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Simple postfix relay host for your Docker containers. Based on Alpine Linux.
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docker-postfix

Simple postfix relay host for your Docker containers. Based on Alpine Linux.

Project update

Notice, that while this commits are old, there project is not dead. It's simply considered feature complete. You will find the latest version of the code on Dockerhub (https://hub.docker.com/r/boky/postfix). If you do have any suggestions, feel free to clone and post a merge.

Description

This image allows you to run POSTFIX internally inside your docker cloud/swarm installation to centralise outgoing email sending. The embedded postfix enables you to either send messages directly or relay them to your company's main server.

This is a server side POSTFIX image, geared towards emails that need to be sent from your applications. That's why this postfix configuration does not support username / password login or similar client-side security features.

IF YOU WANT TO SET UP AND MANAGE A POSTFIX INSTALLATION FOR END USERS, THIS IMAGE IS NOT FOR YOU. If you need it to manage your application's outgoing queue, read on.

TL;DR

To run the container, do the following:

docker run --rm --name postfix -e "ALLOWED_SENDER_DOMAINS=example.com" -p 1587:587 boky/postfix

You can now send emails by using localhost:1587 as your SMTP server address. Of course, if you haven't configured your example.com domain to allow sending from this IP (see openspf), your emails will most likely be regarded as spam.

All standard caveats of configuring the SMTP server apply -- e.g. you'll need to make sure your DNS entries are updated properly if you don't want your emails marked as spam.

Please note that the image uses the submission (587) port by default. Port 25 is not exposed on purpose, as it's regularly blocked by ISP or already occupied by other services.

Configuration options

The following configuration options are available:

ENV vars
$HOSTNAME = Postfix myhostname
$RELAYHOST = Host that relays your msgs
$RELAYHOST_USERNAME = An (optional) username for the relay server
$RELAYHOST_PASSWORD = An (optional) login password for the relay server
$MYNETWORKS = allow domains from per Network ( default 127.0.0.0/8,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16 )
$ALLOWED_SENDER_DOMAINS = domains sender domains
$MASQUERADED_DOMAINS = domains where you want to masquerade internal hosts

HOSTNAME

You may configure a specific hostname that the SMTP server will use to identify itself. If you don't do it, the default Docker host name will be used. A lot of times, this will be just the container id (e.g. f73792d540a5) which may make it difficult to track your emails in the log files. If you care about tracking at all, I suggest you set this variable, e.g.:

docker run --rm --name postfix -e HOSTNAME=postfix-docker -p 1587:587 boky/postfix

RELAYHOST, RELAYHOST_USERNAME and RELAYHOST_PASSWORD

Postfix will try to deliver emails directly to the target server. If you are behind a firewall, or inside a corporation you will most likely have a dedicated outgoing mail server. By setting this option, you will instruct postfix to relay (hence the name) all incoming emails to the target server for actual delivery.

Example:

docker run --rm --name postfix -e RELAYHOST=192.168.115.215 -p 1587:587 boky/postfix

You may optionally specifiy a relay port, e.g.:

docker run --rm --name postfix -e RELAYHOST=192.168.115.215:587 -p 1587:587 boky/postfix

Or an IPv6 address, e.g.:

docker run --rm --name postfix -e 'RELAYHOST=[2001:db8::1]:587' -p 1587:587 boky/postfix

If your end server requires you to authenticate with username/password, add them also:

docker run --rm --name postfix -e RELAYHOST=mail.google.com -e RELAYHOST_USERNAME=hello@gmail.com -e RELAYHOST_PASSWORD=world -p 1587:587 boky/postfix

RELAYHOST_TLS_LEVEL

Define relay host TLS connection level. See http://www.postfix.org/postconf.5.html#smtp_tls_security_level for details. By default, the permissive level ("may") is used, which basically means "use TLS if available" and should be a sane default in most cases.

This level defines how the postfix will connect to your upstream server.

MESSAGE_SIZE_LIMIT

Define the maximum size of the message, in bytes. See more in Postfix documentation.

By default, this limit is set to 0 (zero), which means unlimited. Why would you want to set this? Well, this is especially useful in relation with RELAYHOST setting. If your relay host has a message limit (and usually it does), set it also here. This will help you "fail fast" -- your message will be rejected at the time of sending instead having it stuck in the outbound queue indefenetly.

MYNETWORKS

This implementation is meant for private installations -- so that when you configure your services using docker compose you can just plug it in. Precisely because of this reason and the prevent any issues with this postfix being inadvertently exposed on the internet and then used for sending spam, the default networks are reserved for private IPv4 IPs only.

Most likely you won't need to change this. However, if you need to support IPv6 or strenghten the access further, you can override this setting.

Example:

docker run --rm --name postfix -e "MYNETWORKS=10.1.2.0/24" -p 1587:587 boky/postfix

ALLOWED_SENDER_DOMAINS

Due to in-built spam protection in Postfix you will need to specify sender domains -- the domains you are using to send your emails from, otherwise Postfix will refuse to start.

Example:

docker run --rm --name postfix -e "ALLOWED_SENDER_DOMAINS=example.com example.org" -p 1587:587 boky/postfix

INBOUND_DEBUGGING

Enable additional debugging for any connection comming from MYNETWORKS. Set to a non-empty string (usually "1" or "yes") to enable debugging.

MASQUERADED_DOMAINS

If you don't want outbound mails to expose hostnames, you can use this variable to enable Postfix's address masquerading. This can be used to do things like rewrite lorem@ipsum.example.com to lorem@example.com.

Example:

docker run --rm --name postfix -e "ALLOWED_SENDER_DOMAINS=example.com example.org" -e "MASQUERADED_DOMAINS=example.com" -p 1587:587 boky/postfix

DKIM

This image is equiped with support for DKIM. If you want to use DKIM you will need to generate DKIM keys yourself. You'll need to create a folder for every domain you want to send through Postfix and generate they key(s) with the following command, e.g.

mkdir -p /host/keys; cd /host/keys

for DOMAIN in example.com example.org; do
    # Generate a key with selector "mail"
    opendkim-genkey -b 2048 -h rsa-sha256 -r -v --subdomains -s mail -d $DOMAIN
    # Fixes https://github.com/linode/docs/pull/620
    sed -i 's/h=rsa-sha256/h=sha256/' mail.txt
    # Move to proper file
    mv mail.private $DOMAIN.private
    mv mail.txt $DOMAIN.txt
done
...

opendkim-genkey is usually in your favourite distribution provided by installing opendkim-tools or opendkim-utils.

Add the created <domain>.txt files to your DNS records. Afterwards, just mount /etc/opendkim/keys into your image and DKIM will be used automatically, e.g.:

docker run --rm --name postfix -e "ALLOWED_SENDER_DOMAINS=example.com example.org" -v /host/keys:/etc/opendkim/keys -p 1587:587 boky/postfix

Extending the image

If you need to add custom configuration to postfix or have it do something outside of the scope of this configuration, simply add your scripts to /docker-init.db/: All files with the .sh extension will be executed automatically at the end of the startup script.

E.g.: create a custom Dockerfile like this:

FROM boky/postfix
LABEL maintainer="Jack Sparrow <jack.sparrow@theblackpearl.example.com>"
ADD Dockerfiles/additional-config.sh /docker-init.db/

Build it with docker and your script will be automatically executed before Postfix starts.

Or -- alternately -- bind this folder in your docker config and put your scripts there. Useful if you need to add additional config to your postfix server or override configs created by the script.

For example, your script could contain something like this:

#!/bin/sh
postconf -e "address_verify_negative_cache=yes"

Security

Postfix will run the master proces as root, because that's how it's designed. Subprocesses will run under the postfix account which will use UID:GID of 100:101. opendkim will run under account 102:103.

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