Using an external reverse proxy

One of Mailu use cases is as part of a larger services platform, where maybe other Web services are available than Mailu Webmail and admin interface.

In such a configuration, one would usually run a frontend reverse proxy to serve all Web contents based on criteria like the requested hostname (virtual hosts) and/or the requested path. Mailu Web frontend is disabled in the default setup for security reasons, it is however expected that most users will enable it at some point. Also, due to Docker Compose configuration structure, it is impossible for us to make disabling the Web frontend completely available through a configuration variable. This guide was written to help users setup such an architecture.

There are basically three options, from the most to the least recommended one:

  • have Mailu Web frontend listen locally and use your own Web frontend on top of it
  • use Traefik in another container as central system-reverse-proxy
  • override Mailu Web frontend configuration
  • disable Mailu Web frontend completely and use your own

All options will require that you modify the docker-compose.yml file.

Have Mailu Web frontend listen locally

The simplest and safest option is to modify the port forwards for Mailu Web frontend and have your own frontend point there. For instance, in the front section of Mailu docker-compose.yml, use local ports 8080 and 8443 respectively for HTTP and HTTPS:

front:
  # build: nginx
  image: mailu/nginx:$VERSION
  restart: always
  env_file: .env
  ports:
    - "127.0.0.1:8080:80"
    - "127.0.0.1:8443:443"
    ...
  volumes:
    - "$ROOT/certs:/certs"

Then on your own frontend, point to these local ports. In practice, you only need to point to the HTTPS port (as the HTTP port simply redirects there). Here is an example Nginx configuration:

server {
  listen 443;
  server_name mymailhost.tld;

  # [...] here goes your standard configuration

  location / {
    proxy_pass https://localhost:8443;
  }
}

Because the admin interface is served as /admin and the Webmail as /webmail you may also want to use a single virtual host and serve other applications (still Nginx):

server {
  # [...] here goes your standard configuration

  location /webmail {
    proxy_pass https://localhost:8443/webmail;
  }

  location /admin {
    proxy_pass https://localhost:8443/admin;
  }

  location /main_app {
    proxy_pass https://some-host;
  }

  location /other_app {
    proxy_pass https://some-other-host;
  }

  location /local_app {
    root /path/to/your/files;
  }

  location / {
    return 301 $scheme://$host/main_app;
  }
}

Finally, you might want to serve the admin interface on a separate virtual host but not expose the admin container directly (have your own HTTPS virtual hosts on top of Mailu, one public for the Webmail and one internal for administration for instance).

Here is an example configuration :

server {
  listen <public_ip>:443;
  server_name yourpublicname.tld;
  # [...] here goes your standard configuration

  location /webmail {
    proxy_pass https://localhost:8443/webmail;
  }
}

server {
  listen <internal_ip>:443;
  server_name yourinternalname.tld;
  # [...] here goes your standard configuration

  location /admin {
    proxy_pass https://localhost:8443/admin;
  }

}

Depending on how you access the front server, you might want to add a proxy_redirect directive to your location blocks:

proxy_redirect https://localhost https://your-domain.com;

This will stop redirects (301 and 302) sent by the Webmail, nginx front and admin interface from sending you to localhost.

Traefik as reverse proxy

Traefik is a popular reverse-proxy aimed at containerized systems. As such, many may wish to integrate Mailu into a system which already uses Traefik as its sole ingress/reverse-proxy.

As the mailu/front container uses Nginx not only for HTTP forwarding, but also for the mail-protocols like SMTP, IMAP, etc, we need to keep this container around even when using another HTTP reverse-proxy. Furthermore, Traefik is neither able to forward non-HTTP, nor can it easily forward HTTPS-to-HTTPS. This, however, means 3 things:

  • mailu/front needs to listen internally on HTTP rather than HTTPS
  • mailu/front is not exposed to the outside world on HTTP
  • mailu/front still needs SSL certificates (here, we assume letsencrypt) for a well-behaved mail service

This makes the setup with Traefik a bit harder: Traefik saves its certificates in a proprietary JSON file, which is not readable by Nginx in the front-container. To solve this, your acme.json needs to be exposed to the host or a docker-volume. It will then be read by a script in another container, which will dump the certificates as PEM files, readable for Nginx. The front container will automatically reload Nginx whenever these certificates change.

To set this up, first set TLS_FLAVOR=mail in your .env. This tells mailu/front not to try to request certificates using letsencrypt, but to read provided certificates, and use them only for mail-protocols, not for HTTP. Next, in your docker-compose.yml, comment out the port lines of the front section for port …:80 and …:440. Add the respective Traefik labels for your domain/configuration, like

labels:
  - "traefik.enable=true"
  - "traefik.port=80"
  - "traefik.frontend.rule=Host:$TRAEFIK_DOMAIN"

Note

Please don’t forget to add TRAEFIK_DOMAIN=[...] TO YOUR .env

If your Traefik is configured to automatically request certificates from letsencrypt, then you’ll have a certificate for mail.your.doma.in now. However, mail.your.doma.in might only be the location where you want the Mailu web-interfaces to live — your mail should be sent/received from your.doma.in, and this is the DOMAIN in your .env? To support that use-case, Traefik can request SANs for your domain. Lets add something like

[acme]
  [[acme.domains]]
    main = "your.doma.in" # this is the same as $TRAEFIK_DOMAIN!
    sans = ["mail.your.doma.in", "webmail.your.doma.in", "smtp.your.doma.in"]

to your traefik.toml. You might need to clear your acme.json, if a certificate for one of these domains already exists.

You will need some solution which dumps the certificates in acme.json, so you can include them in the mailu/front container. One such example is mailu/traefik-certdumper, which has been adapted for use in Mailu. You can add it to your docker-compose.yml like:

certdumper:
  restart: always
  image: mailu/traefik-certdumper:$VERSION
  environment:
  # Make sure this is the same as the main=-domain in traefik.toml
  # !!! Also don’t forget to add "TRAEFIK_DOMAIN=[...]" to your .env!
    - DOMAIN=$TRAEFIK_DOMAIN
  volumes:
    - "/data/traefik:/traefik"
    - "$ROOT/certs:/output"

Assuming you have volume-mounted your acme.json put to /data/traefik on your host. The dumper will then write out /data/traefik/ssl/your.doma.in.crt and /data/traefik/ssl/your.doma.in.key whenever acme.json is updated. Yay! Now let’s mount this to our front container like:

volumes:
  - "$ROOT/overrides/nginx:/overrides"
  - /data/traefik/ssl/$TRAEFIK_DOMAIN.crt:/certs/cert.pem
  - /data/traefik/ssl/$TRAEFIK_DOMAIN.key:/certs/key.pem

Override Mailu configuration

If you do not have the resources for running a separate reverse proxy, you could override Mailu reverse proxy configuration by using a Docker volume. Simply store your configuration file (Nginx format), in /mailu/nginx.conf for instance.

Then modify your docker-compose.yml file and change the front section to add a mount:

front:
  build: nginx
  image: mailu/nginx:$VERSION
  restart: always
  env_file: .env
  ports:
    [...]
  volumes:
    - "$ROOT/certs:/certs"
    - "$ROOT/nginx.conf:/etc/nginx/nginx.conf"

You can also download the example configuration files:

Disable completely Mailu reverse proxy

You can simply disable Mailu reverse proxy by removing the front section from the docker-compose.yml and use your own means to reverse proxy requests to the proper containers.

Be careful with this method as resolving container addresses outside the Docker Compose structure is a tricky task: there is no guarantee that addresses will remain after a restart and you are almost certain that addresses will change after every upgrade (and whenever containers are recreated).