A lightweight, fast and thread-safe python3 milter on top of sdgathman/pymilter for basic Access Control (ACL) scenarios. The milter consumes policies from LDAP based on a specialized schema. Alternatively an already present schema can be uses as well. In this case custom queries with trivial templating support must be used:
- %client_addr% = IPv(4|6) address of SMTP client
- %sasl_user% = user name of SASL authenticated user
- %from% = RFC5321.from
- %from_domain% = RFC5321.from_domain
- %rcpt% = RFC5321.rcpt
- %rcpt_domain% RFC5321.rcpt_domain
In the case, one already has a LDAP server running with the amavis schema, for e.g. the 'amavisWhitelistSender' attribute could be reused. The filtering direction (inbound or outbound) can be simply controlled by swapping the %from% and %rcpt% placeholders within the LDAP query template. Please have a look at the docker-compose.yml example below.
The connection to the LDAP server is always persistent: one LDAP-bind is shared among all milter-threads, which makes it more efficient due to less communication overhead (which also implies transport encryption with TLS). Thus, LDAP interactions with 2 msec. and less are realistic, depending on your environment like network round-trip-times or the load of your LDAP server. A very swag LDAP setup is to use a local read-only LDAP replica, which syncs over network with a couple of LDAP masters: OpenLDAP does it for free!. On the one hand, this aproach eliminates network round trip times while reading from a UNIX-socket, and on the other, performance bottlenecks on a shared, centralized and (heavy) utilized LDAP server.
Yes, there´s one! Check out the LDAP-directory ;-)
In case of OpenLDAP, just use the unique overlay. Another aproach to ensure attribute uniqueness is to do it in some programmatic way which is more complex but more flexible.
The intention of this project is to deploy the milter ALWAYS AND ONLY as an OCI compliant container. In this case it´s docker. The main reason is that I´m not interested (and familiar with) in building distribution packages like .rpm, .deb, etc.. Furthermore I´m not really a fan of 'wild and uncontrollable' software deployments like: get the code, compile it and finaly install the results 'somewhere' in the filesystem. In terms of software deployment docker provides wonderful possibilities, which I don´t want to miss anymore... No matter if in development, QA or production stage.
The following docker-compose file demonstrates how such a setup could be orchestrated on a single docker host or on a docker swarm cluster. In this context we use postfix as our milter-capable MTA and OpenLDAP as local LDAP replica.
version: '3'
volumes:
lam_socket:
openldap_spool:
openldap_socket:
services:
openldap:
image: "your/favorite/openldap/image"
restart: unless-stopped
hostname: openldap
volumes:
- "./config/openldap:/etc/openldap:rw"
- "openldap_spool:/var/openldap-data:rw"
- "openldap_socket:/socket:rw"
ldap-acl-milter:
depends_on:
- openldap
image: "ldap-acl-milter/debian:19.02_master"
restart: unless-stopped
environment:
# default: info. Possible: info, warning, error and debug
LOG_LEVEL: debug
#LDAP_SERVER: ldap://ldap-slave.example.local:389
LDAP_SERVER: ldapi:///socket//slapd//slapd
LDAP_BINDDN: uid=lam,ou=applications,dc=example,dc=org
LDAP_BINDPW: TopSecret123!%&
LDAP_BASE: ou=users,dc=example,dc=org
# This example LDAP query is for inbound filtering
# where the 'mail' attribute equals to the recipient
# and the 'amavisWhitelistSender' attribute the eligible sender
#LDAP_QUERY: (&(mail=%rcpt%)(amavisWhitelistSender=%from%))
#LDAP_QUERY: (&(mail=%rcpt%)(|(amavisWhitelistSender=*@%from_domain%)(amavisWhitelistSender=%from%)))
# LDAP_QUERY: (&(|(mail=%rcpt%)(mail=*@%rcpt_domain%))(|(amavisWhitelistSender=*@%from_domain%)(amavisWhitelistSender=%from%)))
# This enables the use of own ldap-acl-milter LDAP schema. Default: False
# Setting MILTER_SCHEMA: true disables the LDAP_QUERY parameter!
#MILTER_SCHEMA: 'true'
# If MILTER_SCHEMA_WILDCARD_DOMAIN is set to true, the milter allows *@domain
# as valid sender/recipient addresses in LDAP.
# This only works if MILTER_SCHEMA is enabled!
MILTER_SCHEMA_WILDCARD_DOMAIN: 'False'
# default: test. Possible: test, reject
#MILTER_MODE: 'reject'
MILTER_NAME: some-another-milter-name
# Default: UNIX-socket located under /socket/ldap-acl-milter
# https://pythonhosted.org/pymilter/namespacemilter.html#a266a6e09897499d8b1ae0e20f0d2be73
#MILTER_SOCKET: inet6:8020
#MILTER_REJECT_MESSAGE: Message rejected due to security policy
#MILTER_TMPFAIL_MESSAGE: Message temporary rejected. Please try again later ;)
# Expect authentication information from LDAP like allowedClientAddr,
# allowedSaslUser or allowedx509CN. This is usefull if the milter handles
# outbound email traffic, where senders must authenticate before submission.
# Default: false (inbound mode)
#MILTER_EXPECT_AUTH: 'true'
# Blank or comma separated list of valid email recipients to whitelist (default: empty)
#MILTER_WHITELISTED_RCPTS: '[email protected],[email protected]'
# Allow null-sender (<>) for bounces/DSNs (default: disabled)
#MILTER_ALLOW_NULL_SENDER: 'true'
# Enable recipient count limits (default: disabled)
#MILTER_MAX_RCPT_ENABLED: 'true'
#MILTER_MAX_RCPT: 1
# Enable DKIM checks (default: disabled).
# This enables the milter to use the
# sender address placed in the 5322.from header
# within policy checks, unless the DKIM authentication results
# are invalid.
# Enabling this feature also requires a DKIM validating milter
# BEFORE the ldap-acl-milter!
#MILTER_DKIM_ENABLED: 'true'
hostname: ldap-acl-milter
volumes:
- "lam_socket:/socket/:rw"
- "openldap_socket:/socket/slapd:ro"
postfix:
depends_on:
- ldap-acl-milter
image: "your/favorite/postfix/image"
restart: unless-stopped
hostname: postfix
ports:
- "25:25"
volumes:
- "./config/postfix:/etc/postfix:rw"
- "lam_socket:/socket/ldap-acl-milter/:rw"
- "openldap_socket:/socket/slapd:ro"
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