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LDAP Authentication

Introduction

Getting started with ldap and docker-mailserver we need to take 3 parts in account:

  • postfix for incoming & outgoing email
  • dovecot for accessing mailboxes
  • saslauthd for SMTP authentication (this can also be delegated to dovecot)

Variables to Control Provisioning by the Container

Have a look at the ENV page for information on the default values.

LDAP_QUERY_FILTER_*

Those variables contain the LDAP lookup filters for postfix, using %s as the placeholder for the domain or email address in question. This means that...

  • ...for incoming email, the domain must return an entry for the DOMAIN filter (see virtual_alias_domains).
  • ...for incoming email, the inboxes which receive the email are chosen by the USER, ALIAS and GROUP filters.
    • The USER filter specifies personal mailboxes, for which only one should exist per address, for example (mail=%s) (also see virtual_mailbox_maps)
    • The ALIAS filter specifies aliases for mailboxes, using virtual_alias_maps, for example (mailAlias=%s)
    • The GROUP filter specifies the personal mailboxes in a group (for emails that multiple people shall receive), using virtual_alias_maps, for example (mailGroupMember=%s).
    • Technically, there is no difference between ALIAS and GROUP, but ideally you should use ALIAS for personal aliases for a singular person (like ceo@example.org) and GROUP for multiple people (like hr@example.org).
  • ...for outgoing email, the sender address is put through the SENDERS filter, and only if the authenticated user is one of the returned entries, the email can be sent.
    • This only applies if SPOOF_PROTECTION=1.
    • If the SENDERS filter is missing, the USER, ALIAS and GROUP filters will be used in in a disjunction (OR).
    • To for example allow users from the admin group to spoof any sender email address, and to force everyone else to only use their personal mailbox address for outgoing email, you can use something like this: (|(memberOf=cn=admin,*)(mail=%s))
Example

A really simple LDAP_QUERY_FILTER configuration, using only the user filter and allowing only admin@* to spoof any sender addresses.

- ENABLE_LDAP=1 # with the :edge tag, use ACCOUNT_PROVISIONER
- LDAP_START_TLS=yes
- ACCOUNT_PROVISIONER=LDAP
- LDAP_SERVER_HOST=ldap.example.org
- LDAP_SEARCH_BASE=dc=example,dc=org"
- LDAP_BIND_DN=cn=admin,dc=example,dc=org
- LDAP_BIND_PW=mypassword
- SPOOF_PROTECTION=1

- LDAP_QUERY_FILTER_DOMAIN=(mail=*@%s)
- LDAP_QUERY_FILTER_USER=(mail=%s)
- LDAP_QUERY_FILTER_ALIAS=(|) # doesn't match anything
- LDAP_QUERY_FILTER_GROUP=(|) # doesn't match anything
- LDAP_QUERY_FILTER_SENDERS=(|(mail=%s)(mail=admin@*))

DOVECOT_*_FILTER & DOVECOT_*_ATTRS

These variables specify the LDAP filters that dovecot uses to determine if a user can log in to their IMAP account, and which mailbox is responsible to receive email for a specific postfix user.

This is split into the following two lookups, both using %u as the placeholder for the full login name (see dovecot documentation for a full list of placeholders). Usually you only need to set DOVECOT_USER_FILTER, in which case it will be used for both filters.

  • DOVECOT_USER_FILTER is used to get the account details (uid, gid, home directory, quota, ...) of a user.
  • DOVECOT_PASS_FILTER is used to get the password information of the user, and is in pretty much all cases identical to DOVECOT_USER_FILTER (which is the default behaviour if left away).

If your directory doesn't have the postfix-book schema installed, then you must change the internal attribute handling for dovecot. For this you have to change the pass_attr and the user_attr mapping, as shown in the example below:

- DOVECOT_PASS_ATTRS=<YOUR_USER_IDENTIFIER_ATTRIBUTE>=user,<YOUR_USER_PASSWORD_ATTRIBUTE>=password
- DOVECOT_USER_ATTRS=<YOUR_USER_HOME_DIRECTORY_ATTRIBUTE>=home,<YOUR_USER_MAILSTORE_ATTRIBUTE>=mail,<YOUR_USER_MAIL_UID_ATTRIBUTE>=uid,<YOUR_USER_MAIL_GID_ATTRIBUTE>=gid

Note

For DOVECOT_*_ATTRS, you can replace ldapAttr=dovecotAttr with =dovecotAttr=%{ldap:ldapAttr} for more flexibility, like for example =home=/var/mail/%{ldap:uid} or just =uid=5000.

A list of dovecot attributes can be found in the dovecot documentation.

Defaults 
- DOVECOT_USER_ATTRS=mailHomeDirectory=home,mailUidNumber=uid,mailGidNumber=gid,mailStorageDirectory=mail
- DOVECOT_PASS_ATTRS=uniqueIdentifier=user,userPassword=password
- DOVECOT_USER_FILTER=(&(objectClass=PostfixBookMailAccount)(uniqueIdentifier=%n))
Example

Setup for a directory that has the qmail-schema installed and uses uid:

- DOVECOT_PASS_ATTRS=uid=user,userPassword=password
- DOVECOT_USER_ATTRS=homeDirectory=home,qmailUID=uid,qmailGID=gid,mailMessageStore=mail
- DOVECOT_USER_FILTER=(&(objectClass=qmailUser)(uid=%u)(accountStatus=active))

The LDAP server configuration for dovecot will be taken mostly from postfix, other options can be found in the environment section in the docs.

DOVECOT_AUTH_BIND

Set this to yes to enable authentication binds (more details in the dovecot documentation). Currently, only DN lookup is supported without further changes to the configuration files, so this is only useful when you want to bind as a readonly user without the permission to read passwords.

SASLAUTHD_LDAP_FILTER

This filter is used for saslauthd, which is called by postfix when someone is authenticating through SMTP (assuming that SASLAUTHD_MECHANISMS=ldap is being used). Note that you'll need to set up the LDAP server for saslauthd separately from postfix.

The filter variables are explained in detail in the LDAP_SASLAUTHD file, but unfortunately, this method doesn't really support domains right now - that means that %U is the only token that makes sense in this variable.

When to use this and how to avoid it

Using a separate filter for SMTP authentication allows you to for example allow noreply@example.org to send email, but not log in to IMAP or receive email: (&(mail=%U@example.org)(|(memberOf=cn=email,*)(mail=noreply@example.org)))

If you don't want to use a separate filter for SMTP authentication, you can set SASLAUTHD_MECHANISMS=rimap and SASLAUTHD_MECH_OPTIONS=127.0.0.1 to authenticate against dovecot instead - this means that the DOVECOT_USER_FILTER and DOVECOT_PASS_FILTER will be used for SMTP authentication as well.

Configure LDAP with saslauthd 
- ENABLE_SASLAUTHD=1
- SASLAUTHD_MECHANISMS=ldap
- SASLAUTHD_LDAP_FILTER=(mail=%U@example.org)

Secure Connection with LDAPS or StartTLS

To enable LDAPS, all you need to do is to add the protocol to LDAP_SERVER_HOST, for example ldaps://example.org:636.

To enable LDAP over StartTLS (on port 389), you need to set the following environment variables instead (the protocol must not be ldaps:// in this case!):

- LDAP_START_TLS=yes
- DOVECOT_TLS=yes
- SASLAUTHD_LDAP_START_TLS=yes

Active Directory Configurations (Tested with Samba4 AD Implementation)

In addition to LDAP explanation above, when Docker Mailserver is intended to be used with Active Directory (or the equivelant implementations like Samba4 AD DC) the following points should be taken into consideration:

  • Samba4 Active Directory requires a secure connection to the domain controller (DC), either via SSL/TLS (LDAPS) or via StartTLS.
  • The username equivalent in Active Directory is: sAMAccountName.
  • proxyAddresses can be used to store email aliases of single users. The convention is to prefix the email aliases with smtp: (e.g: smtp:some.name@example.com).
  • Active Directory is used typically not only as LDAP Directory storage, but also as a domain controller, i.e., it will do many things including authenticating users. Mixing Linux and Windows clients requires the usage of RFC2307 attributes, namely uidNumber, gidNumber instead of the typical uid. Assigning different owner to email folders can also be done in this approach, nevertheless there is a bug at the moment in Docker Mailserver that overwrites all permissions when starting the container. Either a manual fix is necessary now, or a temporary workaround to use a hard-coded ldap:uidNumber that equals to 5000 until this issue is fixed.
  • To deliver the emails to different members of Active Directory Security Group or Distribution Group (similar to mailing lists), use a user-patches.sh script to modify ldap-groups.cf so that it includes leaf_result_attribute = mail and special_result_attribute = member. This can be achieved simply by:

The configuration shown to get the Group to work is from here and here.

# user-patches.sh

...
grep -q '^leaf_result_attribute = mail$' /etc/postfix/ldap-groups.cf || echo "leaf_result_attribute = mail" >> /etc/postfix/ldap-groups.cf
grep -q '^special_result_attribute = member$' /etc/postfix/ldap-groups.cf || echo "special_result_attribute = member" >> /etc/postfix/ldap-groups.cf
...
  • In /etc/ldap/ldap.conf, if the TLS_REQCERT is demand / hard (default), the CA certificate used to verify the LDAP server certificate must be recognized as a trusted CA. This can be done by volume mounting the ca.crt file and updating the trust store via a user-patches.sh script:
# user-patches.sh

...
cp /MOUNTED_FOLDER/ca.crt /usr/local/share/ca-certificates/
update-ca-certificates
...

The changes on the configurations necessary to work with Active Directory (only changes are listed, the rest of the LDAP configuration can be taken from the other examples shown in this documentation):

# If StartTLS is the chosen method to establish a secure connection with Active Directory.
- LDAP_START_TLS=yes
- SASLAUTHD_LDAP_START_TLS=yes
- DOVECOT_TLS=yes

- LDAP_QUERY_FILTER_USER=(&(objectclass=person)(mail=%s))
- LDAP_QUERY_FILTER_ALIAS=(&(objectclass=person)(proxyAddresses=smtp:%s))
# Filters Active Directory groups (mail lists). Additional changes on ldap-groups.cf are also required as shown above.
- LDAP_QUERY_FILTER_GROUP=(&(objectClass=group)(mail=%s))
- LDAP_QUERY_FILTER_DOMAIN=(mail=*@%s)
# Allows only Domain admins to send any sender email address, otherwise the sender address must match the LDAP attribute `mail`.
- SPOOF_PROTECTION=1
- LDAP_QUERY_FILTER_SENDERS=(|(mail=%s)(proxyAddresses=smtp:%s)(memberOf=cn=Domain Admins,cn=Users,dc=*))

- DOVECOT_USER_FILTER=(&(objectclass=person)(sAMAccountName=%n))
# At the moment to be able to use %{ldap:uidNumber}, a manual bug fix as described above must be used. Otherwise %{ldap:uidNumber} %{ldap:uidNumber} must be replaced by the hard-coded value 5000.
- DOVECOT_USER_ATTRS==uid=%{ldap:uidNumber},=gid=5000,=home=/var/mail/%Ln,=mail=maildir:~/Maildir
- DOVECOT_PASS_ATTRS=sAMAccountName=user,userPassword=password
- SASLAUTHD_LDAP_FILTER=(&(sAMAccountName=%U)(objectClass=person))

LDAP Setup Examples

Basic Setup 
version: '3.8'
services:
  mailserver:
    image: docker.io/mailserver/docker-mailserver:latest
    container_name: mailserver
    hostname: mail
    domainname: example.com

    ports:
      - "25:25"
      - "143:143"
      - "587:587"
      - "993:993"

    volumes:
      - ./docker-data/dms/mail-data/:/var/mail/
      - ./docker-data/dms/mail-state/:/var/mail-state/
      - ./docker-data/dms/mail-logs/:/var/log/mail/
      - ./docker-data/dms/config/:/tmp/docker-mailserver/
      - /etc/localtime:/etc/localtime:ro

    environment:
      - ENABLE_SPAMASSASSIN=1
      - ENABLE_CLAMAV=1
      - ENABLE_FAIL2BAN=1
      - ENABLE_POSTGREY=1

      # >>> Postfix LDAP Integration
      - ENABLE_LDAP=1 # with the :edge tag, use ACCOUNT_PROVISIONER
      - ACCOUNT_PROVISIONER=LDAP
      - LDAP_SERVER_HOST=ldap.example.org
      - LDAP_BIND_DN=cn=admin,ou=users,dc=example,dc=org
      - LDAP_BIND_PW=mypassword
      - LDAP_SEARCH_BASE=dc=example,dc=org
      - LDAP_QUERY_FILTER_DOMAIN=(|(mail=*@%s)(mailAlias=*@%s)(mailGroupMember=*@%s))
      - LDAP_QUERY_FILTER_USER=(&(objectClass=inetOrgPerson)(mail=%s))
      - LDAP_QUERY_FILTER_ALIAS=(&(objectClass=inetOrgPerson)(mailAlias=%s))
      - LDAP_QUERY_FILTER_GROUP=(&(objectClass=inetOrgPerson)(mailGroupMember=%s))
      - LDAP_QUERY_FILTER_SENDERS=(&(objectClass=inetOrgPerson)(|(mail=%s)(mailAlias=%s)(mailGroupMember=%s)))
      - SPOOF_PROTECTION=1
      # <<< Postfix LDAP Integration

      # >>> Dovecot LDAP Integration
      - DOVECOT_USER_FILTER=(&(objectClass=inetOrgPerson)(mail=%u))
      - DOVECOT_PASS_ATTRS=uid=user,userPassword=password
      - DOVECOT_USER_ATTRS==home=/var/mail/%{ldap:uid},=mail=maildir:~/Maildir,uidNumber=uid,gidNumber=gid
      # <<< Dovecot LDAP Integration

      # >>> SASL LDAP Authentication
      - ENABLE_SASLAUTHD=1
      - SASLAUTHD_MECHANISMS=ldap
      - SASLAUTHD_LDAP_FILTER=(&(mail=%U@example.org)(objectClass=inetOrgPerson))
      # <<< SASL LDAP Authentication

      - ONE_DIR=1
      - SSL_TYPE=letsencrypt
      - PERMIT_DOCKER=host

    cap_add:
      - NET_ADMIN
Kopano / Zarafa 
version: '3.8'

services:
  mailserver:
    image: docker.io/mailserver/docker-mailserver:latest
    container_name: mailserver
    hostname: mail
    domainname: example.com

    ports:
      - "25:25"
      - "143:143"
      - "587:587"
      - "993:993"

    volumes:
      - ./docker-data/dms/mail-data/:/var/mail/
      - ./docker-data/dms/mail-state/:/var/mail-state/
      - ./docker-data/dms/config/:/tmp/docker-mailserver/

    environment:
      # We are not using dovecot here
      - SMTP_ONLY=1
      - ENABLE_SPAMASSASSIN=1
      - ENABLE_CLAMAV=1
      - ENABLE_FAIL2BAN=1
      - ENABLE_POSTGREY=1
      - SASLAUTHD_PASSWD=

      # >>> SASL Authentication
      - ENABLE_SASLAUTHD=1
      - SASLAUTHD_LDAP_FILTER=(&(sAMAccountName=%U)(objectClass=person))
      - SASLAUTHD_MECHANISMS=ldap
      # <<< SASL Authentication

      # >>> Postfix Ldap Integration
      - ENABLE_LDAP=1 # with the :edge tag, use ACCOUNT_PROVISIONER
      - ACCOUNT_PROVISIONER=LDAP
      - LDAP_SERVER_HOST=<yourLdapContainer/yourLdapServer>
      - LDAP_SEARCH_BASE=dc=mydomain,dc=loc
      - LDAP_BIND_DN=cn=Administrator,cn=Users,dc=mydomain,dc=loc
      - LDAP_BIND_PW=mypassword
      - LDAP_QUERY_FILTER_USER=(&(objectClass=user)(mail=%s))
      - LDAP_QUERY_FILTER_GROUP=(&(objectclass=group)(mail=%s))
      - LDAP_QUERY_FILTER_ALIAS=(&(objectClass=user)(otherMailbox=%s))
      - LDAP_QUERY_FILTER_DOMAIN=(&(|(mail=*@%s)(mailalias=*@%s)(mailGroupMember=*@%s))(mailEnabled=TRUE))
      # <<< Postfix Ldap Integration

      # >>> Kopano Integration
      - ENABLE_POSTFIX_VIRTUAL_TRANSPORT=1
      - POSTFIX_DAGENT=lmtp:kopano:2003
      # <<< Kopano Integration

      - ONE_DIR=1
      - SSL_TYPE=letsencrypt
      - PERMIT_DOCKER=host

    cap_add:
      - NET_ADMIN