Forward-Only Mail-Server with LDAP

Building a Forward-Only Mail-Server

A forward-only mail-server does not have any local mailboxes. Instead, it has only aliases that forward emails to external email accounts (for example to a Gmail account). You can also send email from the localhost (the computer where docker-mailserver is installed), using as sender any of the alias addresses.

The important settings for this setup (on mailserver.env) are these:

PERMIT_DOCKER=host
ENABLE_POP3=
ENABLE_CLAMAV=0
SMTP_ONLY=1
ENABLE_SPAMASSASSIN=0
ENABLE_FETCHMAIL=0

Since there are no local mailboxes, we use SMTP_ONLY=1 to disable dovecot. We disable as well the other services that are related to local mailboxes (POP3, ClamAV, SpamAssassin, etc.)

We can create aliases with ./setup.sh, like this:

./setup.sh alias add <alias-address> <external-email-account>

Authenticating with LDAP

If you want to send emails from outside the mail-server you have to authenticate somehow (with a username and password). One way of doing it is described in this discussion. However if there are many user accounts, it is better to use authentication with LDAP. The settings for this on mailserver.env are:

ENABLE_LDAP=1
LDAP_START_TLS=yes
LDAP_SERVER_HOST=ldap.example.org
LDAP_SEARCH_BASE=ou=users,dc=example,dc=org
LDAP_BIND_DN=cn=mailserver,dc=example,dc=org
LDAP_BIND_PW=pass1234

ENABLE_SASLAUTHD=1
SASLAUTHD_MECHANISMS=ldap
SASLAUTHD_LDAP_SERVER=ldap.example.org
SASLAUTHD_LDAP_START_TLS=yes
SASLAUTHD_LDAP_BIND_DN=cn=mailserver,dc=example,dc=org
SASLAUTHD_LDAP_PASSWORD=pass1234
SASLAUTHD_LDAP_SEARCH_BASE=ou=users,dc=example,dc=org
SASLAUTHD_LDAP_FILTER=(&(uid=%U)(objectClass=inetOrgPerson))

My LDAP data structure is very basic, containing only the username, password, and the external email address where to forward emails for this user. An entry looks like this:

add uid=username,ou=users,dc=example,dc=org
uid: username
objectClass: inetOrgPerson
sn: username
cn: username
userPassword: {SSHA}abcdefghi123456789
email: external-account@gmail.com

This structure is different from what is expected/assumed from the configuration scripts of docker-mailserver, so it doesn't work just by using the LDAP_QUERY_FILTER_... settings. Instead, I had to use a custom configuration (via user-patches.sh). I created the script docker-data/dms/config/user-patches.sh, with content like this:

#!/bin/bash

rm -f /etc/postfix/{ldap-groups.cf,ldap-domains.cf}

postconf \
    "virtual_mailbox_domains = /etc/postfix/vhost" \
    "virtual_alias_maps = ldap:/etc/postfix/ldap-aliases.cf texthash:/etc/postfix/virtual" \
    "smtpd_sender_login_maps = ldap:/etc/postfix/ldap-users.cf"

sed -i /etc/postfix/ldap-users.cf \
    -e '/query_filter/d' \
    -e '/result_attribute/d' \
    -e '/result_format/d'
cat <<EOF >> /etc/postfix/ldap-users.cf
query_filter = (uid=%u)
result_attribute = uid
result_format = %s@example.org
EOF

sed -i /etc/postfix/ldap-aliases.cf \
    -e '/domain/d' \
    -e '/query_filter/d' \
    -e '/result_attribute/d'
cat <<EOF >> /etc/postfix/ldap-aliases.cf
domain = example.org
query_filter = (uid=%u)
result_attribute = mail
EOF

postfix reload

You see that besides query_filter, I had to customize as well result_attribute and result_format.