in the file system
  • passdb: where to find the user’s hashed password

By default Dovecot will run two queries at your database. One for the userdb that gets information like the user ID, group ID, home directory and quota. And another for the passdb that gets the hashed password.

The “userdb” section already reads:

userdb {
   driver = sql
   args = /etc/dovecot/dovecot-sql.conf.ext
}

Apparently Dovecot uses an SQL database lookup to get that information. And it refers to the dovecot-sql.conf.ext file for more information. Let’s see…

/etc/dovecot/dovecot-sql.conf.ext

You will find this file well documented although all configuration directives are commented out. Add these lines at the bottom of the file:

driver = mysql
connect = \
  host=127.0.0.1 \
  dbname=mailserver \
  user=mailserver \
  password=x893dNj4stkHy1MKQq0USWBaX4ZZdq
user_query = SELECT email as user, \
  concat('*:bytes=', quota) AS quota_rule, \
  '/var/vmail/%d/%n' AS home, \
  5000 AS uid, 5000 AS gid \
  FROM virtual_users WHERE email='%u'
password_query = SELECT password FROM virtual_users WHERE email='%u'
iterate_query = SELECT email AS user FROM virtual_users

Backslashes

Ending a line with a backslash (\) means that it is continued on the next line. It keeps the configuration more readable.

What these lines mean:

  • driver: the kind of database. MariaDB is the same kind as MySQL.
  • connect: where to find the MySQL database and how to access it (username, password)
  • user_query: an SQL query that returns the user name (=the email address), the quota, the home directory, user ID and group ID.
  • password_query: this SQL query just gets the password hash from the database
  • iterate_query: ‘doveadm’ uses this query to get a list of all users. That allows you to use the “doveadm user ‘*'” command later.

The user_query gets several pieces of information from the database. Let’s look at it one by one:

  • email AS user
    It gets the the email field from the database which corresponds to the user name. Dovecot expects it in the user field so we set an alias to “user”.
  • userdb_quota_rule
    This is the user’s quota in bytes. Think of it as the maximum possible space on disk that the user can occupy. As documented Dovecot expects the quota in a special format like “*:bytes=10000” if the user should not be able to store more than 10,000 bytes. That’s why we begin the string with ‘*:bytes=’.
  • userdb_home
    This leads to the directory where all emails and various control files for this user are located. The placeholder ‘%d’ replaces the domain and ‘%n’ the user part. So for John that makes it “/var/vmail/example.org/john”.
  • userdb_uid and userdb_gid
    Those are the user ID and group ID of vmail user – 5000 for both. Dovecot uses it to set the permissions of files it creates. As all users share the same system user “vmail” this is just a static number.s

Fix permissions

Make sure that only root can access the SQL configuration file so nobody else is reading your database access passwords:

chown root:root /etc/dovecot/dovecot-sql.conf.ext
chmod go= /etc/dovecot/dovecot-sql.conf.ext

Restart Dovecot from the shell:

systemctl restart dovecot

Look at your /var/log/mail.log logfile. You should see:

... Dovecot v2.3.13 (f79e8e7e4) starting up for imap, lmtp, sieve, pop3 (core dumps disabled)

If you get any error messages please double-check your configuration files.

2 thoughts on “Setting up Dovecot

  • 2021-11-24 at 12:08
    Permalink

    A tiny note:

    “userdb_quota_rule […]
    userdb_home[…]
    userdb_uid and userdb_gid[…]”

    It is possible to get password an user information in just one query using userdb prefetch (dovecot.conf). In this case the query has to return the described userdb fields.

    But the above method uses two queries (password_query and user_query), in that case the fields are just:
    quota_rule, home, uid, gid
    without any userdb prefix (anyway, the above query ist right).

    Reply
    • 2021-11-25 at 17:43
      Permalink

      Thanks for the comment. In fact I used to recommend prefetching several years ago. However there was a specific reason that made it fail. But I can’t find it in my notes any more. Have you tried prefetching and still got a working setup?

      Reply

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