docker-mailserver/docs/content/config/security/ssl.md

There are multiple options to enable SSL:

• using letsencrypt (recommended)
• using self-signed certificates with the provided tool
• using your own certificates

After installation, you can test your setup with checktls.com.

Let's encrypt (recommended)

To enable Let's Encrypt on your mail server, you have to:

• get your certificate using letsencrypt client
• add an environment variable SSL_TYPE with value letsencrypt (see docker-compose.yml.dist)
• mount your whole letsencrypt folder to /etc/letsencrypt
• the certs folder name located in letsencrypt/live/ must be the fqdn of your container responding to the hostname command. The full qualified domain name (fqdn) inside the docker container is build combining the hostname and domainname values of the docker-compose file, e. g.: hostname: mail; domainname: myserver.tld; fqdn: mail.myserver.tld

You don't have anything else to do. Enjoy.

Example using docker for letsencrypt

Make a directory to store your letsencrypt logs and configs.

In my case

mkdir -p /home/ubuntu/docker/letsencrypt/log 
mkdir -p /home/ubuntu/docker/letsencrypt/etc/letsencrypt
cd /home/ubuntu/docker/letsencrypt

Now get the certificate (modify mail.myserver.tld) and following the certbot instructions. This will need access to port 443 from the internet, adjust your firewall if needed

docker run --rm -ti -v $PWD/log/:/var/log/letsencrypt/ -v $PWD/etc/:/etc/letsencrypt/ -p 443:443 deliverous/certbot certonly --standalone -d mail.myserver.tld

You can now mount /home/ubuntu/docker/letsencrypt/etc/ in /etc/letsencrypt of docker-mailserver

To renew your certificate just run (this will need access to port 443 from the internet, adjust your firewall if needed)

docker run --rm -ti -v $PWD/log/:/var/log/letsencrypt/ -v $PWD/etc/:/etc/letsencrypt/ -p 443:443 deliverous/certbot renew

Example using docker, nginx-proxy and letsencrypt-nginx-proxy-companion

If you are running a web server already, it is non-trivial to generate a Let's Encrypt certificate for your mail server using certbot, because port 80 is already occupied. In the following example, we show how docker-mailserver can be run alongside the docker containers nginx-proxy and letsencrypt-nginx-proxy-companion.

There are several ways to start nginx-proxy and letsencrypt-nginx-proxy-companion. Any method should be suitable here. For example start nginx-proxy as in the letsencrypt-nginx-proxy-companion documentation:

docker run -d -p 80:80 -p 443:443 \
    --name nginx-proxy \
    -v /path/to/certs:/etc/nginx/certs:ro \
    -v /etc/nginx/vhost.d \
    -v /usr/share/nginx/html \
    -v /var/run/docker.sock:/tmp/docker.sock:ro \
    --label com.github.jrcs.letsencrypt_nginx_proxy_companion.nginx_proxy \
    jwilder/nginx-proxy

Then start letsencrypt-nginx-proxy-companion:

docker run -d \
    -v /path/to/certs:/etc/nginx/certs:rw \
    -v /var/run/docker.sock:/var/run/docker.sock:ro \
    --volumes-from nginx-proxy \
    jrcs/letsencrypt-nginx-proxy-companion

Start the rest of your web server containers as usual.

Start another container for your mail.myserver.tld. This will generate a Let's Encrypt certificate for your domain, which can be used by docker-mailserver. It will also run a web server on port 80 at that address.:

docker run -d \
    --name webmail \
    -e "VIRTUAL_HOST=mail.myserver.tld" \
    -e "LETSENCRYPT_HOST=mail.myserver.tld" \
    -e "LETSENCRYPT_EMAIL=foo@bar.com" \
    library/nginx

You may want to add -e LETSENCRYPT_TEST=true to the above while testing to avoid the Let's Encrypt certificate generation rate limits.

Finally, start docker-mailserver with path/to/certs/mail.mydomain.tld mounted to /etc/letsencrypt/live/mail.mydomain.tld

Example using the letsencrypt certificates on a Synology NAS

Version 6.2 and later of the Synology NAS DSM OS now come with an interface to generate and renew letencrypt certificates. Navigation into your DSM control panel and go to Security, then click on the tab Certificate to generate and manage letsencrypt certificates. Amongst other things, you can use these to secure your mail server. DSM locates the generated certificates in a folder below /usr/syno/etc/certificate/_archive/. Navigate to that folder and note the 6 character random folder name of the certificate you'd like to use. Then, add the following to your docker-compose.yml declaration file:

volumes:
      - /usr/syno/etc/certificate/_archive/YOUR_FOLDER/:/tmp/ssl 
...
environment:
      - SSL_TYPE=manual
      - SSL_CERT_PATH=/tmp/ssl/fullchain.pem
      - SSL_KEY_PATH=/tmp/ssl/privkey.pem

DSM-generated letsencrypt certificates get auto-renewed every three months.

Self-signed certificates (testing only)

You can easily generate a self-signed SSL certificate by using the following command:

docker run -ti --rm -v "$(pwd)"/config/ssl:/ssl -h mail.my-domain.com -t tvial/docker-mailserver generate-ssl-certificate

# Press enter
# Enter a password when needed
# Fill information like Country, Organisation name
# Fill "my-domain.com" as FQDN for CA, and "mail.my-domain.com" for the certificate.
# They HAVE to be different, otherwise you'll get a `TXT_DB error number 2`
# Don't fill extras
# Enter same password when needed
# Sign the certificate? [y/n]:y
# 1 out of 1 certificate requests certified, commit? [y/n]y

# will generate:
# config/ssl/mail.my-domain.com-key.pem (used in postfix)
# config/ssl/mail.my-domain.com-req.pem (only used to generate other files)
# config/ssl/mail.my-domain.com-cert.pem (used in postfix)
# config/ssl/mail.my-domain.com-combined.pem (used in courier)
# config/ssl/demoCA/cacert.pem (certificate authority)

Note that the certificate will be generate for the container fqdn, that is passed as -h argument. Check the following page for more information regarding postfix and SSL/TLS configuration.

To use the certificate:

• add SSL_TYPE=self-signed to your container environment variables
• if a matching certificate (files listed above) is found in config/ssl, it will be automatically setup in postfix and dovecot. You just have to place them in config/ssl folder.

Custom certificate files

You can also provide your own certificate files. Add these entries to your docker-compose.yml:

volumes:
  - /etc/ssl:/tmp/ssl:ro
environment:
- SSL_TYPE=manual
- SSL_CERT_PATH=/tmp/ssl/cert/public.crt
- SSL_KEY_PATH=/tmp/ssl/private/private.key

This will mount the path where your ssl certificates reside as read-only under /tmp/ssl. Then all you have to do is to specify the location of your private key and the certificate.

Please note that you may have to restart your mailserver once the certificates change.

Testing certificate

From your host:

docker exec mail openssl s_client -connect 0.0.0.0:25 -starttls smtp -CApath /etc/ssl/certs/

or

docker exec mail openssl s_client -connect 0.0.0.0:143 -starttls imap -CApath /etc/ssl/certs/

And you should see the certificate chain, the server certificate and:

Verify return code: 0 (ok)

Plain text access

Not recommended for purposes other than testing.

Just add this to config/dovecot.cf:

ssl = yes
disable_plaintext_auth=no

These options in conjunction mean:

ssl=yes and disable_plaintext_auth=no: SSL/TLS is offered to the client, but the client isn't required to use it. The client is allowed to login with plaintext authentication even when SSL/TLS isn't enabled on the connection. This is insecure, because the plaintext password is exposed to the internet.