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

title
Security | TLS (aka SSL)

There are multiple options to enable SSL:

• Using letsencrypt (recommended)
• Using Caddy
• Using Traefik
• Using self-signed certificates with the provided tool
• Using your own certificates

After installation, you can test your setup with:

• checktls.com
• testssl.sh

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)

• 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 fqdn (full qualified domain name) inside the docker container is built combining the hostname and domainname values of the docker-compose file, eg:

  services:
    mail:
      hostname: mail
      domainname: myserver.tld
      fqdn: mail.myserver.tld
  

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

Example using Docker for Let's Encrypt

1. Make a directory to store your letsencrypt logs and configs. In my case:

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

2. Now get the certificate (modify mail.myserver.tld) and following the certbot instructions.

3. This will need access to port 80 from the internet, adjust your firewall if needed:

   docker run --rm -it \
     -v $PWD/log/:/var/log/letsencrypt/ \
     -v $PWD/etc/:/etc/letsencrypt/ \
     -p 80:80 \
     certbot/certbot certonly --standalone -d mail.myserver.tld
   

4. 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 -it \
     -v $PWD/log/:/var/log/letsencrypt/ \
     -v $PWD/etc/:/etc/letsencrypt/ \
     -p 80:80 \
     -p 443:443 \
     certbot/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 --detach \
  --name nginx-proxy \
  --restart always \
  --publish 80:80 \
  --publish 443:443 \
  --volume /server/letsencrypt/etc:/etc/nginx/certs:ro \
  --volume /etc/nginx/vhost.d \
  --volume /usr/share/nginx/html \
  --volume /var/run/docker.sock:/tmp/docker.sock:ro \
  jwilder/nginx-proxy

Then start nginx-proxy-letsencrypt:

docker run --detach \
  --name nginx-proxy-letsencrypt \
  --restart always \
  --volume /server/letsencrypt/etc:/etc/nginx/certs:rw \
  --volumes-from nginx-proxy \
  --volume /var/run/docker.sock:/var/run/docker.sock:ro \
  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 the mailserver with the docker-compose.yml. Make sure your mount path to the letsencrypt certificates is correct.

Inside your /path/to/mailserver/docker-compose.yml (for the mailserver from this repo) make sure volumes look like below example:

volumes:
  - maildata:/var/mail
  - mailstate:/var/mail-state
  - ./config/:/tmp/docker-mailserver/
  - /server/letsencrypt/etc:/etc/letsencrypt/live

Then: /path/to/mailserver/docker-compose up -d mail

Example using Docker, nginx-proxy and letsencrypt-nginx-proxy-companion with docker-compose

The following docker-compose.yml is the basic setup you need for using letsencrypt-nginx-proxy-companion. It is mainly derived from its own wiki/documenation.

version: "2"

services:
  nginx: 
    image: nginx
    container_name: nginx
    ports:
      - 80:80
      - 443:443
    volumes:
      - /mnt/data/nginx/htpasswd:/etc/nginx/htpasswd
      - /mnt/data/nginx/conf.d:/etc/nginx/conf.d
      - /mnt/data/nginx/vhost.d:/etc/nginx/vhost.d
      - /mnt/data/nginx/html:/usr/share/nginx/html
      - /mnt/data/nginx/certs:/etc/nginx/certs:ro
    networks:
      - proxy-tier
    restart: always

  nginx-gen:
    image: jwilder/docker-gen
    container_name: nginx-gen
    volumes:
      - /var/run/docker.sock:/tmp/docker.sock:ro
      - /mnt/data/nginx/templates/nginx.tmpl:/etc/docker-gen/templates/nginx.tmpl:ro
    volumes_from:
      - nginx
    entrypoint: /usr/local/bin/docker-gen -notify-sighup nginx -watch -wait 5s:30s /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf
    restart: always

  letsencrypt-nginx-proxy-companion:
    image: jrcs/letsencrypt-nginx-proxy-companion
    container_name: letsencrypt-companion
    volumes_from:
      - nginx
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
      - /mnt/data/nginx/certs:/etc/nginx/certs:rw
    environment:
      - NGINX_DOCKER_GEN_CONTAINER=nginx-gen
      - DEBUG=false
    restart: always

networks:
  proxy-tier:
    external:
      name: nginx-proxy

The second part of the setup is the actual mail container. So, in another folder, create another docker-compose.yml with the following content (Removed all ENV variables for this example):

version: '2'
services:
  mail:
    image: tvial/docker-mailserver:latest
    hostname: ${HOSTNAME}
    domainname: ${DOMAINNAME}
    container_name: ${CONTAINER_NAME}
    ports:
    - "25:25"
    - "143:143"
    - "465:465"
    - "587:587"
    - "993:993"
    volumes:
    - ./mail:/var/mail
    - ./mail-state:/var/mail-state
    - ./config/:/tmp/docker-mailserver/
    - /mnt/data/nginx/certs/:/etc/letsencrypt/live/:ro
    cap_add:
    - NET_ADMIN
    - SYS_PTRACE
    restart: always

  cert-companion:
    image: nginx
    environment:
      - "VIRTUAL_HOST="
      - "VIRTUAL_NETWORK=nginx-proxy"
      - "LETSENCRYPT_HOST="
      - "LETSENCRYPT_EMAIL="
    networks:
      - proxy-tier
    restart: always

networks:
  proxy-tier:
    external:
      name: nginx-proxy

The mail container needs to have the letsencrypt certificate folder mounted as a volume. No further changes are needed. The second container is a dummy-sidecar we need, because the mail-container do not expose any web-ports. Set your ENV variables as you need. (VIRTUAL_HOST and LETSENCRYPT_HOST are mandandory, see documentation)

Example using the Let's Encrypt 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.

Caddy

If you are using Caddy to renew your certificates, please note that only RSA certificates work. Read #1440 for details. In short for Caddy v1 the Caddyfile should look something like:

https://mail.domain.com {
  tls yourcurrentemail@gmail.com {
    key_type rsa2048
  }
}

For Caddy v2 you can specify the key_type in your server's global settings, which would end up looking something like this if you're using a Caddyfile:

{
  debug
  admin localhost:2019
  http_port 80
  https_port 443
  default_sni mywebserver.com
  key_type rsa4096
}

If you are instead using a json config for Caddy v2, you can set it in your site's TLS automation policies:

{
  "apps": {
    "http": {
      "servers": {
        "srv0": {
          "listen": [
            ":443"
          ],
          "routes": [
            {
              "match": [
                {
                  "host": [
                    "mail.domain.com",
                  ]
                }
              ],
              "handle": [
                {
                  "handler": "subroute",
                  "routes": [
                    {
                      "handle": [
                        {
                          "body": "",
                          "handler": "static_response"
                        }
                      ]
                    }
                  ]
                }
              ],
              "terminal": true
            },
          ]
        }
      }
    },
    "tls": {
      "automation": {
        "policies": [
          {
            "subjects": [
              "mail.domain.com",
            ],
            "key_type": "rsa2048",
            "issuer": {
              "email": "email@email.com",
              "module": "acme"
            }
          },
          {
            "issuer": {
              "email": "email@email.com",
              "module": "acme"
            }
          }
        ]
      }
    }
  }
}

The generated certificates can be mounted:

volumes:
  - ${CADDY_DATA_DIR}/certificates/acme-v02.api.letsencrypt.org-directory/mail.domain.com/mail.domain.com.crt:/etc/letsencrypt/live/mail.domain.com/fullchain.pem
  - ${CADDY_DATA_DIR}/certificates/acme-v02.api.letsencrypt.org-directory/mail.domain.com/mail.domain.com.key:/etc/letsencrypt/live/mail.domain.com/privkey.pem

EC certificates fail in the TLS handshake:

CONNECTED(00000003)
140342221178112:error:14094410:SSL routines:ssl3_read_bytes:sslv3 alert handshake failure:ssl/record/rec_layer_s3.c:1543:SSL alert number 40
no peer certificate available
No client certificate CA names sent

Traefik

Traefik is an open-source Edge Router which handles ACME protocol using lego.

Traefik can request certificates for domains through the ACME protocol (see Traefik's documentation about its ACME negotiation & storage mechanism). Traefik's router will take care of renewals, challenge negotiations, etc.

Traefik v2

(For Traefik v1 see next section)

Traefik's V2 storage format is natively supported if the acme.json store is mounted into the container at /etc/letsencrypt/acme.json. The file is also monitored for changes and will trigger a reload of the mail services. Lookup of the certificate domain happens in the following order:

1. $SSL_DOMAIN
2. $HOSTNAME
3. $DOMAINNAME

This allows for support of wild card certificates: SSL_DOMAIN=*.example.com. Here is an example setup for docker-compose:

version: '3.8'
services:
  mail:
    image: tvial/docker-mailserver:stable
    hostname: mail
    domainname: example.com
    volumes:
    - /etc/ssl/acme-v2.json:/etc/letsencrypt/acme.json:ro
    environment:
      SSL_TYPE: letsencrypt
      # SSL_DOMAIN: "*.example.com" 
  traefik:
    image: traefik:v2.2
    restart: always
    ports:
    - "80:80"
    - "443:443"
    command:
    - --providers.docker
    - --entrypoints.web.address=:80
    - --entrypoints.web.http.redirections.entryPoint.to=websecure
    - --entrypoints.web.http.redirections.entryPoint.scheme=https
    - --entrypoints.websecure.address=:443
    - --entrypoints.websecure.http.middlewares=hsts@docker
    - --entrypoints.websecure.http.tls.certResolver=le
    - --certificatesresolvers.le.acme.email=admin@example.net
    - --certificatesresolvers.le.acme.storage=/acme.json
    - --certificatesresolvers.le.acme.httpchallenge.entrypoint=web
    volumes:
    - /var/run/docker.sock:/var/run/docker.sock:ro
    - /etc/ssl/acme-v2.json:/acme.json

  whoami:
    image: containous/whoami
    labels:
    - "traefik.http.routers.whoami.rule=Host(`mail.example.com`)"

This setup only comes with one caveat: The domain has to be configured on another service for traefik to actually request it from lets-encrypt (whoami in this case).

Traefik v1

If you are using Traefik v1, you might want to push your Traefik-managed certificates to the mailserver container, in order to reuse them. Not an easy task, but fortunately, youtous/mailserver-traefik is a certificate renewal service for docker-mailserver.

Depending of your Traefik configuration, certificates may be stored using a file or a KV Store (consul, etcd...) Either way, certificates will be renewed by Traefik, then automatically pushed to the mailserver thanks to the cert-renewer service. Finally, dovecot and postfix will be restarted.

Self-Signed Certificates (Testing Only)

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

docker run -it --rm -v "$(pwd)"/config/ssl:/tmp/docker-mailserver/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 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](http://www.mad-hacking.net/documentation/linux/applications/mail/using-ssl-tls-postfix-courier.xml).

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.

!!! note You may have to restart your mailserver once the certificates change.

Testing a Certificate is Valid

• 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)

In addition, to verify certificate dates:

docker exec mail openssl s_client \
  -connect 0.0.0.0:25 \
  -starttls smtp \
  -CApath /etc/ssl/certs/ \
  2>/dev/null | openssl x509 -noout -dates

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/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.

Importing Certificates Obtained via Another Source

If you have another source for SSL/TLS certificates you can import them into the server via an external script. The external script can be found here: external certificate import script.

The steps to follow are these:

1. Transport the new certificates to ./config/ssl (/tmp/ssl in the container)
2. You should provide fullchain.key and privkey.pem
3. Place the script in ./config/ (or /tmp/docker-mailserver/ inside the container)
4. Make the script executable (chmod +x tomav-renew-certs.sh)
5. Run the script: docker exec mail /tmp/docker-mailserver/tomav-renew-certs.sh

If an error occurs the script will inform you. If not you will see both postfix and dovecot restart.

After the certificates have been loaded you can check the certificate:

openssl s_client \
  -servername mail.mydomain.net \
  -connect 192.168.0.72:465 \
  2>/dev/null | openssl x509

# or

openssl s_client \
  -servername mail.mydomain.net \
  -connect mail.mydomain.net:465 \
  2>/dev/null | openssl x509

Or you can check how long the new certificate is valid with commands like:

export SITE_URL="mail.mydomain.net"
export SITE_IP_URL="192.168.0.72" # can also be `mail.mydomain.net`
export SITE_SSL_PORT="993" # imap port dovecot

##works: check if certificate will expire in two weeks 
#2 weeks is 1209600 seconds
#3 weeks is 1814400
#12 weeks is 7257600
#15 weeks is 9072000

certcheck_2weeks=`openssl s_client -connect ${SITE_IP_URL}:${SITE_SSL_PORT} \
  -servername ${SITE_URL} 2> /dev/null | openssl x509 -noout -checkend 1209600`

####################################
#notes: output can be
#Certificate will not expire
#Certificate will expire
####################

What does the script that imports the certificates do:

1. Check if there are new certs in the /tmp/ssl folder.
2. Check with the ssl cert fingerprint if they differ from the current certificates.
3. If so it will copy the certs to the right places.
4. And restart postfix and dovecot.

You can of course run the script by cron once a week or something. In that way you could automate cert renewal. If you do so it is probably wise to run an automated check on certificate expiry as well. Such a check could look something like this:

## code below will alert if certificate expires in less than two weeks
## please adjust varables! 
## make sure the mail -s command works! Test!

export SITE_URL="mail.mydomain.net"
export SITE_IP_URL="192.168.2.72" # can also be `mail.mydomain.net`
export SITE_SSL_PORT="993" # imap port dovecot
export ALERT_EMAIL_ADDR="bill@gates321boom.com"

certcheck_2weeks=`openssl s_client -connect ${SITE_IP_URL}:${SITE_SSL_PORT} \
  -servername ${SITE_URL} 2> /dev/null | openssl x509 -noout -checkend 1209600`

####################################
#notes: output can be
#Certificate will not expire
#Certificate will expire
####################

#echo "certcheck 2 weeks gives $certcheck_2weeks"

##automated check you might run by cron or something
## does tls/ssl certificate expire within two weeks?

if [ "$certcheck_2weeks" = "Certificate will not expire" ]; then
  echo "all is well, certwatch 2 weeks says $certcheck_2weeks"
  else
    echo "Cert seems to be expiring pretty soon, within two weeks: $certcheck_2weeks"
    echo "we will send an alert email and log as well"
    logger Certwatch: cert $SITE_URL will expire in two weeks
    echo "Certwatch: cert $SITE_URL will expire in two weeks" | mail -s "cert $SITE_URL expires in two weeks " $ALERT_EMAIL_ADDR 
fi