docker-mailserver

Table of Contents

Building a simple mailserver
Using docker-mailserver behind proxy

Information
Configuration of the used proxy software
Configuration of the backend (dovecot and postfix)

Building a simple mailserver

WARNING: Adding the docker network's gateway to the list of trusted hosts, e.g. using the network or connected-networks option, can create an open relay, for instance if IPv6 is enabled on the host machine but not in Docker. (#1405)

We are going to use this docker based mailserver:

First create a directory for the mailserver and get the setup script:

mkdir -p /var/ds/mail.example.org
cd /var/ds/mail.example.org/

curl -o setup.sh \
    https://raw.githubusercontent.com/tomav/docker-mailserver/master/setup.sh
chmod a+x ./setup.sh

Create the file docker-compose.yml with a content like this:
```
version: '2'

services:
  mail:
    image: docker.io/mailserver/docker-mailserver:latest
    hostname: mail
    domainname: example.org
    container_name: mail
    ports:
    - "25:25"
    - "587:587"
    - "465:465"
    volumes:
    - ./data/:/var/mail/
    - ./state/:/var/mail-state/
    - ./config/:/tmp/docker-mailserver/
    - /var/ds/wsproxy/letsencrypt/:/etc/letsencrypt/
    environment:
    - PERMIT_DOCKER=network
    - SSL_TYPE=letsencrypt
    - ONE_DIR=1
    - DMS_DEBUG=1
    - SPOOF_PROTECTION=0
    - REPORT_RECIPIENT=1
    - ENABLE_SPAMASSASSIN=0
    - ENABLE_CLAMAV=0
    - ENABLE_FAIL2BAN=1
    - ENABLE_POSTGREY=0
    cap_add:
    - NET_ADMIN
    - SYS_PTRACE
```
For more details about the environment variables that can be used, and their meaning and possible values, check also these:
- https://github.com/tomav/docker-mailserver#environment-variables
- https://github.com/tomav/docker-mailserver/blob/master/.env.dist
Make sure to set the proper domainname that you will use for the emails. We forward only SMTP ports (not POP3 and IMAP) because we are not interested in accessing the mailserver directly (from a client). We also use these settings:
- PERMIT_DOCKER=network because we want to send emails from other docker containers.
- SSL_TYPE=letsencrypt because we will manage SSL certificates with letsencrypt.
We need to open these ports on the firewall: 25, 587, 465
```
ufw allow 25
ufw allow 587
ufw allow 465
```
On your server you may have to do it differently.

Pull the docker image:

docker pull docker.io/mailserver/docker-mailserver:latest

Now generate the DKIM keys with ./setup.sh config dkim and copy the content of the file config/opendkim/keys/domain.tld/mail.txt on the domain zone configuration at the DNS server. I use bind9 for managing my domains, so I just paste it on example.org.db:

mail._domainkey IN      TXT     ( "v=DKIM1; h=sha256; k=rsa; "
        "p=MIIBIjANBgkqhkiG9w0BAQEFACAQ8AMIIBCgKCAQEAaH5KuPYPSF3Ppkt466BDMAFGOA4mgqn4oPjZ5BbFlYA9l5jU3bgzRj3l6/Q1n5a9lQs5fNZ7A/HtY0aMvs3nGE4oi+LTejt1jblMhV/OfJyRCunQBIGp0s8G9kIUBzyKJpDayk2+KJSJt/lxL9Iiy0DE5hIv62ZPP6AaTdHBAsJosLFeAzuLFHQ6USyQRojefqFQtgYqWQ2JiZQ3"
        "iqq3bD/BVlwKRp5gH6TEYEmx8EBJUuDxrJhkWRUk2VDl1fqhVBy8A9O7Ah+85nMrlOHIFsTaYo9o6+cDJ6t1i6G1gu+bZD0d3/3bqGLPBQV9LyEL1Rona5V7TJBGg099NQkTz1IwIDAQAB" )  ; ----- DKIM key mail for example.org

Add these configurations as well on the same file on the DNS server:

mail      IN  A   10.11.12.13

; mailservers for example.org
    3600  IN  MX  1  mail.example.org.

; Add SPF record
          IN TXT "v=spf1 mx ~all"

Then don't forget to change the serial number and to restart the service.

Get an SSL certificate from letsencrypt. I use wsproxy for managing SSL letsencrypt certificates of my domains:
```
cd /var/ds/wsproxy
ds domains-add mail mail.example.org
ds get-ssl-cert myemail@gmail.com mail.example.org --test
ds get-ssl-cert myemail@gmail.com mail.example.org
```
Now the certificates will be available on /var/ds/wsproxy/letsencrypt/live/mail.example.org.

Start the mailserver and check for any errors:

apt install docker-compose
docker-compose up mail

Create email accounts and aliases with SPOOF_PROTECTION=0:
```
./setup.sh email add admin@example.org passwd123
./setup.sh email add info@example.org passwd123
./setup.sh alias add admin@example.org myemail@gmail.com
./setup.sh alias add info@example.org myemail@gmail.com
./setup.sh email list
./setup.sh alias list
```
Aliases make sure that any email that comes to these accounts is forwarded to my real email address, so that I don't need to use POP3/IMAP in order to get these messages. Also no anti-spam and anti-virus software is needed, making the mailserver lighter.

Or create email accounts and aliases with SPOOF_PROTECTION=1:

./setup.sh email add admin.gmail@example.org passwd123
./setup.sh email add info.gmail@example.org passwd123
./setup.sh alias add admin@example.org admin.gmail@example.org
./setup.sh alias add info@example.org info.gmail@example.org
./setup.sh alias add admin.gmail@example.org myemail@gmail.com
./setup.sh alias add info.gmail@example.org myemail@gmail.com
./setup.sh email list
./setup.sh alias list

This extra step is required to avoid the 553 5.7.1 Sender address rejected: not owned by user error (the account used for setting up gmail is admin.gmail@example.org and info.gmail@example.org )

Send some test emails to these addresses and make other tests. Then stop the container with Ctrl+c and start it again as a daemon: docker-compose up -d mail.
Now save on Moodle configuration the SMTP settings and test by trying to send some messages to other users:
- SMTP hosts: mail.example.org:465
- SMTP security: SSL
- SMTP username: info@example.org
- SMTP password: passwd123

Using docker-mailserver behind proxy

Information

If you are hiding your container behind a proxy service you might have discovered that the proxied requests from now on contain the proxy IP as the request origin. Whilst this behavior is technical correct it produces certain problems on the containers behind the proxy as they cannot distinguish the real origin of the requests anymore.

To solve this problem on TCP connections we can make use of the proxy protocol. Compared to other workarounds that exist (X-Forwarded-For which only works for HTTP requests or Tproxy that requires you to recompile your kernel) the proxy protocol:

it is protocol agnostic (can work with any layer 7 protocols, even when encrypted).
it does not require any infrastructure changes
nat-ing firewalls have no impact it
it is scalable The is only one condition: both endpoints of the connection MUST be compatible with proxy protocol.

Luckily dovecot and postfix are both Proxy-Protocol ready softwares so it depends only on your used reverse-proxy/loadbalancer.

Configuration of the used proxy software

The configuration depends on the used proxy system. I will provide the configuration examples of traefik v2 using IMAP and SMTP with implicit TLS. Feel free to add your configuration if you achived the same goal using different proxy software below:

traefik v2

Truncated configuration of traefik itself:

version: '3.7'
services:
  reverse-proxy:
    image: traefik:v2.4
    container_name: docker-traefik
    restart: always
    command:
      - "--providers.docker"
      - "--providers.docker.exposedbydefault=false"
      - "--providers.docker.network=proxy"
      - "--entrypoints.web.address=:80"
      - "--entryPoints.websecure.address=:443"
      - "--entryPoints.smtp.address=:25"
      - "--entryPoints.smtp-ssl.address=:465"
      - "--entryPoints.imap-ssl.address=:993"
      - "--entryPoints.sieve.address=:4190"
    ports:
      - "25:25"
      - "465:465"
      - "993:993"
      - "4190:4190"
[...]

Truncated list of neccessary labels on the mailserver container:

version: '2'
services:
  mail:
    image: docker.io/mailserver/docker-mailserver:release-v7.2.0
    restart: always
    networks:
      - proxy
    labels:
      - "traefik.enable=true"
      - "traefik.tcp.routers.smtp.rule=HostSNI(`*`)"
      - "traefik.tcp.routers.smtp.entrypoints=smtp"
      - "traefik.tcp.routers.smtp.service=smtp"
      - "traefik.tcp.services.smtp.loadbalancer.server.port=25"
      - "traefik.tcp.services.smtp.loadbalancer.proxyProtocol.version=1"
      - "traefik.tcp.routers.smtp-ssl.rule=HostSNI(`*`)"
      - "traefik.tcp.routers.smtp-ssl.entrypoints=smtp-ssl"
      - "traefik.tcp.routers.smtp-ssl.service=smtp-ssl"
      - "traefik.tcp.services.smtp-ssl.loadbalancer.server.port=465"
      - "traefik.tcp.services.smtp-ssl.loadbalancer.proxyProtocol.version=1"
      - "traefik.tcp.routers.imap-ssl.rule=HostSNI(`*`)"
      - "traefik.tcp.routers.imap-ssl.entrypoints=imap-ssl"
      - "traefik.tcp.routers.imap-ssl.service=imap-ssl"
      - "traefik.tcp.services.imap-ssl.loadbalancer.server.port=10993"
      - "traefik.tcp.services.imap-ssl.loadbalancer.proxyProtocol.version=2"
      - "traefik.tcp.routers.sieve.rule=HostSNI(`*`)"
      - "traefik.tcp.routers.sieve.entrypoints=sieve"
      - "traefik.tcp.routers.sieve.service=sieve"
      - "traefik.tcp.services.sieve.loadbalancer.server.port=4190"
[...]

Keep in mind that it is neccessary to use port 10993 here. More information below at dovecot configuration.

Configuration of the backend (`dovecot` and `postfix`)

The following changes can be achived completely by adding the content to the appropriate files by using the projects function to overwrite config files.

Changes for postfix can be applied by adding the following content to config/postfix-main.cf:

postscreen_upstream_proxy_protocol = haproxy

and to config/postfix-master.cd:

submission/inet/smtpd_upstream_proxy_protocol=haproxy
smtps/inet/smtpd_upstream_proxy_protocol=haproxy

Changes for dovecot can be applied by adding the following content to config/dovecot.cf:

haproxy_trusted_networks = <your-proxy-ip>, <optional-cidr-notation>
haproxy_timeout = 3 secs
service imap-login {
  inet_listener imaps {
    haproxy = yes
    ssl = yes
    port = 10993
  }
}

Note that port 10993 is used here to avoid conflicts with internal systems like postscreen and amavis as they will exchange messages on the default port and obviously have a different origin then compared to the proxy.

Home

This project is licensed under the MIT license.

Building a simple mailserver

Using docker-mailserver behind proxy

Information

Configuration of the used proxy software

Configuration of the backend (dovecot and postfix)

Configuration of the backend (`dovecot` and `postfix`)