Chapter 1. Configuring and maintaining a Dovecot IMAP and POP3 server

Dovecot is a high-performance mail delivery agent (MDA) with a focus on security. You can use IMAP or POP3-compatible email clients to connect to a Dovecot server and read or download emails.

Key features of Dovecot:

  • The design and implementation focuses on security
  • Two-way replication support for high availability to improve the performance in large environments
  • Supports the high-performance dbox mailbox format, but also mbox and Maildir for compatibility reasons
  • Self-healing features, such as fixing broken index files
  • Compliance with the IMAP standards
  • Workaround support to bypass bugs in IMAP and POP3 clients

1.1. Setting up a Dovecot server with PAM authentication

Dovecot supports the Name Service Switch (NSS) interface as a user database and the Pluggable Authentication Modules (PAM) framework as an authentication backend. With this configuration, Dovecot can provide services to users who are available locally on the server through NSS.

Use PAM authentication if accounts:

  • Are defined locally in the /etc/passwd file
  • Are stored in a remote database but they are available locally through the System Security Services Daemon (SSSD) or other NSS plugins.

1.1.1. Installing Dovecot

The dovecot package provides:

  • The dovecot service and the utilities to maintain it
  • Services that Dovecot starts on demand, such as for authentication
  • Plugins, such as server-side mail filtering
  • Configuration files in the /etc/dovecot/ directory
  • Documentation in the /usr/share/doc/dovecot/ directory

Procedure

  • Install the dovecot package:

    # dnf install dovecot
    Note

    If Dovecot is already installed and you require clean configuration files, rename or remove the /etc/dovecot/ directory. Afterwards, reinstall the package. Without removing the configuration files, the dnf reinstall dovecot command does not reset the configuration files in /etc/dovecot/.

1.1.2. Configuring TLS encryption on a Dovecot server

Dovecot provides a secure default configuration. For example, TLS is enabled by default to transmit credentials and data encrypted over networks. To configure TLS on a Dovecot server, you only need to set the paths to the certificate and private key files. Additionally, you can increase the security of TLS connections by generating and using Diffie-Hellman parameters to provide perfect forward secrecy (PFS).

Prerequisites

  • Dovecot is installed.
  • The following files have been copied to the listed locations on the server:

    • The server certificate: /etc/pki/dovecot/certs/server.example.com.crt
    • The private key: /etc/pki/dovecot/private/server.example.com.key
    • The Certificate Authority (CA) certificate: /etc/pki/dovecot/certs/ca.crt
  • The hostname in the Subject DN field of the server certificate matches the server’s Fully-qualified Domain Name (FQDN).

Procedure

  1. Set secure permissions on the private key file:

    # chown root:root /etc/pki/dovecot/private/server.example.com.key
    # chmod 600 /etc/pki/dovecot/private/server.example.com.key
  2. Generate a file with Diffie-Hellman parameters:

    # openssl dhparam -out /etc/dovecot/dh.pem 4096

    Depending on the hardware and entropy on the server, generating Diffie-Hellman parameters with 4096 bits can take several minutes.

  3. Set the paths to the certificate and private key files in the /etc/dovecot/conf.d/10-ssl.conf file:

    1. Update the ssl_cert and ssl_key parameters, and set them to use the paths of the server’s certificate and private key:

      ssl_cert = </etc/pki/dovecot/certs/server.example.com.crt
      ssl_key = </etc/pki/dovecot/private/server.example.com.key
    2. Uncomment the ssl_ca parameter, and set it to use the path to the CA certificate:

      ssl_ca = </etc/pki/dovecot/certs/ca.crt
    3. Uncomment the ssl_dh parameter, and set it to use the path to the Diffie-Hellman parameters file:

      ssl_dh = </etc/dovecot/dh.pem
    Important

    To ensure that Dovecot reads the value of a parameter from a file, the path must start with a leading < character.

Additional resources

  • /usr/share/doc/dovecot/wiki/SSL.DovecotConfiguration.txt

1.1.3. Preparing Dovecot to use virtual users

By default, Dovecot performs many actions on the file system as the user who uses the service. However, configuring the Dovecot back end to use one local user to perform these actions has several benefits:

  • Dovecot performs file system actions as a specific local user instead of using the user’s ID (UID).
  • Users do not need to be available locally on the server.
  • You can store all mailboxes and user-specific files in one root directory.
  • Users do not require a UID and group ID (GID), which reduces administration efforts.
  • Users who have access to the file system on the server cannot compromise their mailboxes or indexes because they cannot access these files.
  • Setting up replication is easier.

Prerequisites

  • Dovecot is installed.

Procedure

  1. Create the vmail user:

    # useradd --home-dir /var/mail/ --shell /usr/sbin/nologin vmail

    Dovecot will later use this user to manage the mailboxes. For security reasons, do not use the dovecot or dovenull system users for this purpose.

  2. If you use a different path than /var/mail/, set the mail_spool_t SELinux context on it, for example:

    # semanage fcontext -a -t mail_spool_t "<path>(/.*)?"
    # restorecon -Rv <path>
  3. Grant write permissions on /var/mail/ only to the vmail user:

    # chown vmail:vmail /var/mail/
    # chmod 700 /var/mail/
  4. Uncomment the mail_location parameter in the /etc/dovecot/conf.d/10-mail.conf file, and set it to the mailbox format and location:

    mail_location = sdbox:/var/mail/%n/

    With this setting:

    • Dovecot uses the high-performant dbox mailbox format in single mode. In this mode, the service stores each mail in a separate file, similar to the maildir format.
    • Dovecot resolves the %n variable in the path to the username. This is required to ensure that each user has a separate directory for its mailbox.

Additional resources

  • /usr/share/doc/dovecot/wiki/VirtualUsers.txt
  • /usr/share/doc/dovecot/wiki/MailLocation.txt
  • /usr/share/doc/dovecot/wiki/MailboxFormat.dbox.txt
  • /usr/share/doc/dovecot/wiki/Variables.txt

1.1.4. Using PAM as the Dovecot authentication backend

By default, Dovecot uses the Name Service Switch (NSS) interface as the user database and the Pluggable Authentication Modules (PAM) framework as the authentication backend.

Customize the settings to adapt Dovecot to your environment and to simplify administration by using the virtual users feature.

Prerequisites

  • Dovecot is installed.
  • The virtual users feature is configured.

Procedure

  1. Update the first_valid_uid parameter in the /etc/dovecot/conf.d/10-mail.conf file to define the lowest user ID (UID) that can authenticate to Dovecot:

    first_valid_uid = 1000

    By default, users with a UID greater than or equal to 1000 can authenticate. If required, you can also set the last_valid_uid parameter to define the highest UID that Dovecot allows to log in.

  2. In the /etc/dovecot/conf.d/auth-system.conf.ext file, add the override_fields parameter to the userdb section as follows:

    userdb {
      driver = passwd
      override_fields = uid=vmail gid=vmail home=/var/mail/%n/
    }

    Due to the fixed values, Dovecot does not query these settings from the /etc/passwd file. As a result, the home directory defined in /etc/passwd does not need to exist.

Additional resources

  • /usr/share/doc/dovecot/wiki/PasswordDatabase.PAM.txt
  • /usr/share/doc/dovecot/wiki/VirtualUsers.Home.txt

1.1.5. Completing the Dovecot configuration

Once you have installed and configured Dovecot, open the required ports in the firewalld service, and enable and start the service. Afterwards, you can test the server.

Prerequisites

  • The following has been configured in Dovecot:

    • TLS encryption
    • An authentication backend
  • Clients trust the Certificate Authority (CA) certificate.

Procedure

  1. If you want to provide only an IMAP or POP3 service to users, uncomment the protocols parameter in the /etc/dovecot/dovecot.conf file, and set it to the required protocols. For example, if you do not require POP3, set:

    protocols = imap lmtp

    By default, the imap, pop3, and lmtp protocols are enabled.

  2. Open the ports in the local firewall. For example, to open the ports for the IMAPS, IMAP, POP3S, and POP3 protocols, enter:

    # firewall-cmd --permanent --add-service=imaps --add-service=imap --add-service=pop3s --add-service=pop3
    # firewall-cmd --reload
  3. Enable and start the dovecot service:

    # systemctl enable --now dovecot

Verification

  1. Use a mail client, such as Mozilla Thunderbird, to connect to Dovecot and read emails. The settings for the mail client depend on the protocol you want to use:

    Table 1.1. Connection settings to the Dovecot server

    ProtocolPortConnection securityAuthentication method

    IMAP

    143

    STARTTLS

    PLAIN[a]

    IMAPS

    993

    SSL/TLS

    PLAIN[a]

    POP3

    110

    STARTTLS

    PLAIN[a]

    POP3S

    995

    SSL/TLS

    PLAIN[a]

    [a] The client transmits data encrypted through the TLS connection. Consequently, credentials are not disclosed.

    Note that this table does not list settings for unencrypted connections because, by default, Dovecot does not accept plain text authentication on connections without TLS.

  2. Display configuration settings with non-default values:

    # doveconf -n

Additional resources

  • firewall-cmd(1) man page

1.2. Setting up a Dovecot server with LDAP authentication

If your infrastructure uses an LDAP server to store accounts, you can authenticate Dovecot users against it. In this case, you manage accounts centrally in the directory and, users do not required local access to the file system on the Dovecot server.

Centrally-managed accounts are also a benefit if you plan to set up multiple Dovecot servers with replication to make your mailboxes high available.

1.2.1. Installing Dovecot

The dovecot package provides:

  • The dovecot service and the utilities to maintain it
  • Services that Dovecot starts on demand, such as for authentication
  • Plugins, such as server-side mail filtering
  • Configuration files in the /etc/dovecot/ directory
  • Documentation in the /usr/share/doc/dovecot/ directory

Procedure

  • Install the dovecot package:

    # dnf install dovecot
    Note

    If Dovecot is already installed and you require clean configuration files, rename or remove the /etc/dovecot/ directory. Afterwards, reinstall the package. Without removing the configuration files, the dnf reinstall dovecot command does not reset the configuration files in /etc/dovecot/.

1.2.2. Configuring TLS encryption on a Dovecot server

Dovecot provides a secure default configuration. For example, TLS is enabled by default to transmit credentials and data encrypted over networks. To configure TLS on a Dovecot server, you only need to set the paths to the certificate and private key files. Additionally, you can increase the security of TLS connections by generating and using Diffie-Hellman parameters to provide perfect forward secrecy (PFS).

Prerequisites

  • Dovecot is installed.
  • The following files have been copied to the listed locations on the server:

    • The server certificate: /etc/pki/dovecot/certs/server.example.com.crt
    • The private key: /etc/pki/dovecot/private/server.example.com.key
    • The Certificate Authority (CA) certificate: /etc/pki/dovecot/certs/ca.crt
  • The hostname in the Subject DN field of the server certificate matches the server’s Fully-qualified Domain Name (FQDN).

Procedure

  1. Set secure permissions on the private key file:

    # chown root:root /etc/pki/dovecot/private/server.example.com.key
    # chmod 600 /etc/pki/dovecot/private/server.example.com.key
  2. Generate a file with Diffie-Hellman parameters:

    # openssl dhparam -out /etc/dovecot/dh.pem 4096

    Depending on the hardware and entropy on the server, generating Diffie-Hellman parameters with 4096 bits can take several minutes.

  3. Set the paths to the certificate and private key files in the /etc/dovecot/conf.d/10-ssl.conf file:

    1. Update the ssl_cert and ssl_key parameters, and set them to use the paths of the server’s certificate and private key:

      ssl_cert = </etc/pki/dovecot/certs/server.example.com.crt
      ssl_key = </etc/pki/dovecot/private/server.example.com.key
    2. Uncomment the ssl_ca parameter, and set it to use the path to the CA certificate:

      ssl_ca = </etc/pki/dovecot/certs/ca.crt
    3. Uncomment the ssl_dh parameter, and set it to use the path to the Diffie-Hellman parameters file:

      ssl_dh = </etc/dovecot/dh.pem
    Important

    To ensure that Dovecot reads the value of a parameter from a file, the path must start with a leading < character.

Additional resources

  • /usr/share/doc/dovecot/wiki/SSL.DovecotConfiguration.txt

1.2.3. Preparing Dovecot to use virtual users

By default, Dovecot performs many actions on the file system as the user who uses the service. However, configuring the Dovecot back end to use one local user to perform these actions has several benefits:

  • Dovecot performs file system actions as a specific local user instead of using the user’s ID (UID).
  • Users do not need to be available locally on the server.
  • You can store all mailboxes and user-specific files in one root directory.
  • Users do not require a UID and group ID (GID), which reduces administration efforts.
  • Users who have access to the file system on the server cannot compromise their mailboxes or indexes because they cannot access these files.
  • Setting up replication is easier.

Prerequisites

  • Dovecot is installed.

Procedure

  1. Create the vmail user:

    # useradd --home-dir /var/mail/ --shell /usr/sbin/nologin vmail

    Dovecot will later use this user to manage the mailboxes. For security reasons, do not use the dovecot or dovenull system users for this purpose.

  2. If you use a different path than /var/mail/, set the mail_spool_t SELinux context on it, for example:

    # semanage fcontext -a -t mail_spool_t "<path>(/.*)?"
    # restorecon -Rv <path>
  3. Grant write permissions on /var/mail/ only to the vmail user:

    # chown vmail:vmail /var/mail/
    # chmod 700 /var/mail/
  4. Uncomment the mail_location parameter in the /etc/dovecot/conf.d/10-mail.conf file, and set it to the mailbox format and location:

    mail_location = sdbox:/var/mail/%n/

    With this setting:

    • Dovecot uses the high-performant dbox mailbox format in single mode. In this mode, the service stores each mail in a separate file, similar to the maildir format.
    • Dovecot resolves the %n variable in the path to the username. This is required to ensure that each user has a separate directory for its mailbox.

Additional resources

  • /usr/share/doc/dovecot/wiki/VirtualUsers.txt
  • /usr/share/doc/dovecot/wiki/MailLocation.txt
  • /usr/share/doc/dovecot/wiki/MailboxFormat.dbox.txt
  • /usr/share/doc/dovecot/wiki/Variables.txt

1.2.4. Using LDAP as the Dovecot authentication backend

Users in an LDAP directory can usually authenticate themselves to the directory service. Dovecot can use this to authenticate users when they log in to the IMAP and POP3 services. This authentication method has a number of benefits, such as:

  • Administrators can manage users centrally in the directory.
  • The LDAP accounts do not require any special attributes. They only need to be able to authenticate to the LDAP server. Consequently, this method is independent from the password storage scheme used on the LDAP server.
  • Users do not need to be available locally on the server through the Name Service Switch (NSS) interface and the Pluggable Authentication Modules (PAM) framework.

Prerequisites

  • Dovecot is installed.
  • The virtual users feature is configured.
  • Connections to the LDAP server support TLS encryption.
  • RHEL on the Dovecot server trusts the Certificate Authority (CA) certificate of the LDAP server.
  • If users are stored in different trees in the LDAP directory, a dedicated LDAP account for Dovecot exists to search the directory. This account requires permissions to search for Distinguished Names (DNs) of other users.

Procedure

  1. Configure the authentication backends in the /etc/dovecot/conf.d/10-auth.conf file:

    1. Comment out include statements for auth-*.conf.ext authentication backend configuration files that you do not require, for example:

      #!include auth-system.conf.ext
    2. Enable LDAP authentication by uncommenting the following line:

      !include auth-ldap.conf.ext
  2. Edit the /etc/dovecot/conf.d/auth-ldap.conf.ext file, and add the override_fields parameter as follows to the userdb section:

    userdb {
      driver = ldap
      args = /etc/dovecot/dovecot-ldap.conf.ext
      override_fields = uid=vmail gid=vmail home=/var/mail/%n/
    }

    Due to the fixed values, Dovecot does not query these settings from the LDAP server. Consequently, these attributes also do not have to be present.

  3. Create the /etc/dovecot/dovecot-ldap.conf.ext file with the following settings:

    1. Depending on the LDAP structure, configure one of the following:

      • If users are stored in different trees in the LDAP directory, configure dynamic DN lookups:

        dn = cn=dovecot_LDAP,dc=example,dc=com
        dnpass = password
        pass_filter = (&(objectClass=posixAccount)(uid=%n))

        Dovecot uses the specified DN, password, and filter to search the DN of the authenticating user in the directory. In this search, Dovecot replaces %n in the filter with the username. Note that the LDAP search must return only one result.

      • If all users are stored under a specific entry, configure a DN template:

        auth_bind_userdn = cn=%n,ou=People,dc=example,dc=com
    2. Enable authentication binds to the LDAP server to verify Dovecot users:

      auth_bind = yes
    3. Set the URL to the LDAP server:

      uris = ldaps://LDAP-srv.example.com

      For security reasons, only use encrypted connections using LDAPS or the STARTTLS command over the LDAP protocol. For the latter, additionally add tls = yes to the settings.

      For a working certificate validation, the hostname of the LDAP server must match the hostname used in its TLS certificate.

    4. Enable the verification of the LDAP server’s TLS certificate:

      tls_require_cert = hard
    5. Set the base DN to the DN where to start searching for users:

      base = ou=People,dc=example,dc=com
    6. Set the search scope:

      scope = onelevel

      Dovecot searches with the onelevel scope only in the specified base DN and with the subtree scope also in subtrees.

  4. Set secure permissions on the /etc/dovecot/dovecot-ldap.conf.ext file:

    # chown root:root /etc/dovecot/dovecot-ldap.conf.ext
    # chmod 600 /etc/dovecot/dovecot-ldap.conf.ext

Additional resources

  • /usr/share/doc/dovecot/example-config/dovecot-ldap.conf.ext
  • /usr/share/doc/dovecot/wiki/UserDatabase.Static.txt
  • /usr/share/doc/dovecot/wiki/AuthDatabase.LDAP.txt
  • /usr/share/doc/dovecot/wiki/AuthDatabase.LDAP.AuthBinds.txt
  • /usr/share/doc/dovecot/wiki/AuthDatabase.LDAP.PasswordLookups.txt

1.2.5. Completing the Dovecot configuration

Once you have installed and configured Dovecot, open the required ports in the firewalld service, and enable and start the service. Afterwards, you can test the server.

Prerequisites

  • The following has been configured in Dovecot:

    • TLS encryption
    • An authentication backend
  • Clients trust the Certificate Authority (CA) certificate.

Procedure

  1. If you want to provide only an IMAP or POP3 service to users, uncomment the protocols parameter in the /etc/dovecot/dovecot.conf file, and set it to the required protocols. For example, if you do not require POP3, set:

    protocols = imap lmtp

    By default, the imap, pop3, and lmtp protocols are enabled.

  2. Open the ports in the local firewall. For example, to open the ports for the IMAPS, IMAP, POP3S, and POP3 protocols, enter:

    # firewall-cmd --permanent --add-service=imaps --add-service=imap --add-service=pop3s --add-service=pop3
    # firewall-cmd --reload
  3. Enable and start the dovecot service:

    # systemctl enable --now dovecot

Verification

  1. Use a mail client, such as Mozilla Thunderbird, to connect to Dovecot and read emails. The settings for the mail client depend on the protocol you want to use:

    Table 1.2. Connection settings to the Dovecot server

    ProtocolPortConnection securityAuthentication method

    IMAP

    143

    STARTTLS

    PLAIN[a]

    IMAPS

    993

    SSL/TLS

    PLAIN[a]

    POP3

    110

    STARTTLS

    PLAIN[a]

    POP3S

    995

    SSL/TLS

    PLAIN[a]

    [a] The client transmits data encrypted through the TLS connection. Consequently, credentials are not disclosed.

    Note that this table does not list settings for unencrypted connections because, by default, Dovecot does not accept plain text authentication on connections without TLS.

  2. Display configuration settings with non-default values:

    # doveconf -n

Additional resources

  • firewall-cmd(1) man page

1.3. Setting up a Dovecot server with MariaDB SQL authentication

If you store users and passwords in a MariaDB SQL server, you can configure Dovecot to use it as the user database and authentication backend. With this configuration, you manage accounts centrally in a database, and users have no local access to the file system on the Dovecot server.

Centrally managed accounts are also a benefit if you plan to set up multiple Dovecot servers with replication to make your mailboxes highly available.

1.3.1. Installing Dovecot

The dovecot package provides:

  • The dovecot service and the utilities to maintain it
  • Services that Dovecot starts on demand, such as for authentication
  • Plugins, such as server-side mail filtering
  • Configuration files in the /etc/dovecot/ directory
  • Documentation in the /usr/share/doc/dovecot/ directory

Procedure

  • Install the dovecot package:

    # dnf install dovecot
    Note

    If Dovecot is already installed and you require clean configuration files, rename or remove the /etc/dovecot/ directory. Afterwards, reinstall the package. Without removing the configuration files, the dnf reinstall dovecot command does not reset the configuration files in /etc/dovecot/.

1.3.2. Configuring TLS encryption on a Dovecot server

Dovecot provides a secure default configuration. For example, TLS is enabled by default to transmit credentials and data encrypted over networks. To configure TLS on a Dovecot server, you only need to set the paths to the certificate and private key files. Additionally, you can increase the security of TLS connections by generating and using Diffie-Hellman parameters to provide perfect forward secrecy (PFS).

Prerequisites

  • Dovecot is installed.
  • The following files have been copied to the listed locations on the server:

    • The server certificate: /etc/pki/dovecot/certs/server.example.com.crt
    • The private key: /etc/pki/dovecot/private/server.example.com.key
    • The Certificate Authority (CA) certificate: /etc/pki/dovecot/certs/ca.crt
  • The hostname in the Subject DN field of the server certificate matches the server’s Fully-qualified Domain Name (FQDN).

Procedure

  1. Set secure permissions on the private key file:

    # chown root:root /etc/pki/dovecot/private/server.example.com.key
    # chmod 600 /etc/pki/dovecot/private/server.example.com.key
  2. Generate a file with Diffie-Hellman parameters:

    # openssl dhparam -out /etc/dovecot/dh.pem 4096

    Depending on the hardware and entropy on the server, generating Diffie-Hellman parameters with 4096 bits can take several minutes.

  3. Set the paths to the certificate and private key files in the /etc/dovecot/conf.d/10-ssl.conf file:

    1. Update the ssl_cert and ssl_key parameters, and set them to use the paths of the server’s certificate and private key:

      ssl_cert = </etc/pki/dovecot/certs/server.example.com.crt
      ssl_key = </etc/pki/dovecot/private/server.example.com.key
    2. Uncomment the ssl_ca parameter, and set it to use the path to the CA certificate:

      ssl_ca = </etc/pki/dovecot/certs/ca.crt
    3. Uncomment the ssl_dh parameter, and set it to use the path to the Diffie-Hellman parameters file:

      ssl_dh = </etc/dovecot/dh.pem
    Important

    To ensure that Dovecot reads the value of a parameter from a file, the path must start with a leading < character.

Additional resources

  • /usr/share/doc/dovecot/wiki/SSL.DovecotConfiguration.txt

1.3.3. Preparing Dovecot to use virtual users

By default, Dovecot performs many actions on the file system as the user who uses the service. However, configuring the Dovecot back end to use one local user to perform these actions has several benefits:

  • Dovecot performs file system actions as a specific local user instead of using the user’s ID (UID).
  • Users do not need to be available locally on the server.
  • You can store all mailboxes and user-specific files in one root directory.
  • Users do not require a UID and group ID (GID), which reduces administration efforts.
  • Users who have access to the file system on the server cannot compromise their mailboxes or indexes because they cannot access these files.
  • Setting up replication is easier.

Prerequisites

  • Dovecot is installed.

Procedure

  1. Create the vmail user:

    # useradd --home-dir /var/mail/ --shell /usr/sbin/nologin vmail

    Dovecot will later use this user to manage the mailboxes. For security reasons, do not use the dovecot or dovenull system users for this purpose.

  2. If you use a different path than /var/mail/, set the mail_spool_t SELinux context on it, for example:

    # semanage fcontext -a -t mail_spool_t "<path>(/.*)?"
    # restorecon -Rv <path>
  3. Grant write permissions on /var/mail/ only to the vmail user:

    # chown vmail:vmail /var/mail/
    # chmod 700 /var/mail/
  4. Uncomment the mail_location parameter in the /etc/dovecot/conf.d/10-mail.conf file, and set it to the mailbox format and location:

    mail_location = sdbox:/var/mail/%n/

    With this setting:

    • Dovecot uses the high-performant dbox mailbox format in single mode. In this mode, the service stores each mail in a separate file, similar to the maildir format.
    • Dovecot resolves the %n variable in the path to the username. This is required to ensure that each user has a separate directory for its mailbox.

Additional resources

  • /usr/share/doc/dovecot/wiki/VirtualUsers.txt
  • /usr/share/doc/dovecot/wiki/MailLocation.txt
  • /usr/share/doc/dovecot/wiki/MailboxFormat.dbox.txt
  • /usr/share/doc/dovecot/wiki/Variables.txt

1.3.4. Using a MariaDB SQL database as the Dovecot authentication backend

Dovecot can read accounts and passwords from a MariaDB database and use it to authenticate users when they log in to the IMAP or POP3 service. The benefits of this authentication method include:

  • Administrators can manage users centrally in a database.
  • Users have no access locally on the server.

Prerequisites

  • Dovecot is installed.
  • The virtual users feature is configured.
  • Connections to the MariaDB server support TLS encryption.
  • The dovecotDB database exists in MariaDB, and the users table contains at least a username and password column.
  • The password column contains passwords encrypted with a scheme that Dovecot supports.
  • The passwords either use the same scheme or have a {pw-storage-scheme} prefix.
  • The dovecot MariaDB user has read permission on the users table in the dovecotDB database.
  • The certificate of the Certificate Authority (CA) that issued the MariaDB server’s TLS certificate is stored on the Dovecot server in the /etc/pki/tls/certs/ca.crt file.

Procedure

  1. Install the dovecot-mysql package:

    # dnf install dovecot-mysql
  2. Configure the authentication backends in the /etc/dovecot/conf.d/10-auth.conf file:

    1. Comment out include statements for auth-*.conf.ext authentication backend configuration files that you do not require, for example:

      #!include auth-system.conf.ext
    2. Enable SQL authentication by uncommenting the following line:

      !include auth-sql.conf.ext
  3. Edit the /etc/dovecot/conf.d/auth-sql.conf.ext file, and add the override_fields parameter to the userdb section as follows:

    userdb {
      driver = sql
      args = /etc/dovecot/dovecot-sql.conf.ext
      override_fields = uid=vmail gid=vmail home=/var/mail/%n/
    }

    Due to the fixed values, Dovecot does not query these settings from the SQL server.

  4. Create the /etc/dovecot/dovecot-sql.conf.ext file with the following settings:

    driver = mysql
    connect = host=mariadb_srv.example.com dbname=dovecotDB user=dovecot password=dovecotPW ssl_ca=/etc/pki/tls/certs/ca.crt
    default_pass_scheme = SHA512-CRYPT
    user_query = SELECT username FROM users WHERE username='%u';
    password_query = SELECT username AS user, password FROM users WHERE username='%u';
    iterate_query = SELECT username FROM users;

    To use TLS encryption to the database server, set the ssl_ca option to the path of the certificate of the CA that issued the MariaDB server certificate. For a working certificate validation, the hostname of the MariaDB server must match the hostname used in its TLS certificate.

    If the password values in the database contain a {pw-storage-scheme} prefix, you can omit the default_pass_scheme setting.

    The queries in the file must be set as follows:

    • For the user_query parameter, the query must return the username of the Dovecot user. The query must also return only one result.
    • For the password_query parameter, the query must return the username and the password, and Dovecot must use these values in the user and password variables. Therefore, if the database uses different column names, use the AS SQL command to rename a column in the result.
    • For the iterate_query parameter, the query must return a list of all users.
  5. Set secure permissions on the /etc/dovecot/dovecot-sql.conf.ext file:

    # chown root:root /etc/dovecot/dovecot-sql.conf.ext
    # chmod 600 /etc/dovecot/dovecot-sql.conf.ext

Additional resources

  • /usr/share/doc/dovecot/example-config/dovecot-sql.conf.ext
  • /usr/share/doc/dovecot/wiki/Authentication.PasswordSchemes.txt

1.3.5. Completing the Dovecot configuration

Once you have installed and configured Dovecot, open the required ports in the firewalld service, and enable and start the service. Afterwards, you can test the server.

Prerequisites

  • The following has been configured in Dovecot:

    • TLS encryption
    • An authentication backend
  • Clients trust the Certificate Authority (CA) certificate.

Procedure

  1. If you want to provide only an IMAP or POP3 service to users, uncomment the protocols parameter in the /etc/dovecot/dovecot.conf file, and set it to the required protocols. For example, if you do not require POP3, set:

    protocols = imap lmtp

    By default, the imap, pop3, and lmtp protocols are enabled.

  2. Open the ports in the local firewall. For example, to open the ports for the IMAPS, IMAP, POP3S, and POP3 protocols, enter:

    # firewall-cmd --permanent --add-service=imaps --add-service=imap --add-service=pop3s --add-service=pop3
    # firewall-cmd --reload
  3. Enable and start the dovecot service:

    # systemctl enable --now dovecot

Verification

  1. Use a mail client, such as Mozilla Thunderbird, to connect to Dovecot and read emails. The settings for the mail client depend on the protocol you want to use:

    Table 1.3. Connection settings to the Dovecot server

    ProtocolPortConnection securityAuthentication method

    IMAP

    143

    STARTTLS

    PLAIN[a]

    IMAPS

    993

    SSL/TLS

    PLAIN[a]

    POP3

    110

    STARTTLS

    PLAIN[a]

    POP3S

    995

    SSL/TLS

    PLAIN[a]

    [a] The client transmits data encrypted through the TLS connection. Consequently, credentials are not disclosed.

    Note that this table does not list settings for unencrypted connections because, by default, Dovecot does not accept plain text authentication on connections without TLS.

  2. Display configuration settings with non-default values:

    # doveconf -n

Additional resources

  • firewall-cmd(1) man page

1.4. Configuring replication between two Dovecot servers

With two-way replication, you can make your Dovecot server high-available, and IMAP and POP3 clients can access a mailbox on both servers. Dovecot keeps track of changes in the index logs of each mailbox and solves conflicts in a safe way.

Perform this procedure on both replication partners.

Note

Replication works only between server pairs. Consequently, in a large cluster, you need multiple independent backend pairs.

Prerequisites

  • Both servers use the same authentication backend. Preferably, use LDAP or SQL to maintain accounts centrally.
  • The Dovecot user database configuration supports user listing. Use the doveadm user '*' command to verify this.
  • Dovecot accesses mailboxes on the file system as the vmail user instead of the user’s ID (UID).

Procedure

  1. Create the /etc/dovecot/conf.d/10-replication.conf file and perform the following steps in it:

    1. Enable the notify and replication plug-ins:

      mail_plugins = $mail_plugins notify replication
    2. Add a service replicator section:

      service replicator {
        process_min_avail = 1
      
        unix_listener replicator-doveadm {
          mode = 0600
          user = vmail
        }
      }

      With these settings, Dovecot starts at least one replicator process when the dovecot service starts. Additionally, this section defines the settings on the replicator-doveadm socket.

    3. Add a service aggregator section to configure the replication-notify-fifo pipe and replication-notify socket:

      service aggregator {
        fifo_listener replication-notify-fifo {
          user = vmail
        }
        unix_listener replication-notify {
          user = vmail
        }
      }
    4. Add a service doveadm section to define the port of the replication service:

      service doveadm {
        inet_listener {
          port = 12345
        }
      }
    5. Set the password of the doveadm replication service:

      doveadm_password = replication_password

      The password must be the same on both servers.

    6. Configure the replication partner:

      plugin {
        mail_replica = tcp:server2.example.com:12345
      }
    7. Optional: Define the maximum number of parallel dsync processes:

      replication_max_conns = 20

      The default value of replication_max_conns is 10.

  2. Set secure permissions on the /etc/dovecot/conf.d/10-replication.conf file:

    # chown root:root /etc/dovecot/conf.d/10-replication.conf
    # chmod 600 /etc/dovecot/conf.d/10-replication.conf
  3. Enable the nis_enabled SELinux Boolean to allow Dovecot to open the doveadm replication port:

    setsebool -P nis_enabled on
  4. Configure firewalld rules to allow only the replication partner to access the replication port, for example:

    # firewall-cmd --permanent --zone=public --add-rich-rule="rule family="ipv4" source address="192.0.2.1/32" port protocol="tcp" port="12345" accept"
    # firewall-cmd --permanent --zone=public --add-rich-rule="rule family="ipv6" source address="2001:db8:2::1/128" port protocol="tcp" port="12345" accept"
    # firewall-cmd --reload

    The subnet masks /32 for the IPv4 and /128 for the IPv6 address limit the access to the specified addresses.

  5. Perform this procedure also on the other replication partner.
  6. Reload Dovecot:

    # systemctl reload dovecot

Verification

  1. Perform an action in a mailbox on one server and then verify if Dovecot has replicated the change to the other server.
  2. Display the replicator status:

    # doveadm replicator status
    Queued 'sync' requests        0
    Queued 'high' requests        0
    Queued 'low' requests         0
    Queued 'failed' requests      0
    Queued 'full resync' requests 30
    Waiting 'failed' requests     0
    Total number of known users   75
  3. Display the replicator status of a specific user:

    # doveadm replicator status example_user
    username        priority  fast sync  full sync  success sync  failed
    example_user    none      02:05:28   04:19:07   02:05:28      -

Additional resources

  • dsync(1) man page
  • /usr/share/doc/dovecot/wiki/Replication.txt

1.5. Automatically subscribing users to IMAP mailboxes

Typically, IMAP server administrators want Dovecot to automatically create certain mailboxes, such as Sent and Trash, and subscribe the users to them. You can set this in the configuration files.

Additionally, you can define special-use mailboxes. IMAP clients often support defining mailboxes for special purposes, such as for sent emails. To avoid that the user has to manually select and set the correct mailboxes, IMAP servers can send a special-use attribute in the IMAP LIST command. Clients can then use this attribute to identify and set, for example, the mailbox for sent emails.

Prerequisites

  • Dovecot is configured.

Procedure

  1. Update the inbox namespace section in the /etc/dovecot/conf.d/15-mailboxes.conf file:

    1. Add the auto = subscribe setting to each special-use mailbox that should be available to users, for example:

      namespace inbox {
        ...
        mailbox Drafts {
          special_use = \Drafts
          auto = subscribe
        }
      
        mailbox Junk {
          special_use = \Junk
          auto = subscribe
        }
      
        mailbox Trash {
          special_use = \Trash
          auto = subscribe
        }
      
        mailbox Sent {
          special_use = \Sent
          auto = subscribe
        }
        ...
      }

      If your mail clients support more special-use mailboxes, you can add similar entries. The special_use parameter defines the value that Dovecot sends in the special-use attribute to the clients.

    2. Optional: If you want to define other mailboxes that have no special purpose, add mailbox sections for them in the user’s inbox, for example:

      namespace inbox {
        ...
        mailbox "Important Emails" {
          auto = <value>
        }
        ...
      }

      You can set the auto parameter to one of the following values:

      • subscribe: Automatically creates the mailbox and subscribes the user to it.
      • create: Automatically creates the mailbox without subscribing the user to it.
      • no (default): Dovecot neither creates the mailbox nor does it subscribe the user to it.
  2. Reload Dovecot:

    # systemctl reload dovecot

Verification

  • Use an IMAP client and access your mailbox.

    Mailboxes with the setting auto = subscribe are automatically visible. If the client supports special-use mailboxes and the defined purposes, the client automatically uses them.

Additional resources

1.6. Configuring an LMTP socket and LMTPS listener

SMTP servers, such as Postfix, use the Local Mail Transfer Protocol (LMTP) to deliver emails to Dovecot. If the SMTP server runs:

  • On the same host as Dovecot, use an LMTP socket
  • On a different host, use an LMTP service

    By default, the LMTP protocol is not encrypted. However, if you configured TLS encryption, Dovecot uses the same settings automatically for the LMTP service. SMTP servers can then connect to it using the LMTPS protocol or the STARTTLS command over LMTP.

Prerequisites

  • Dovecot is installed.
  • If you want to configure an LMTP service, TLS encryption is configured in Dovecot.

Procedure

  1. Verify that the LMTP protocol is enabled:

    # doveconf -a | egrep "^protocols"
    protocols = imap pop3 lmtp

    The protocol is enabled, if the output contains lmtp.

  2. If the lmtp protocol is disabled, edit the /etc/dovecot/dovecot.conf file, and append lmtp to the values in the protocols parameter:

    protocols = ... lmtp
  3. Depending on whether you need an LMTP socket or service, make the following changes in the service lmtp section in the /etc/dovecot/conf.d/10-master.conf file:

    • LMTP socket: By default, Dovecot automatically creates the /var/run/dovecot/lmtp socket.

      Optional: Customize the ownership and permissions:

      service lmtp {
        ...
        unix_listener lmtp {
          mode = 0600
          user = postfix
          group = postfix
        }
        ...
      }
    • LMTP service: Add a inet_listener sub-section:

      service lmtp {
        ...
        inet_listener lmtp {
          port = 24
        }
        ...
      }
  4. Configure firewalld rules to allow only the SMTP server to access the LMTP port, for example:

    # firewall-cmd --permanent --zone=public --add-rich-rule="rule family="ipv4" source address="192.0.2.1/32" port protocol="tcp" port="24" accept"
    # firewall-cmd --permanent --zone=public --add-rich-rule="rule family="ipv6" source address="2001:db8:2::1/128" port protocol="tcp" port="24" accept"
    # firewall-cmd --reload

    The subnet masks /32 for the IPv4 and /128 for the IPv6 address limit the access to the specified addresses.

  5. Reload Dovecot:

    # systemctl reload dovecot

Verification

  1. If you configured the LMTP socket, verify that Dovecot has created the socket and that the permissions are correct:

    # ls -l /var/run/dovecot/lmtp
    srw-------. 1 postfix postfix 0 Nov 22 17:17 /var/run/dovecot/lmtp
  2. Configure the SMTP server to submit emails to Dovecot using the LMTP socket or service.

    When you use the LMTP service, ensure that the SMTP server uses the LMTPS protocol or sends the STARTTLS command to use an encrypted connection.

Additional resources

  • /usr/share/doc/dovecot/wiki/LMTP.txt

1.7. Disabling the IMAP or POP3 service in Dovecot

By default, Dovecot provides IMAP and POP3 services. If you require only one of them, you can disable the other to reduce the surface for attack.

Prerequisites

  • Dovecot is installed.

Procedure

  1. Uncomment the protocols parameter in the /etc/dovecot/dovecot.conf file, and set it to use the required protocols. For example, if you do not require POP3, set:

    protocols = imap lmtp

    By default, the imap, pop3, and lmtp protocols are enabled.

  2. Reload Dovecot:

    # systemctl reload dovecot
  3. Close the ports that are no longer required in the local firewall. For example, to close the ports for the POP3S and POP3 protocols, enter:

    # firewall-cmd --remove-service=pop3s --remove-service=pop3
    # firewall-cmd --reload

Verification

  • Display all ports in LISTEN mode opened by the dovecot process:

    # ss -tulp | grep dovecot
    tcp  LISTEN 0  100  0.0.0.0:993  0.0.0.0:*  users:(("dovecot",pid=1405,fd=44))
    tcp  LISTEN 0  100  0.0.0.0:143  0.0.0.0:*  users:(("dovecot",pid=1405,fd=42))
    tcp  LISTEN 0  100     [::]:993     [::]:*  users:(("dovecot",pid=1405,fd=45))
    tcp  LISTEN 0  100     [::]:143     [::]:*  users:(("dovecot",pid=1405,fd=43))

    In this example, Dovecot listens only on the TCP ports 993 (IMAPS) and 143 (IMAP).

    Note that Dovecot only opens a port for the LMTP protocol if you configure the service to listen on a port instead of using a socket.

Additional resources

  • firewall-cmd(1) man page

1.8. Enabling server-side email filtering using Sieve on a Dovecot IMAP server

You can upload Sieve scripts to a server using the ManageSieve protocol. Sieve scripts define rules and actions that a server should validate and perform on incoming emails. For example, users can use Sieve to forward emails from a specific sender, and administrators can create a global filter to move mails flagged by a spam filter into a separate IMAP folder.

The ManageSieve plugin adds support for Sieve scripts and the ManageSieve protocol to a Dovecot IMAP server.

Warning

Use only clients that support using the ManageSieve protocol over TLS connections. Disabling TLS for this protocol causes clients to send credentials in plain text over the network.

Prerequisites

  • Dovecot is configured and provides IMAP mailboxes.
  • TLS encryption is configured in Dovecot.
  • The mail clients support the ManageSieve protocol over TLS connections.

Procedure

  1. Install the dovecot-pigeonhole package:

    # dnf install dovecot-pigeonhole
  2. Uncomment the following line in /etc/dovecot/conf.d/20-managesieve.conf to enable the sieve protocol:

    protocols = $protocols sieve

    This setting activates Sieve in addition to the other protocols that are already enabled.

  3. Open the ManageSieve port in firewalld:

    # firewall-cmd --permanent --add-service=managesieve
    # firewall-cmd --reload
  4. Reload Dovecot:

    # systemctl reload dovecot

Verification

  1. Use a client and upload a Sieve script. Use the following connection settings:

    • Port: 4190
    • Connection security: SSL/TLS
    • Authentication method: PLAIN
  2. Send an email to the user who has the Sieve script uploaded. If the email matches the rules in the script, verify that the server performs the defined actions.

Additional resources

  • /usr/share/doc/dovecot/wiki/Pigeonhole.Sieve.Plugins.IMAPSieve.txt
  • /usr/share/doc/dovecot/wiki/Pigeonhole.Sieve.Troubleshooting.txt
  • firewall-cmd(1) man page

1.9. How Dovecot processes configuration files

The dovecot package provides the main configuration file /etc/dovecot/dovecot.conf and multiple configuration files in the /etc/dovecot/conf.d/ directory. Dovecot combines the files to build the configuration when you start the service.

The main benefit of multiple config files is to group settings and increase readability. If you prefer a single configuration file, you can instead maintain all settings in /etc/dovecot/dovecot.conf and remove all include and include_try statements from that file.

Additional resources

  • /usr/share/doc/dovecot/wiki/ConfigFile.txt
  • /usr/share/doc/dovecot/wiki/Variables.txt